IEEE Power System Paper-Coordination Control of Statcom and Ultc of Power Transformers

COORDINATION CONTROL OF STATCOM AND ULTC OF POWER TRANSFORMERS Mojtaba Khederzadeh (1) (1) Power & Water University of Technology, IRAN

ABSTRACT STATCOM provides the opportunity to improve power quality and reliability due to its fast response, and has the functional capability to handle dynamic conditions, such as transient stability and power oscillation damping in addition to providing voltage regulation. When both STATCOM and under-load tap changers (ULTCs) are used to control system voltage, the STATCOM reacts to the voltage deviation faster than the ULTC. If the STATCOM output reaches the maximum capacity limit, it loses active control and behaves similar to a shunt reactor/capacitor bank. Keeping reactive power reserve in an STATCOM during steady-state operation is always needed to provide reactive power requirements during emergencies. This paper presents a new control strategy to limit the steady-state reactive-power output of the STATCOM to a desired value during the steady-state voltage range based on a variable ULTC reference voltage control scheme. When STATCOM settles to a new operating point following a disturbance, the reference voltage of ULTC will be effectively changed based on the STATCOM output; thereby activates slow voltage regulators to return back and resetting the STATCOM to be within the steady-state margin. Keywords: STATCOM, Under-Load Tap Changer (ULTC), Power Transformer, Voltage Regulation.

1 INTRODUCTION Voltage magnitude of a substation bus is generally controlled by the under load tap changer (ULTC) of the power transformer and several capacitor banks/reactors; the transformer changes its tap position to control the lower side voltage magnitude directly, whereas the capacitor banks/reactors affect the higher side voltage magnitude indirectly by changing the amount of reactive power demand at the bus. Although components of the ULTC control system are simple devices, its overall system is complex due to the presence of nonlinearity such as time delay, dead band, etc., which is indispensable to limit the number of tap changes [1]. The Static Synchronous Compensator (STATCOM) is a shunt device of the Flexible AC Transmission Systems (FACTS) family using power electronics to control power flow and improve transient stability on power grids [2]. The STATCOM regulates voltage at its terminal by controlling the amount of reactive power injected into or absorbed from the power system. When system voltage is low, the STATCOM generates reactive power (STATCOM capacitive). When system voltage is high, it absorbs reactive power (STATCOM inductive). While the application of continuously-controlled reactive compensation can have many benefits for faster phenomena in a power system, interaction of such a device with ULTC as a slow response device creates issues in steady-state and quasi steady-state operation. When both STATCOM and ULTC are used to control

system voltage, the STATCOM reacts to the voltage deviation faster than the ULTC. Therefore, if a bus voltage is controlled by both a STATCOM and an ULTC transformer without coordination, there is no chance for the transformer to participate in controlling the bus voltage, except when the STATCOM is in its limit. This makes the coordination control between the two devices complicated. There has been an approach to improve the voltage profile and reduce the number of tap operations by coordinating the ULTC and STATCON (Static Condensor) [3]. In [3] the concepts of coordinating a STATCON as a fast reactive-power compensator with local voltage-var control devices such as the ULTCs and capacitor banks for long-term voltage-var management are discussed. The authors introduce the three objectives as the concepts of long-term voltage-var management; the resetting a STATCON by simple reactive power runback function so that it would be available for the next dynamic event on the system; improving the overall system voltage profile by coordinating the STATCON with local ULTCs and/or capacitor banks; and reducing the ULTC tap movements by coordinating the STATCON with local ULTCs and/or capacitor banks. They used a simple gain Kbias for ULTC control, its value is chosen based on a complicated criterion considering system short circuit MVA and transformer leakage impedance. Some references consider the coordination between SVC and ULTC. It is worth noting that there is a difference between STATCOM and SVC. Although, the STATCOM performs the same function as the SVC, at

