DRAFT
Technical standard for stand-alone PV systems using inverters
Date: Friday, 20 June 2003
Authors:
J. Muñoz, E. Lorenzo.
Organisation:
Instituto de Energía Solar.
Universidad Politécnica de Madrid.
Address:
ETSI Telecomunicación.
Ciudad Universitaria.
E-28040 Madrid.
Telephone: (34-91) 544 10 60
Facsimile: (34-91) 544 63 41
E-mail: info@ies-def.upm.es
Web: www.ies.upm.es
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INDEX
1. Introduction
4
2. Specification for inverters and performance
6
2.1 AC Loads
6
2.2 Inverters: principles of operation
10
2.3 Reliability
2.3.1 DC side
2.3.2 AC side
2.3.2.1 Power capacity
2.3.2.2 Regulation
2.3.2.3 Harmonic distortion
2.3.2.4 Protections
12
12
14
14
15
16
17
2.4 Safety
2.4.1 Protection against direct contact
2.4.2 Protection against indirect contact
2.4.2.1 AC circuit
2.4.2.2 DC circuit
2.4.3 Additional protection against both direct and
indirect contact in DC side
18
19
20
21
30
2.5 Energy and power conditioning performance
2.5.1 Power efficiency
2.5.2 Energy efficiency and sizing
33
33
35
2.6 User friendliness
37
2.7 Installation and maintenance
38
2.8 References
39
3. Inverter standard
40
31
3.1 Scope
40
3.2 Inverter and AC load requirements
3.2.1 Performance
3.2.2 Protections
3.2.3 Installation
3.2.3.1 General requirements
3.2.3.2 Earthing sytems
3.2.3.3 Floating systems
3.2.3.4 DC circuit
40
40
42
42
42
43
43
44
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1.
INTRODUCTION
DC/AC converters, or inverters, are being increasingly employed in stand-alone
PV systems for rural electrification purposes. Their use allows the users to access the
commodities offered by the standard AC consumer market. When added to small Solar
Home Systems (< 200W), by far the today most extended PV application, inverters
allow for the use of a variety of small domestic appliances: colour television, video
tapes, personal computers, water pumps, mixers, drills, etc. When added to larger PV
systems, often employed for health centres, schools or high income houses, inverters
also allow for relatively powerful appliances: refrigerators, irons, tools for handicrafts,
etc.
But, together with this undeniable advantage, the use of inverters also
encompasses some complexities mainly associate to three different aspects. First,
inverters provide AC voltages (125 or 220 V) over 50 V, which represent a potential
danger for persons, so that PV systems including inverters must also provide protection
means against electric shock. Second, inverters are relatively expensive equipment
whose failures dramatically affect the long run operational costs of the whole PV
system, and also the PV user’s satisfaction, so that reliability must be ensured. And,
third, standard AC appliances are designed to operate on the particular conditions of the
conventional grid (well regulated voltage and frequency, low voltage harmonic
distortion, large surge power capability, etc.), which are sometimes not maintained by
inverters. That represents a risk of improper functioning, even damage, for the
appliances, which should be taken into consideration when designing the corresponding
PV system.
This report results from work, which has been funded by the European
Commission under XXXX contracts[1] and presents a standard for stand-alone PV
systems using inverters. This proposal standard is intended to provide a quality
reference for procurement specification issued by National Governments, donors and
investors. In addition, it is intended to be useful as a design guideline for manufacturers
and installers.
This proposed standard should be understood as a follow-up of the “Universal
Technical Standard for Solar Home Systems”[2] (from now UTSfSHS), previously
developed under Thermie-B contract. This previous standard applies exclusively to
entirely DC SHS, but it fully keeps its validity for all the aspects related with the DC
part of the here considered PV systems, i.e. PV generator, support structure, battery,
charge regulator and DC loads. Because of that, the present document is restricted to
only deal with the AC part, i.e., inverter and AC load. On the same line of the previous
standard, the present document contains a discussion of inverter performance (section 2)
at the lights of the following criteria: reliability, safety, energy performance, user
friendliness, and installation and maintenance. This discussion provides the basis for
deriving the technical requirements, which are classified into three categories:
Compulsory, Recommended and Suggested, in order to allow the here proposed
standard to be adapted to the particular conditions of each country:
Compulsory requirements (C) are those which could directly affect safety or
reliability. Failure to meet these requirements could lead to personal injuries or to
inverter failure, and they are therefore intended to constitute a minimum core of
requirements that must be fulfilled.
-4-
Recommended requirements (R) are those which would normally lead to
optimisation. Most of these requirements are universally applicable, and failure to meet
them would normally lead to a cost increase. However, because economic
considerations can depend on local conditions, the application of these requirements
must be reviewed for each particular case.
Suggested requirements (S) are those which might be expected to produce a
sound installation. However, it should be noted that any judgement of soundness is
essentially subjective, so the suggested requirements given here may have been
influenced by the personal experience of the authors, and their applicability should also
be reviewed for each particular case.
(Note: The symbols C, R and S are used in this document to specify the
compulsory, recommended and suggested character of each recommendation, according
to the above classification.)
This report has been divided into two main sections:
Section 2 contains a discussion of inverter performance, which provides the
basis for deriving and classifying the requirements. In many cases, several
specifications can result from different criteria, e.g., almost all the safety requirements
concern the installation. However, we will only deal with them in the first discussion in
which they appear.
Section 3 presents the requirements in a formal way, which is suitable for use in
contract documents for PV rural electrification programmes.
Inverters are been used in PV rural electrification for about 20 years ago. Former
inverters performed badly in terms of, both, reliability and energy efficiency, which has
prevented their diffusion during long time. However, inverter technology has
consistently evolved from these early days, and good products are commercially
available from the last 5 years. This technology improvement has removed the matter of
solar inverters from the realm of research and demonstration projects to the much larger
of widely extended use. In this sense, inverters can be considered as relatively recent PV
rural electrification products. Not surprising, available literature is sparing in notices
about inverter field experience. Attempting to compensate this lack of systematic
feedback from the field, we have performed an extensive laboratory testing campaign
with a representative sample of 12 different inverters bought in the current market, and
we have reviewed similar activities being performed by other laboratories[3] , [4] .
Moreover, for the analysis of the safety of persons we largely have relied on the
international standard IEC 60364[5] .
No doubt, systematic quality assurance procedures should comprise not only the
definition of technical standards, but also the definition of procedures for testing
prototypes or samples, in order to analyse their adherence to the specifications of the
selected standard. The present report is restricted to the definition of a standard for
inverters. However, it is important to mention that the possibility of adding test
procedures to verify the proposed technical requirements has been present in the
author’s mind. Such test procedures are the object of a different report[11] and have been
developed under consideration that they could be applied in common electricity
laboratories within the reach of the local organisations, which carry out electrification
programmes.
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2.
SPECIFICATION FOR INVERTERS AND PERFORMANCE
Inverters convert power from DC to AC. Stand-alone PV systems can
incorporate small inverters just to power a particular appliance or larger inverters to
power a full AC line, so that a large variety of different situations can be imagined.
However, cost and reliability reasons have lead to the current predominance of single
phase, fixed frequency inverters associated to low or medium power (>50 W, <5000 W)
individual PV systems. For our present purposes, that can be well characterised by open
circuit voltages, at standard test conditions, below 120 V DC, and AC distribution lines
shorter than 400 m. The first imply that DC voltages never represent a potential danger
for persons, while the second means that AC lines can be effectively isolated from the
earth, because leakage parasitic capacitive currents are negligible. Both features
contribute to facilitate the provision of protective means on the AC side, and define the
framework for all the cases considered in this standard. Definition of specifications for
other cases, like variable frequency inverters for PV pumps, or central inverters
powering local networks for villages, is not here considered and remains for future
work.
2.1
AC LOADS
Obviously, inverter requirements are closely related with the electrical
characteristics of the load to be powered. For example, an inverter powering an AC
motor must be able to deliver the inrush current and to operate with low power factors.
Because of that, AC loads must be properly characterised prior to any inverter
consideration.
When powered by an ideal AC voltage source, i.e. pure sinusoidal voltage and
null internal impedance, AC loads can be characterised in accordance with their power
consumption and current drain, by using the following electrical parameters:
The rated power, defined as the power consumed by the load in steady state
operation.
The surge power is the maximum power consumed by the load during starting.
The starting power required by some loads can reach up to 5 or 6 times the
corresponding rated power. As an example, the Figure 1 shows the power demanded by
an electric drill during starting when is supplied by the utility grid.
The power factor (PF) is defined as the ratio of the active (or real) power
consumed by the load to the apparent power (product of rms (root-mean-square)
voltage and rms current). The power factor of real loads is a combination of two
components. On one side, when the load current is sinusoidal (linear load), the power
factor is equal to the cosines of the displacement angle between voltage and current
waveforms (a pure resistive load has a PF=1, and an ideal inductor or capacitor have a
PF=0). On the other side, when the load is non-linear the current waveform is distorted
and the power factor is also lower than 1. The Figure 2 shows an example of the current
drain of two fluorescent lamps with different ballasts (electromagnetic and electronic),
each one shows the predominance of one power factor component. For the same real
power, the lower the power factor the bigger the required current and, therefore, the
greater the difficulty for the inverter. Motors, non-linear loads, etc. can have power
factors below 0.5. But, fortunately they can be corrected by means of adequate
capacitors and filters. Hence:
-6-
• Reactive and non-linear loads, especially fluorescent lamps,
should have power factors > 0.9. (R)
2000
Surge power
(1800 W)
Power [W]
1500
1000
Rated power
(225 W)
500
0
0
1
2
Time [s]
3
4
Figure 1. Power consumption during the starting of an electric
drill supplied by the grid.
The Total Harmonic Distortion or THD (see definition in annex 1) gives the
harmonics content of the current drained by non-linear loads, such as switch-mode
power supplies (TV, computer, etc.), electronic ballast for fluorescent lighting, variable
speed motors, half-wave rectifiers (e.g. hair dryer), etc, which draw current only during
a controlled time of the voltage period. For example, the distortion for current
waveform of Figure 2b reaches 140%.
0.2
200
0.2
0
0.0
0
0.0
Voltage [V]
-200
-400
0.00
-0.2
Voltage
0.01
Current
0.02 0.03
Time [s]
-0.4
0.04
0.4
PF=0.54
-200
-400
0.00
(a)
-0.2
Current
Voltage
0.01
0.02 0.03
Time [s]
Current [A]
200
Voltage [V]
400
PF=0.42
Current [A]
0.4
400
-0.4
0.04
(b)
Figure 2. Voltage and current waveforms of two fluorescent lamps
whose power factors are lower than 1. (a) Electromagnetic ballast
(displaced current). (b) Electronic ballast (distorted current).
