Static Synchronous Series Compensator (SSSC) is a power quality FACTS device that employs a voltage source converter connected in series to a transmission line through a transformer or multilevel inverters. The SSSC works like the STATCOM (STATic Synchronous COMpensator), except that it is serially connected instead of 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. SSSC can be used to reduce the equivalent line impedance and enhance the active power transfer capability of the line. When operated with a proper energy supply, the SSSC can also inject a voltage component, which is of the same magnitude but opposite in phase angle with the voltage developed across the line. Thus, the effect of the voltage drop on power transmission is offset. As a main advantage, SSSCs are inherently neutral to sub-synchronous resonance. This factsheet focuses on voltage levels above 110 kV.
Static Synchronous Series Compensator (SSSC) are part of the family of series controllers within FACTS devices. Two variants are possible:
- the conventional SSSC, connected to the transmission line through a transformer, and
- the transformerless SSSC, connected to the transmission line through multilevel inverters (such as modular transformerless SSSC).
Components & enablers
Conventional SSSC are also known as ASC (Advanced Series Compensator) 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 include a single-phase, modular-SSSC injecting a leading or lagging voltage in quadrature with the line current. It can increase or decrease power flows on a circuit and perform dynamic services. The device can operate at multiple voltage levels due to this feature and lack of transformer.
Advantages & field of application
The use of SSSC provide typical advantages related to load flow control, that can also be realised by other technologies like PST or shunt and also offers additional special functions like:
- Better controllability of power flow since the SSSC possess the inherent capability to decrease as well as to increase (real) power flow almost linearly in the line.
- Receiving end voltage regulation of a radial line: The SSSC can be used to regulate the end-voltage of a radial line by controlling the degree of series compensation so as to keep the end-voltage constant in face of changing load and load power factor.
- Improvement of transient stability and dynamic stability (power oscillation damping): the SSSC is controlled to force the line current and power flow to increase, by rapid injection of the necessary compensating voltage, when the relevant machines in the system accelerate, and, conversely, force the line current and transmitted power to decrease when the machines decelerate.
Specific technologies (such as modular SSSC) present additional advantages such as scalability, due to their modular nature, speed of deployment allowing annual adjustment counteracting scenario uncertainty.
Technology Readiness Level
Conventional SSSC: TRL 7-System prototype demonstration in operational environment
Transformerless SSSC: TRL 7-System prototype demonstration in operational environment (pilot deployed in North America at 115 kV and operational since February 2019).
Research & Development
Current fields of research: Generic research related to FACTS includes a variety of domain from power electronics to applications. One could mention power electronic topologies and control, exploration of new type of semiconductors replacing silicon, mitigation of power quality impacts of large scale power electronics application, more user-friendly interfaces, standardisation, evaluation of cost and benefits through demonstrations, coordinated control of multiple FACTS devices and relocatable FACTS.
In addition to these generic topics, when focusing more particularly on SSSC, research is conducted to detect in advance sub synchronous resonance (SSR) events and mitigate them using transformerless SSSC or novel damping controllers with conventional SSSC. Some research is focusing on enhancing the low voltage ride through capability of wind turbines using a combination of SSSC and controllable series braking resistor.
Focusing specifically on modular transformerless SSSC, real time congestion management, power quality management, offshore network control and voltage regulation are the fields to be further developed in the future.
Best practice performance
- Rated system voltage: 220 kV
- Rated Reactive power: 100-400 MVAR with voltage level 400 kV
- 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.
Best practice application
The application foresees the SSSC functionalities validation, the case study definition enabling the equipment behaviour validation (in normal operation and during contingencies in the grid) by using a power flow simulation software. It is complemented by a simplified case study to analyse the behaviour of the SSSC in electromagnetic short-circuit simulations.
For the SSSC behaviour validation the reference grid includes two transmission lines representing the 400 kV Transmission System with two parallel lines representing the 220 kV lines where the problem to be solved is located, the impedance and the transmission capacity of the 220 kV lines is different. The SSSC behaviour under short circuit is based on a grid model considering that the SSSC has to support 40 kA of short circuit current across the coupling transformer.
The SSSC can solve some of the overload problems detected in the 220 kV grid of the Spanish Electrical System. It has proven to be particularly convenient in old lines (with low capacity) with a power flow very much influenced by wind power production.
At times of high wind infeed and hydro production, 220 kV high voltage lines were overloaded in Torres del Segre in Spain. To relief the congestions, the system operator was obliged to reduce renewable production output at certain instance of time.
A 50 MVAr conventional SSSC was installed including control equipment, magnetic elements for grid coupling, by-pass switch, thyristor and a local SCADA.
Construction of a new 220 kV has been avoided, efficiency of the existing infrastructure has been improved with enhanced dispatching of power flows and an increase in renewable energy integration has been achieved.
Use of SSSC for solving problems associated with Nigerian 330 kV longitudinal power network using voltage magnitude as performance metrics.
Modeling of power system and SSSC modeling producing two sets of non-linear algebraic equations solved simultaneously using Newton-Raphson algorithm method and implemented using MATLAB.
Results of power flow analysis of Nigerian 330 kV transmission network without SSSC showed that, there was voltage limit violation of ±10% at bus 16 Gombe (0.8973p.u). The results with incorporation of SSSC showed that, the SSSC was effective in eliminating voltage limit violation and reduced network active power loss by more than 5% of base case (93.87 MW). Therefore, SSSC is effective in solving steady-state problems of longitudinal power systems.
For the trial installation, three transformerless SSSC units were installed on the Cashla – Ennis 110 kV line. Once the trial validated (installation of the unit done safely), two transformerless SSSC units were installed on the first tower coming out of the Cashla substation in County Galway. The remaining unit was installed on the first tower coming out of the Ennis substation on the same circuit. This pilot lasted for a year verifying that communications with the devices did not impact systems used to report primary faults and can change reactance as specified. The full functionality of the devices was assessed including switching units from full Capacitive Reactance Injection mode to full Inductive Reactance Injection mode.
Use of modular transformerless SSSC to enable real-time power flow control on grids.
During system faults, the devices performed as expected and entered a bypass mode under fault conditions, no unexpected interactions with the normal protection system. During the testing, Ireland was hit by the tail end of Hurricane Ophelia on Monday 16th October 2017 (Status Red wind warning – the highest threat level possible). Despite strong gusts of up to 156 km an hour on land, there was no structural damage caused to the devices. The units did not cause any damage to the transmission infrastructure. All units remained fully operational through this period.
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