At present time the main loads of electric networks are asynchronous motors, various distribution or conversion transformers, semiconductor conversion devices, etc.
Such a load in the process of operation is a consumer of reactive power, which, swinging between the source, is consumed on the creation of electromagnetic fields and creates an additional load of equipment for production, transmission and distribution of electricity. Sharp varying nature of electricity consumption is accompanied by voltage fluctuations at the load assemblies.
Load application with a nonlinear volt-ampere characteristic is accompanied by a generation of non-sinusoidal distortions into the supply network, these distortions adversely affect all electrical equipment of the power object:
- increased heating of transmission and distribution equipment, increase of active losses in conductor and dielectric materials;
- vibration, unstable operation of engines;
- false activation of relay protection and emergency controls devices;
- electromagnetic interference in measurement equipment and control devices;
- unauthorized triggering of switching equipment;
- a possibility of occurrence of resonant phenomena in a reactive power compensation.
Static Thyristor Compensators of Reactor Power (SVC) are one of the devices securing an increase of works effectiveness and energy saving of power transmission and distribution systems.
SVC 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 SVC for application at traction substations of electrified railways.
Depending on object installation SVC 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 busbars of loads
- Furnace productivity increase
- Reduction of electrodes consumption and lining/fettle.
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 existing standards’ requirements as per main indicators of power energy quality SVC 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. SVC’s paying back ability is from 1 up to 3 years.
Thus, by analogy with environment protection SVC 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 SVC configuration includes a set of filters of higher harmonics – filter compensating circuits (FCC), permanently connected to a network or commutating/switching 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 trace out a load current or a reactive power in an energy system.
SVC 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 SVC’s equipment protection, control and signaling of failures and can be modified under specific customer’s requirements. Reaction time of SVC 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.
SVC has a level of automation securing its operation without constant staff presence. SVC control is carried out with a remote control device or with automatic process control system through external interface.
Rated power and scheme of SVC 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.
Type scheme of SVC for ASF
When STC using on high-voltage electricity transmission lines, the higher its connection point is, the higher SVC’s effectiveness is. SVC 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.
Type SVC scheme for electricity transmission lines and its regulating characteristic
Maximum effect happens when SVC is connected directly to electricity transmission lines or to busbars of high-voltage substation – at this SVC 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.
Single-line scheme of controlled shunt reactor of transformer type
SVC Power Equipment
Static Capacitor Banks
- application of capacitors with power 700 – 1000 kVar for voltage up to 14 kV, outside installation, with built-in element fuses and discharge resistors.
- delivery in completion with capacitor blocks with a necessary set of insulators and busbars and current transformer of unbalance protection.
SVC DELIVERY SET
- High-voltage anti-parallel thyristor valve
- Cooling system
- Compensating reactors
- Static capacitor banks and filter reactors
SVC automatic control and protection system
Thyristor valve (TV)
Thyristor valve is a main element of SVC through which the current of compensating reactors is regulated and a reactive power generated in the network.
Thyristor valve consists of thyristor modules, which are independent structural units. Each such module contains a group of thyristors connected in a counter-parallel circuit, the number of which is determined by a rated current and rated voltage of SVC.
Each pair of thyristors has a separate control circuit and a damping chain. A control light signal is applied to a control cell, which converts them into an electrical signal that allows thyristors to be switched on. A control over the state of thyristors is also carried out via the light communication channel. In the control cells there is a protection of thyristors from non-admissible overloads.
Optical monitoring system and thyristors control
It performs the functions of transmitting the control signal to thyristors and transmitting of a state control signal of thyristors in a reverse direction, which provides a high-speed signaling about faults of the thyristors or its control cells. It has a high reliability and stability to electromagnetic interferences.
Cooling system of thyristor valves
A cooling system provides heat dissipation/transfer from the power elements of thyristor valve to ensure a predetermined temperature range during operation. Cooling systems are divided into air and water, depending on the applied coolant.
Water cooling system of thyristor valve:
- it is used for more intensive heat dissipation/transfer, with a power of transferred losses up to 300 kW
- carries out deionization of water
- produces a continuous monitoring of pressure, consumption, temperature and conductivity of water
Control and Protection System
A control and protection system of SVC consists of a control cabinet (CC) and a relay protection cabinet (RPC), which is based on programmable electronic relays. All CC functions are implemented in a digital format in the controller board using a high-speed signal processor.
High performance of a control system is achieved through the use of software and hardware algorithms. The control system has an increased noise immunity.
The following is implemented in a control system
- regulating circuit as per reactive current/power of feed line
- voltage-keeping circuit on substation buses
- high-speed channel limiting large voltage deviations
- protection from increase/reduction of voltage
- a protection of thyristor-reactor group from over current and overload
- a protection of filter compensating circuits (FCC) from over currents, overload, unbalanced currents in branches of static capacitor banks
SVC is equipped with filter and compensating reactors. Filter reactors are connected in series to static capacitor bank and form a FCC tuned for a certain resonant frequency. The compensating reactors are connected in parallel to FCC and in series with TV, forming a TRG for rapid control of reactive power generated in the network.
At SVC production – dry air reactors with air core for outdoor installation are used.
Static Thyristor Compensator of reactive power of modular design with air cooling system.
Static Capacitor Banks
During static capacitor bank production for SVC – the capacitors of power up to 1000 kVAr are used with a voltage up to 14 kV, outside installation, with built-in element fuses and discharge resistors. Static capacitor banks are delivered as a complete set of capacitor blocks with a necessary set of insulators and busbar and current transformer of unbalanced protection.
Thyristor valve, cooling system and automatic control system of SVC are placed in a room with automatic microclimate support. Compensating reactors and FCC are placed in the open air.
EFFECT OF SVC IMPLEMENTATION
- power factor correction cos φ
- reduction of losses during electrical power transmission and distribution
- reduction of loading of electrical power transmission and distribution equipment
- impact reduction of higher harmonic components of current and voltage
- improvement of production indicators, stabilization of technological process
- increase of electrical grids operation reliability
- improvement of production indicators, stabilization of the technological process
- reliability increase of electrical networks
- increase of power equipment service life