This load characterisation approach presents the advantage of being independent
of the voltage source, and is very useful when the conventional AC grid is concerned,
because this grid is (or must be) close an ideal voltage source. However, the same is not
true for the here considered inverters, because their internal impedance use to be
significant, when compared with the impedance of the load, and also because the
voltage waveform is not necessarily sinusoidal. As a matter of fact, square and other
waveforms are also present in the current market and are acceptable in many practical
-7-
situations. It follows that the above defined electrical parameters can depend not only on
the characteristics of the load, but also on the characteristics of the inverter. For
example, because its internal impedance, an inverter powering an AC motor can limit
the inrush current well below the peak value demanded by the motor when it is powered
by the grid, and still be able of properly starting the motor; hence, such an inverter
should be accepted despite its inability to provide the characteristic peak power
associated to the grid operation. The Figure 3 shows an example of this. The graphic
represents the power consumption of the same electric drill of Figure 1 when is supplied
by a tested inverter. On the same lines, non-sinusoidal waveforms can be reach in odd
harmonics, even with pure resistive loads, but corresponding inverters should be
accepted providing that such harmonics do not hamper the proper operation of the
concerned loads.
1800
Surge power (grid)
Power [W]
1500
1200
Surge power (inverter)
900
Rated power (225W)
600
300
0
1
2
3
4
5
Time [s]
Figure 3. Comparison between the power consumption during the
starting of the electric drill supplied by the grid and by a tested
inverter.
An alternative way of load specification, more appropriate for our present
purposes, consists on the precise definition of all the particular elements composing the
load. For example, a XXX inch, XXX W television (or, even, the television model XXX
from XXX, or similar); a standard XXX W drill; etc. Then, the inverter specifications
can focus on simply assure the proper operation of all these elements, disregarding more
detailed parameters, like harmonic distortion. Moreover, some information describing
the expected patterns of the load’s use is also required, in order to determine, both, the
simultaneously allowed load elements, and the energy requirements. The first is needed
for inverter specification, while the second should be know for PV system sizing
purposes. The Table 1 can help on that load specification way. From this information is
possible to derive a characteristic day, composed by several periods of equal expected
AC loads. For practical reasons, no more than 3 or 4 periods are recommended.
Appliance
Rated power
Hours of daily operation
Table 1. AC load specification.
-8-
Timetable or incompatibilities
As an example
Let us imagine we must select an inverter for supply the AC load defined in
Table 2. The sum of rated power of all elements is 300W, while the minimum rated
power of the inverter must be equal to 200W. The last option implies the user must be
aware of the need of load management and does not use the electric drill when other
load is being operated. Besides, we have verified the inverter is able to start up the drill
(Figure 3) and we have chosen an electric drill with a corrected power factor (close to 1)
in order to avoid selecting a bigger inverter. The DC energy required for system sizing
is composed by the load consumption (500 Wh·day-1) plus inverter energy losses, whose
calculation will be discussed in section 2.5.2.
Appliance Rated power [W] Hours of daily operation Timetable [Hour] or incompatibilities
TV
40
3
20,21,22
Lighting
60
3
21,22,23
Electric Drill
200
1
Anytime, excepting from 20 to 23 hours
Table 2. Example of AC load definition.
To help on the explanation of the work behind this proposed standard, Table 3
presents the AC loads we have used in the above mentioned inverter testing campaign.
Load type
Manufacturer’s
specification
Measured
(220-230 V AC sine waveform)
Rated Power
PF
Rated Power
PF
- Incandescent lamps
25-150 W
NA
25-150 W
1
- Fluorescent lamps (electromagnetic ballasts)
10-20 W
NA
30-80 VA
0.33-0.42
NA
NA
600 VA
0.15
1-5 kW
NA
1-5 kW
1
34 W
NA
60 VA
0.6
- Video
NA
NA
34 VA
0.46
- Computer (CPU + display)
NA
NA
190 VA
0.63
11-32 W
NA
20-50 VA
0.43-0.57
- Hair-dryer (half-wave operation)
600 W
NA
830 VA
0.7
- Stereo system (CD and radio)
16 W
NA
8 VA
0.82
60 W
NA
97 VA
Linear loads
- Induction coil
- Resistors (water heaters)
Non-linear loads
- TV
- Fluorescent lamps (electronic ballasts)
Motors
- Electric fan
- Electric drill
550 W
NA
- Water pump
858 VA
0.23
- Grain mill
2 kVA
0.98
- Fridge
250 W
NA
230 VA
940 VA
973 VA
Unloaded.
Table 3. Electrical characteristics of AC loads used in the inverter
testing campaign.
-9-
(*)
390 VA
Notes: NA=Not available from manufacturer’s specifications.
(*)
(*)
0.73
0.98
0.34
0.77
0.74
2.2
INVERTERS: PRINCIPLES OF OPERATION
Inverters convert DC to AC by using electronic switches that periodically
reverse the polarity of the electricity supplied to the load. Single-phase inverters are
usually based on the use of a H-bridge switch topology. This circuit is composed by
four controlled switches arranged as Figure 4a shows, including the battery and the AC
load. Operated in this simplest manner, the bridge will generate a square wave by
closing S1 and S4 while opening S2 and S3 for one half cycle, and then, closing S2 and S3
and opening S1 and S4 for the second half cycle. A square waveform is this way
generated (Figure 4b). Then, a transformer can be added if a different voltage is
required at the load.
(a)
(b)
Figure 4. (a) H-bridge switch arrangement. (b) Switching pattern
that generates a square wave.
Square voltages waveforms are rich in odd harmonics of low order, which may
not be suitable for supplying some AC appliances. Because they are of low frequency
the filtering of these harmonics involves large and costly filters. An additional
disadvantage of the previous topology is the load voltage depends on the battery one,
which varies with the state-of-charge, discharged current and temperature, so that
additional regulator features can be required. Both disadvantages may be overcome by
operating the switches of the bridge at a higher frequency to generate waveforms with
several pulses per half-cycle. Pulse Width Modulated (PWM) inverters generally drive
the switches at a constant switching frequency and the ratio of time for which the switch
is ON to the period is varied to control the load voltage. PWM techniques are used for
both DC/DC conversion to transform DC power between one unregulated voltage to
another controlled and also in DC/AC conversion. As the number of pulses increases,
harmonics content is displaced to high frequency components well separated from the
fundamental, facilitating their removal by a suitable filter.
Commercial inverters result from different combinations of bridge converters,
transformers and filters. In principle, the more sinusoidal and the more regulated the
output waveform the more complex and the more expensive the inverter.
Tested inverters can be classified according with different criteria. Using as
criterion the output voltage waveform, three kinds of inverters can be identified: square
-10-
wave, modified square wave (also called modified sine-wave), and sine wave inverters
(see Figure 5).
0.00
0.02
Time [s]
(a)
0.04
400
200
0
-200
-400
400
200
0
-200
-400
0.00
Voltage [V]
Voltage [V]
Voltage [V]
400
200
0
-200
-400
0.02
Time [s]
0.04
0.00
(b)
0.02
Time [s]
0.04
(c)
Figure 5. (a) Sine wave inverter. (b) Modified square or modified
sine wave inverter. (c) Square-wave inverter.
According with the technology used to voltage elevation, two types of inverters
have been tested:
-
Transformer-based inverters. These inverters use a low frequency transformer of
iron, heavy and large (see Figure 6a). Advantages are DC/AC isolation, good
surge ability and good efficiency.
-
High frequency inverters. These inverters elevate the low voltage of batteries
using a DC/DC converter (see Figure 6b). Such converters use switches
commutated at high frequency. Because of the high frequency (typically >20
kHz) they require a ferrite transformer, which is light and small. Disadvantages
are poor surge ability and poor efficiency with reactive loads.
(a)
(b)
Figure 6. Types of tested inverter topologies. (a) Transformerbased inverters. (b) High-frequency inverters.
Table 4 shows the characteristics of inverters tested in the laboratory campaign
performed to develop the present standard.
-11-
Inverter
Output voltage
waveform
Voltage elevation
topology
DC voltage Rated power
[V]
[VA]
1
Modified square
High-frequency
12
140
2
Square
Transformer-based
24
150
3
Sine wave
“
12
150
4
Sine wave
“
12
200
5
Modified square
“
12
200
6
Modified square
“
12
250
7
Modified square
“
12
450
8
Modified square
“
12
500
9
Modified square
High-frequency
12
500
10
Sine wave
Transformer-based
24
900
11
Sine wave
“
24
1200
12
Sine wave
“
48
5000
Table 4. Inverters tested in the laboratory testing campaign.
2.3
RELIABILITY
This section focuses on the capacity of the inverter to properly operate the loads,
and to avoid failures in case of fault conditions.
2.3.1 DC side
Stand alone PV systems incorporate charge regulators to protect the battery
against, both, overcharging and deep discharging. In order to preserve the last:
• The inverter should be preferably connected downwards of the
charge regulator output. (R)
Then, the range of DC voltages at the input of the inverter depends on the
protection algorithm of the charge regulator, and also on the voltage losses on the
charge regulator and wiring. The inverter must be capable of supply AC in all such
voltage range. For that, the following requirement is here proposed:
• Inverters connected to the charge regulator must ensure safe
starting and operation of all the specified load in the voltage
range permitted by the charge regulator. (C)
This requirement is conservative enough to cope with the rules given at the
UTSfSHS for charge regulator and wiring. Inverters generally have their own undervoltage and over-voltage protections. For that, they automatically turn out when the
input voltage falls out of a certain range defined by two thresholds. It is worth to
mention that the lower thresholds of all the tested inverters have been found between
9.6 to 10.5 V, while the upper thresholds have been found between 15 to 16.3 V. In
other words, all of them can easily cope with the above proposed requirement.
Obviously, the connection of the inverter at the charge regulator output implies
that the last must be able to support all the current required by the first. It should be
-12-
recognised that this is not always possible in practice. For example, when adding AC
facilities to an already existing PV system, the current demand of the new inverter can
sometimes surpass the capacities of the original charge regulator. Hence, this possibility
of directly connect the inverter to the battery should also be envisaged, but still
protecting the battery. Then:
• Direct connection of the inverter to the battery can be permitted.
Then, the inverter must protect the battery against deep
discharging according with the same rules established for the
charge regulator. (C)
It is important to insist on that the above mentioned lower thresholds of the
under-voltage internal protection of the tested inverters are too low to cope with this
requirement, and need of severe corrections before to be directly connected to a battery.
See reference [12] for a more detailed discussion on the load-disconnect voltages
required for SHS batteries.
Potentially dangerous for the inverter is the sudden cut off of the DC supply.
