A detailed model for the STATCOM using ATPdraw

A Detailed Model for the STATCOM using ATPdraw M. M. I. El-Shamoty

Ebrahim A. Badran, IEEE Member

Mohamed Saad El-Morcy

Electrical Engineering Department, Faculty of Engineering, Mansoura University, Mansoura, Egypt E-Mail: [email protected]

Technology Department, Academy of Special Studies, WU, St. 51, Ras-Elbar, Egypt E-Mail: [email protected]

SEGAS, LNG Plant, Private Free Zone, Damietta Port, Damietta, Egypt E-Mail: [email protected]

Abstract- This paper introduces a detailed model for the STATic COMpensator (STATCOM). The proposed model consists of two six-pulse converters connected in series on the ac side. The dc sides of the two converters are connected in parallel and share the same dc capacitor. The phase control technique is implemented. The main parts of the compensator system are modeled as separate parts. So, the model can be used as a benchmark model for the STATCOM using the ATPdraw of the ElectroMagnetic Transients Program (EMTP-ATP). The proposed model verification has been undertaken by the use of published results. Key Words: Modeling, FACTS, STATCOM, EMTP, ATPdraw

I- INTRODUCTION Recently, there has been a significant interest in reactive power as one of several ancillary services required to ensure system reliability. System operators and researchers have been looking for appropriate mechanisms for reactive power provision in the context of deregulation. Reactive power is tightly related to bus voltages throughout a power network, and hence reactive power services have a significant effect on system security. Insufficient reactive power supply can result in voltage collapse, which has been one of the reasons for some recent major blackouts (e.g. Canada-US and Sweden blackouts in 2003) [1]. The computer simulation of power systems has presented many challenges and opportunities over the years. Power system engineers always try to improve modeling techniques and to apply computer technology to design study tools that meet the analysis requirements [2]. In simulation studies including FACTS (Flexible AC Transmission System), detailed three-phase FACTS models may be required as well as simplified models. Detailed three-phase studies should include all necessary elements of FACTS together with its non-linearities. The details of the control blocks should be all modeled, representing all necessary firing pulses for each of the valves. The STATCOM as a shunt-connected static var compensator is one of those complicated power electronic devices to be modeled [3]. For this purpose, in this paper, a detailed model for the STATCOM has been manipulated, implemented and tested using the ATPDraw program. The presented STATCOM detailed model includes the GTO and diode valves together with their necessary snubber circuits,

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which make this model a valuable tool in a power system design process. The control system does not use typical dq transformation to obtain dc signals for control process. Instead, the control system use simple input variable measurements, similar to a Static Var Compensator (SVC) control system, which is a different way to treat the STATCOM control circuit design. The STATCOM operating and control limits are included into the model, which makes this model suitable for steady state and transient stability studies. The presented detailed model is not a novel model since it realistically represents the practical device; however, their implementation into the ATPDraw proved to be a challenging task. The model produced the expected results only when certain snubber circuits were used, due to numerical oscillation problems of the trapezoidal integration method. Moreover, the STATCOM model was included into a realistic test system as opposed to the usual two-bus test systems found in the literature. II- THE EXISTING STATCOM CONFIGURATIONS, CONTROL, AND MODELS Many STATCOM configurations have been reported in the literature. These configurations can be classified into; two-level, three-level, and multi-level STATCOM. In [4], an 80 MVAR STATCOM has been developed and applied successfully in Japan. In order to reduce the harmonics, a 48-pulse multiple STATCOM is employed. The output voltage is very similar to the sine wave. In [5], a 100 MVAR STATCOM has been presented in USA. The output is also nearly sinusoidal waveform. In [6] high-pulse schemes based on three-level STATCOM are introduced. Utilising two parallel units of two-level inverter and a current sharing inductor leads to low order harmonic profile outputs equivalent to a 48pulse STATCOM. In [7] a five-level STATCOM is introduced, where the voltage level can be increased without series connection of thyristors and the harmonics can be greatly reduced without filters. A multi-level STATCOM was presented in [8], which enables regulating the bus voltage and controlling the capacitor voltages without using any transformers. The majority of STATCOM systems in operation are based on the two-level STATCOM. They mostly operate

at a high-pulse number to increase the order of their harmonics, thus reduce the size of passive harmonic filters. Recently, there has been recognition that the multi-level has distinct advantages over the two-level STATCOM and several research groups have begun studies to apply such multi-level STATCOM to make it more compact and economical. However, increasing the number of levels in high voltage applications increases the number of commutation loops, so that an excess complex control should be needed to maintain equally charged capacitors. Best understanding of the STATCOM configurations requires good understanding of the control systems. The STATCOM control strategies can be classified into Fundamental Frequency Switching (FFS), in which the switching of each semiconductor device is one turn-on and off per power cycle, and Pulse-Width-Modulation (PWM), in which the semiconductor switch is turned on and off at a rate higher than the power frequency. Many STATCOM models have been reported in the literature. These models are used to validate the STATCOM topology, control, or benefits [3], and [7-10]. Some of these models [3], and [9-10] focus on the detailed modeling of the power electronic switches, the control systems, and the valve firing circuits. Acha presented a PWM control implementation in PSCAD/EMTDC and applied it on a two-level distribution STATCOM [9]. In [10] a PWM distribution STATCOM simulation is reported. Manitoba HVDC Research Center has introduced many STATCOM models using the PSCAD/EMTDC [3]. In [11] a reduced model for the PWM-controlled STATCOM using PSCAD is reported. This model is based on the function than on configuration. Indeed, some STATCOM complex detailed models are already found within PSCAD/EMTDC. These models include the two-level and the three-level STATCOM. All existing models are of a high grade of complexity. Fine details of the data of all existing power electronic elements are needed. To overcome such complexity, this paper introduces a benchmark model which is more based on the STATCOM function blocks. The model is separated into several parts connected by simple signals. Each part can be used separately in any other universal model.

