Grid Connection Equipment and Technology – Best Practice and
Philosophy
CONTENTS
1
1.1
1.2
INTRODUCTION
1
GENERATOR GRID CONNECTION - PRINCIPLES
1
1
2
SYNCHRONIZATION AND CONNECTION
1
3
SYNCHRONIZATION
1
3.1 VOLTAGE FLUCTUATION
3.2 SYNCHRONISATION LIMITS
3.3 SYNCHRONISATION OF INDUCTION MACHINES
3.4 SYNCHRONISATION OF SYNCHRONOUS MACHINES
3.5 VARIABLE SPEED DRIVES
3.6 INTERCONNECTION PROTECTION
3.7 INTERCONNECTION PROTECTION OUTLINE
3.7.1 UNDER AND OVER VOLTAGE PROTECTION
3.7.2 UNDER AND OVER FREQUENCY PROTECTION
3.7.3 TIME DELAY ON VOLTAGE AND FREQUENCY PROTECTION
3.8 ISLANDING
3.9 HAZARDS OF ISLANDING
3.9.1 UNEARTHED OPERATION
3.9.2 LOWER FAULT LEVELS
3.9.3 OUT OF SYNCHRONISATION RECLOSURE
3.9.4 VOLTAGE LEVELS
3.9.5 QUALITY OF SUPPLY
3.9.6 RISK TO MAINTENANCE PERSONNEL
3.9.7 ISLANDING SUMMARY
3.10 LOSS OF MAINS PROTECTION
3.10.1 RATE OF CHANGE OF FREQUENCY
3.10.2 VOLTAGE VECTOR SHIFT
3.10.3 INTER TRIPPING
3.10.4 OTHER POSSIBLE METHODS
2
2
2
2
3
4
4
4
4
5
5
5
5
5
5
6
6
6
6
6
6
7
7
7
4
7
INTER CONNECTION PROTECTION IMPLEMENTATION
4.1.1 INTERCONNECTION PROTECTION SETTINGS
4.2 INTERCONNECTION PROTECTION TESTING
4.3 OTHER SYSTEM TESTING
7
8
8
1
Introduction
The issues to be covered in this section are:
Grid connection principles
Synchronisation requirements and methods
Interconnection protection
Islanded operation – meaning and hazards
Loss of Mains protection
Protection implementation and testing
1.1
Generator Grid Connection - Principles
The distribution network should not be unacceptably affected by the generator. That is to say,
the quality of supply to customers should be maintained at all times. This is the prime DNO
requirement.
Conversely, the generating equipment is not to be damaged by the distribution system. The
generator should be able to operate, and export power to the grid as intended.
Safety should be maintained at all times for the generator, distribution system and customers
2
Synchronization and Connection
A generator must be synchronized with the grid prior to connection.
Specific regulations must be followed for the interconnection arrangements and protection,
for example G59 or G83 in the UK.
Other requirements that must be complied with include:
Electrical safety,
earthing,
voltage regulation,
harmonics
3
Synchronization
Synchronisation means the minimisation of the difference in:
magnitude (voltage)
frequency
phase angle
between the corresponding phases of the generator output and the grid supply prior to the
connection of the two supplies. Synchronisation MUST occur before a generator can be
connected to the grid
Synchronisation can be achieved either manually or automatically, Automatic synchronisation
is preferred. It is very unlikely that new installations will include only manual
synchronisation. If manual synchronisation is suggested for a particular reason, its safe and
reliable operation should be seriously considered and implemented carefully.
DR-S6-RevA.doc
Section 6
Page 1
3.1
Voltage Fluctuation
There will normally be a voltage fluctuation on the distribution line when a generator is
synchronised. During Synchronisation of a generator, the induced voltage fluctuation on the
grid should:
1.
2.
3.2
not normally exceed 3% at the Point of Common Coupling, and
should meet the requirements for voltage step and flicker
Synchronisation limits
The normal limits specified for allowing synchronisation are:
Phase angle
+/- 20 degrees
Maximum voltage difference 7%
Maximum slip frequency
0.44 %
These are the maximum settings of a synch check relay
Synch Check relay
A “synch check” relay must be used to check that the synchronisation of the generator and the
grid is within the specified limits. The relay must operate on at least two, and preferably all
three phases to ensure phase rotation is correct. The use of synch check relays does not apply
to induction generators.