That can happen in normal operation, when the charge regulator disconnect the load to
protect the battery, or due to unusual service conditions, like accidental disconnection of
the battery during maintenance or when a fuse blows. Then:
• The inverter, and also the load, must be protected against sudden
cut off of the DC supply, in all possible conditions of the
specified load. (C)
Other unusual service conditions, as derived from a defective charge regulator,
can originate DC input voltages outside the specified range. So:
• The inverter must resist without damage any input voltage in the
range from -25% to 33% of the nominal input voltage (9 to 16 V
for 12V inverters). (R)
On the other hand, voltage drops on DC wiring of the inverter should be limited,
because they can negatively affect its efficiency and operation. Hence:
• The section of DC cables must cause less than 1% of voltage
losses between the inverter input and its DC connection point.
This applies at the maximum current condition. (C)
• The inverter DC terminals must allow for a secure and
mechanically strong electrical connection. They must have low
electrical resistance, leading to voltage losses less than 0.5% of
nominal voltage. This applies for each individual terminal at the
maximum current condition. (C)
The DC current demanded by one phase inverters use to have a ripple
component, which can be significant, specially when the inverter operates near its rated
power. As an example, the Figure 7 shows the DC current of a tested sine wave inverter
supplying a resistive load. As can be seen, this waveform resembles a sine wave whose
frequency is 100 Hz (twice the output one). The mean current of this waveform is 90A,
while the rms one reaches 120A. The latter must be used to size the wiring and the
protective fuses. Besides, this current ripple induces a ripple in DC voltage which can
interfere the proper operation of the inverter itself and other components connected to
the batteries, such as the charge regulator. Then:
-13-
• Notwithstanding the above requirements, the maximum voltage
drop between battery and inverter must not interfere the correct
operation of any balance-of-system component and the inverter
itself. (C)
250
50
40
Current
Voltage
150
30
100
20
50
10
0
0
0.00
0.01
0.02
0.03
DC voltage [V]
DC current [A]
200
0.04
Time [s]
Figure 7. DC current and voltage waveforms at the input terminals
of inverter number 12 supplying a resistive load at rated power.
2.3.2 AC side
2.3.2.1 Power capacity
Inverter and loads are generally characterised by their respective rated powers.
Obviously, the PV system design should ensure that:
• The rated power of the inverter should be equal or greater than
the sum of the rated power of all the simultaneously allowed
individual loads. (C)
However, this simple condition do not suffices to ensure proper operation.
Starting load requirements must also be matched by surge power inverter capabilities.
That can always no be anticipated from the manufacturer’s information because, as
mentioned in 2.1, load starting way can be affected by the inverter itself, so that the
compatibility must be ensured for each inverter-specified load pair. It is reasonable to
assume that all starting loads will not be started simultaneously, therefore:
• The inverter must ensure safe starting of whichever individual
load, in any normal operating condition, i.e., with the rest of the
simultaneously allowed individual loads maintained on steady
state operation. (C)
In some practical situations, AC loads cannot be precisely anticipated. Think, for
example, on the provision of standard PV systems to a group of schools whose AC
equipment is from diverse origin. In such cases, it can be of interest to specify the surge
inverter capabilities by simply referring to the inverter itself, and disregarding the load.
For example, the GEF/World Bank-Assisted China PV project[13] suggests that:
-14-
• The inverter must operate safely for at least one minute at 125%
of rated power. Moreover, it should provide 150% of rated power
for at least two seconds to facilitate starting of motors and other
high capacitance loads. (S)
2.3.2.2 Regulation
AC loads should generally be operated at fixed voltage and frequency. Several
problems can rise from both, over voltage and under voltage: load damage, not-ignition,
poor luminous flux of lamps, etc. Besides, frequency variations can affect equipment
using frequency as reference for their operation, such as internal clocks or timers.
Inverter regulation means its capacity to maintain the AC voltage and frequency
output close to the nominal values, face fluctuations of the DC input voltage and load
power demand. As a representative example, Figure 8 shows the AC output voltage
versus the DC input voltage for three different tested inverters operating at a fixed
resistive load. Clearly, two of them behave poorly, which justify the following
requirements:
• Fluctuations of the rms value of the AC output voltage must be
less than 10% of the nominal value in any steady state condition
of the specified load, and for all the allowed DC input voltage
range. (C)
• Fluctuations of the rms value of the AC output voltage must be
less than 5% of the nominal value in any steady state condition
of the specified load, and for all the allowed DC input voltage
range. (R)
290
Inverter 7
AC voltage [V]
270
Inverter 8
Inverter 4
250
230
210
190
170
10
11
12
13
14
15
16
DC voltage [V]
Figure 8. AC output voltage versus the DC input voltage for three
different tested inverters operating at a fixed resistive load.
Similarly, for frequency regulation:
• Fluctuations of the frequency of the AC output voltage must be
less than 2% of the nominal value in any steady state condition
of the specified load, and for all the allowed DC input voltage
range. (C)
-15-
• Fluctuations of the frequency of the AC output voltage must be
less than 1% of the nominal value in any steady state condition
of the specified load, and for all the allowed DC input voltage
range. (R)
It is worth to mention that this frequency limits are been chosen from the
conventional utility standards and all the tested inverters comply them comfortably.
2.3.2.3 Harmonic distortion
In principle, voltage harmonics can be to the detriment of the correct operation
of AC loads. Potential impacts are: overheating, premature failure, energy losses,
interference, disturbance, etc. Even when powered by a pure sinusoidal voltage source,
non-linear loads, such as switch-mode power supplies (TV, computer, etc.), electronic
ballast for fluorescent lamps, etc. draw current only during a controlled period of the
power voltage waveform, as Figure 9 shows. That produces current harmonics that, in
turns, can originate voltage harmonics due to the voltage drops in the upstream
impedance of the power source. This is a way current harmonics drained by loads are
generally undesirable and tend to be limited in relevant international standards[14] , [15] .
Internal impedance of inverters is usually larger than the corresponding to the
conventional grid, so that voltage harmonics content tend to increase with inverter
operation, even when they are of sine wave type. As an example, the Figure 10a shows
the voltage and current waveforms of a sine wave tested inverter supplying a non-linear
load. The voltage distortion reaches 11%. The same inverter supplying resistive loads
has a total harmonic distortion lower than 2% (see Figure 10b). And, obviously, the
situation becomes worsened when the inverter is not of sinusoidal type (Figure 11).
1.6
2
Current [A]
Current [A]
THDI=111%
0
-4
0.00
0.8
0.0
-0.8
-2
0.01
0.02
Time [s]
(a)
0.03
-1.6
0.00
2
THDI=124%
Current [A]
4
0.01 0.02
Time [s]
0.03
1
THDI=44%
0
-1
-2
0.00
0.01 0.02
Time [s]
(b)
0.03
(c)
Figure 9. Current drained by several non-linear loads supplied by
the utility grid. (a) Electronic ballasts. (b) TV. (c) Hair-dryer.
The limitation of harmonics is still an open question even when the grid is
concerned. Because of that, any attempts of quantify the maximum allowable harmonics
content would here lack of practical sense. Instead of that, we propose to simply ensure
the proper operation of the specified load, and to avoid radio frequency interference,
always keeping a prime for the lower harmonic content of the sine wave inverters.
Then:
-16-
• Despite voltage distortion, the inverter must ensure the proper
operation of all the specified load. (C)
• Despite voltage distortion, the inverter must no produce radio
frequency interference in any operation condition. (C)
• Sine wave inverters should be preferred over non-sine wave
ones. Then, the total voltage harmonic distortion with linear
loads must not exceed 5%. (R)
400
200
1
200
1.5
0
0
0
0.0
Current
-200
-400
0.00
-1
0.01
0.02
0.03
Voltage [V]
2
Voltage
Current [A]
Voltage [V]
400
THDV=1.7%, THDI=2.1%
Current
-200
-400
0.00
-2
0.04
Voltage
3.0
-1.5
0.01
0.02
0.03
Current [A]
THDV=11%, THDI=52%
-3.0
0.04
Time [s]
Time [s]
(b)
(a)
Figure 10. (a) Voltage and current waveforms of tested sine wave
inverter supplying a non-linear load (lamps with electronic
ballasts). (b) The same inverter supplying a linear load
(incandescent lamps).
THDV=27%, THDI=50%
400
2.0
Current
200
1.0
0
0.0
-200
-400
0.00
Current [A]
Voltage [V]
Voltage
-1.0
0.01
0.02
0.03
-2.0
0.04
Time [s]
Figure 11. Voltage and current waveforms of a tested modified sine
wave inverter supplying a non-linear load (fluorescent lamps with
electronic ballast).
2.3.2.4 Protections
Despite the restrictions imposed to the specified load, unexpected service
conditions can occur, for example, because other than allowed loads or because wiring
faults. If not specific protection is provided, that can damage the inverter and also the
permitted loads. To avoid this:
-17-
• The inverter must be able to resist the sudden disconnection of
all the simultaneously allowed individual loads. (C)
• The inverter must be able to resist any possible output overload,
and also short circuit condition. (C)
• The inverter must be able to protect the load in any possible
output overload, and also short circuit condition. (C)
2.4
SAFETY
AC voltages over 50 V represent a potential danger for persons, so PV systems
including inverters must provide protection against electric shock. For this, we have
relied on the international standard IEC 60364[5] , which offers a wide variety of
protective schemes. It must point out the adoption of this standard depends on each
particular country, which implies that, in some countries even the conventional
electrical installations may be away from its safety requirements. Besides, if we take
into account the benefits of the electrical safety are not perceived immediately, we can
understand the difficulty applying measures of protection against electric shock.
Because of that, we have tried to explain this situation and to propose several easy
guidelines to improve the safety, even, without reaching the strict compliance of the
IEC 60364. The IEC 60364 distinguishes the protection against two types of contact:
direct contact, defined as the contact with live parts during normal service, and indirect
contact, defined as the contact with exposed conductive parts, e.g., metal casings, which
are not normally live, but they can become energised under fault conditions. All the
protective schemes are based on the general rule that dangerous live parts must not be
accessible and exposed conductive parts must not become dangerous, neither on normal
service nor on fault conditions. In general, the analysis and application of protection for
personal safety is not an easy task, although this can be simplified if we consider a
particular case. For our present purposes, this standard is restricted to PV systems with
the following characteristics:
1. Open circuit voltages at standard test conditions less than 120 V DC[6] .
This condition implies DC voltages are considered as “extra-low voltages”
according to the IEC 60364 and the protection against indirect contact in DC
is ensured provided DC circuits are isolated from AC ones with a degree of
safety equivalent to that of an isolating transformer[9] .
2. AC distribution lines are shorter than 400 meters. This ensures that AC lines
can be effectively isolated from the earth because leakage parasitic currents
between cables and earth are negligible. This wiring length has been chosen
to satisfy the condition proposed in IEC 603641 for the nominal inverter
voltages (120 or 220 V AC).
