REACTIVE POWER SUPPORT FROM GENERATORS AND VOLTAGE CONTROL
PRINCIPAL APPLICATION DETAILS AND POINTS TO TAKE INTO CONSIDERATION
DEFINITIONS:
1-
Overexcited operation: Lagging operation of the generator to provide MVARs to the grid.
2-
Underexcited operation: Leading operation of the generator to draw MVARs from the grid.
OPERATION PRINCIPLES WHILE PROVIDING REACTIVE POWER SUPPORT
The generation company, must regulate the high side bus bar voltage of the related power plant, by
utilizing the reactive power capacity of each unit at the related power plant, according to a set point, and
within a tolerance band*, both defined by the Regional and/or National Control Center. The generation
company, while realizing this responsibility, should be in accordance with the operating principles
mentioned below. Main aim should be to regulate the high side bus bar voltage of the power plant by
utilizing an automated outer loop. (where inner loop is the AVR)
a) Regulating high side bus bar voltage of the power plant via an Automated Outer Loop.
Defined bus bar
voltage set point
Bus
Voltage
V/Q Ref Voltage
or MVAR
Ref. Set
Plant Bus bar
Voltage
Controller
Active/reactive
loading of the other
units
Gen. Terminal
Voltage Ref.
(Raise/Lower)
Generator
Terminal
AVR
Q, MVAR
G
Step-Up
Trafo
System
Other units
High side Bus
Figure 1: Principle block diagram for bus voltage control: Outer loop
*To evaluate the success of the bus voltage regulation, the tolerance band will be defined by TEİAŞ.
In this configuration, the droop of voltage control, ΔV/ΔQ, (i.e., 0%, integral control or 4%, proportional
control), should be evaluated with TEİAŞ when necessary, taking into account the facility’s network
connection, to eliminate unintentional MVAR sharing/swings between electrically close plants.
A remarkable point that must be taken into consideration in this configuration is to operate “individual
units”, mainly on Generator Terminal Voltage Regulation or on Slow Reactive Power Control** mode,
and configure the outer loop slower than the inner loop, to assign Terminal Voltage Reference (raiselower pulses) or MVAR reference to each individual unit.
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Reactive Power Support and Voltage Control, principal application details and points to take into consideration, 17.12.2010
b) Regulating high side bus bar voltage via Line Drop Compensation Method (IEEE Std. 421.5)
According the
defined set point
and tolerance Raise
Lower
Generator
Terminal It
Voltaj
Ref. Set
(pu)
AVR
G
Vt
Vc= | Vt + jIt.Xc |, Xc<0
Vc
Step-Up
Trafo,Xt
System
Current/Voltage
Measurement
Other units
High side Bus
Figure 2: Principle block diagram for bus voltage control: Line Drop Compensation
This configuration is appropriate for plants where each unit has a separate step-up transformer. Mainly,
this is an implemented but not activated function in modern digital excitation systems. By compensating
around 50% of the step up transformer impedance (0.5*Xt <= Xc <=0.7*Xt) an effective bus voltage
control can be achieved. However to determine the most appropriate compensation level and for the right
configuration, manufacturer or engineering support might be necessary. The control structure in Figure 1
can be thought to be more suitable for general operation, than configuration in Figure 2, if specific
knowledge is not present.
** What is mentioned by Slow Reactive Power Control on individual unit basis is that the reactive power controller
on the unit basis must be configured much slower than the AVR loop. A reactive power set point change that will
reach steady state in 25-30 second can be accepted as slow MVAR control. In addition, the plant bus voltage
controller should be configured in such a way that it should be slower than the inner loops. Fast Reactive Power
Controllers or Power Factor Controllers on unit basis, must not be utilized.
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Reactive Power Support and Voltage Control, principal application details and points to take into consideration, 17.12.2010
c) Regulating high side bus bar voltage by Control Room Operator Supervision.
Defined bus
bar voltage
set point
Raise
Lower
Voltage
Ref. Set
Terminal
Voltage Ref.
Generator
Terminal
AVR
Q, MVAR
G
Step-Up
Trafo
Control
Bus Voltage
Room
Position of OnLoadTap Operator
System
Changer , if exists ****
Other units
Active/Reactive
Loading of other
units
**** OnLoad Tap Changer can either be used in Manual Mode by the operator or in
Auto mode to regulate high side voltage.