The requirement for a synch check relay in addition to automatic synchronisation systems is:
1.
2.
3.3
to act as a backup during synchronisation and
to act as a backup to ensure a generator will not connect to a dead distribution line
Synchronisation of Induction machines
An Induction generator is easily synchronised and connected with the grid. It just needs to be
run up to synchronous speed and connected BUT there will be a large inrush current and
consequent drop in voltage. Some standards require that there is limited inrush current
The two main methods of synchronization are:
1.
2.
Use an electronic soft start unit to “motor” the generator up to the synchronous speed.
This method can be used to control inrush current. A standard motor controller is used to
run the generator up to the synchronous speed and is then bypassed by a contactor.
Use the turbine shaft power (with the control valve very slightly open), to mechanically
drive the generator up to synchronous speed.
Once the generator is at the synchronous speed, the generator may be connected to the grid.
It is best if the generator speed is slightly above the synchronous speed as the generator will
then export power as it slows down to the synchronous speed.
3.4
Synchronisation of Synchronous Machines
A Synchronous machine needs to be running at synchronous speed and has the additional
requirement that the generator’s output voltage waveform should be “in phase” with the grid
DR-S6-RevA.doc
Section 6
Page 2
voltage waveform. Put another way, the phase angle between the generator output and the
grid supply must be within specified limits.
Not only should the generator’s output voltage waveform be “in phase” with the grid voltage
waveform but, during the process of synchronizing, the rate of change in phase angle between
the grid and the generator must be within specified limits. A “Synch Check” relay is used to
ensure that the generator is connected to the grid only when it is in synch.
Synchronisation can be achieved by either:
1.
2.
3.5
Controlling the turbine power so that the generator speed will slowly pass through the
synchronous speed and hoping that the voltages will pass through being in phase – this
needs good turbine speed control.
Control of the load on the generator to achieve synchronisation. This can practically be
achieved with an Electronic Load Control (ELC) system. ELC systems are described in
another section of this guide. This method of synchronization permits a very smooth an
accurate way of connecting the generator to the grid.
Variable Speed Drives
A variable speed drive arrangement can be used to maintain a constant output frequency
whilst allowing the generator speed to vary. This can be useful with low head reaction
turbines which only run efficiently if the speed of the turbine matches the flow – if the flow
changes, the speed of the turbine needs to change to maintain efficiency. Such turbines
include the Propellor turbine and the Archemedies screw type turbines (such as one
implemented by GP Electronics at the Dart Country Park).
Using a variable speed drive, the turbine can be run at it’s optimum operating speed for the
available flow and head. A pair of variable speed motor drives are used to connected back-toback with one half of the system running in regenerative breaking mode.
Fig.3.1 - Variable speed drive example
DR-S6-RevA.doc
Section 6
Page 3
3.6
Interconnection Protection
Interconnection Protection describes the protection required between the generator and the
distribution system. The Interconnection Protection is required to ensure the principles
outlined in the Introduction for generator connection are complied with. The required
Interconnection Protection will depend on a number of factors, including:
1.
2.
3.
3.7
Individual and cumulative generation capacity
Generator type
Relative size compared to captive load
Interconnection Protection Outline
The primary types of interconnection protection required for embedded generators include:
Under and Over Voltage
Under and Over Frequency
Loss of Mains (Islanding) protection includes:
ROCOF
“true” ROCOF
Voltage Vector Shift
Intertripping
Neutral Voltage Displacement
3.7.1 Under and over voltage protection
Under and over voltage protection is required to:
1.
2.
Ensure the grid system is operating within specification and is suitable for connection
(e.g. it will not damage the generator).
Act as a backup Loss of Mains protection
Typical settings:
Under voltage -10% of nominal voltage
Over voltage +10% of nominal voltage
3.7.2 Under and over frequency protection
Under and over frequency protection is required to:
1.
2.