Both features contribute to facilitate the provision of protective means and
define the framework for all the cases considered in this standard. Next sections discuss
the measures of protection against direct and indirect contact in AC and DC circuits
assuming they are isolated each other with a degree of safety equivalent to that of an
1
The product of wiring length, in meters, by the nominal AC voltage, in volts, must be lower
than 10 , with a maximum of 500 meters.
5
-18-
isolating transformer. Finally, the section 2.4.3 discusses the validity of such protections
when the inverter does not fulfil this condition.
2.4.1
Protection against direct contact
This protection must avoid the accidental contact with live parts during normal
service ensuring they are not accessible. In normal conditions, the protection against
direct contact is required if the voltage exceeds 25 V AC or 60 V DC[5] . This implies
the protection is always required in AC (inverter voltages widely exceeds 25 V AC),
while in DC depends on the system configuration. In practise, conventional PV systems
with 12 or 24 V nominal DC voltages should not require this protection because output
circuit voltages of the respective PV generators should not exceed 60 V DC. In any
case:
• Protection against direct contact in AC side must be provided.
(C)
• Protection against direct contact in DC side must be provided
when the output circuit voltage of the PV generator, at STC,
exceeds 60 V. (C)
The measures of protection by obstacles or by placing out of reach ensure a
partial protection because they only avoid casual or chance contacts. Therefore, this
kind of protection should be only applied in locations accessible by qualified persons,
which are aware of conductive parts are live parts and must not be voluntarily touched.
Then:
• The protection against direct contacts by means of obstacles or
placing out of reach must be applied only in locations with
restricted access to unauthorised persons. (R)
In contrast, protective means based on the insulation of live parts (e.g., cable
covers) and barriers or enclosures provide a more secure protection because they can
only be removed by destruction. Hence:
• In locations with unrestricted access, the protection against
direct contacts should be ensured by means of the insulation of
live parts, barriers or enclosures. (R)
• The insulation of live parts must be able to withstand any
mechanical, chemical, electrical or thermal stresses under
normal service conditions. (C)
• The barriers or enclosures must provide a minimum degree of
protection of IP 2X or IP XXB, or, if they are placed in an easily
accessible location, IP 4X or IP XXD. (R)
Additionally, in the AC side, the use of residual current devices (RCDs) of high
sensibility (≤30mA) is recognised as an additional protection in cases where other
measures of protection fail or carelessness by the users. It must point out that the use of
RCDs is not recognised as sole means of protection and does not obviate the need to
apply one of the previous measures against direct contacts. Hence:
• Residual current devices of high sensibility (≤ 30 mA) should be
used in AC side. (R)
-19-
2.4.2
Protection against indirect contact
The protection against indirect contacts is intended to prevent the contact with
exposed conductive parts, which are not normally live, but they can become energised
in the case of an insulation fault. The danger of an electric shock depends mainly on the
magnitude and duration of the touch current flow through the human body[7] . In
practise, the protective measures against indirect contact use as design criterion the
maximum allowable touch voltage (product of the touch current and the body
impedance) as a function of time. In this regard, the so-called “conventional touch
voltage limit”[8] is defined as the maximum value of the touch voltage that is permitted
to be maintained indefinitely. In normal conditions, conventional touch voltage limits
are 50 V for AC and 120 V for DC2. A voltage that does not exceed one of these limits
is defined as an “extra-low voltage” according to the IEC 60364, which ensures the
protection against indirect contact provided the extra-low voltage circuit is separated
from other higher voltages circuits with a degree of safety equivalent to that of an
isolating transformer[9] . While the extra-low voltage condition is fulfilled by the DC
circuit of the PV systems covered in this standard (VOC,STC < 120 V DC), the same is not
true for the AC one because output inverter voltages widely exceed 50 V AC, which
requires additional measures of protection.
Equipment classification
Class I
Class II
Table 5. Classes of equipment according the level of protection
against electric shock.
In this point, it is worth to remind which are the levels of protection of the
electrical equipment against electric shock, which will be useful in the later discussion.
Namely:
Class I equipment. This equipment has exposed conductive parts in which a
dangerous touch voltage can appears in the event of an insulation fault. Examples of
typical Class I loads are washing machine, fridge, iron, etc. Class I equipment is marked
by the symbol displayed in Table 5. This equipment should provide a means of
connecting its exposed conductive parts to a protective conductor (earthing system) or
to an unearthed equipotential bonding (floating system). Both configurations are
discussed below.
Class II equipment. This equipment relies in a doubled or reinforced insulation,
which prevents the presence of a dangerous touch voltage. This degree of insulation is
considered by the IEC 60364 as sole means of protection against indirect contact.
Examples of typical Class II loads are TV, radio, electric fan, hair dryer, etc. Class II
equipment is marked by the symbol of a square inside other square (see Table 5). In this
document, Class II equipment are represented likewise, but including additional
graphics in the internal square referred to the type of equipment (inverter, load, etc.).
2
In special cases (medical installations, moist conditions, etc.), these limits are reduced to 25 V
AC and 60 V DC.
-20-
2.4.2.1 AC circuit
The measures of protection against indirect contact rely in either earthing or
floating systems. Earthing systems are connected to earth and the protection that applies
is the automatic disconnection of supply. This is the most common and widespread
measure of protection against indirect contact. Following IEC 60364 recommendations:
• Protection by automatic disconnection of supply must be used in
AC excepting other measures of protection against indirect
contact are applied. (C)
In contrast, floating systems are isolated from earth. The relevant protections are
Class II or equivalent insulation, and electrical separation. While the first one relies on a
doubled or reinforced insulation, the second one relies on an isolated AC network where
several precautions are adopted to avoid earth faults. The difference between both
configurations is the last one permits the additional possibility of using Class I AC
loads. Despite the name “electrical separation” has sense in the framework of the utility
grid (the floating AC network and the earthed grid are galvanically separated), we have
preferred to keep the same nomenclature.
Once we have reached this point, the question does not take a long time in
arising. If both, earthing and floating systems, may be equally safe from the point of
view of personnel safety, and both allow the use of Class I loads: what is the right
choice, an earthing or a floating system?. Although national standards and regulations
have the last word, it is worth to propose several recommendations and criteria to select
the suitable system configuration.
Floating systems has the advantage of the continuity of service (the first fault
causes small leakage currents and the system may continue in operation) and, above
this, they do not incurred in additional costs for the provision of earthing equipment
(earth electrode, conductors, etc.). Earthing systems also have several advantages, such
as the improvement in the protection against induced overvoltages3 or the reduction in
radiated electromagnetic interference. When size and system costs increase, the need of
protection against overvoltages is every time more stressed and the relative incurred
costs in earthing equipment become less and less important. Thus:
• Floating systems should be used in small PV systems, in those
where AC appliances are operated occasionally, or when using
only Class II AC loads. (R)
• Earthing systems should be used in medium and large PV
systems where the power supply is mainly provided in AC for all
kinds of loads. (R)
Earthing systems
The protection against indirect contact consists in connecting to earth the
exposed conductive parts, e.g., metal casings of Class I loads, and usually, a live
conductor. If a fault develops between a live part and an exposed metal part, and the
prospective touch voltage exceeds the conventional touch voltage limit of 50 V AC, a
3
See, for example, the reference [16] for a more detailed discussion on lightning and overvoltage
protection in PV systems.
-21-
protective device must automatically disconnect the supply to avoid the persistence of
this voltage for a time sufficient to cause a risk of harmful physiological effect in a
human being. It is worth to stress that when the touch voltage is less than the
conventional touch voltage limit, the disconnection of supply is not required from the
viewpoint of protection against indirect contact. Table 6 displays the required
disconnecting times according the prospective AC touch voltage[10] .
Prospective AC touch voltage [V]
≤ 50
75
100
125
220
Disconnecting time [s]
Not required
0.60
0.40
0.33
0.18
Table 6. Required disconnecting times according the prospective
touch voltage.
The IEC 60364 defines three earthing configurations, which are identified by
means of two letters. The first letter indicates the earthing condition of the neutral (T:
earthed. I: unearthed), and the second one, the earthing condition of the exposed
conductive parts (T: directly earthed. N: connected to the neutral). In single-phase
inverters, it is not strictly correct to talk about of live (L) or neutral (N), because, in
principle, both conductors are identical and interchangeable. However, this
nomenclature will be kept, choosing anyone as neutral. The Figure 12 shows each
earthing configuration, where it is supposed the inverter is Class II to simplify the
drawings. The conductor used to connect the metal parts to earth is so-called protective
conductor or PE.
L
N
Class II
Inverter
L
N
Class II
Inverter
PE
L
N
Class II
Inverter
PE
PE
Class I load
Earthing of the neutral
conductor and Class I
equipment
(a)
Class I load
Earthing of the Earthing of the
neutral
Class I equipment
Class I load
Earthing of the
Class I equipment
(b)
(c)
Figure 12. Examples of earthing systems. (a) TN-S. (b) TT. (c) IT.
In the TN configuration, the neutral is earthed and the metal casings of Class I
loads are connected to the neutral by means of the PE. Despite TN configuration allows
using the neutral as protective conductor, this possibility is not considered here because
it precludes the use of RCDs as protective means. Thus, only the so-called TN-S
configuration, where neutral and PE are different conductors, will be taken into account.
In the TT configuration, the neutral and the metal casings are earthed separately.
If an insulation fault develops between the live conductor and a metal casing, the fault
current is limited by the resistance of earth electrodes. The suitable protective device is
-22-
the RCD since the fault current use to be small and returns to the circuit through the
earth. As this configuration requires two separate and independent earth electrodes, its
use is not too recommendable for small PV systems.
In the IT configuration, only the metal casings are earthed. The first insulation
fault between a live conductor and earth or a metal casing is not usually dangerous
(excepting in extremely long or defective AC distribution lines) and the power supply
disconnection is not required. This is the main advantage of this earthing system
provided the fault is located and eliminated as soon as possible, which permits
continued the use of the installation. The detection and the location of the first fault
require an insulation monitoring device and technical maintenance, respectively, which
can only be justified in applications where the continuity of service is essential, such as
power plants. If a second fault develops, and the first one has not been eliminated, the
power supply must be disconnected, which does not take the advantages of this
configuration.