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voltages lower than the normal voltage regulation range, the STATCOM can generate more reactive power than the SVC. This is due to the fact that the maximum capacitive power generated by a SVC is proportional to the square of the system voltage (constant susceptance) while the maximum capacitive power generated by a STATCOM decreases linearly with voltage (constant current). This ability to provide more capacitive reactive power during a fault is one important advantage of the STATCOM over the SVC. In addition, the STATCOM will normally exhibits a faster response than the SVC because with the Voltage-Sourced Converter (VSC), the STATCOM has no delay associated with the thyristor firing (in the order of 4 ms for a SVC). Reference [4] proposes the coordinated control system between the SVC and ULTC of the distribution substation. This control reserves the SVC operating margin without increasing the tap position; however, the resetting of SVC output reactive power has not been taken into consideration. In [5], the proposed SVC control strategy is to limit the steady-state reactive-power output of the SVC to a desired value during the steady-state voltage range by using two regulation slopes and a combination of the fixed-voltage reference control and floating-voltage reference control. This method needs switching points and different droop characteristics that are not desirable for practical purposes. In [6] an artificial neural network (ANN)-based coordination control scheme for under load tap changing (ULTC) transformer and STATCOM is proposed. The objective of the coordination controller is to minimize both the amount of tap changes of the transformer and STATCOM output while maintaining an acceptable voltage magnitude at the substation bus. The coordination controller is designed to substitute for a classical ULTC mechanism by utilizing active and reactive powers, tap position, and STATCOM output. An ANN is used as a classifier for tap positions and trained by a proposed iterative condensed nearest neighbor (ICNN) rule. This method needs a lot data for training the ANN and is case-specific. In this paper, a new simple and robust control strategy to coordinate the STATCOM output and ULTC operation is proposed to limit the steady-state reactive-power output of the STATCOM to a desired value during the steady-state voltage range based on a variable ULTC reference voltage control scheme. When STATCOM settles to a new operating point following a disturbance, the reference voltage of ULTC will be effectively changed based on the STATCOM output, hence the ULTC is forced to return back and supply the required reactive power from the source and resetting the STATCOM to be within the steady-state margin. The simulation results show the potential of the method to effectively make the STATCOM available for further system dynamics.

2 COORDINATION CONTROLLER Figure 1 shows the scheme of the proposed controller. After a load change as a disturbance, the STATCOM supplies/absorbs required reactive power very quickly (nearly within 3 msec) to keep the load bus voltage at the specified value, and then stabilizes at a steady-state operating point. In order to make the STATCOM available for further system changes, the coordination controller forces the ULTC to activate and be set at a tap position appropriate to nearly zero STATCOM output, while keeping the load bus voltage at the desired value. Figure 2 shows the coordination controller flow diagram. As can be seen from this figure, a dead zone block is used to prevent oscillatory operation around the desired operating point. The Dead Zone block generates zero output within a specified region, called its dead zone. The lower and upper limits of the dead zone are specified as the Start of dead zone and End of dead zone parameters. The block output depends on the input and dead zone:

• If the input is within the dead zone (greater than the lower limit and less than the upper limit), the output is zero.

• If the input is greater than or equal to the upper limit, the output is the input minus the upper limit.

• If the input is less than or equal to the lower limit, the output is the input minus the lower limit.

The reference voltage of the power transformer is the load bus voltage that needs to be kept at a desired value, for example 1 p.u.. When STATCOM output is nearly zero, the reference voltage is the load bus voltage, otherwise, the reference voltage is higher or lower than the load bus voltage, and hence the ULTC is forced to compensate the reactive power already supplied by STATCOM. By each tap changing, STATCOM decreases its output, so the final reference value approaches the load bus voltage.

Figure 1 Proposed Controller Scheme

Load Bus Voltage

Tap

STATCOM Reactive Power

Coordination Controller

STATCOM Load

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3 SIMULATION RESULTS In order to show the efficiency of the coordination controller, a sample power system as Figure 3 is used. The simulation is done by MATLAB/Simulink ver. 7.4. As can be seen from this figure, a 25 kV distribution network supplies power to a 36 MW /10 Mvar load (0.964 PF lagging) from a 120 kV, 1000 MVA system and a 120kV/25 kV ULTC regulating transformer. Reactive power compensation is provided at load bus by a 15 Mvar capacitor bank. ULTC transformer implements a three-phase regulating transformer rated 47 MVA, 120 kV/25 kV, Wye/ Delta, with the ULTC connected on the high voltage side (120 kV). The ULTC transformer is used to regulate system voltage at 25 kV bus B4. Voltage regulation is performed by varying the transformer turn ratio. This is obtained by connecting on each phase, a tapped winding (regulation winding) in series with each 120/sqrt(3) kV winding. Nine (9) ULTC switches allow selection of 8 different taps (tap positions 1 to 8, plus tap 0 which provides nominal 120kV/25 kV ratio). A reversing switch included in the ULTC allows reversing connections of the regulation winding so that it is connected either additive (positive tap positions) or subtractive (negative tap positions). For a fixed 25 kV secondary voltage, each tap provides a voltage correction of +/-0.01875 pu or +/-1.875% of nominal 120 kV voltage. Therefore, a total of 17 tap

