Reactive Power Compensation
We always in practice to reduce reactive power to improve system efficiency . This are acceptable at some level. If system is purely resistively or capacitance it make cause some problem in Electrical system.
- Alternating systems supply or consume two kind of power: real power and reactive power. Methods of Reactive Power Compensation are states bellow.
Reactive power (VAR) compensation is defined as the management of reactive power to improve the performance of ac systems.
There are two aspects:- a) Load Compensation –
The main objectives are to :- (i) increase the power factor of the system (ii) to balance the real power drawn from the system (iii) compensate voltage regulation (iv) to eliminate current harmonics.
b) Voltage Support – The main purpose is to decrease the voltage fluctuation at a given terminal of transmission line. Therefore the VAR compensation improves the stability of ac system by increasing the maximum active power that can be transmitted.
What is Reactive Power ?
- Power is referred as the product of voltage and current i.e. power = V x I The portion of electricity that establishes and sustains the electric and magnetic fields of alternating-current equipment.
- Reactive power must be supplied to most types of magnetic equipment, such as motors and transformers.
- In an ac transmission, when the voltage and current go up and down at the same time, only real power is transmitted and when there is a time shift between voltage and current both active and reactive power are transmitted.
Methods of Reactive Power Compensation
- Synchronous motors are occasionally used to provide dynamic voltage support to the power system as they provide mechanical power to their load.
- Some combustion turbines and hydro units are designed to allow the generator to operate without its mechanical power source simply to provide the reactive power capability to the power system when the real power generation is unavailable or not needed.
- Synchronous machines that are designed exclusively to provide reactive support are called synchronous condensers.
Static VAR Compensators :
- A static VAR compensator (or SVC) is an electrical device for providing reactive power on transmission networks. The term “static” refers to the fact that the SVC has no moving parts (other than circuit breakers and disconnects, which do not move under normal SVC operation).
- The SVC is an automated impedance matching device, designed to bring the system closer to unity power factor. If the power system’s reactive load is capacitive(leading), the SVC will use reactors (usually in the form of thyristor-Controlled Reactors) to consume vars from the system, lowering the system voltage.
- Under inductive (lagging) conditions, the capacitor banks are automatically switched in, thus providing a higher system voltage.
Static synchronous compensators (STATCOM) :
- Statcoms generate a sine wave from a voltage source, normally a DC capacitor. Hence they are referred to as Voltage Source Converters (VSC).
- Statcom’s use IGBT semiconductors which can also switch off. A series of cascaded and individually controlled bi-directional inverter bridges are turned on or off, as required, to generate an output voltage waveform. The number of modules determines the smoothness of the voltage waveform.
- The Statcom is shunt connected to the supply through a reactor and is therefore a controlled current source. Statcom acts like a capacitor or reactor depending on the control. The output waveform can be also adjusted continuously. to dynamically inject or filter harmonic currents for active filtering.
- The Statcoms response time is typically around 3 ms to 5 ms, making it faster and more flexible to the system needs.
Series compensation (SC)
- Series capacitors compensate for the inductive line impedance to increase transmission system power transfer capacity and voltage and angular stability during transients. The spark gap and MOV are the key technologies to protect the capacitors from over voltages.
- The whole capacitor bank is at the line potential, so it needs to be insulated from ground, and is difficult to measure the key parameters in the system. The first level of protection is the MOV, which is fast, but has limited protective capability.
- The second level is the spark gap, which needs to be triggered. The last level is “tripping” of the SC, which is done by closing the bypass circuit breaker. Therefore control and protection dominate the design requirements for series compensation systems.
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