Reactive Power Control Devices Based on Modern Power Electronics

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REACTIVE POWER CONTROL DEVICES BASED ON MODERN POWER ELECTRONICS

Reactive power control (compensation) devices are designed to ensure stable levels of electric power voltage by maintaining predetermined voltage levels at network control points. In some cases, especially for inter-system and system backbone linkages with long-distance power transmission, these devices are also expected to maintain predetermined levels of static and dynamic stability of electric power systems and load stability.

Reactive power compensation devices include the following units:

Static shunt capacitor banks (SCB);
Shunting reactors;
Harmonic filters;
Static thyristor compensators (STC).

Background

Reactive power control devices come in two types: static devices and power devices.

Static devices:
This group includes simple static shunt capacitor banks and shunting reactors that perform graduated control of reactive power, reactor groups commutated by vacuum switches, controlled shunting reactors, static thyristor compensators, static reactive power compensators using modern power electronics (powerful IGBT transistors) – STATCOM (see Figure 1.) Working principles of aforementioned devices are covered in the existing literature in great detail, and will not be addressed here. Figure 1 shows a diagram of STATCOM as a basic element of FACTS static devices.

Electric power devices:
This group includes synchronized and asynchronized compensators. The former is a well-researched and commonly used device, so there is no need to add any further details here. Asynchronized compensators have two windings on the rotor and a special vector system for excitation control – see Figure 2.

Technical characteristics of reactive power control devices and their areas of application:

№	Name	Description	Application	Makers
1.	Reactor groups and commutated switches	Adjustable reactors connected to tertiary winding of autotransformers by vacuum switches with 5000 to 10000 commutations and switch activation time of Δt=0,02-0,12 seconds .	These devices are used for compensating charging capacity of grid lines and load buses in order to keep voltage levels within established limits. Their purpose is to ensure adjustable voltage (reactive power) control when power levels transmitted by gridlines do not exceed natural loading. Their optimal application is within power distribution networks. May also be used in parallel connection with capacitor banks.	Russia and abroad
2.	Controlled shunting reactor with magnetic bias field via direct current	Design is based on a special oil-cooled transformer. The common core houses power winding of the reactor, compensating winding and control winding; thyristor rectifier and a filter are located outside of reactor case. Response time of the reactor depends on the degree of excitation and disexcitation by the magnetic bias field as well as rectifier capacity.	CSR units are designed for throttling reactive power (voltage) when power levels transmitted by gridlines do not exceed natural loading. They can be installed at grid lines and at substation busbars, and are not designed to support stability. Their optimal use is at distribution networks. May also be used in parallel connection with capacitor banks.	Russia
3.	Static thyristor compensators	STCs include an air-cooled reactor and a thyristor valve with air or water cooling; both of these form thyristor groups with throttling of thyristor ignition angle. Capacitor bank and occasionally filter compensating circuits are connected parallel to STC. They are connected to the high-voltage grid network at tertiary winding of low-voltage autotransformers or step-up transformers. Minimal value of the time constant of reactive power control is τρΣ=0,01-0,02c	STC provide voltage (reactive power) control whether power levels transmitted by gridlines do or do not exceed natural loading. They also enhance stability and limits of power transmitted by power lines. Their optimal application is at distribution and mainline networks as well as inter-system linkages to ensure in-depth control of reactive power and enhance stability. They are not effective in “weak” networks.	Russia and abroad
4.	Static reactive power compensator based on voltage converter (STATCOM)	Consists of a voltage converter built with power transistors that ensures generation and consumption of reactive power within the range of ±100% of installed capacity without additional power reactors and capacitor banks. Connected to the high-voltage grid at tertiary winding of low-voltage autotransformer or a separate step-up high or low voltage transformer.	Used to ensure dynamic stabilization of voltage, enhance power line transmission capacity, minimize voltage fluctuations, increase stability during transitional electromechanical processes and improve damping of variations within electrical power systems. Can be used in any electric power networks, but particularly effective in “weak” networks.	Russia and abroad
5.	Synchronized compensators	Includes synchronized units and an exciter, and a modified version with brushless excitation is also available. Can provide control of reactive power of 100% output and 30% to 50% of consumption. Is able to handle substantial overloading (2x or 3x current overload within 30 seconds).	Used to control voltage and increase limits of static and dynamic stability as well as increase of transmission capacity. There are some limits with regard to its use in networks that require in-depth ( ±100%) control of reactive power. Can be used in any power networks.	Russia and abroad
6.	Asynchronized compensators	Includes asynchronized electric AC machines and static frequency converters. Has two or more rotor windings, which ensure the ability to control reactive power within ±100% of variation. Has the ability to control the value and phase of the voltage vector in the electric power system. Can withstand significant current overload (by a factor of two or three) during 300 seconds. Can also operate with a variable revolution frequency with a shaft flywheel in order to increase the limits of dynamic parameters of electric energy systems.	Used to control voltage, increase the limits of static and dynamic stability, enhance power line transmission capacity, and improve power system damping. Can be used in any power networks, and is especially effective in “weak” systems.	Russia and abroad