STATIC SYNCHRONOUS SERIES COMPENSATOR (SSSC)

Thursday, May 31, 2012

Static Synchronous Series Compensator (SSSC) is a modern power quality FACTS device that employs a voltage source converter connected in series to a transmission line through a transformer. The SSSC operates like a controllable series capacitor and series inductor. The primary difference is that its injected voltage is not related to the line intensity and can be managed independently. This feature allows the SSSC to work satisfactorily with high loads as well as with lower loads.

The Static Synchronous Series Compensator has three basic components:

a.    Voltage Source Converter (VSC) – main component
b.    Transformer – couples the SSSC to the transmission line
c.     Energy Source – provides voltage across the DC capacitor and compensate for device losses  
Static Synchronous Series Compensator (SSSC)
Static Synchronous Series Compensator (SSSC) Diagram
Operation and Capabilities

Static synchronous series compensator works like the STATCOM, except that it is serially connected instead of shunt. It is able to transfer both active and reactive power to the system, permitting it to compensate for the resistive and reactive voltage drops – maintaining high effective X/R that is independent of the degree of series compensation. However, this is costly as a relatively large energy source is required.

On the other hand, if control is limited to reactive compensation then a smaller supply should be enough. In this case only the voltage is controllable because the voltage vector forms 90ยบ with the line intensity. Subsequently, the serial injected voltage can advance or delay the line current, meaning, the SSSC can be uniformly controlled in any value.

The SSSC when operated with the proper energy supply can inject a voltage component, which is of the same magnitude but opposite in phase angle with the voltage developed across the line. As a result, the effect of the voltage drop on power transmission is offset. In addition, the static synchronous series compensator provides fast control and is inherently neutral to sub-synchronous resonance.

Modes of Operation

Generally, the line reactance is constant but its net effect can be controlled through voltage injection. For instance, the line current decreases as the inductive reactance compensation level increases from 0% to 100%. Meanwhile, the line current increases with the capacitive reactance compensation level from 0% to 33%. 
SSSC Modes of Operation
SSSC Modes of Operation
It can be noted that the static synchronous series compensator does not only increase the transferable power but it can also decrease it, by simply reversing the polarity of the injected voltage. This reversed polarity voltage is fed directly to the line voltage drop as if the line impedance was increased.

In short, the effects of reactance compensation on normalized power flow in the transmission line are as follows:

·        When the emulated reactance is capacitive, the active and reactive power flow increases and the effective reactance decreases as the reactance compensation increases in the positive direction.
·      When the emulated reactance is inductive, the active and reactive power flow decrease and the effective reactance increases as the reactance compensation increases in the negative direction

SSSC and DVR

The SSSC’s counterpart is the Dynamic VoltageRegulator (DVR). Although both are utilized for series voltage sag compensation, their operating principles differ from each other. The static synchronous series compensator injects a balance voltage in series with the transmission line. On the other hand, the DVR compensates the unbalance in supply voltage of different phases. Also, DVRs are usually installed on a critical feeder supplying the active power through DC energy storage and the required reactive power is generated internally without any means of DC storage.

Applications and Advantages

The SSSC is typically applied to correct the voltage during a fault in the power system. However, it also has several advantages during normal conditions:

·      Power factor correction through continuous voltage injection and in combination with a properly structured controller.
·         Load balancing in interconnected distribution networks.
·         It can also help to cover the capacitive and reactive power demand.
·         Power flow control.
·         Reduces harmonic distortion by active filtering.

References:
El-Zonkoly A. (2006). Optimal Sizing of SSSC Controllers to Minimize Transmission Loss and a Novel Model of SSSC to Study Transient Response.
Gupta S., Pachar, R., and Tiwari, H. (2010). Study on Major Issues and Their Impact on DVR System Performance.

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I am an Electrical Engineer with a Masters Degree in Business Administration. My interest is in Power Quality and Protective Relaying. I have been working in an electric distribution utility for about seven years now as a Planning & Design Engineer. I handle PQ studies, power system analysis and capital budgeting for company projects.