positions, including tap 0, allow a voltage variation from 0.85 pu (102 kV) to 1.15 pu (138 kV) by steps of 0.01875 pu (2.25 kV). The positive-sequence voltage measured at bus B2 is provided as input to the voltage regulator (input 'Vmeas' of the transformer blocks). The reference voltage is set to 1.0 pu. In order to start simulation with 25-KV voltages close to 1.0 pu at bus B4, the initial tap position is set at -1, so that the transformer is boosting the voltage by a factor 1/(1-0.01875)=1.019. The tap transition is performed by temporarily short-circuiting two adjacent transformer taps through resistors (5 ohm resistances and 60 ms transition time). The phasor model is built with current sources emulating the transformer impedance which depends on winding resistances, leakage reactances and tap position. The model use a voltage regulator that generates pulses at the 'Up' or 'Down' outputs and orders a tap change either in the positive or negative direction. The voltage regulation depends on the specified dead band (DB = two times the voltage step or 0.0375 pu). This means that the maximum voltage error at bus B4 should be 0.01875 pu. As long as the maximum tap number is not reached (-8 or +8), voltage should stay in the range: (Vref-DB/2< V<1.04+DB/2) = (1.021< V< 1.059). As tap selection is a relatively slow mechanical process (4 sec per tap as specified in the 'Tap selection time' parameter of the block menus), the simulation Stop time is set to 2 minutes (120 s). STATCOM rating is 15 MVA and its droop is 3%. Figure 4 shows the simulation results of the sample network without the fixed capacitors. At t=20 Sec, the breaker closes and a 18MW/5Mvar load connects to the load bus; this causes the voltage of bus B4 decreases and STATCOM generates the required capacitive reactive power to keep the bus voltage at the desired value. After some delay, the coordination controller instructs ULTC to change and STATCOM decreases its output.

Yg/Delta (D1) 47 MVA 120/25 kV

Phasors

pow ergui

Vm (pu) m

A

B

C

a

b

c

Three-Phase OLTCRegulating Transformer

(Phasor Type)

A B C

a b c

Three-Phase Breaker

0

Tap2

Scope1

V_REF

Tap2

m1

[V_REF]

[m1]

[V1_B4]

Dead Zone

A

B

C

a

b

c

B4

A

B

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a

b

c

B3

A B C 25 kV36 MW10 Mvar

A B C 25 kV18 MW5 Mvar 1

A B C

25 kV10 Mvar 1

N

A

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120 kV

A

B

C

A

B

C

1000 MVA

Trip

m

A B C

STATCOM100 MVA

STATCOM1

<Tap>

<Qm (pu)>

Figure 3 Sample system set-up in Simulink

Load Bus Voltage in p.u. ULTC

Reference Voltage

STATCOM Reactive Power Output in p.u.

Dead Zone Block

Figure 2 Coordination Controller Flow Diagram

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0 20 40 60 80 100 120-8

-6

-4Tap #

0 20 40 60 80 100 120-1

0

1Q_STATCOM

0 20 40 60 80 100 1200

1

2 V B4 (pu)

0 20 40 60 80 100 1200.95

1

Time

V B3 (pu)

Figure 4 Coordination of STATCOM and ULTC without fixed capacitor

This procedure continues until the STATCOM output reaches to nearly zero. As can be seen from Figure 4, the voltage of bus B4 is kept constant due to the STATCOM fast operation, while the voltage of HV changes for voltage regulation. At t=60 Sec, the breaker opens and the load is disconnected, STATCOM absorbs reactive power, and avoids voltage increase at the bus B4. In this case, ULTC also operates and helps STATCOM to return back to the standby condition. In this case, the initial tap position for nearly zero STATCOM output at the start of simulation is -4.

Figure 5 shows another case with fixed capacitors. In this case, the initial tap position is -1. As can be deduced from this figure, the ULTC and STATCOM perfectly operate in coordination with each other. Figure 6 shows the STATCOM and ULTC operation without coordination. As can be seen from this figure, STATCOM remains at its limit and jumps from a high capacitive to a high inductive value at t=60 Sec. This indicates the necessity to coordinate the ULTC operation with STATCOM.