High Side Bus
Figure 3: Principle block diagram for bus voltage control: Operator supervision
A remarkable point to take into consideration in this configuration, where operator supervision is the
basis, is that individual units without an automatic outer control loop, must be operated on Generator
Terminal Voltage Regulation mode (voltage supporting operation, AVR Auto Mode). When high side bus
voltage control is realized by operator supervision, unless otherwise stated by TEİAŞ, individual units
must not be operated on Reactive Power Control or Power Factor Control modes.*** During
instantaneous voltage dips or during a situation where a voltage collapse might occur, automatic AVR
action of the individual units and continuation of the voltage support is vital and can only be realized in
this manner.
In the end, what is expected with the utilization of three different control structures mentioned here is that,
units must use their compulsory reactive capability portion, between nominal and minimum generation
levels, when required, to regulate the high side bus voltage. Whether the units reached maximum
compulsory MVAR levels or not, will be checked unless bus voltage is regulated within the defined
tolerance band.
For all control structures, as a general principle, the continuous operation band of generator terminal
voltage must not be less than %95-%105 of nominal terminal voltage. V/f Limiter and V/f Trip values are
to be set as >=% 107~108.
In case different control structures are utilized, the control structure must be presented to TEİAŞ, as a
principle block diagram for the evaluation of appropriateness.
*** For units or generation facilities below certain capacities, Reactive Power Support Service, final decision being up to TEIAS,
may be obtained via these operating modes by maintaining a set reactive power or power factor output, instead of voltage control.
Especially for small units being connected to stiff buses, these modes might offer ease of operation.
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Reactive Power Support and Voltage Control, principal application details and points to take into consideration, 17.12.2010
PERMISSIONS/PRECAUTIONS BEFORE TESTING
AND
POINTS TO TAKE INTO CONSIDERATION
1. An approval from TEİAŞ National or Regional Load Dispatch Centre should be obtained.
2. Unit under test, being independent of all other outer loop controls, must be operated on Generator
Terminal Voltage Regulation Mode (AVR Auto Mode) and reactive power loading will be
realized via issuing raise/lower commands to the generator terminal voltage set point. For units,
where there is an on load tap changer on the step-up transformer, reactive power loading can be
realized via tap changing and issuing raise/lower pulses to the terminal voltage set point.
3. Bus voltage deviations due to testing, should not exceed ~ ± % 2-3 of the nominal voltage. In
order to achieve this, remaining units of the power plant may be used to regulate the voltage
accordingly or coordination with the Regional Control Center may be realized.
4. In order to be able to reach the maximum MVAR loading of the unit, related tests
(overexcited/underexcited) should be started taking into account the bus bar voltage level (below
nominal/over nominal).
5. If it is possible for the unit under test to reach nominal load, tests are to be realized at nominal
load. Tests are to be carried out under normal operating conditions, the unit is generally
experiencing (hydrogen pressure etc...) and any changes that will temporarily affect the test results
in positive/negative manner should be avoided. The idea is to test the unit at “as is” operating
conditions. Of course permanent improvements are welcome.
6. Generator capability curve and settings of related protection functions (V/f limiter, V/f trip, Over
Excitation Limiter, Over Excitation Trip, Stator Current Limiter, Over Volts trip, Under
Excitation Limiter, Loss of Excitation Trip, Under Voltage trip etc... ) must be obtained and
analyzed before the tests and must be attached to the test report.
7. Principle block diagram of the control structure used to regulate the high side bus of the plant is to
be attached to the report.
8. Following the completion of tests, Reactive Power Support Service Test Report must be filled and
signed by the parties.
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Reactive Power Support and Voltage Control, principal application details and points to take into consideration, 17.12.2010
REACTIVE POWER SUPPORT SERVICE, TEST PROCEDURE
1) While the unit is at nominal load,
Increase the generator terminal voltage set point (change the step-up transformer tap position), gradually,
until one of the below is realized:
(a) Generator capability curve limits are reached,
(b) V/f limiter becomes active, (maximum allowable continuous operation generator terminal voltage
(% 105 – 110)
(c) Over Excitation Limiter becomes active,
(d) Stator Current Limiter becomes active,
(e) Maximum allowable auxiliary equipment voltage is reached, (if the unit under test has a direct
influence on the auxiliary/house load voltage and if it is not regulated by some other means)
Determine/note down explicitly and clearly the reason / protection function / setting that hinders the unit
to take up more MVARs. After 30 minutes of operation at this MVA loading, fill up Table 1.