With regard to personal safety, the three earthing configurations are equivalent
and no one is preferred over another. However, taking account the previous discussion
and disregarding other factors:
• The TN-S earthing configuration is preferred. (R)
Measures of protection in the TN-S earthing configuration
The protection against indirect contact in a TN-S configuration operates in this
way. If an insulation fault develops between the live conductor and a metal casing
(Figure 13a), a short circuit live-neutral develops across the PE (the current is limited by
the resistance of loop cables and output inverter impedance), and the load power supply
is disconnected by either over-current protective devices, such as fuses or circuit
breakers, or the inverter itself (activation of short-circuit protection). A RCD can also
ensure this disconnection because the fault current goes through the live conductor but
returns through the PE. As the RCD operates by detecting the difference in current
between live and neutral, it must be placed downstream the connecting point of the
neutral to the protective conductor in order to ensure its proper operation (see Figure
13). Besides, the RCD only operates when the insulation fault takes place in the AC
loads. For example, if the insulation fault develops between the live conductor and the
metal casing of a Class I inverter, the fault current does not got through the RCD and
the disconnection of supply must rather rely in over-current devices (placed upstream
the connecting point of the neutral to the PE) or in the inverter self-protection against
short-circuit (see Figure 13b). Hence:
• The automatic disconnection of supply can be provided by fuses,
over current circuit breakers, residual current devices, or by the
inverter itself (activation of short-circuit protection). (R)
For this purpose, we define the “disconnecting current” as the fault current that
activates the relevant protective device. When the disconnection is provided by overcurrent circuit breakers or RCDs, it is sufficient to ensure that the disconnecting current
is, at least, equal to the smallest current causing their instantaneous operation. When the
protective device is a fuse, the disconnecting current is that ensuring the operation in a
time not exceeding 5 seconds.
The prospective touch voltage, in volts, is approximately equal to the product of
the fault current, in amps, and the resistance of the protective conductor between the
-23-
points A and B, in ohms (see Figure 13). The exact calculation of this touch voltage is
not an easy task because it depends on the particular characteristics of the system:
output inverter voltage, internal inverter impedance, and the section and length of live
and protective conductors. However, it is permissible to dispense with this calculation
just ensuring the following condition:
• The protective conductor must be sized to ensure a voltage drop
lower than 50 V between any Class I equipment and the
connecting point to the earthing of the system. This applies at
the disconnecting current. (R)
Insulation fault
Live
RCD
Class II
Inverter
A
Earthing of
the system
Overcurrent
device
Neutral
PE
L
N
Class I
Inverter
B
Insulation
fault
RCD
PE
Class I load
Earthing of
the system
(a)
(b)
Figure 13. Fault current path (dotted line) when an insulation fault
develops between the live conductor and the metal casing of :
(a) Class I load. (b) Class I inverter.
The previous requirement is equivalent to the measure of protection so-called
“supplementary equipotential bonding” by the IEC 60364 and is very useful for
standardisation purposes because it is independent of other system characteristics and
allows to state conventional disconnecting times irrespective of the touch voltage.
Besides, it greatly facilitates the protection against indirect contact and does not impose
strict requirements to the protective conductor, which is shown with the following
example.
As an example
Let us imagine the worst case possible of an inverter of 120 V AC with a rated
power of 5 kVA, and a surge power equal to three times the rated one during 5 seconds.
In this case, the inverter itself provides the disconnection of supply, being the
disconnecting current equal to 125 A (3 x 5000 VA / 120 V). The maximum resistance
of the protective conductor between points A and B that ensures a touch voltage less
than 50 V is therefore:
R AB ≤
50 V
= 0.4 Ω
125 A
Using copper cables (specific resistance equal to 0.01724 Ω·mm2/m at 20 ºC),
the Table 7 shows the minimum required sections for different wiring lengths. These
sections can even be reduced to very low values using RCDs. For example, if the
sensibility of the RCD is 500 mA, the required sections are ¡250 times! lesser than the
-24-
displayed in Table 7. Needless to say, these sections must be compatible with the
conductor ampacity in order to reduce the risk of overheating.
Length [m]
Required sections [mm2]
10
> 0.43
50
> 2.15
100
> 4.31
Table 7. Example of required sections for the protective conductor for
different wiring lengths, which avoid a touch voltage exceeding 50V at
a disconnecting current of 125A.
Insulation
fault
RCD
Class II
inverter
PE
RB
VC
Class I load
RE
IF
Figure 14. Fault current path (dotted line) in case of an insulation
fault between the live conductor and earth in a TN-S system.
The IEC 60364 suggests that, in exceptional cases, an insulation fault between
the live conductor and earth can develop (see Figure 14), increasing the voltage of metal
casings and PE over the earth. To ensure this voltage does not exceed the conventional
touch voltage limit, the IEC 60364 proposes to ensure the following condition:
VC = I F ⋅ R B =
V0
⋅ R B ≤ 50 V
RB + RE
⇒
RB
50
≤
R E V0 − 50
Where VC is the touch voltage, IF is the fault current, RB is the earthing
resistance, RE is the minimum resistance of the insulation fault between live and earth,
and V0 is the AC output voltage (120 or 230 V). Voltages, currents and resistances are
given in V, A, and Ω, respectively. However, this condition has a doubtful usefulness in
practise because it must anticipate the earthing resistance of the insulation fault, which
is, in principle, unknown. One possible alternative is to correlate the earthing resistance
with the fault current that activates the relevant protective device (fuse, over-current
circuit-breaker or RCD) like this:
• The earthing resistance of the buried electrode must ensure a
touch voltage less than 50 V. This applies at the disconnecting
current. (R)
-25-
As an example
Let us imagine a PV system composed by a 230V/50Hz inverter whose rated
power is equal to 5 kVA. This system uses as protective means over-current circuit
breakers, which have a disconnecting current equal to the rated AC one, that is 21.74 A
(5000 VA / 230 V). Then, the earthing resistance limit is:
RB ≤
50 V
= 2.3 Ω
21.74 A
The use of over-current protective devices requires a very low earth resistance,
which may be difficult to achieve in practise and ensuring during the lifetime of the
system. For example, using a RCD of 500 mA, a worst earthing resistance is permitted
(RB ≤ 100 Ω), which is an additional reason to encourage the use of these protective
devices.
Floating systems
Class II equipment or equivalent insulation
If the inverter and the AC loads are Class II, the protection against indirect
contacts in AC side is ensured and no additional requirements are needed. This “Class II
system” is displayed in Figure 15. In general, if the inverter, the loads or both, are not
Class II, we have an additional possibility for applying this protection adding a
supplementary insulation during the installation process, providing a degree of safety
equivalent to Class II. For example, in small PV systems, it is very common to find
Class I inverters supplying Class II appliances (TV, radio, etc.), which can be converted
to Class II providing a supplementary inverter insulation, for example, locking the
inverter inside a wood box under key.
Class II
inverter
Class II load
Class II load
Figure 15. Class II system ensures the protection against indirect
contacts.
Strict compliance of the IEC 60364 would require to carry out an insulation test
to verify its quality, which is difficult of performing in rural conditions. Instead of that,
the simple common sense should suffice to judge the box protection capability.
Enclosures with at least IP 2X or IP XXB (stop face of fingers), normal robustness,
well-ventilation, lock and warnings are minimum requirements. It is supposed the
operator is aware of accessible metal parts locked inside the box can be energised owing
to an insulation fault and must not be touched while the inverter is operating. For
-26-
example, the Figure 16 shows a homemade inverter box made with wood of 3 cm
thickness and padlocked. Hence:
• If the inverter is not Class II, a degree of safety equivalent may
be provided during the installation locking the inverter inside an
insulated box under key. (R)
If loads are Class I, the conversion to Class II is much more difficult since they
normally should be directly handled by the user and exposed conductive parts are
always accessible (fridge, iron, etc.). In this case, the use of Class I loads must rather
rely on the protection by electrical separation.
Figure 16. Homemade inverter box made with wood.
Electrical separation
An ideal isolated AC network from earth ensures the protection against indirect
contacts because if a person comes into contact with a live part, the potential shock
current does not have a returning path to the circuit (Figure 17a). However, an ideal
isolation does not exist in practise owing to leakage impedance between the live
conductors and earth, and the fault current always finds a returning path (Figure 17b).
For a given AC inverter voltage, the leakage impedance depends on the length of AC
distribution lines, which must be kept as short as possible. As commented above, the
limit has been fixed in 400 m according to the IEC 60364 requirements.
Inverter
Inverter
Live
Neutral
IF = 0
Leakage
impedance
IF > 0
Earth
Earth
(b)
(a)
Figure 17. (a) Ideal isolated AC network. (b) Real isolated AC
network.
-27-
Additionally, some precautions must be taken into account in order to reduce as
much as possible the occurrence of an insulation fault between live parts and earth.
Firstly, the possibility of an insulation fault between the AC wiring and earth must be
unlikely, for example, using double insulated cable (H07RN-F type or similar quality)
or providing a complementary insulation to cable with a single insulation cover. The
Figure 18 shows an example of both solutions. Secondly, the live conductors must not
be connected to earth. Hence:
• The AC wiring must be installed to reduce the possibility of
earthing faults, for example, using doubled insulated cables or
providing a complementary insulation (plastic pipes, insulated
conduits, etc.) to cables with a single insulation cover. (R)
• AC live conductors must not be intentionally earthed. (C)
Conductor
Insulation
Insulation
(a)
(b)
Figure 18. Fault-proof AC wiring. (a) Doubled insulated cables.
(b) Single insulated cables in insulated conduits or pipes.
Despite the adoption of the previous precautions, if two or more Class I
equipment are present, there is a likelihood of electric shock if two insulation faults
develop between different live conductors and metal casings and a person
simultaneously come into contact with both (Figure 19). In contrast, when there is only
one Class I equipment, for example, a Class I inverter supplying Class II loads, there is
no danger of indirect contact. So:
• If there is only one Class I AC equipment the protection against
indirect contact is ensured, and its metal casing should not be
connected to any protective conductor. (R)
• If there are two or more Class I AC equipment, their metal
casings must be interconnected by means of an unearthed
equipotential bonding. (R)
Needless to say, although Class II loads can have metal casings, they must not
be connected to this equipotential bonding since a dangerous voltage can never appear.
In principle, an alternative solution to this unearthed equipotential bonding could be the
separation of Class I equipment a distance sufficient to avoid the simultaneously access
to different exposed metal parts. However, this poses an additional risk because the first
fault in any Class I load can earth the circuit. Think, for example, in a washing machine
with buried metal pipes connected to the metal enclosure.
The Figure 20 displays the topology of the isolated network with both Class I
and Class II loads. In the event of two faults between conductors of different polarity
and Class I loads, a short-circuit takes place through the equipotential conductor, which
must trip a protective device, such as an over-current circuit breaker or a fuse, or the
-28-
inverter short-circuit protection. As commented above, the following requirements
ensure the protection against indirect contact:
• The automatic disconnection of supply can be provided by fuses,
over current circuit breakers, or by the inverter itself (activation
of short-circuit protection). (R)
• The unearthed equipotential conductor must be sized to ensure a
voltage drop lower than 50 V between simultaneously accessible
Class I equipment. This applies at the disconnecting current. (R)
Class II
inverter
Second fault
First fault
Class I load
Class I load
Figure 19. Risk of electric shock between two simultaneously
accessible metal casings. The dotted line represents the fault current
path.
