Reactive power(VARs) is defined as the amount of power that remains unused and gets generated within an AC circuit or system by the reactive components. This is sometimes called imaginary power. A reactive circuit supplies the amount of power back to the supply which it has consumed thus, the average consumed power of the circuit will be zero.
If any line show 300 Ampere load and if Power Factor is 0.90 i.e. 270 Amp is Active and 30 Amp is Reactive. And if Power Factor is 0.70 i.e. 210 Amp is active and 90 Amp is reactive.
Definition of Reactive Power compensation
The compensation of reactive power of the circuit is quite important as it is associated with the value of the power factor. The reactive power compensation corresponds to the controlling of reactive power to increase the performance characteristics of the AC system. There are some methods by which the power factor of the system can be improved and hence these are regarded as methods of reactive power compensation.
We understand reactive compensation by water glass example. As shown in figure, to improve active power(Kw) quality, it’s require to compensate reactive power(VARs) in the system.
Calculation of Reactive Power
Reactive Power=S, Active Power=P, Apparent Powe=Q
S=Square root(P2 + Q2 )
Why Reactive power control in power system?
Ideal power system is one, where frequency, voltage are constant at every supply point, system is free from harmonics and power factor would be unit.
Reactive power control is required to improve the quality of power supply in ac power systems. Better utilization of existing equipment to defer new equipment purchases.
If supply of reactive power(VARs) is more than the demand it results in high voltages in the system. If supply of reactive power(VARs) is less than demand, it results in low voltages in the system. For maintaining proper voltage profile in the system, it is necessary that supply of VARs matches with the demand of VARs.
Do transmission line produce or consume reactive power?
A transmission line always produces reactive power(VARs) in proportion to the square of the voltage applied. At the same time it also consumes reactive power(VARs) in proportion to the square of the current carried by it.
That means consumption of VARs increases if load increases & decreases if load decreases. The load at which VARs produced by the line is equal to the VARs consumed by it is known as surge impedance load (SIL).
Need for reactive power compensation
Reactive power from supply to the reactor in a way that in the first quarter cycle of the AC signal, a capacitor stores the power while in the second quarter cycle, the stored power gets back to the AC source. This to and from movement of the reactive power between the source and load must be controlled.
Also, the loads in industrial equipment like induction motors, induction furnaces, arc, etc. are the ones that operate at poor power factor while fluorescent tubes, fans, etc. that operate at low power factor requires quite a large amount of reactive power hence the level of voltage at the load terminals get reduced.
Due to this reason, the power factor of the system must be necessarily improved using some specific methods.With reactive power compensation, transmission efficiency is increased. Along with this, the steady-state and temporary over-voltages can be regulated that resultantly avoids blackouts.
The demand for this reactive power is mainly originated from the inductive load connected to the system. These inductive loads are generally electromagnetic circuits of electric motors, electrical transformers, the inductance of transmission and distribution networks, induction furnaces, fluorescent lightings, etc.
This reactive power should be properly compensated otherwise, the ratio of actual power consumed by the load, to the total power i.e. vector sum of active and reactive power, of the system becomes quite less.
This ratio is alternatively known as the electrical power factor, and a lower ratio indicates a poor power factor of the system. If the power factor of the system is poor, the ampere burden of the transmission, distribution network, transformers, alternators and other types of equipment connected to the system, becomes high for required active power.
Hence reactive power compensation becomes so important. This is commonly done by a capacitor bank at electrical substation.
Sources of Reactive power
Synchronous Generators – Synchronous machines can be made to generate or absorb reactive power depending upon the excitation (a form of generator control) applied.
Synchronous Compensators – Certain smaller generators, once run up to speed and synchronized to the system, can be declutched from their turbine and provide reactive power without producing real power.
Capacitive and Inductive Compensators-These are devices that can be connected to the system to adjust voltage levels.A capacitive compensator produces an electric field thereby generating reactive power whilst an inductive compensator produces a magnetic field to absorb reactive power.
Compensation devices are available as either capacitive or inductive alone or as a hybrid to provide both generation and absorption of reactive power.
Overhead Lines and Underground Cables-Overhead lines and underground cables, when operating at the normal system voltage, both produce strong electric fields and so generate reactive power. When current flows through a line or cable it produces a magnetic field which absorbs reactive power.
A lightly loaded overhead line is a net generator of reactive power whilst a heavily loaded line is a net absorber of reactive power. In the case of cables designed for use at 220 or 400kV the reactive power generated by the electric field is always greater than the reactive power absorbed by the magnetic field and so cables are always net generators of reactive power.
Methods of Reactive Power Compensation
Traditionally, rotating synchronous condensers and fixed or mechanically switched capacitors or inductors have been used for reactive power compensation.To effectively manage the voltage profile in the network reactive power must be provided in the right amount at the right location, and at the right time.
In recent years, static VAR compensator employing thyristor switched capacitors and thyristor controlled reactors to provide or absorb the required reactive power have been developed.
A low value of power factor requires large reactive power and this affects the voltage level. Hence in order to compensate for the reactive power, the power factor of the system must be improved.Thus, the methods for reactive power compensation are nothing but the methods by which poor power factors can be improved. The methods are as follows:
capacitor banks
synchronous condensers
static VAr compensators
Capacitor Banks
In this method, a bank of capacitors forms a connection across the load. As we know that the capacitor takes the leading reactive power, thus this causes the decrease in power taken from the source. This resultantly improves the value of the power factor of the system. This is further classified as series and shunt compensation.
These categories are mainly based on the methods of connecting the capacitor bank with the system. Among these two categories, shunt capacitors are more commonly used in the power system of all voltage levels.
There are some specific advantages of using shunt capacitors such as,
It reduces the line current of the system.
It improves the voltage level of the load.
It also reduces system Losses.
It improves the power factor of the source current.
It reduces the load of the alternator.
It reduces capital investment per megawatt of the Load.
Synchronous Condensers
Exciting a synchronous motor above a fixed level makes it work as a synchronous condenser or capacitor. It is designed to provide dynamic correction of power factors over the range of its excitation.
Initially, when the synchronous motor is under excited then it functions as a lagging power factor thus reactive power is absorbed. While under overexcited conditions, the leading power factor comes into action and starts generating reactive power thus acts as a capacitor.
Synchronous condensers have played a major role in voltage and reactive power control for more than 50 years. The field current is adjusted to either generate or absorb reactive power as required by the ac system. The machine can provide continuous reactive power control when used with the proper automatic exciter circuit.
Synchronous condensers are rarely used today because they require substantial foundations and a significant amount of starting and protective equipment. They also contribute to the short circuit current and they cannot be controlled fast enough to compensate for rapid load changes.
Static VAr Compensaters
Static VAR compensaters (with the advantage of faster response) consist of standard reactive power shunt elements (reactors and capacitors) which are controlled to provide rapid and variable reactive power. They can be grouped into two basic categories, the thyristor-switched capacitor and the thyristor-controlled reactor.
It is abbreviated as SVC and shows improved system stability, reduction in line losses, maintaining the variation within limits. It has shunt reactors and shunt capacitors. Shunt reactors and thyristor-controlled reactors are used for limiting the voltage rise at no load or low load conditions while static capacitors and thyristor switched capacitors are used for preventing the voltage sag at peak load conditions.
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