di/dt due to valve switching. The STATCOM control circuit is also modeled in great details.

Fig. 1. Basic building blocks of the STATCOM circuit

The control of the STATCOM is achieved by variations in the switching angle of the controlled semiconductor switches. So that, the fundamental component of the STATCOM output voltage lags or leads the AC system bus voltage by a few degrees. This causes real power to transiently flow in or out of the converter. Thus changing the DC capacitor voltage and consequently the magnitude of the STATCOM output voltage. The modeled twelvepulse STATCOM circuit is shown in Fig. 2. The circuit consists of two six-pulse converters connected in series on the AC side of the circuit with a 150 MVAR, 230 KV / 6 KV summing and intermediate transformer.

III- THE PROPOSED STATCOM MODEL In this paper, the STATCOM is modeled as a threephase, twelve-pulse voltage-sourced converter that is connected to the AC system through an appropriate transformer. The basic building block of the 150 MVAR, 230 KV STATCOM circuit is the six-pulse converter shown in Fig. 1. It is important to emphasize that the STATCOM model, includes all GTO valves and diodes, with the required snubber circuits used to reduce dv/dt and

Fig. 2. The Twelve-pulse STATCOM model in ATPdraw

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The DC sides of the two converters are connected in parallel and share the same DC capacitor. They can also be connected in series on the DC side in twelve-pulse operation, with twice the DC voltage and, consequently, twice the converter output AC voltage. However, this kind of connection is avoided due to the fact that the two DC buses (capacitors) must have equal voltages. From the AC side, the converters are connected in series through the series connection of the line-side windings of the intermediate transformer to provide appropriate cancellation of the characteristic harmonics. It is possible to connect the converters in parallel on the AC side, but that would require transformers with special windings and would increase the total cost of the STATCOM [12]. Fig. 3 shows the elements of the voltage regulation system. The voltages from the NETWORK are measured and have been inputted to the synchronization unit to generate a set of balanced phase voltages, then they have been inputted to the gate pulse generator (GPG) to generate the proper turn-on and turn-off signals. The voltage regulator unit model is shown in Fig. 4. It is shown that the measured voltages have been send to the RMS calculator device (TACS device with code 66). The per-unit value is calculated and compared with the reference voltage. The output is, then, inputted to the integrator which gives the general firing angle. The GPG uses this angle to generate the GTO valves firing angles. The concept of the GPG is adapted from [12] and modeled as shown in Fig. 5. The measured voltages are tracked for its zero crossing and the voltage synchronizing unit is used to generate set of balanced voltages.

Fig. 3. Block diagram of the voltage regulation system

Fig. 4. The ATPdraw model for voltage regulator unit

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The scheme shown in Fig. 5 generates the pulses for the six-pulse converter connected to the Y transformer winding. Whereas, the pulses are shifted by 1.39 ms, i.e., 300, for the six-pulse converter connected to the ∆ transformer winding. This technique used to produce the firing pulses is known as Equidistant Firing Pulse Control and is used in balanced power systems. The voltage synchronizing block, similar to the one explained in [12], is designed to track the actual phase voltages at the compensated bus and to generate a set of balanced voltages that truly follow the phase voltages of the compensated bus, in its voltage magnitude and phase angle. This set follows the zero crossings of the reference voltage and is used as an input to the gate pulse generator. The voltage synchronization unit can be represented to provide the reference voltages for firing pulses by taking the positive sequence fundamental line-line voltages. The input to this unit are the actual phase voltages. Fig. 6 shows the ATPdraw model used for determining the positive sequence component of the fundamental line-line voltages. IV- VERIFICATION OF THE PROPOSED STATCOM MODEL The STATCOM model is verified by comparing its outputs with those published in [15]. The single line diagram of the test system used for this verification is shown in Fig. 7. The test system was picked from [12] and modeled using ATPdraw as shown in Fig. 8. The test system operates at 230 kV. It consists of a 245.5 MW, 13.8 kV synchronous generator with automatic voltage regulator (AVR) and a ∆-Y Step-up transformer. The infinite bus has been modeled by its corresponding Thevenin equivalents; an ideal voltage source and a coupled impedance. The test system contains several transmission lines of various lengths, where the long lines are modeled as distributed parameters lines, while the short length lines are modeled as 3-phase π-equivalents. There are two loads modeled as impedance loads, while the corresponding step-down transformers are simply modeled using their leakage reactance's. A passive filter is connected to the compensated bus (Bus14) to prevent the 11th, 13th and 23rd and other higher harmonics. At fundamental frequency, the filter has a capacitive rating with 65 MVAR. Firstly, the system has been verified without using the STATCOM. Fig. 9 illustrates the load increasing in two steps at 5 seconds and 6 seconds. The voltage profile indicates to unacceptable voltage drops, where the system can not be restored to acceptable values. The comparison in this figure between the published and the simulation waveforms shows the validity of the test system modeling.