Time
Generator
MW
Generator
MVAR
Generator
Terminal
Voltage
(kV)
High
Side
Bus
Voltage
(kV)
Field
Current
(Amp)
Field
Voltage
(Volts)
Stator
Current
(kAmp)
Aux./
House
Load
voltage
(kV)
Beginning
of the test
End of the
test
What is the reason that the unit can not take up more lagging MVARs ?:
Table 1: Variables to be recorded for overexcited operation
2) While the unit is at nominal load,
Decrease the generator terminal voltage set point (change the step-up transformer tap position), gradually,
until one of the below is realized:
(a) Under Excitation Limiter becomes active,
(b) Minimum allowable continuous operation generator terminal voltage is reached, (<= %95)
(c) Generator capability curve limits are reached,
(d) Stator Current Limiter becomes active,
(e) Minimum allowable auxiliary equipment voltage is reached, (if the unit under test has a direct
influence on the auxiliary/house load voltage and if it is not regulated by some other means)
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Reactive Power Support and Voltage Control, principal application details and points to take into consideration, 17.12.2010
Determine/note down explicitly and Cleary the reason / protection function / setting that hinders the unit
to take up more MVARs. After 30 minutes of operation at this MVA loading, fill up Table 2.
Time
Generator
MW
Generator
MVAR
Generator
Terminal
Voltage
(kV)
High
Side
Bus
Voltage
(kV)
Field
Current
(Amp)
Field
Voltage
(Volts)
Stator
Current
(kAmp)
Aux./
House
Load
voltage
(kV)
Beginning
of the test
End of the
test
What is the reason that the unit can not take up more leading MVARs ?:
Table 2: Variables to be recorded for underexcited operation
3) Repeat Steps 1 and 2 at minimum load, and at two more levels between Minimum and Nominal Load
(in total, four different loading including the nominal load). Fill up the necessary tables accordingly.
POINTS TO TAKE INTO CONSIDERATION
1. During the tests, if compulsory overexcited and underexcited MVAR portions could not be
reached due to the generator terminal voltage operating range, (i.e. V/f limiter, overvolts
protection...), tests should be repeated by re-evaluating and modifying if necessary the limiter
settings for a permanent improvement, by changing the on load tap changer position or postponing
to a new date for the off-load tap changer position change.
2. During the tests, if compulsory overexcited and underexcited MVAR portions could not be
reached within %10 tolerance of these values, other than the reasons mentioned above Item 1,
(other limiters and protection functions), technical reasons and solutions (protection setting
coordination etc...) must be mentioned and applied. Tests are to be repeated after improvements
were realized.
3. During the tests, if the maximum MVAR loading, beyond the compulsory overexcited and
underexcited MVAR portions, differ from the values defined in the generator capability curve
more than 10% of these values, technical reasons must be mentioned.
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Reactive Power Support and Voltage Control, principal application details and points to take into consideration, 17.12.2010
REACTIVE POWER SUPPORT SERVICE TEST PROTOCOL, INFO SHEET
1. Date
:........................................
2. Name of the Power Plant
:........................................
3. Code/Number of the unit under test
:........................................
4. Unit’s nominal power (as defined in the Acceptance
Protocol or Generation License) (Pnom)
:..….….…..… MW
5. Type of the turbine (Hydro, Gas, Steam...)
:.....................
6. Unit’s minimum continuous operation loading level
:.........…….… MW
7. Generator’s nominal MVA
:.........…….… MVA
8. Generator’s nominal terminal voltage
:.........…….… kV
9. Generator rated field (rotor)
current/voltage
:.........…….… Amp;
:.........…… Volt
10. Excitation system (exciter) rated
current/voltage
:.........…….… Amp;
:.........…… Volt
11. Nominal voltage of the generator high side
bus
:.........…….… kV
12. Nominal power factor of the generator
:.........…….…
13. Type of generator cooling
(direct air / water-air / water-hydrogen)
:.........…….…
14. Compulsory Overexcited MVARs
(Qmax +)
:...................... MVar
15. Compulsory Underexcited MVARs
(Q max -)
:...................... MVar
16. Step-Up Transformer
Nominal Primary (low) and Secondary(high) Voltage
:.........…….…kV / .........…….……kV
17. Step-Up Transformer impedance (%);
X/R Ratio; and nominal MVA
:%...........…;
18. Tap position of Step-up Transformer
During the test
:.........…….…
X/R=.........; ..…..MVA
19. Protection and Limiter Settings (Value/Time):
V/f limiter, V/f trip, Over Excitation Limiter, Over Excitation Trip, Stator Current
Limiter, Over Voltage Trip, Under Excitation Limiter, Loss of Excitation, Under Voltage
Trip
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Reactive Power Support and Voltage Control, principal application details and points to take into consideration, 17.12.2010