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Over the last two decades, advancements in static reactive compensation (STATCOM) technology based on voltage source converter (VSC) concepts have produced significant benefits. STATCOM systems offer rapid response to system events, improved voltage and power system stabilization, enhanced reliability, real and reactive power flow control, and increased power transfer limits. This article provides an overview of one recent application.

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The STATCOM system at Vermont Electric Power Co.'s Essex 115kV substation was installed to compensate for heavy increases in summertime electric usage (see photo , on page 41). These increases rendered the existing system increasingly vulnerable to failure in the event of problems elsewhere on the transmission system. The STATCOM system also provides enhanced power quality indirectly to a nearby industrial facility that is sensitive to voltage disturbances experienced at the substation.

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The STATCOM system has a rated capacity of +133/-41 MVA. It consists of two groups of 43 MVA voltage source converters (VSCs) and two sets of 24 MVA shunt capacitors (see Fig. 1 , on page 41).

Each VSC group consists of three sets of 12.5 MVA VSC modules and a small 5 MVA harmonic filter, with a nominal phase-to-phase AC voltage of 3.2kV and a DC link voltage of 6kV. The 43 MVA STATCOM groups of VSCs connect to the 115kV system via two 43 MVA, 3.2kV to 115kV 3-phase inverter transformers. The 24 MVA shunt capacitors are connected directly at the 115kV level.

The main power semiconductor devices incorporated in the VSC design are 6-in. gate turn-off thyristors, rated at 6kV and 6 kA. These devices are used in each VSC module, forming a 3-level inverter circuit that reduces the harmonic current, as compared to a 2-level design.

The inverter regenerates the snubber energy to the DC capacitors in the inverter circuit for lower-loss operation. Five-pulse PWM (pulse-width modulation) controls the inverter, which further decreases the harmonics as compared to 3-pulse or 1-pulse PWM. This results in the AC side requirement for only a small high-pass harmonic filter (5 MVA at 3.2kV for each of the STATCOM groups). Other benefits of this system include:

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  • Rapid response to disturbances.
  • Smooth voltage control over a wide range of operating conditions.
  • Voltage support in the substation region, thus improving transmission reliability and local power quality.
  • A significant amount of built-in redundancy (i.e., any one or more of the 12.5 MVA VSC modules or 43 MVA groups can be out of service while all others remain in operation at full-rated capability).
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As Fig. 1 illustrates, the system design is entirely redundant for optimum operating flexibility and efficiency. Each VSC-based STATCOM group and each shunt capacitor bank are supplied to a 115kV bus via 115kV SF-6 gas circuit breakers (GCBs). A main disconnect switch connects the entire STATCOM system to the Essex substation's 115kV ring bus position.

Conclusion

As the worldwide electric utility industry moves forward with deregulation, utility transmission systems are reaching their limits, making the need for reliable power greater than ever. This example demonstrates how STATCOM technology can enhance transmission system control, reliability, and operation, and also improve distribution-system power quality.

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Dr. Gregory Reed is general manager of the Power Systems Division at Mitsubishi Electric Power Products Inc. You can reach him at gregory.reed@meppi.com .

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Dr. Reed would like to thank Laurie Thomas, of the Vermont Electric Power Co., for her support in the writing of this article and her significant contributions to the Essex project .

- STATCOM Technology in Brief

STATCOM systems essentially consist of a DC voltage source behind self-commutated inverters using insulated gate bipolar transistor (IGBT), gate turn-off (GTO), or gate commutated turn-off (GCT) thyristors and an interconnecting transformer. The voltage source inverter set connects to the power system via a multi-winding or two-winding inverter transformer, depending upon the application.

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The figure on the top shows the basic STATCOM configuration. The inverter and DC voltage source can be modeled as a variable voltage source, as shown in the equivalent circuit (bottom figure). The power system also can be modeled as a voltage source. An inductor representing the leakage reactance of the transformer connects the two voltage sources. The output voltage phase of the thyristor-based inverter, V - i , is controlled in the same way as the system voltage, V - s .

The compact design of STATCOM systems takes up approximately 1/3 the area and 1/5 the volume of conventional static var compensation (SVC) systems.