It must point out that only a RCD placed at the beginning of the AC distribution
line is not able to provide the supply disconnection because the fault current path is
between live and neutral. Although the IEC 60364 does not consider this possibility, a
RCD could be used as alternative protective device to provide the supply disconnection
of Class I AC loads connecting the equipotential conductor to a live conductor upstream
the RCD. This configuration is identical to a TN-S configuration removing the earthing
of the system. Hence:
• The power supply disconnection can alternatively be provided by
a RCD placed at the beginning of the AC distribution line
connecting the unearthed equipotential conductor to a live
conductor upstream the RCD. (S)
Fault-proof wiring and length ≤ 400 m
Class II
inverter
Class I load
Class I load
Class II load
Unearthed equipotential bonding
Figure 20. Topology of an isolated AC network that provides
protection against indirect contacts.
-29-
2.4.2.2 DC circuit
As commented above, this standard is restricted to systems with PV generators
having output circuit voltages, at STC, not exceeding 120 V DC[6] . Thus, the DC
voltage is considered as “extra-low voltage” and the protection against indirect contact
is ensured if the DC circuit is separated from the AC one with a degree of safety
equivalent to that of an isolating transformer[9] . This condition is intended to prevent
the appearance of higher voltages in DC side coming from the AC one. Hence:
• The inverter must provide DC/AC separation with a degree of
safety equivalent to that of an isolating transformer. (R)
• The inverter shall be selected or erected in accordance with
requirements of Class II equipment or equivalent insulation. (R)
On one side, although most of tested inverters have low frequency output
transformers with separated windings, only three among them meet this degree of
isolation (marked with a relevant standard), while the rest remain unspecified. However,
there are not apparent difficulties to ensure this degree of isolation, excepting the
additional cost of such transformers, which should be overcame by the benefits in
simplicity and flexibility of the protective measures that can be applied in DC side.
On the other side, none of tested inverters are marked with the symbol of Class
II equipment, and only one has a non-conductive casing (although this is not an
essential requirement for Class II equipment). The question that underlies the Class II
requirement is a metal casing may put in contact the AC circuit with the DC one, even if
an isolating transformer separates them. Being aware of the possible difficulty to ensure
the Class II requirement, it is worth to propose an alternative to this condition, so:
• Alternatively to the Class II inverter requirement, at least one
circuit, whether the AC or the DC one inside a Class I inverter,
must be separated from the metal casing with a degree of
insulation equivalent to Class II equipment. (C)
In this way, the metal inverter casing must be considered, for the purposes of the
application of measures of protection against electric shock proposed in this standard, as
an exposed-metal part belonged to the circuit of which is not separated. For example, if
the metal casing is separated from the AC circuit, it must be subjected to the protection
against electric shock applied in DC side, and vice versa. In principle, this requirement
may be easily fulfilled by the AC circuit using, for example, a doubled insulated cable
connected to the secondary of the output transformer and AC outlets with the same
degree of insulation. In the DC circuit, this requirement may be more difficult of fulfil
because the power semiconductors are usually in contact with the metal casing, directly
or through a heat sink. Additionally, we must also take into account the manufacturer’s
configuration. For example, the tested inverter number 3 has an isolating output
transformer, while the metal casing and the DC negative conductor are connected
together.
Additionally to the inverter, DC and AC wiring must be separated likewise
providing, at least, the same degree of isolation existing between the input and output
windings of an isolating transformer. Among the different solutions that can be adopted
to ensure this requirement, it is preferable to physically separate DC and AC live
conductors, for example, using different conduits or wiring paths. The PV system
displayed in the Figure 21 shows an example of the latter, where the DC wiring is
-30-
outside the home and physically separated from the AC one, which is inside it (the
battery and the inverter are connected through the wall). Hence:
• DC and AC live conductors must preferably be physically
separated. (R)
If the inverter and the wiring ensure a safe DC/AC electrical separation, the DC
circuit may be either floating (so-called safety extra-low voltage or SELV) or earthed
(so-called protective extra-low voltage or PELV). The latter permits connecting the
metal casings of Class I DC equipment and/or a DC live conductor to earth, which can
be required by other reasons different of protecting against indirect contact, e.g., to
improve the protection against overvoltages, and does not exclude the possibility of
using the same earthing that the AC one.
Battery
AC wiring
Inverter
Figure 21. Example of physical separation between DC and AC
wiring.
2.4.3
Additional protection against both direct and indirect contact in DC side
If the inverter or the wiring does not ensure a safe DC/AC isolation, the extralow voltage condition for the DC circuit is useless and additional measures of protection
against both direct and indirect contact must be provided. In this case, the DC voltage is
so-called functional extra-low voltage or FELV.
Regarding the protection against direct contact, all DC live part must be
subjected to this protection regardless of the nominal DC voltage because the first
insulation fault poses a great risk of electric shock if person comes into contact with
these parts. The Figure 22a shows an example of this. The fault current returns to the
circuit through earth, which is not eliminated by any protective means. Even a RCD of
high sensibility (≤30 mA) recommended against this kind of contact is useless to detect
the fault current because it returns to the circuit upstream its location.
Regarding the protection against indirect contact, the IEC 60364 requires
exposed DC conductive parts must be subjected to the same protection applied in AC
side. In other words, metal casings of Class I DC equipment must be connected to the
protective conductor in an earthing system or to the unearthed equipotential bonding in
a floating one. However, the application of such protection poses several difficulties.
Firstly, although the first insulation fault is not dangerous, it increases the voltage
-31-
between the DC live conductors and the metal casings of Class I DC equipment up to
the AC output voltage (see Figure 22a). Thus, Class I DC equipment must withstand, at
least, the same insulating requirements that Class I AC equipment. If a second
insulation fault develops, and the first one has not been eliminated, this may cause,
depending on its location, either a short-circuit live-neutral, which must activate the
relevant protection, or a DC short-circuit (see Figure 22b). In the last case, the
automatic disconnection of supply could be provided, for example, by over-current
devices or RCDs type B (sensible to DC fault currents) placed between the battery and
the inverter, causing the inverter disconnection. These complexities led as to propose, in
the absence of a more detailed experimental study on this subject, the following solution
to ensure the protection against both direct and indirect contact:
• If the inverter does not provide a safe DC/AC separation, the
protection against both direct and indirect contact in DC side
should be ensured using Class II equipment or equivalent
insulation. (R)
First fault
Live
RCD
Neutral
Inverter
PE
Earth
Class I DC
equipment
Class I AC
load
Earthing of the
system
(a)
Inverter
Live
RCD
Battery
First fault
PE
Second
fault
Class I DC
equipment
Earthing of the
system
(b)
Figure 22. (a) Risk of direct contact in a FELV system, in which the
AC side is earthed. (b) DC short-circuit as result of two insulation
faults.
-32-
Neutral
2.5
ENERGY AND POWER CONDITIONING PERFORMANCE
The energy performance of an inverter depends on the efficiency with which it
converts DC to AC. The knowledge of power efficiency under different operating
conditions (DC input voltage, load size and type) and patterns of the load’s use are
required to calculate the energy requirements for sizing purposes.
2.5.1
Power efficiency
The inverter power efficiency, ηi, quantifies how well the DC input power is
converter in useful AC power. It is defined as the ratio between AC output power (PAC)
and DC input power (PDC):
P
PAC
ηi = AC =
PDC PAC + Power losses
Among other factors, the inverter efficiency depends on load type and size. Once
the load has been defined (section 2.1), the inverter efficiency for each particular
application can be easily measured using a sample of the elements composing the load.
When AC loads cannot be precisely anticipated, the inverter efficiency can be specified
using a standard load. For this purpose, we have used as reference conventional
incandescent lamps (PF=1).
The modelling of efficiency curves is an unavoidable step for predict the energy
performance and for inverter comparison purposes. Among the different approaches that
can be adopted, we have used a descriptive model that provides a good equilibrium
between precision and simplicity[17] :
ηi =
p0
p0 + ( k0 + k1 ⋅ p0 + k 2 ⋅ p02 )
Where p0 is the ratio between the AC output power and the rated one (0≤ p0≤1),
and the terms in parenthesis represent the inverter power losses. The parameters k0, k1
and k2 must be obtained for each particular inverter. This model has the advantage that
parameters k0, k1 and k2 have physical meaning according the type of losses involved in
the power conversion: k0 represents the self-consumption losses, which are independent
of the load, k1 represents the losses that varies linearly with the current (e.g., diodes),
and k2 represents the Joule losses, which are proportional to the square of the current
(voltage drops across the wiring, transformers, switching semiconductors, etc.). A
parameter additional, k0’, is used to quantify the self-consumption losses with no load
condition (0 ≤ k0’≤ k0). If model parameters k0, k1 and k2 are unknown, they can be
calculated by means of three points of power efficiency curve. For example, using the
efficiency at levels 10%, 50% and 100% of the inverter rated power, they can be
obtained using the following relationships:
k0 =
5 1
1 1
1 1
−
+
36 η10% 4 η50% 9 η100%
k1 = −
k2 =
5 1
33 1
4 1
+
−
−1
12 η10% 12 η50% 3 η100%
5 1
5 1
20 1
−
+
18 η10% 2 η50%
9 η100%
-33-
The average power efficiency with resistive loads obtained in the inverter testing
campaign is represented in the Figure 23. It has been calculated using the average
values of model parameters obtained by each particular inverter. According to this
average performance:
• Inverter power efficiency with resistive loads must be greater
than 0.8, when the output power is greater than 0.1 times the
rated power and at nominal DC voltage. (R)
1.0
(a) Average efficiency
Power efficiency
( k0=0.016 / k1=0.023 / k2=0.22)
0.9
0.8
0.7
(b) Inverter 2
(c) Inverter 4
0.6
0
0.2
0.4
po
0.6
0.8
1
Figure 23. Power efficiency curves with resistive loads obtained in
the inverter testing campaign (a) Average efficiency. (b) Tested
inverter with the worst self-consumption losses (k0=0.0528). (c)
Inverter with the worst Joule losses (k2=0.4583).
When the load is disconnected (p0=0), the power efficiency is zero, and the selfconsumption losses quantify the power demanded by the inverter. The Table 8
summarises the self-consumption model parameters of tested inverters. The power
consumption with no load condition (parameter k0’) ranges from 1.44 W (inverter 5) up
to 11.25 W (inverter 9). Depending on the energy required by the load, these losses
could be unacceptable.
Inverter k0 [%]
k’0 [%]
Inverter
k0 [%]
k’0 [%]
1
1.37
1.37
7
1.77
0.17
2
5.28
5.28
8
1.20
0.11
3
1.36
1.36
9
2.25
2.25
4
0.90
0.90
10
1.17
0.26
5
0.72
0.72
11
0.84
0.18
6
1.63
1.63
12
0.74
0.10
Table 8. Self-consumption of tested inverters expressed as a
percentage of the inverter rated power.