Ensure the grid system is operating within specification and is suitable for connection
(e.g. it will not damage the generator)
Act as a backup Loss of Mains protection
Typical settings:
Under frequency -6% of nominal frequency
Over frequency +1% of nominal frequency
DR-S6-RevA.doc
Section 6
Page 4
3.7.3 Time delay on voltage and frequency protection
A time delay is usually allowed for the operation of voltage and frequency protection. This
time delay is to minimise nuisance tripping due to grid system disturbances. The maximum
time delay must be less than the minimum operating time of auto-reclose devices used on the
distribution line. Typical maximum time delay is 0.5 secs – this includes the operation time of
any relays and breakers in the control and protection system. Generator operators should
make use of this time delay to minimise nuisance tripping.
3.8
Islanding
An Islanded situation is where the generator(s) and some local loads have become
disconnected from the main distribution system. The usual situation is a section of distribution
line (or the complete distribution line or lines), becomes disconnected from the main
distribution / transmission system. The generator(s) output may balance with the load
demand.
3.9
Hazards of Islanding
The hazards created during islanded operation are:
1.
2.
3.
4.
5.
6.
Unearthed operation of the distribution system
Lower fault levels
Out of synchronisation reclosure
Voltage levels outside statutory limits
Reduction in quality of supply
Risk to maintenance personnel
3.9.1 Unearthed operation
Usually the distribution line is only earthed at the grid sub station. At the generating site the
high voltage (HV) side of the transformer is in delta configuration with no earth. When the
distribution line, and its generator(s), are disconnected from the grid sub-station the line is not
earthed. This is a potential hazard as line voltages may become excessive in a fault. This is
detected by Neutral Voltage Displacement protection
3.9.2 Lower Fault Levels
In an islanded condition the fault level will be reduced. The fault level contribution from the
grid sub station has been lost. The reduction in fault level will affect the operation of
protection on the distribution line. There may not be sufficient current generated in a fault
condition to operate the protection.
3.9.3 Out of Synchronisation Reclosure
During islanded operation the island will not maintain synchronisation with the grid system.
When the grid system attempts to reconnect the islanded distribution line to the grid system
there will an out of synchronisation siituation at the reconnection point. If out of
synchronisation reclosure occurs, there will be sudden and potentially large power flows and
voltage disturbances. This may cause damage to distribution, generator and consumer
equipment
DR-S6-RevA.doc
Section 6
Page 5
The use of synch check or dead line check relays on auto reclose breakers will protect against
out of synchronisation reclosure. If auto reclosers are to be used on distribution lines that have
emebedded generators connected to them, the reclose time should be at least twice the
maximum disconnection time of any generator protection systems.
3.9.4 Voltage Levels
The voltage levels of an islanded distribution line are controlled by the generator voltage
regulators, the generated power and the load demand. Voltage levels may go outside statutory
or normal operating limits.
3.9.5 Quality of supply
The quality of supply of an islanded distribution line will generally be reduced. It will be
subject to voltage variations as there is limited generating capacity and may be little control
on the output of this capacity.
3.9.6 Risk to Maintenance Personnel
There is a risk to the distribution system maintenance personnel from islanded operation. For
example, a line may be disconnected for maintenance purposes and the maintenance
personnel may assume the line will become, or remain, dead. Should a generator continue to
be connected, or become connected later, this will be a hazard for them.
3.9.7 Islanding summary
Due to the hazards listed the operation of an islanded situation is generally not allowed. There
may be an advantage to consumers, and generating companies, to allow islanded operation.
This is the maintenance of supply when a line has been disconnected. This advantage is small
and the hazards outweigh the advantages. It may be allowed in special situations with
appropriate additional protection
3.10
Loss of Mains Protection
Loss of Mains (LoM) Protection is designed to sense when the generator, and any local loads,
have become disconnected from the main grid system that is to say, when an island has been
created.
Normal forms of Loss of Mains protection for G59 sized systems are:
1.
2.
3.
Rate of Change of Frequency – usually referred to by the terms RoCoF or dF/dT
Voltage Vector Shift – usually referred to by the term dV
Inter tripping
3.10.1 Rate of Change of Frequency
When an island occurs, there is usually an imbalance between the generator output and local
captive load. The generator will therefore speed up or slow down. This causes a change of
frequency. The change of frequency can be detected by a Rate of Change of Frequency relay
(RoCoF or dF/dT). Settings of the relay required depend upon the relative size of generation,
captive load and system inertia. The first two items are difficult to quantify. A typical setting
is 0.15 Hz/second.
