System Connection of very large variable speed drives for compressor
purposes
K. Krüger, M. Loskarn, C. Unger, R. Witzmann
and contractual agreements on an overall
Summary
viable and economic system design is
For environmental as well as for economic
highlighted.
reasons, large compressors with a power
rating of 10 MW and more are increasingly
Using
fed by electric variable speed drives. The
commissioned plants, possible mitigation
connection of such drives to the public
measures as well as general experiences
medium or high voltage grid requires a
will be presented. To ensure that every
thorough
entire
time certain limits for system disturbances
system mainly in respect to voltage
will be met, filters have to be co-ordinated
stability and harmonics to avoid any
with actual and future system conditions
disturbances of adjacent loads.
taking into account possible outage as well
consideration
of
the
examples
from
recently
as loading conditions of the feeding grid.
The
paper
summarises
the
basic
The
consideration
of
transient
operational behaviour of such compressor
disturbances such as starting or stopping
drives with special consideration of system
the drive are important especially for the
perturbations. Steady state and transient
design of the auxiliaries. Examples from
fundamental
simulations show typical problems which
frequency
interaction
between the feeding system and the drive
are
discussed.
Harmonics
and
interharmonics produced by the drives are
described and the impact of imposed limits
have
to
be
taken
into
account.
SYSTEM CONNECTION OF VERY LARGE VARIABLE SPEED DRIVES FOR COMPRESSOR PURPOSES
K. Krüger, M. Loskarn, C. Unger, R. Witzmann
Siemens AG, Erlangen, Germany
Abstract - For environmental as well as for
economic reasons, large compressors with a
power rating of 10 MW and more are increasingly
fed by electric variable speed drives. The
connection of such drives to the public medium or
high voltage grid requires a thorough consideration
of the entire system mainly in respect to voltage
stability and harmonics to avoid any disturbances
of adjacent loads. The paper summarizes the basic
operational behaviour of such compressor drives
with special consideration of system perturbations.
Using examples from recently commissioned
plants, possible mitigation measures as well as
general experiences will be presented. To ensure
that everytime certain limits for system
disturbances will be met, filters have to be coordinated with actual and future system conditions
taking into account possible outage as well as
loading conditions of the feeding grid. The
consideration of transient disturbances such as
starting or stopping the drive are important
especially for the design of the auxiliaries.
Examples from simulations show typical problems
which have to be taken into account.
This paper discusses the principal considerations
necessary for the connection of large drives to the
I. INTRODUCTION
transmission system. Discussion focuses on
meeting the requirements of power factor and
compatibility of harmonic and interharmonic levels
in the system. Examples from recently
commissioned plants are presented.
During the recent years, electric drives became a
serious alternative to gas turbines mainly for
compressor applications in the range of several ten
Megawatts. Main advantages of electric variable
speed drives are e.g. significantly lower
maintenance costs and the lack of any local air
emissions (1). Such, traditional gas-turbine drivers
are gradually being replaced by modern power
electronics-based solutions.
The technical and economical constraints for such
large drives limit the design possibilities. Most
drives feature a 12-pulse thyristor rectifier on the ac
system side, a current source dc-link and a 3- or 6phase synchronous motor fed by an inverter as
shown in Figure 1. A high-speed compressor is
connected either directly to the motor or via a gear.
To avoid disturbing other loads and interfering with
the operation of sensitive equipment, power quality
requirements are set by the feeding utility to
achieve long term trouble-free system operation. In
most cases, special measures will be necessary to
meet these requirements (2), (3). At the present
time, passive filter circuits are typically used to
meet in particular power factor and harmonic limits.
MV or HV power supply
converter transformer
Ddy5
12 pulse rectifier
dc-link
12 pulse inverter
M
synchronous
motor
Figure 1: General configuration for multimegawatt
variable speed drives
II. STEADY - STATE FUNDAMENTAL FREQUENCY SYSTEM REACTIONS
Fundamental currents on the ac system side of the
drive are mainly determined by the load
characteristic. Therefore, as a first step, motor
torque and speed values have to be derived from
possible operational conditions such as different
flow, pressure or physical properties of the material
to be transported. Normally, these different
operating conditions translate into an envelope in a
shaft power-versus-speed diagram as depicted for
an example in Figure 2. Generally, these curves
z
follow a P = rpm law with z normally in a range
between 2 and 3 .
This diagram acts as a connection between the
industrial process and the electrical power supply
part of the installation. It will be taken as a basis for
the design of the entire drive train. The second
important input variable influencing mainly the
system reaction of the drive converter is the
minimum ac operating voltage determining the
firing angle reserve and thereby the reactive power
1,0
power factor
demand of the drive. Such, large operating voltage
variations may cause additional costs for larger
filters, transformer coils etc. The reactive power
demand at nominal voltage for the converter driving
a motor load according to Figure 2 is shown in
Figure 3 for a minimum design voltage of 90 % of
the nominal value. The power factor values are
given in Figure 4. At full speed, a power factor
better than 0.9 could be achieved by reducing
voltage variations to less than 5 %.
min. load
0,8
max. load
0,6
0,4
0,2
0,0
0
active power (MW)
25
2000
4000
6000
speed (rpm)
20
min. load
max. load
15
Figure 4: Drive power factor
III. TRANSIENT FUNDAMENTAL - FREQUENCY
SYSTEM REACTIONS
10
5
For the design of the auxiliaries of a compressor
plant, transient voltage variations due to the drive
operation have to be limited in order to avoid any
disturbances mainly of the LV load connected.
