How to choose the right static compensator for your application [Part 8/8: Static var compensators (SVC)]

After the introduction of static compensators in the first three articles of this series, this eighth and last article will discuss features and applications of static var compensators.

Static var compensators (SVC for short), also called static var systems (SVS for short) have been around since the 1970s. Description of their topology and operating principle can be found as far back as 1976. Since then, a large number of SVCs have been commissioned around the world, making SVCs a well-proven technology.

They were developed thanks to the technological evolution of thyristor valves in the 1970s to take care of the problems in the electric power system created by fast changing reactive power demand or by highly dynamic loads that conventional solutions like mechanically switched capacitor banks (MSC for short), mechanically switched reactors (MSR for short) or synchronous condensers could not handle.

Functions

SVCs combine the technical advantages of thyristor switched compensation devices with the cost-effectiveness of mechanically switched compensation devices to form an economical real-time compensator with a single controller without the drawbacks of conventional solutions.

Modern SVCs can take care of several power quality problems, provide ancillary services and support the development of clean energy by combining different control functions in a single device.

According to their use and the control functions implemented, SVCs can be classified into three types.

• Industrial SVCs are installed for example in steel mills, mines, oil & gas facilities and railway electrification systems. They are mainly used to improve power factor, reduce voltage fluctuations, increase production efficiency, reduce harmonic distortion, load balancing and improve installations’ voltage profile.
• Renewables SVCs are installed for example in wind farms and solar power plants. They are mainly used to control reactive power and maintain the voltage level at the point of common coupling, and to reduce the voltage fluctuation caused by power variation during generation, stabilizing the electric power system.
• Transmission and distribution (or utility) SVCs are installed by electric utilities. They are large size SVCs, up to 1000 kV and hundreds of Mvar, mainly used to improve grid availability and the available active power, improve power factor, suppress voltage fluctuations, control voltage unbalance and reduce the loss of reactive power.

Markets and applications

SVCs can be applied to medium or large applications in a wide range of segments.

They have many high voltage potential applications where their use offers many benefits.

• Solar inverters and wind turbine generators.
• Transmission and distribution substations and lines.
• Installations with fast changing reactive power demand like electric arc furnaces (EAF), ladle furnaces (LF) and ball mills.
• Highly dynamic loads (power factor fluctuates rapidly or in big steps) like rolling mills, cranes, shredders, hoists, winders, crushers, presses and conveyors.
• Railway electrification systems (trains and trams).
• Modulated phase controllers, cycloconverters and thyristor-controlled AC voltage regulators.
• Hot-dip galvanization and electrogalvanization lines.
• Loads with low power factor like underground high voltage cable (UHVC) networks, lightly loaded transformers, etc.

Design

An SVC is a power electronics-based shunt compensation device connected in 3-wire electric power systems in parallel with the equipment generating the power quality problems or that has issues to comply with grid code and energy efficiency requirements. The SVC behaves as a controlled impedance providing any kind of current waveform (in terms of phase, amplitude and frequency) in real time (typical reaction time is under 5 milliseconds and typical overall response time is under 10 milliseconds).

Depending on design, the SVC is formed by several mechanically (contactor or circuit breaker) switched and thyristor switched compensation devices connected to a common control & protection system and HMI.

The mechanically switched compensation devices used in SVCs are mechanically switched capacitor banks (MSC). The thyristor switched compensation devices used include thyristor controlled reactors (TCR) and thyristor switched capacitor banks (TSC).

The most common operating voltage range for SVCs is 3 kV up to 36 kV as they are built using high voltage thyristor valves. It is possible to connect them to higher voltages using a suitable step-up transformer.

Components

The components of an SVC can be divided into the ones forming the passive part of the device and the ones forming the active part of the device.

Passive part

The main components of the passive part are:

• Step-up transformer: It enables the use of medium voltage thyristor valves by connecting the medium voltage and the high voltage electric power system.
• TCR reactors: They provide inductive reactive power by point-on-wave control (smooth adjustable output) from minimum current to full rated current. They absorb reactive power to decrease system voltage.
• MSC banks: They are usually tuned filter capacitor banks. They provide capacitive reactive power at fundamental frequency and they absorb the harmonic currents generated by the equipment and the TCR reactors.
• TSC banks: They provide capacitive reactive power by fast ON/OFF switching (output in blocks, no current or full rated current). They generate reactive power to increase system voltage.
• Switchgear: Circuit breakers, contactors, earthing switches and disconnectors allow connection and maintenance of TCRs, MSCs and TSCs. CTs and VTs are used for the measurement of currents and voltages. Surge arresters protect medium voltage components.

