Static Thyristor Compensators of Reactor Power (STC) are one of the devices securing an increase of works effectiveness and energy saving of power transmission and distribution systems.
STC are designed in two main modifications: for industrial plants of arc steel-melting furnaces (ASF) type and for thyristor drives of rolling mills and for high-voltage power transmission lines. Also there is a special performance of STC for application at traction substations of electrified railways.
Depending on object installation STC application effectiveness is determined by its realization of the following functions:
For industrial plants and traction substations of rail ways
- Voltage fluctuations reduction
- Power factor correction
- Load balancing
- Higher harmonics currents reduction
For arc steel-melting furnaces
- Significant reduction of voltage fluctuations (flicker) in a supply network
- Powerful furnaces connection possibility to energy systems with low power of short circuit
- Average power factor correction
- Reduction of higher harmonics currents, flowing into energy system
- Symmetrization of currents, consumed from network
- Voltage stabilization at load busbars
- Furnace productivity increase
- Reduction of electrodes consumption and lining
For power transmission lines
- Increase of static and dynamic stability of transmission
- Voltage deviation reduction at big disturbances in a system
- Voltage stabilization
- Inside overvoltage limitation
- Increase of transmitting ability of electrical power transmission due to stability improvement at a big transmitted power
- Filtration of higher harmonics currents
Except meeting of GOST 13109-97 requirements as per main indicators of power energy quality STC fulfill unloading of network transformers and feed lines of energy transmission from reactive power, therefore, they decrease the value of rms current and active losses in them, what permits to increase transmitting active power without new equipment installation. STC's paying back ability is from 1 up to 3 years.
Thus, by analogy with environment protection STC are a kind of "purifying systems" for energetic media, restoring energy quality spoiled by consumers and reducing active losses for its transmission.
Scheme and functional concept
Main scheme of configuration of STC includes a set of filters of higher harmonics – filter compensating circuits (FCC), permanently connected to a network or commutating by circuit-breakers, and three phases of reactors controlled by thyristors (thyristor-reactor group, TRG), which are switched on in parallel to them into a triangle. TRG's ignition angle can be changed very quickly in such a way that the current in a reactor will track out a load current or a reactive power in an energy system.
STC control and protection system secures a quick compensation of reactive power of load and maintaining of adjusted parameter in accordance with the specified setting, fulfills STC equipment protection, control and signaling of failures and can be modified under specific customer's requirements. Reaction time of STC control system for adjustable parameter change is 5 ms for loads of ASF type and 25-100 ms for general purpose industrial loads and grid substations.
STC has a level of automation securing its operation without constant staff presence. STC control is carried out with remote control device or with automatic process control system through external interface.
Rated power and scheme of STC are chosen for a definite object depending on power supply system parameters, type and power of compensating load and energy quality requirements and fulfilled functions. For each individual case there is a calculation of required power of TRG and FCC and their composition is determined.
When STC using on high-voltage electricity transmission lines, the higher its connection point is, the higher STC's effectiveness is. STC equipment is usually applied for voltage class from 10 up to 35 kV and connected either through a special step down transformer to substation busbars, or to a tertiary winding of substation autotransformer.
Maximum effect happens when STC is connected directly to electricity transmission lines or to busbars of high-voltage substation – at this STC can realize a range of system functions relating to operation regimes of electricity transmission lines. In this case it is expedient to use the so called controlled shunt reactor of transformer type, combined in itself a set down transformer and TRG. The high-voltage winding of controlled shunt reactor of transformer type (grid winding) is carried out for the required voltage class; a secondary control winding has 100% magnetic coupling with a grid winding and carried out for the voltage class, which is optimal for thyristor valve loading switched on in parallel with control winding.
- Capacitors are used with power 700 - 1000 kVar, voltage up to 14 kV, outside installation, with built-in sectional fuses and discharge resistors.
- They are supplied in a set as capacitor blocks with a necessary set of insulators and busbars, and imbalance protection current transformer.
STC delivery set
- High-voltage anti-parallel thyristor valve
- Water cooling system
- Compensating reactors
- Capacitor batteries and reactors of filters
- STC automatic control and protection system
It is possible to supply filter-compensating devices as a set of filter-compensating circuits commuting by circuit-breakers. Supply volume of filter-compensating devices has:
- Capacitor batteries and reactors of filter-compensating circuits
- Automatic control and protection cabinet
Thyristor valve, cooling system and automatic control system of STC are placed in a closed heated room. Compensating reactors and filter-compensating circuits are placed outside of the building at an open area.