Figure 5 shows some calculation results in respect
to fundamental voltage changes caused by a large
drive. Voltage sags as well as swells may affect
mainly electronic systems, lighting and small
variable speed drives. In the case, the disturbances
induced by the operation of the large drive exceed
the withstand capabilities of some auxiliaries, there
are several ways to make the different loads
supplied by one power system compatible to each
other:
0
0
2000
4000
6000
speed (rpm)
Figure 2: Drive shaft power demand
reactive power (MVAr)
25
20
15
10
min load
max. load
5
0
0
2000
4000
− reduce the voltage changes caused by the large
drive
− reduce the sensitivity of the disturbed load
− protect individual sensitive loads by certain
measures such as UPS systems
6000
speed (rpm)
Figure 3: Drive reactive power demand
In most cases, only the drive as source of the
disturbance will be considered in respect to any
mitigation measures. However, in many cases, the
other two alternatives might be less costly and may
be implemented easily if considered early enough
in the planning process.
0.10
0.05
0.00
-0.05
0
10
20
30
40
50
-0.10
time(sec)
Figure 5: Voltage variation during drive start
IV. HARMONICS AND INTERHARMONICS
Variable speed drives with a power rating of many
megawatts are equipped with 12-pulse rectifiers
20
min. load
15
max. load
The harmonic and interharmonic voltage level
10
5
0
0
2000
the system reach levels of several promille of the
rated voltage. Especially in the former case, only
some individual speed points are critical which can
be avoided easily by the drive control.
Figure 8: Typical system impedance
impedance in Ohms
25th harmonic current (Amp
with the typical 12-pulse characteristic harmonics.
It has to be taken into account, that the percentage
of the individual harmonics varies significantly from
one operating point to an other (Figure 6).
4000
6000
speed (rpm)
100
10
1
0,1
10
Figure 6: Harmonic current versus speed
Additionally, especially on weak systems, the
harmonic injection of the drive itself depends on
the filter configuration requiring appropriate
modelling tools especially in the case of
unfavourable system conditions (4).
percentage of fundamental current
For the consideration of the harmonic injection into
the system, non-characteristic harmonics have to
be taken into account as well. There are many
factors influencing the occurrence of such noncharacteristics. First of all, remaining 6-pulse
th th
th
th
characteristics (5 /7 , 17 , 19 etc.) are caused
mainly by an imperfect cancellation of the
harmonics of both bridges due to unsymmetries in
the 3-winding transformer. Unsymmetries in the
feeding voltage can lead additionally to triplen
rd
th
harmonics (3 , 9 etc.). The level of even
harmonics is generally quite small and has to be
considered in the case of poorly damped
resonances only. Figure 7 shows a typical
harmonic spectrum of maximum harmonics to be
considered for the investigation of the harmonic
levels due to the drive operation in the system.
100
1000
10000
frequency in Hertz
achieved in the feeding system depends not only
on the injection but also on the impedance of the
system at the respective frequency. The
impedance of the feeding system may vary
significantly versus frequency and can not be
determined by the fault level only (Figure 8).
Additionally, different switching and loading
conditions may lead to substantially varying
impedances. The thorough determination of the
system impedance under different conditions is a
serious task required before the beginning of any
project with large converters involved. The
following main parameters should be considered:
− actual and future status of network expansion
− different generation and loading conditions (e.g.
maximum winter and minimum summer load)
− outage conditions
Figure 9: Example for an harmonic impedance
envelope
The most suitable way of presenting the results is
by definition of impedance envelopes at the point of
140
10
120
8
100
6
80
4
60
40
2
20
0
1
7
13
19
25
31
37
43
49
order number
Figure 7: Typical harmonic spectrum of a 12pulse variable speed drive
Interharmonics are caused by the incomplete
smoothing of the dc current and occur at
frequencies varying with the motor frequency.
Interharmonics are mainly important in the case of
a ripple-control system run by the supply company.
Also flicker may be an issue under unfavourable
conditions. In both cases, interharmonic voltages in
0
0
10
20
30
40
50
R in Ohm
common coupling (PCC) for as much frequencies
as possible (5). At least, all odd order numbers
should be covered. To cover uncertainties in the
model, a certain frequency band around the
frequency under consideration should be
considered in the calculations. Figure 9 shows an
example for an impedance envelope for one
individual frequency under
different switching conditions.
consideration
of
harmonic limits for rare overload conditions, or
p.f. limits for rare low-load conditions)
− applying fixed Var instead of p.f. limits for lowload operating conditions
V. IMPOSED LIMITS
Independent on the drive characteristic, the
permissible level of interaction with the ac system
is limited according to the requirements of the
feeding utility. In many cases, maximum limits for
reactive power exchange, harmonic current
injection and/or resulting harmonic voltage
distortion must be held typically at the PCC and
require any special filter measures. Most common
standards are summarized in Figure 10.
