How to choose the right static compensator for your application [Part 6/8: Thyristor switched / controlled reactors (TSR / TCR)]

After the introduction of static compensators in the first three articles of this series, this sixth article will discuss features and applications of thyristor switched reactors and thyristor controlled reactors.

Thyristor switched reactors (TSR for short) and thyristor controlled reactors (TCR for short) have been around since the 1970s. Description of their topology and operating principle can be found as far back as 1974.

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 reactors (MSR for short) could not handle.

Functions

TSRs and TCRs are suitable solutions for applications that require real time transient free inductive reactive power compensation. They provide an stable and accurate inductive reactive current flow without the drawbacks of conventional solutions, reducing system losses and stabilising the system voltage.

Modern TSRs and TCRs 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.

Markets and applications

TSRs and TCRs 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.

• Underground high voltage cable (UHVC) networks.
• Long and lightly loaded overhead transmission and distribution lines.
• Installations with fast changing reactive power demand like electric arc furnaces (EAF), ladle furnaces (LF) and ball mills.
• Solar inverters and wind turbine generators.
• Highly dynamic loads (power factor fluctuates rapidly or in big steps) like rolling mills, cranes, hoists, winders, crushers, shredders, presses, conveyors and head & band saws.
• Railway electrification systems (trains and trams).
• Modulated phase controllers, cycloconverters and thyristor-controlled AC voltage regulators.
• Compensation of cables, wires or mains capacitance in industrial installations.

Design

TSRs and TCRs are power electronics-based shunt compensation devices 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. They behave as controlled impedances 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).

The most common operating voltage range for TSRs and TCRs 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 a TSR or a TCR 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 and the high voltage electric power system.
• Reactors: They are connected to the thyristor valves and provide inductive reactive power. They absorb reactive power to decrease system voltage.
• Switchgear: It takes care of the measurement of currents and voltages (CTs and VTs), allows the maintenance of the TSR / TCR (earthing switches and disconnectors), and protects medium voltage components (surge arresters).

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.
• Cooling system: Cooling for the thyristor valves.
• Control system: Real-time operation control ensuring response to system’s requirements.
• Protection system: Real-time protection system designed to detect system faults and abnormalities and disconnect the TSR / TCR from the rest of the electric power system if necessary.
• HMI: Monitors the condition of the TSR / TCR and communicates with customers’ SCADA system. It can also provide remote monitoring and analysis capability if integrated into an IIoT software platform.

Reactors

The reactors?used by TSRs or TCRs are usually three-phase assemblies, normally connected in a delta arrangement to provide partial cancellation of harmonics. The reactors are usually air-core dry type and split into two halves, with the thyristor valve connected between them. This protects the thyristor valve from damage due to flashovers, lightning strikes, etc.

Thyristor valves (TV)

Thyristor valves consist of inverse-parallel-connected pairs of thyristors connected in series with the reactors controlling the current to manage the power flow. They are usually installed in a dedicated building or in a modified shipping container. Cooling is usually provided by a tailor made cooling system.

They are designed for dynamic reactive power compensation applications. The switching is done without transient currents (normally associated with the switching of electromechanical contactors or breakers). TVs allow an unlimited number of switching operations without applying significant stress to the components of the system, including the reactors.

Operating principle

Thyristor controlled reactors

A TCR is a reactance connected in series with a bidirectional phase-controlled thyristor valve. The effective reactance is varied continuously (using point-on-wave control) by full to zero conduction operation of the thyristor valve.

TCRs produce a smooth current output when fully conducting (firing angle 90°) or fully blocking (firing angle 180°) and a distorted current output when the firing angle is between 90° and 180°. The maximum current is obtained when the firing angle is 90°, at which point the TCR is said to be in "full conduction".

TCRs offer the ability to dynamically adjust inductive reactive power. This adjustment is performed automatically by the control system based on the dynamic system requirements. TCRs do not generate transients, but they generate harmonic currents at firing angles above 90°. These harmonics need to be dampened by using harmonic filter capacitor banks.

Thyristor switched reactors

TSRs and TCRs have similar construction and features. A TSR is a special case of a TCR in which the variable firing-angle control is not used. Instead, the TSR is operated in two states only, fully conducting (firing angle 90°) or fully blocking (firing angle 180°).

TSRs ensure a very rapid availability of inductive reactive power in the electric power system without generating harmonics, as when the firing angle is 90°, the resultant steady state current will be a smooth sinusoidal signal.

Features

The most typical features of TSRs and TCRs 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 TSRs and TCRs in parallel for higher power outputs.
• Considerably simplified valve electronics design by using light-triggered thyristors (LTTs).
• Overall response time <10 milliseconds.
• 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 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 TSRs and TCRs can be summarized as:

• Thyristor valves are used to switch smoothly the current guaranteeing one network cycle response time in all conditions.
• No mechanical movements of contacts when switching, this means no arcing or sparking takes place and no audible noise is produced.
• Capability to deliver instantaneous and precise inductive reactive power compensation.
• Increased electric power system efficiency by allowing the flow of more active power.
• They limit the voltage level increase on the electric power system (they can decrease the system voltage during low load periods).
• Simple dimensioning and installation.
• Compliance with the strictest power quality standards and grid codes.

Comparison with conventional solutions

The next article of this series will discuss features and applications of thyristor switched capacitor banks (TSC) and thyristor switched filter banks (TSF).

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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.