-34-
As an example
Let us suppose a Solar Home System (SHS), whose typical daily energy
consumption is 150 Wh·day-1, adds the inverter 2 (rated power=150W) to supply a 40W
color TV during 1 hour. The inverter efficiency at this power level is ηi(p0=0.266)≈0.8
(the efficiency curve is displayed in Figure 23b), so the DC energy required when the
TV operates is (40W/0.8)x1h=50Wh. If the inverter operates full time the DC energy
required during the rest of the day reaches 7.92Wx23h≈182Wh·day-1. In other words,
the SHS does not produce enough energy even to keep the inverter operating.
Besides, as the inverter is sized to satisfy the peak demand, it usually runs most
of time at partial load where the reduction of efficiency owing to self-consumption
losses can be considerable. Thus, taking into account the testing results displayed in
Table 8:
• Notwithstanding the above requirements, the inverter selfconsumption losses in normal operation must be lower than 1%
of rated power at DC nominal voltage. (R)
2.5.2
Energy efficiency and sizing
For PV system sizing purposes, it is necessary to calculate the DC energy
required by the inverter to supply the specified AC load. This calculation requires the
knowledge of the power efficiency and some information describing the expected
pattern of load’s use. From this data, we can derive a characteristic daily load profile
composed by several periods of expected AC loads and determine the points on the
power efficiency curve where the inverter will be operating during such periods. While
the power efficiency can be easily measured in the laboratory, the same cannot be said
of the daily load profiles because of the random nature of energy consumption patterns.
Magnitude and temporal distribution of power consumption only can be exactly knew in
certain applications, such as remote telecommunication systems, lighting street, etc.
From now, let us supposing the required information is available.
The daily energy efficiency, ηe, is defined as the ratio between the energy
consumed by the AC load, EAC, and the energy to be supplied by the PV system, EDC:
ηe =
E AC
E DC
On an hourly basis, it can be calculated in this simple way:
24
∑ PACh [W ]
ηe =
E AC [Wh]
P
+ PAC 2 + ... + PAC 24
= h =1
= AC 1
24
P
PAC 24
E DC [Wh]
P
[W ]  AC 1 PAC 2
∑  ACh  η + η + ... + η
ηih 
i1
i2
i 24
h =1 
Where h is the hour of the day (h=1, 2, …, 24), and PACh and PA,h/ηih are,
respectively, the AC load power consumption and the DC power required by the
inverter in each hour. The application of this equation requires the knowledge of the
inverter power efficiency and the hourly load profile. The last one can be defined by
means of 24 hourly load factors (F1, F2,…, F24), which indicate the fraction of the daily
energy consumed in each hour. The relationship between each hourly load factor, the
relevant hourly AC power and the daily energy consumption is given by:
-35-
E ACh [Wh] = Fh ⋅ E AC [Wh] = PACh [W ]·1h
Where EACh is the hourly AC energy consumption. Obviously, the addition of the
24 hourly factors verifies their addition is equal to 1 (F1+F2+ …+ F24=1). Using the
previous equations and the power efficiency model defined in the section 2.5.1, we
obtain the following equation for the daily energy efficiency:
ηe =
l
l + d0 + d1 ⋅ l + d 2 ⋅ l 2
The parameter l is the normalised daily AC energy consumption, defined as the
ratio between the daily energy consumption and the maximum energy that the inverter
can provide in one hour:
l=
E AC [Wh]
PNOM [W ]·1h
Where PNOM is the inverter rated power. Similarly to the power efficiency model,
the parameters d0, d1, and d2 represent the daily energy losses involved in the DC/AC
conversion:
d0
Self-consumption losses: d0 = 24·k0 . For hours with Fh=0 (no load
condition) k0 must be substituted by k’0 if they are different.
d1
Linear losses: d1 = k1 .
d2
Quadratic losses: d 2 = k 2 · ∑ Fh2 .
24
h =1
Among these parameters, the self-consumption losses may have great influence
on the energy efficiency, especially, if the AC loads are used for short periods and the
inverter is continuously draining power. Hence:
• The daily energy losses caused by the inverter self-consumption,
in the periods when loads are switched off, must be lower than
20% of daily AC energy consumption. (R)
The previous requirements may be achieved either shutting down the inverter
when loads are disconnected (k0’=0) or having lower self-consumption losses. For
example, several inverters have a stand-by mode that reduces the power demand when
the loads are disconnected (k0’< k0), although this is not usually available in small units.
As an example
Let us continue with the example of table 2. The AC energy consumption of the
specified load was EAC=500 Wh·day-1, while the daily load profile and the relevant
hourly load factors are displayed in Figure 24.
-36-
200
Hourly load factors different from zero.
Fh=(PACh [W]x1h) / 500 [Wh]
PAC [W]
160
120
F12=0.4
F20=0.08
F21=0.2
F22=0.2
F23=0.12
80
40
0
1
4
7
10
13
16
19
24
∑ Fh2 = 0.2608
22
Hour of the day
h =1
Figure 24. Example of daily load profile and hourly load factors.
Supposing that AC appliances behave as resistive loads, and we have selected
the inverter number 4 whose efficiency curve is shown in Figure 23c (model
parameters: k0=0.0090, k1=0.0062 and k2=0.4583), we can calculate the daily energy
efficiency like this:
PNOM = 200W
ηe =
⇒ l = 2.5
2.5
2.5 + 24 x0.0090 + 0.0062 x 2.5 + 0.4583 x0.2608 x 2.5 2
= 0.719
Finally, the required daily DC energy for sizing purposes is calculated as:
E DC =
E AC
= 695 Wh
ηe
The inverter self-consumption is 1.8 W (k0xPNOM), causing no-load energy losses
equal to 34.2 Wh·day-1 (1.8Wx19h). These losses, around 7% of EAC, have not very
influence in energy efficiency. For example, supposing the inverter is switched off
during this time, the resulting energy efficiency is:
ηe =
2.6
2. 5
2.5 + 5 x0.0090 + 0.0062 x 2.5 + 0.4583 x0.2608 x 2.5 2
= 0.756
USER FRIENDLINESS
Inverter must display information to users indicating its operating condition.
Basically, it must provide the following readings:
• If the inverter AC voltage is present at the output, this should be
indicated by means of a visual indicator. (R)
• If the inverter protects the battery against deep discharging, the
disconnection of AC loads by this reason must be indicated by
means of a visual indicator. (R)
-37-
• If the inverter is shutting down by unusual service conditions,
e.g., short-circuit, the relevant activated protection must be
indicated by means of visual indicators. (R)
• In order to save energy, manual activation of informative
displays may be provided. (S)
2.7
INSTALLATION AND MAINTENANCE
The inverter installation should be made according manufacturer
recommendations, although, in general, the following guidelines may be applied:
• The inverter must be located in a well-ventilated and dry place.
(R)
• The inverter must be away from heat sources and battery
gassing, and the ventilation spaces have not to be obstructed. (C)
• Inverter should be placed in an easily accessible location with
restricted access to unauthorised persons. (C)
-38-
2.8
[1]
REFERENCES
“Certification and Standardisation Issues for a Sustainable PV Market in
Developing Countries”. JOULE-C JOR3980275.
“XXXX”.
[2]
“Universal technical standard for solar home systems”. Thermie B SUP 995-96,
EC-DGXVII, 1998.
[3]
R. H. Bonn, J. Ginn and S. Gonzalez. "Stand-Alone Generic Test Plan". Sandia
National Laboratories (1998). www.sandia.gov
[4]
F. Chenlo and N. Martín. "Testing procedures and acceptance criteria for standalone photovoltaic inverters". CIEMAT (1995).
[5]
IEC Standard 60364. “Electrical installations of buildings”. Part 4-41: Protection
for safety - Protection against electric shock.
[6]
IEC Standard 60364. “Electrical installations of buildings”. Part 7-712:
Requirements for special installations or locations - Solar photovoltaic (PV)
power supply systems.
[7]
IEC Standard 60479. “Effects of current on human beings and livestock - Part 1:
General aspects”.
[8]
IEC Standard 60050. “International Electrotechnical Vocabulary”. Chapter 826:
Electrical Installations of buildings.
[9]
IEC Standard 60742. “Isolating transformers and safety isolating transformers.
Requirements”.
[10]
IEC/TR3 61200-413. “Electrical installation guide - Clause 413: Explanatory
notes to measures of protection against indirect contact by automatic
disconnection of supply”.
[11]
“Testing for inverters in PV stand-alone systems”. Internal IES report (2000).
[12]
P. Díaz and E. Lorenzo. “Solar home system battery and charge regulator
testing”. Progress in Photovoltaics: Research and Applications. Volume 9,
Number 5 (2001). Pages: 363-377.
[13]
GEF/World Bank-Assisted China PV project. “Solar photovoltaic systems and
photovoltaic/wind hybrid systems specifications and qualifying requirements”
(1999).
[14]
IEC Standard 61000. “Electromagnetic compatibility (EMC)”.
[15]
IEEE Standard 519-1992. “IEEE recommended practices and requirements for
harmonic control in electrical power systems”.
[16]
“Lightning and Overvoltage Protection in Photovoltaic and Solar Thermal
Systems”. Thermie-B Programme Action Nº. SME-1662-98-DE.
[17]
M. Jantsch, H. Schmidt and J. Schmid. “Results of the concerted action on
power conditioning and control”. 11th E.C. Photovoltaic Solar Energy
Conference.Montreux (1992).
-39-
3.
INVERTER STANDARD
3.1
SCOPE
This section presents a proposition for stand-alone PV systems using inverters,
which are designed to provide AC power to conventional AC loads, such as electrical
appliances (TV, radio, tools, etc.) and lighting.
This standard is restricted to PV systems with open circuit voltages, at standard
test conditions, below 120 V DC, and AC distribution lines shorter than 400 m. It deals
only with the AC part of these PV systems, i.e., inverter and AC load, while the
“Universal Technical Standard for Solar Home Systems”[2] fully keeps here its validity
for all the aspects related with the DC part, i.e., PV generator, support structure, battery,
charge regulator and DC loads.
This standard exclusively applies to single-phase and voltage source inverters. It
does not apply to other cases, like variable frequency inverters for PV pumps or central
inverters powering local networks for villages, which remain for future work.
The requirements specified in this technical standard are classified according to
their importance: compulsory, recommended or suggested. Each requirement has a key
label composed of two letters and one number (e.g.: CI3). The first letter indicates the
importance of the requirement (i.e. compulsory), and the second letter refers to the
inverter as shown in Table 9. This nomenclature is coherent with the “Universal
Technical Standard for Solar Home Systems”. The number is just an identifier for
reference purposes.
First letter
Second letter
Compulsory
PerFormance
Recommended
Protections
Suggested
Installation
Table 9. Classification of requirements.