For the proposed STATCOM model validation an 150 MVAR STATCOM is connected to Bus14. Fig. 10 shows the voltage at the compensated bus (Bus14). It is noted that the STATCOM performs the required function at the two steps of load variations. The voltage can be restored to the required level when the STATCOM is used. Fig. 11 illustrates the control angle of the STATCOM controller. Also, Fig. 12 illustrates the output reactive power of the STATCOM. These figures ensure the validity of the proposed detailed model. It is clearly seen that the outputs of the proposed model are in good agreement with that of the published results. Fig. 7. The single line diagram of the test system To the STATCOM

Filter

Synchronous machine

Loads

Fig. 8. The test system model using ATPdraw

Fig. 5. The gate pulse generator model using ATPdraw

(a) Published

(b) Simulated

Fig. 9. The load voltage response without the STATCOM

V- CONCLUSIONS Fig. 6. The ATPdraw model for determining the positive sequence component of the fundamental line-line voltages

This paper introduces a detailed model for the STATCOM. The model consists of two six-pulse

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converters. The phase control technique is implemented. The main parts of the compensator are modeled in great details as separate parts. These parts are connected by simple signals. The developed model is verified using published results. The comparison between the published waveforms and the simulation waveforms are in good agreement. It is evident that, each part of the developed model can be used separately in any other universal model. This enables to study in details each part separately. So, the model can be used as a detailed benchmark model for the STATCOM using the ATPdraw.

(a) Published

(b) Simulated

Fig. 12. The reactive power of the STATCOM [2] Om Nayak et al., “Power Electronics Spark New Simulation Challenges”, IEEE Computer Applications In Power, Vol. 15, No. 4, Oct. 2002, pp. 37-44. [3] D. Woodford, “Introduction to PSCAD V3”, Manitoba HVDC Research Center, Jan. 2001. [4] S. Mori et al., “Development Of A Large Static VAR Generator Using Self-Commutated Inverters For Improving Power System Stability”, IEEE Transactions On Power Systems, Vol. 8, No. 1, February 1993, pp. 371-377. [5] A. Edris et al., “Development Of A 100 MVAR Static Condenser For Voltage Control Of Transmission Systems”, IEEE Transactions On Power Delivery, Vol. 10, No. 3, July 1995, pp. 1486-1496. [6] C. J. Hatziadoniu et al., “A Transformerless High-Pulse Static Synchronous Compensator Based On The 3-Level GTOInverter, IEEE Transactions On Power Delivery”, Vol. 13, No. 3, July 1998, pp. 883-888. [7] R. W. Menzies et al., “Advanced Static Compensation Using A Multilevel GTO Thyristor Inverter”, IEEE Transactions On Power Delivery, Vol. 10, No. 2, April 1995, pp. 732-738. [8] R. M. Mathur et al., “Distribution System Compensation Using a New Binary Multilevel Voltage Source Inverter”, IEEE Transactions On Power Delivery, Vol. 14, No. 2, April 1999, pp. 459-464. [9] E. Acha et al., “Modeling and Analysis of Custom Power Systems By PSCAD/EMTDC”, IEEE Transactions On Power Delivery, Vol. 17, No. 1, Jan. 2002, pp. 266-272. [10] A. Martinez et al., “A Benchmark System For Digital TimeDomain Simulation of a Pulse-Width-Modulated DSTATCOM”, IEEE Transactions On Power Delivery, Vol. 17, No. 4, Oct. 2002, pp. 1113-1120. [11] M. H. Abdel-Rahman, et. al, "A Reduced Model for the PWMControlled STATCOM", Proceedings of the 4th International Conf. on Electrical Power Quality and Supply Reliability, Estonia, August 29-31, 2004. [12] Edvina Uzunovic, "EMTP, Transient Stability and Power Flow Models and Controls of VSC Based FACTS Controller", Ph.D. Thesis, The University of Waterloo, Canada, 2001.

(a) Published

(b) Simulated

Fig. 10. Bus14 voltage with the STATCOM connected

(a) Published

(b) Simulated

Fig. 11. The control angle of the STATCOM controller

REFERENCES [1] I. El-Samahy, K. Bhattacharya, and C. A. Cañizares, "A Unified Framework for Reactive Power Management in Deregulated Electricity Markets," Proc. Power Systems Conference and Exposition (PSCE), Atlanta, Georgia, November 2006, 7 pages.

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