Static Synchronous Series Compensator ⚡SSSC

Static Synchronous Series Compensator (SSSC) is a type of power electronic device that is used to improve the stability and control of power systems. It is connected in series with a transmission line and uses voltage source converter technology to inject a controllable voltage into the line. This allows the SSSC to regulate the voltage and power flow in the line, which can help to improve the stability and efficiency of the power system. The SSSC can also be used to mitigate the effects of disturbances such as voltage dips and flicker, and to provide dynamic voltage support during faults.

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Static Synchronous Series Compensator (SSSC) is a type of flexible AC transmission system which consists of a solid-state voltage source inverter coupled with a transformer that is connected in series with a  transmission line. This device can inject an almost sinusoidal voltage in series with the line. This injected voltage could be considered as an inductive or capacitive reactance, which is connected in series with the transmission line. This feature can provide controllable voltage compensation. In addition, SSSC is able to reverse the power flow by injecting a sufficiently large series reactive compensating voltage.

Static Synchronous Series Compensator (SSSC) is a power quality FACTS device that employs a VSC connected in series to a transmission line through a transformer or multilevel inverters. The SSSC works like the STATCOM, except that it is serially connected instead of a shunt. Its output is a series injected voltage, which leads or lags the line current by 90°, thus emulating a controllable inductive or capacitive reactance. The SSSC can be used to reduce or increase the equivalent line impedance and enhance the active power transfer capability of the line. Moreover, the SSSCs are highly controllable devices and can provide further functionalities and services to the energy system.

COMPENSATOR HISTORY

A prototype 1 MVAr STATCON was described by Empire State Electric Energy Research Corporation in 1987.The first production 100 MVAr STATCON made by Westinghouse Electric was installed at the Tennessee Valley Authority Sullivan substation in 1995 and was quickly retired due to obsolescence of its components.

TECHNOLOGY TYPES

SSCCs are part of the family of Series Controllers within FACTS devices. Two variants are possible:

1. The conventional SSSC, connected to the transmission line through a transformer

2. The transformerless SSSC, connected to the transmission line through multilevel inverters (such as modular transformerless SSSCs).

COMPONENTS

Conventional SSSCs are also known as advanced series compensators (ASCs) or GTO-CSC, being the evolution of controlled series compensation (SC) devices. The SSSC consists of a coupling transformer, a GTO VSC and a DC circuit. They act as a controllable voltage source whose voltage magnitude can be in an operating area controlled independently of the line current. The SSSC can be considered functionally as an ideal generator that can be operated with a relatively small DC storage capacitor in a self-sufficient manner to exchange reactive power with the AC system or, with an external DC power supply or energy storage, to also exchange independently controllable active power, analogously to a STATCOM.

Transformerless SSSC solutions typically comprise a single-phase, modular-SSSC injecting a leading or lagging voltage in quadrature with the line current, but include a built-in-bypass to avoid damage of the power electronics resulting from high currents e. g. during a network fault. It can increase or decrease power flows on a circuit and perform dynamic services.

OPERATING PRINCIPLES

SSSC is a reactive power series compensator employing a Voltage-Sourced Converter (VSC) in series with the transmission line. This operating mode emulates a controlled series reactive compensation (such as obtained with the Thyristor Controlled Series Capacitor (TCSC)), but provides wider control range as it can operate equally at capacitive or inductive operating domains as well as it can operate just as voltage source. Thyristor based FACTS controllers, like TCSC, are based on impedance control, so they are dependent on the line current. The SSSC can operate as a voltage source, which guarantee series compensation independent of the line current. 

APPLICATION & ADVANTAGES

The use of SSSCs provides the typical advantages of load flow control that can also be realised by other technologies such as PST, TCSC and partly by a series reactor.

However, both conventional and transformerless SSSCs also offer additional special functions such as:

1. Better controllability of power flow, as SSSCs possess the inherent capability to decrease as well as to increase (real) power flow almost linearly in the circuit.

2. Receiving end voltage regulation of a radial line: In short circuit weak networks by controlling the degree of series compensation to keep the end-voltage constant in the face of changing load and load power factor.

3. Power oscillation damping: the SSSC is a controlled device and can be used to damp wide area low frequency power oscillations.

4. Regional network voltage regulation by the inherent networking of devices applied at different locations in a grid.

5. Phase balancing: Modular SSSC as a single phase device can independently alter the effective phase impedance of a circuit[s] to rebalance power flows.

In addition to these range of applications, the modular SSSC are designed to be easily relocated if needed. Moreover, the amount of load flow shift can be adjusted by adding or removing modules, depending on the network needs. A little effort redeployability for is expected.

PERFORMANCE

Conventional SSSC:

• Rated system voltage: 220 kV

• Rated reactive power: 100-400 MVAR

Transformerless SSSC:

• Rated system voltage: up to 550 kV

• Rated reactive power: modular units are designed to be operated in combination, allowing any reactive power rating to be possible, i.e. 1 – 10 MVAr in size for each module.

WORLD EXAMPLE

New York, United States 2019

Three transformerless SSSCs were installed on the 115 kV Sturgeon Pool – Ohioville line owned by Central Hudson in New York State. Central Hudson sought to gain experience with the technology in advance of a larger installation planned for 2021. This larger installation will add 21% series compensation on a 345 kV line to enable full capacity deliverability of interconnecting generation. The Electric Power Research Institute (EPRI) observed the 2019 installation and evaluated the technology’s functionality.

Design: Use of modular transformerless SSSC to enable real-time power flow control on grids.

Results: The installation process was smooth and without incident. The transformerless SSSC operated effectively in capacitive and inductive injection modes. The devices responded through control commands issued locally through the substation-based interface and remotely from the SCADA / EMS. During system faults, the devices performed as expected and entered a bypass mode under fault conditions. There were no un-expected interactions with the normal protection system.

United Kingdom 2021

NGET are proceeding with five installations in 2020, due for completion in 2021. These projects will contain a total of 375 MVAr of power flow control capability, located along the Fourstones to Harker to Stella West, Penwortham to Kirkby, and Lackenby to Norton circuits.

Design: Use of modular transformerless SSSC to enable real-time power flow control on grids.

Results: These projects are anticipated to increase boundary capabilities by 1.5 gigawatts in total across three transmission network boundaries. National Grid ESO has assessed similar projects at a number of further locations, and these are recommended for progression in the Network Options Assessments report for later years.