Installation requirements have been divided in several subsections because they
should be or not applied depending on the earthing of the system. Additionally, some
inverter features are included here because they are closely related with the system
installation.
3.2
INVERTER AND AC LOAD REQUIREMENTS
3.2.1
Performance
CF1
Inverters connected to the charge regulator must ensure safe starting and
operation of all the specified load in the voltage range permitted by the charge
regulator.
CF2
The rated power of the inverter should be equal or greater than the sum of the
rated power of all the simultaneously allowed individual loads.
-40-
CF3
The inverter must ensure safe starting of whichever individual load, in any
normal operating condition, i.e., with the rest of the simultaneously allowed
individual loads maintained on steady state operation.
CF4
Fluctuations of the rms value of the AC output voltage must be less than 10% of
the nominal value in any steady state condition of the specified load, and for all
the allowed DC input voltage range.
CF5
Fluctuations of the frequency of the AC output voltage must be less than 2% of
the nominal value in any steady state condition of the specified load, and for all
the allowed DC input voltage range.
CF6
Despite voltage distortion, the inverter must ensure the proper operation of all
the specified load.
CF7
Despite voltage distortion, the inverter must no produce radio frequency
interference in any operation condition.
RF1
Reactive and non-linear loads, especially fluorescent lamps, should have power
factors > 0.9.
RF2
Fluctuations of the rms value of the AC output voltage must be less than 5% of
the nominal value in any steady state condition of the specified load, and for all
the allowed DC input voltage range.
RF3
Fluctuations of the frequency of the AC output voltage must be less than 1% of
the nominal value in any steady state condition of the specified load, and for all
the allowed DC input voltage range.
RF4
Sine wave inverters should be preferred over non-sine wave ones. Then, the total
voltage harmonic distortion with linear loads must not exceed 5%.
RF5
Inverter power efficiency with resistive loads must be greater than 0.8, when the
output power is greater than 0.1 times the rated power and at nominal DC
voltage.
RF6
Notwithstanding the above requirements, the inverter self-consumption losses in
normal operation must be lower than 1% of rated power at DC nominal voltage.
RF7
The daily energy losses caused by the inverter self-consumption, in the periods
when loads are switched off, must be lower than 20% of daily AC energy
consumption.
RF8
If the inverter AC voltage is present at the output, this should be indicated by
means of a visual indicator.
RF9
If the inverter protects the battery against deep discharging, the disconnection of
AC loads by this reason must be indicated by means of a visual indicator.
RF10 If the inverter is shutting down by unusual service conditions, e.g., short-circuit,
the relevant activated protection must be indicated by means of visual indicators.
SF1
The inverter must operate safely for at least one minute at 125% of rated power.
Moreover, it should provide 150% of rated power for at least two seconds to
facilitate starting of motors and other high capacitance loads.
SF2
In order to save energy, manual activation of informative displays may be
provided.
-41-
3.2.2
Protections
CP1
The inverter, and also the load, must be protected against sudden cut off of the
DC supply, in all possible conditions of the specified load.
CP2
The inverter must be able to resist the sudden disconnection of all the
simultaneously allowed individual loads.
CP3
The inverter must be able to resist any possible output overload, and also short
circuit condition.
CP4
The inverter must be able to protect the load in any possible output overload, and
also short circuit condition.
RP1
The inverter must resist without damage any input voltage in the range from
-25% to 33% of the nominal input voltage (9 to 16 V for 12 V inverters).
3.2.3
Installation
3.2.3.1 General requirements
CI1
Direct connection of the inverter to the battery can be permitted. Then, the
inverter must protect the battery against deep discharging according with the
same rules established for the charge regulator.
CI2
The section of DC cables must cause less than 1% of voltage losses between the
inverter input and its DC connection point. This applies at the maximum current
condition.
CI3
The inverter DC terminals must allow for a secure and mechanically strong
electrical connection. They must have low electrical resistance, leading to
voltage losses less than 0.5% of nominal voltage. This applies for each
individual terminal at the maximum current condition.
CI4
Notwithstanding the above requirements, the maximum voltage drop between
battery and inverter must not interfere the correct operation of any balance-ofsystem component and the inverter itself.
CI5
Protection against direct contact in AC side must be provided.
CI6
Protection against direct contact in DC side must be provided when the output
circuit voltage of the PV generator, at STC, exceeds 60 V.
CI7
The insulation of live parts must be able to withstand any mechanical, chemical,
electrical or thermal stresses under normal service conditions.
CI8
Protection by automatic disconnection of supply must be used in AC excepting
other measures of protection against indirect contact are applied.
CI9
The inverter must be away from heat sources and battery gassing, and the
ventilation spaces have not to be obstructed.
CI10
Inverter should be placed in an easily accessible location with restricted access
to unauthorised persons.
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RI1
The inverter should be preferably connected downwards of the charge regulator
output.
RI2
The protection against direct contacts by means of obstacles or placing out of
reach must be applied only in locations with restricted access to unauthorised
persons.
RI3
In locations with unrestricted access, the protection against direct contacts
should be ensured by means of the insulation of live parts, barriers or enclosures
RI4
The barriers or enclosures must provide a minimum degree of protection of IP
2X or IP XXB, or, if they are placed in an easily accessible location, IP 4X or IP
XXD.
RI5
Residual current devices of high sensibility (≤ 30 mA) should be used in AC
side.
RI6
Floating systems should be used in small PV systems, in those where AC
appliances are operated occasionally, or when using only Class II AC loads.
RI7
Earthing systems should be used in medium and large PV systems where the
power supply is mainly provided in AC for all kinds of loads.
RI8
The inverter must be located in a well-ventilated and dry place.
3.2.3.2 Earthing sytems
RI9
The TN-S earthing configuration is preferred.
RI10 The automatic disconnection of supply can be provided by fuses, over current
circuit breakers, residual current devices, or by the inverter itself (activation of
short-circuit protection).
RI11 The protective conductor must be sized to ensure a voltage drop lower than 50 V
between any Class I equipment and the connecting point to the earthing of the
system. This applies at the disconnecting current.
RI12 The earthing resistance of the buried electrode must ensure a touch voltage less
than 50 V. This applies at the disconnecting current.
3.2.3.3 Floating systems
CI11
AC live conductors must not be intentionally earthed.
RI13 If the inverter is not Class II, a degree of safety equivalent may be provided
during the installation locking the inverter inside an insulated box under key.
RI14 The AC wiring must be installed to reduce the possibility of earthing faults, for
example, using doubled insulated cables or providing a complementary
insulation (plastic pipes, insulated conduits, etc.) to cables with a single
insulation cover.
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RI15 If there is only one Class I AC equipment the protection against indirect contact
is ensured, and its metal casing should not be connected to any protective
conductor.
RI16 If there are two or more Class I AC equipment, their metal casings must be
interconnected by means of an unearthed equipotential bonding.
RI17 The automatic disconnection of supply can be provided by fuses, over current
circuit breakers, or by the inverter itself (activation of short-circuit protection).
RI18 The unearthed equipotential conductor must be sized to ensure a voltage drop
lower than 50 V between simultaneously accessible Class I equipment. This
applies at the disconnecting current.
SI1
The power supply disconnection can alternatively be provided by a RCD placed
at the beginning of the AC distribution line connecting the unearthed
equipotential conductor to a live conductor upstream the RCD.
3.2.3.4 DC circuit
CI12
Alternatively to the Class II inverter requirement, at least one circuit, whether
the AC or the DC one inside a Class I inverter, must be separated from the metal
casing with a degree of insulation equivalent to Class II equipment.
RI19 The inverter must provide DC/AC separation with a degree of safety equivalent
to that of an isolating transformer.
RI20 The inverter shall be selected or erected in accordance with requirements of
Class II equipment or equivalent insulation.
RI21 DC and AC live conductors must preferably be physically separated.
RI22 If the inverter does not provide a safe DC/AC separation, the protection against
both direct and indirect contact in DC side should be ensured using Class II
equipment or equivalent insulation.
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ANNEX 1. ADDITIONAL REFERENCES
AUTHOR
TITLE
YEAR
IEC
IEC 61683. “Photovoltaic systems – Power conditioners – Procedures for
measuring efficiency”.
2002
Quality Program for Photovoltaics (QuaP-PV)
(http://www.worldbank.org/astae/quappv)
World Bank ASTAE
G. H. Atmaram, James D. Roland. “Training Manual for Quality
Improvement of Photovoltaic Testing Laboratories in Developing
Countries. Chapter 5. Test procedures for PV inverters”. Florida Solar
Energy Center.
2001
M. R.Vervaart, F. D. J. Nieuwenhout. “Solar Home Systems: Manual for
the Design and Modification of Solar Home System Components.
Chapter 4. DC-AC conversion”. ECN -Netherlands Energy Research
Foundation.
Y. Ch. Qin, N. Mohan, R. West, and R. Bonn. “Status and Needs of
Sandia National
Power Electronics for Photovoltaic Inverters”. SAND2002-1535.
Laboratories
(http://www.doe.gov/bridge).
2002
PV GAP Recommended Specification. “Inverters for photovoltaic (PV)
stand-alone systems”.
PV GAP
2000
Blank detail specification.
Annex – Specification and testing procedure.
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ANNEX 2. LIST OF REVIEWERS
NAME
ORGANISATION
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COUNTRY
ANNEX 3. HARMONIC DISTORTION
The harmonic distortion of a periodic waveform indicates the content of higher
sinusoid frequency components or harmonics, which are integer multiples of the
fundamental frequency. Fourier theorem states that any non-sinusoidal periodic function
can be represented by the sum of a DC component and sinusoidal terms, which are of
two types: the first one, at the frequency of the function, is the fundamental, and the rest
are harmonics, at integer multiples of the fundamental. In particular, this theorem can be
applied to AC current and voltage, which can break down as follows:
n =∞
v (t ) = V0 + ∑ 2 ⋅ Vn ⋅ sin( nωt + ψ n )
n =1
n =∞
i (t ) = I 0 + ∑ 2 ⋅ I n ⋅ sin( nωt + ψ n − ϕ n )
n =1
Where:
v(t)
AC voltage.
i(t)
AC current.
V0, I0
DC components.
Vn, In
RMS values of nth harmonic component.
n
nth harmonic component.
ω
Angular frequency of fundamental harmonic (n=1).
ψn
Phase shift of nth harmonic voltage.
ϕn
Phase shift between nth harmonic of current and voltage.
The parameter used in this document to indicate the global distortion is the
Total Harmonic Distortion, THD, which represents the ratio of the harmonic
components of voltage (or current) to the voltage (or current) of the fundamental alone:
n =∞
THDV (%) = 100 x
∑V
n=2
V1
n =∞
THDI (%) = 100 x
2
n
∑I
n=2
2
n
I1
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