0 20 40 60 80 100 120-10

-5

0Tap #

0 20 40 60 80 100 120-1

0

1Q_STATCOM

0 20 40 60 80 100 1200

1

2 V B4 (pu)

0 20 40 60 80 100 1200.95

1

Time (Sec)

V B3 (pu)

Figure 5 Coordination of STATCOM and ULTC with fixed capacitor

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0 20 40 60 80 100 120-3

-2

-1Tap #

0 20 40 60 80 100 120-1

0

1Q_STATCOM

0 20 40 60 80 100 1200

1

2 V B4 (pu)

0 20 40 60 80 100 1200.95

1

Time

V B3 (pu)

Figure 6 Operation of STATCOM and ULTC without coordination (with fixed capacitor)

Figure 7 shows the independent operation of STATCOM and ULTC of the sample network without the fixed capacitor bank. The initial tap is -4 in order to provide the required reactive power. At t=20 Sec. the breaker closes and the bus load increases, and motivating the STATCOM to operate. STATCOM can not supply the required reactive power, so ULTC operates and moves to tap -6. STATCOM output

decreases a small value, but not to standby condition. Figure 8 shows the STATCOM and ULTC coordinated operation for a light load change. The tap initial position is -4 and changes to -5 in order to compensate for STATCOM to return to standby condition. The role of dead zone block is salient here, to avoid oscillatory operation of ULTC around the desired value.

0 20 40 60 80 100 120-6

-5

-4Tap #

0 20 40 60 80 100 120-1

0

1Q_STATCOM

0 20 40 60 80 100 1200

1

2 V B4 (pu)

0 20 40 60 80 100 1200.95

1

Time

V B3 (pu)

Figure 7 Operation of STATCOM and ULTC without coordination (without fixed capacitor)

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0 20 40 60 80 100 120-5

-4.5

-4Tap #

0 20 40 60 80 100 120-0.5

0

0.5Q_STATCOM

0 20 40 60 80 100 1200.5

1

1.5 V B4 (pu)

0 20 40 60 80 100 1200.96

0.98

1

Time (Sec)

V B3 (pu)

Figure 8 Coordination of STATCOM and ULTC for a light load change (without fixed capacitor)

4 CONCLUSIONS This paper proposes a new control scheme for the coordination of a ULTC transformer and a STATCOM installed at the same bus and has the merit of reserving the STATCOM operating margin for emergencies. The proposed coordination controller controls the ULTC in maximizing the capacity margin of the STATCOM without increasing the tap operation and improving the load voltage profile and quality. The coordinated control avoids ULTC oscillatory operation by applying a dead zone block.

5 REFERENCES 1. Calovic, M. S., “Modeling and analysis of under-

load tap-changing transformer control system,” IEEE Trans. on Power Apparatus and Systems, vol. PAS-103, no. 7, pp. 1909–1915, 1984.

2. Hingorani, N. G. and Gyugy, L., "Understanding FACTS", Concepts and Technology of Flexible AC Transmission System. New York: Inst. Elect. Electron. Eng., Inc., 2000.

3. Paserba, J. J., Leonard, D. J., Miller, N. W., Naumann, S. T., Lauby, M. G., and Sener, F. P., "Coordination of a distribution level continuously controlled compensation device with existing substation equipment for long term var

management,” IEEE Trans. Power Del., vol. 9, no. 2, pp. 1034–1040, Apr. 1994.

4. Son, K. M., Moon, K. S., Lee, S. K., and Park, J. K., “Coordination of an SVC with a ULTC reserving compensation margin for emergency control,” IEEE Trans. Power Del., vol. 15, no. 4, pp. 1193–1198, Oct. 2000.

5. Kim, G. W. and Lee, K. Y., “Coordination Control of ULTC Transformer and STATCOM Based on an Artificial Neural Network,” IEEE Trans. Power Sys., vol. 20, no. 2, pp. 580-586, May 2005.

6. Abdel-Rahman, M. H., Youssef, F. M. H., Saber, A. A., “New Static Var Compensator Control Strategy and Coordination with an Under-Load Tap Changer,” IEEE Trans. Power Del., vol. 21, no. 3, pp. 1630–1635, July 2006.

AUTHOR'S ADDRESS Dr. Mojtaba Khederzadeh can be contacted at Electrical Engineering Department, Power & Water University of Technology, Tehranpars, Vafadar Bldv., P. O. Box: 16765-1719, Tehran, IRAN. Email: [email protected]

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