DR-S6-RevA.doc
Section 6
Page 6
3.10.2 Voltage Vector Shift
At the moment when an island occurs, there will be a jump in the voltage waveform as the
current flow is disturbed. The size of the waveform jump will depend upon the current flow
and system characteristics. This jump is known as Voltage Vector Shift. It can be single or
three phase – usually single phase is asked for to detect loss of a single overhead line. Typical
settings are 6 to 8 degrees. Setting requirement will be as the same as given for Rocof,
however, it will depend on the generation and location.
3.10.3 Inter tripping
Inter tripping is a communication connection between the distribution sub-station and the
generator. If there is a trip at the sub-station (e.g. caused by a line fault) this will signal to the
generator to disconnect as an island has formed. Whilst is very reliable, it is expensive to
make arrangements for and implement. This is because it requires fail safe, reliable
communications. In order to arrange for this, the distributed network operator (DNO might
need a fixed leased line. Usually this kind of arrangement is only made for larger generation
schemes of several MW or more.
3.10.4 Other possible methods
Other methods of islanding protection that may be considered for some particular situations
include:
1.
2.
3.
4
Low power or reverse power protection
Fault throwing – causing a line fault which will operate other forms of protection
Reverse VAR protection- detecting a flow of reactive power from the generator to the
grid. Requires the generator to normally import reactive power and have stable power
factor control
Inter connection Protection Implementation
Inter connection protection necessitates the use of standard relays or proprietary systems.
Standard protection relays are available from several manufacturers such as Selco, Areva,
P&B Engineering and SEG/AVK. Nowadays, the equipment is always digital. Proprietary
systems have been developed for specific applications. These are analogue or digital systems.
From a specification view point, always use relays designed and manufactured to IEC
standards e.g. IEC 255 – to ensure performance and compatibility. It is also good practice to
have a direct operation of the breaker / contactor by the protection relay. If relying on a PLC
system then the PLC code may need to be proved and verified which could be time
consuming and expensive.
4.1.1 Interconnection Protection settings
The standards and connection agreement will specify protection settings. Loss of Mains relays
are a common source of nuisance tripping. For example, local grid disturbances (particularly
on weak distribution lines), can cause voltage dips and vector shifts. This may cause problems
for a generator loosing generation time and may be a reason to discuss possible changes to
settings. A common cause of tripping can be a problem when a fault on one distribution line
causes tripping on other lines. Some forms of islanding protection are more subject to
common cause tripping.
DR-S6-RevA.doc
Section 6
Page 7
4.2
Interconnection Protection testing
An on-site test will normally be required by the Network Operator when the system is first
grid connected. Testing is required to prove the operation of the interconnection protection
and verify settings. Testing is undertaken by the generator and witnessed by the Network
Operator engineer. It is the responsibility of the generator to provide a suitable test procedure
and test equipment.
It is normally the Generator operator’s responsibility to maintain and re-test the protection
equipment at regular intervals (typically every 3-5 years). In order to do this, the following
preparations will need to be made before the day of the test:
1.
2.
3.
4.
5.
Discuss and agree tests to be undertaken and test procedures with the Network Operator
engineer before hand
Check through the test form to be completed by the Network Operator engineer
Provide single line protection drawings to the Network Operator engineer
Use calibrated test equipment
Agree where signals are to be injected and protection operation is to be measured
Tests will include:
Over and Under Voltage
Over and Under Frequency
Loss of Mains – ROCOF and / or Voltage vector shift
Loss of supply – disconnect main supply
Loss of phase – disconnect one phase of the supply.
On an LV connected system the Utility can usually pull one supply fuse.
On an HV connected system the Generator may have to add a facility to disconnect one phase
Loss of supply to the protection relay
4.3
Other System Testing
It is the generator operator’s responsibility to undertake testing of all other electrical
equipment. These tests will include at least the following:
1.
2.
3.
4.
5.
6.
7.
Cable integrity, connections and fixing
Insulation resistance
Earthing continuity
Earth system resistance
Control system operation
Sensor operation
Generator AVR and PFC setup and commissioning
A clear record must be made of all tests and the test procedures should comply with the
appropriate legislation. This is a large subject which is beyond the scope of this section.
David Roberts
MorbenHydro
November 2008
DR-S6-RevA.doc
Section 6
Page 8