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The Structure, Characteristics, and Advantages of SSSCs

Published Date
Author Cadence System Analysis

Key Takeaways

  • SSSCs can supply output voltage in quadrature with, and controllable independently of, the line current, thereby controlling the reactive voltage across a power line.

  • Whenever the voltage injected from an SSSC is not in quadrature with the line current, the SSSC compensates for real power.

  • Given their advantageous characteristics, SSSCs are superior to other FACTS controllers.

Electric power transmission

Flexible AC transmission systems (FACTS) controllers are used for transmitted electric power control

For the control of electrical power transmission, both real power compensation and reactive power compensation are required. Flexible AC transmission systems (FACTS) controllers are used for transmitted electric power control. Static synchronous series compensators (SSSCs) are some of the most important FACTS controllers. Similar to STATCOM, but connected in series to the electric power system, these series compensators can be operated with or without an external energy source. 

SSSCs can supply output voltage in quadrature with, and controllable independently of, the line current, thereby controlling the reactive voltage across a power line. They also support real power compensation. 

Let’s take a closer look at an SSSC’s structure, operating characteristics, and advantages. 

The Structure of SSSCs

The key element of an SSSC is the power electronic converter, usually a DC-AC converter or inverter. The converter can be either a voltage source converter or a current source converter. 

In commonly employed voltage source converter-based SSSCs, the converter is switched to supply a required voltage to the power system. The voltage from the converter is injected in series to the line using an injection transformer. Even though the SSSC is duly insulated from the ground, the injection transformer is a source of insulation for the power electronic converter in an SSSC. The transformer primary is rated at the maximum voltage that the converter can supply. 

Usually, the transformer is a step-up transformer, and injects more voltage in series to the power system, thereby reducing the voltage rating of the converter and the DC energy source, if present.  The transformer carries the full line current, including the fault current. If there is a short circuit current or line fault, the SSSC is disconnected from the power system to protect the converter. 

Characteristics of SSSCs

SSSCs can be used for reactive power as well as real power compensation. SSSCs generate reactive power by producing the compensating voltage. They can be used to supply voltage either 90° lagging or leading the line current. When the SSSC injects voltage that leads the line current, it is equivalent to a capacitive reactance connected in series with the power line, and it increases the line current as well as the power flow. When the injected voltage lags behind the line current, it is similar to an inductive reactance connected in series to the line, causing the line current and power flow to decrease. 

An SSSC can be considered a variable impedance type series compensator, as it indirectly varies the line resistance and line reactance by injecting real power and reactive power, respectively. Whenever the voltage injected is not in quadrature with the line current, the SSSC compensates for real power. A DC energy source is required in SSSCs for real power supply—this can be a battery, photovoltaic module, or fuel cell. A DC-link capacitor is connected in parallel to the DC power source to keep the DC voltage constant and to increase the quality of current flowing towards the power electronic converter. 

The Advantages of SSSCs 

Compared to other FACTS controllers, SSSCs are superior due to the following advantages:

  1. They eliminate bulky passive components such as capacitors and inductors. 

  2. They can supply or absorb reactive power. The ability to offer inductive and capacitive operating modes symmetrically is also a benefit.

  3. When connected with a DC power source on the DC side of an SSSC, they can exchange real power to the power system.

SSSCs can also connect to other renewable sources such as wind or any AC source. In such cases, an extra converter is included to convert AC to DC, which precedes the DC-link capacitor in an SSSC’s structure. 

The power electronic converter (which, in most cases, is a DC-AC converter or inverter) is the heart of an SSSC. Cadence’s software is a popular tool for simulating different topologies of power electronic inverters, and is not limited to circuits ranging from rectifiers, choppers, and cyclo converters. 

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