Thyristor switched compensation devices for low and high voltage applications [Part 3/5: Operating principle and main functions]

After the introduction of thyristor switched compensation devices and their design, this third article will discuss the operating principle and main functions of these devices.

Operating principle

TSCs and TSRs use thyristor switch modules (TSM for short) as the switching device. TSMs are designed for real-time (dynamic) reactive power compensation applications. The switching is done at zero voltage level and without transient currents (normally associated with the switching of electromechanical contactors). TSMs allow an unlimited number of switching operations without applying significant stress to the components of the system including reactors and capacitor units.

Zero cross over switching technique is used to turn on and turn off the TSMs. Due to zero cross over switching techniques, the generation of voltage transients (generated during the switching action) can be controlled within safe limits.

TSMs can be operated through a dedicated dynamic power factor controller or by a direct signal from the load. The use of TSMs for operating capacitor banks and shunt reactors offer many advantages compared to the use of conventional electromechanical contactors. Some of the advantages are:

• Fast switching, meaning faster reactive power compensation and voltage stabilization.
• Smooth connection and disconnection of capacitor units or shunt reactors as switching is done without inrush currents or transients.
• Elimination of surge currents.
• No limitations in the number of switching operations for a thyristor compared to a contactor.
• No mechanical movements of contacts when switching, this means no arcing or sparking takes place and no audible switching noise is produced.
• No system perturbation by switching operations due to zero cross over switching techniques. Transients can be controlled within safe limits.
• Simple dimensioning and installation.

Advantages of thyristor switched devices over contactor switched devices

Functions

TSCs and TSRs can correct some power quality problems such as fundamental-frequency reactive power (non-unity displacement factor), voltage variations and voltage fluctuations. They can also support the development of clean energy by power factor correction and the reduction of the energy losses in the electric power system.

The main functions that usually TSCs and TSRs can provide can be summarized in:

• Power factor correction (PFC).
• Reduction of voltage variations (sags and swells).
• Mitigation of voltage fluctuations (flicker).

Power factor correction (PFC)

Low power factor is typically caused when inductive or capacitive loads like motors, transformers, cables or lighting are present in the electric power system. Other contributors to low power factor are harmonic currents produced by nonlinear loads or renewable generators, or the change of load in the electric power system.

TSCs and TSRs detect the phase angle difference caused by inductive or capacitive loads and they generate and inject in real time leading or lagging current into the electric power system, making the phase angle of the current the same as that of the voltage, which brings fundamental power factor to unity.

Reduction of voltage variations (sags and swells)

Some power quality phenomena occur extremely fast requiring the mitigation to be even faster. If the installation is affected by voltage sags or voltage swells created by the loads, TSCs and TSRs with fast response time could be the solution to reduce those voltage variations and protect sensitive loads from those disturbances.

TSCs and TSRs can support the voltage of the installation via reactive power injection. They can monitor the voltage of the electric power system on which they are connected and determine the proper amount of reactive power needed to either raise the system voltage or lower it. They can inject leading reactive power to raise the voltage and lagging reactive power to lower the voltage.

The amount of reduction of voltage variations that TSCs and TSRs can provide to keep the electric power system within its stated voltage tolerance level depends on system parameters like impedances and cannot be specified alone. The device can only be specified for the rated capacity of current and reactive power that can be used for the reduction of voltage variations.

Mitigation of voltage fluctuations (flicker)

TSCs and TSRs are a very cost-effective solution to deliver instantly voltage fluctuation mitigation functionality. They can mitigate voltage fluctuation caused by the load by providing a rapid detection and injection of the needed reactive current. This prevents the reactive current of the system from over loading the network and causing rapid voltage deviations, also known as voltage fluctuations or flicker.

The amount of mitigation that TSCs and TSRs can provide depends on system parameters like impedances and cannot be specified alone. The device can only be specified for the rated capacity of current and reactive power that can be used for the mitigation of voltage fluctuations.

The next article of this series will discuss thyristor switched capacitor banks (TSC).

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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.