Active part

The main components of the active part are:

• Thyristor valves: High-performance valves built on multilevel valve topology using modular light-triggered thyristors (LTTs) take care of switching the TCR reactors and TSC banks.
• Cooling system: De-ionized water system used for cooling the thyristor valves.
• Control system: Real-time operation control of the SVC ensuring response to system’s requirements.
• Protection system: Real-time protection detecting system faults and abnormalities and disconnecting the SVC from the rest of the electric power system.
• HMI: Monitors SVC condition and communicates with customers’ SCADA system. It can also provide remote monitoring and analysis capability by IIoT.

Operating principle and types

An SVC is a dynamically variable source of reactive power, that absorbs inductive reactive power when voltage is too high and generates capacitive reactive power when voltage is too low. It can provide an inductive and capacitive output and it can be operated over its output range even at low system voltage.

SVCs formed by TCRs and MSCs

The design consists of several parallel branches of TCRs and MSCs. The variation of reactive power is accomplished by controlling the thyristor’s firing instants and, accordingly, the current that flows by the reactance. The TCRs do not generate transients and provide a continuous adjustment of inductive reactive power. The MSCs eliminate the harmonics generated by the equipment they are connected to and generated by the TCRs while providing capacitive reactive power.

SVCs formed by TSCs

TSCs are switched on and off as needed to inject capacitive reactive power step by step instead of being controlled continuously (like TCRs). Switching takes place when the voltage across the thyristor valve is zero, making it virtually transient-free. Disconnection takes place by suppressing the firing pulses to the thyristors which will be blocked when the current reaches zero.

SVCs formed by TCRs, TSCs and MSCs

In this configuration, the control of the SVC is based on measuring the reactive component of the equipment current at the instant of voltage zero. Then, the measured current is used to determine the firing angle so that the SVC absorbs or injects the amount of reactive power required for compensation. With this type of SVC, continuously variable reactive power can be obtained across the entire control range of the device (inductive and capacitive).

Features

The most typical features of SVCs that can be found nowadays in the market include:

• Direct connection to 3-36 kV 50/60 Hz 3-wire electric power systems.
• Simple connection to higher voltages (>36 kV) through suitable step-up transformer.
• Possibility to connect an unlimited amount of TCRs, MSCs and TSCs in parallel for higher power outputs.
• Considerably simplified valve electronics design by using light-triggered thyristors (LTTs).
• Overall response time <10 milliseconds.
• Instantaneous, precise and stepless power factor correction of inductive and capacitive loads.
• Harmonic currents elimination by using tuned MSCs.
• Mitigation of unbalances between phases.
• Optimized long distance signal transfer through fibre optics link between high voltage thyristor disc level and ground control system.
• Built-in protection functions including overcurrent, overvoltage, undervoltage, neutral unbalance (for capacitor banks) and overtemperature.
• Typically using de-ionized water as cooling media in a closed-loop redundant cooling system.
• Redundant design: Redundant discs, valve bases and cooling system pumps and fans. Each thyristor valve has an independent controller. If any valve disc fails, the rest will continue in operation.
• Modular design ready for installation in a building or a container. Mobile options available.
• Power quality monitoring and reporting capabilities.
• Remote asset connectivity, big data processing and analytics by industrial IoT (IIoT) software platforms.

Benefits

Some of the technical and economic benefits of using SVCs can be summarized as:

• Protection of loads from waveform distortions, voltage variations and fluctuations, low power factor and unbalance.
• Increase of transmission and distribution system stability by providing reactive power control, voltage control, power oscillation damping and power transfer capacity increase.
• Ensuring grid code compliance for renewable integration by providing reactive power control, voltage control and fault ride through support.
• Robust voltage support under severe system disturbances where voltage recovery is critical.
• High availability, reliability and efficiency.
• Unaffected by voltage drop, they can provide full output at reduced system voltage.
• Mitigation of transients such as inrush currents and start-up transients of heavy equipment.
• Fast voltage regulation under various load conditions (steady-state and dynamic events).
• Can take care of individual disturbance patterns and automatically adapt to changing load conditions and network topologies.
• Simple dimensioning and installation.
• Compliance with the strictest power quality standards and grid codes including G5/4, IEEE 519, IEC 61000, GOST 13109 and EN 50160.

Comparison with conventional solutions

This was the last article of this series on static compensators, their features and benefits, and how to choose the right one for your application.

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About the author:

Pedro Esteban is a versatile, multicultural and highly accomplished marketing, communications, sales and business development leader who holds since 2002 a broad global experience in sustainable energy transition including renewable energy, energy efficiency and energy storage. Author of over a hundred technical publications, he delivers numerous presentations each year at major international trade shows and conferences. He has been a leading expert at several management positions at General Electric, Alstom Grid and Areva T&D, and he is currently working at Merus Power Plc.