For large converter applications, it may save much
money to start discussing early enough the cost
impact of harmonic and p.f. limits. In many practical
cases, a slight reduction of the conditions where
limits have to be met may reduce substantially the
effort for the filter equipment. The following
contractual changes may be applied without or with
a slight impact to the utilities transmission system
or other loads only, and had already helped in the
past reducing filter costs:
− Shifting the PCC for the purpose of observation
of limits from the border of ownership to the
next node in the system where other loads are
connected
− Excluding rare outage conditions from the
cases where limits have to be met
− Allowing to exceed limits by a certain
percentage for rare operating conditions (e.g.
IEC 61000
voltage distortion
frequency-dependent limits for
individual distortion, THD limit,
identical limits for LV and MV
current distortion
no limits
interharmonics
Communication line
interference (I*T)
frequency-dependent
recommendations
not covered
It has to be taken into account, that the
implementation of a large converter load with filters
in the system may have a substantial impact to
existing installations in the grid even if the
disturbing emissions are quite low under steadystate conditions. The following effects were already
observed:
− Overloading of existing compensation units in
the transmission system due to energization of
the new converter transformer
− Increase/decrease of the voltage distortion
produced by other already existing loads due to
changes in the system resonance conditions
− Voltage sags and swells during drive start and
stop which can not be compensated by the
slower utility voltage regulation
VI. EXAMPLES
A typical drive configuration with associated filters
is shown in Figure 11. The total size of the filter
plant in terms of MVAr depends on the p.f.
requirements. The number of individual filters
required is mainly determined by the harmonic
limits. In many cases, filters are tuned to all lowth
th
th
frequency 6-pulse harmonics (5 , 7 , 11 etc.)
followed by a high-pass filter in order to damp
higher-frequency resonances and to take higherfrequency harmonic currents.
IEEE - 519
frequency-independent limits for individual
distortion, THD limit, limits depend on supply
voltage level
individual and total limits depending on
frequency, voltage level and short-circuit-to-load
ratio
not covered
3 categories between 10.000 and 25.000
Figure 10: Common international standards
VII. CONCLUSIONS
~
~
M
n:
5
7
11
13
Figure 11: Typical filter circuit configuration
Filters may be connected generally either to a
dedicated converter transformer winding or to the
feeding busbar. In most cases, the former
alternative is less expensive in the case of a direct
high-voltage infeed without any intermediate
medium voltage level. The connection of filters to
an additional winding requires a specific design of
the converter transformer in order to optimize the
filter effect.
In the case of an unfavourable system
configuration consisting on an inductive system
impedance together with the capacitance of a
feeding cable, the system may behave as a filter
circuit itself. In this case, single-tuned filters in
parallel to the converter load are unable to filter
harmonics sufficiently especially when the system
resonance condition is close to a powerful
th
th
harmonic as the 11 or 13 . In such cases, the
system impedance has to be increased artificially
by a parallel blocking filter (Figure 12).
utility system
parallel resonance
blocking circuit
13.8 kV
~
filter bank
Large variable speed drives are becoming more
and more attractive, especially for pumping and
compressing applications. Prior to the connection
of such drives to the ac system, careful
investigation of their effects on the feeding system
is required. However in most cases, there are
solutions to reduce these effects to such an extent
that the limits set by the utility will be met. To obtain
the most economical solution for the mitigating
equipment, a close co-operation between utility,
operator and manufacturer of the plant is required.
~
variable speed
drive
M
Figure 12: Filter connection at a dedicated
winding and blocking filter
VIII. REFERENCES
1. Oliver J.A., Samotyj M.J., Dec. 1999
"Electrification of Natural Gas Pipelines – A Great
Opportunity for Two Capital Intensive Industries".
IEEE Transactions on Energy Conversion, Vol. 14,
No. 4
2. Loskarn M., Tost K.D., Unger C., Witzmann R.,
1998
“System Connection of Large Cycloconverter-Fed
Mill Drives – Experiences and Mitigation Methods"
th
12 CEPSI, Pattaya, Thailand
3. Duchon G., Schultz W., Unger C., Voss L.,
Lockley B., Leuw J., 1997
“Experience With the Connection of Large
Variable Speed Compressor Drives to HV Utility
Distribution Systems” IEEE Summer Meeting,
Berlin, Germany
4. B.K.Perkins B.K.
"Steady-state solution of the HVDC converter
including AC/DC system interaction by a direct
method" IEEE paper accepted for publication (not
published yet)
5. Sachs U., Tyll H., Unger C., April 1977
“Network
Models
Enlighten
Harmonics
Investigation,” IEEE Computer Applications in
Power, vol 9, no. 2, pp. 23–26