Methods to Compensation Reactive power and generation
Reactive power compensation and switching of capacitors what is Reactive power compensation technique generation requirement control methods synchronous alternators advantages reactive power compensation in transmission lines
Reactive power compensation and switching of capacitors
a) Reactive power compensation techniques
Definition of reactive power
Reactive power (Q)
It is a consequence of an AC system. Reactive power are used to build up magnetic fields. It is measured in var,kvar,Mvar.
It is a consequence of an AC system. Reactive power are used to build up magnetic fields. It is measured in var,kvar,Mvar.
& calculated as,
reactive power formula:
Q = S x sinφ or P x tanφ
It is the Active Power that contributes to the energy consumed,or transmitted.Reactive Power does not contribute to the energy.It is an inherent part of the ‘‘total power’’which is often referred as “Useless Power”
so, why reactive power is necessary? Problems of reactive power ?Need of Reactive Power in power supply
In electrical networks in which inductive consumers (e.g. motors) are switched on and off, the power factor cos φ often changes with each switching operation. The Power Utilities demand from their consumers that the ratio of the consumed effective power P to the drawn apparent power S means power factor does not fall below a certain value, as the transmission of apparent power is uneconomic.
Resone for reactive power compensation is..
The reactive power of motors,luminescent lamps with series chokes and other inductive loads is therefore frequently compensated by connecting capacitors,in order to reduce the additional load of transformers and lines by the reactive current.
In deciding whether it is more advantageous to compensate individual consumers with fixed capacitors or to provide central compensation units,economic and technical considerations are definitive.Control units for central compensation have a higher price per power unit (kVA). If allowance is made however for the fact that in most operations not all consumers are switched on at the same time, a lower installed capacitor power is often sufficient for central compensation.
Types of reactive power compensation:
1) Individual compensation
 |
| a) Individual compensation of motors |
For individual compensation the capacitors are directly connected to the terminals of the individual consumer (e.g. motor, transformer, induction heater, luminescent lamp) and switched together with these via a common switchgear unit. Single compensation is recommended with large consumers with constant power consumption and long ON-times. They offer the advantage that the lines to the consumers are also relieved of load. The capacitors can frequently be connected directly to the terminals of the individual consumer and be switched on and off with a common switchgear device.
In the case of motors, the capacitors can be connected up- or down stream the motor protection unit (Fig. b). In most cases the capacitor will be connected parallel to the motor (case 1). In this case the motor protection unit should be set to a smaller setting current Ie than the motor rated current Iɴ as the magnitude of the line current falls due to the compensation: Individual compensation of motors
 |
| b) Individual compensation of motors |
Ie = (cos φ1/cos φ2) *In
cos φ1 = power factor of the uncompensated motor
cos φ2 = power factor of the compensated motor
Iɴ = motor rated current
2) Group compensation
 |
| c) Group compensation |
For group compensation each compensation device is assigned to one consumer group. This may consist of motors or also for example of luminescent lamps that are connected to the mains via a contactor or a circuit breaker (Fig. c).
3) Central compensation
 |
| d) Central compensation |
Mostly reactive power control units are used for central compensation which are directly assigned to a main- or sub-distribution station (Fig.d). This is especially advantageous if many consumers with differing power requirements and variable on-times are installed in the network.
Central compensation also offers the advantage that
- compensation device is easy to monitor due its central location,
- any retrospective installation or extension is relatively simple,
- the capacitive power is continuously adapted to the reactive power requirement of the
consumers and - making allowance for a simultaneity factor a lower capacitance is often required than for individual compensation.
how reactive power is generated
Sources of reactive power are given below
b) Generate reactive by switching of capacitors
Capacitors form oscillator circuits together with the inductances of the lines and the transformers. During closing, very high transient currents with higher frequencies may flow. Typical values are 10 ... 30 times the capacitor rated current at frequencies of 2 ... 6 kHz
For this reason, the switching of capacitors represents a very heavy load on switchgear and can result in increased contact burn-off or under adverse conditions even welding of the contacts. Especially when capacitors are switched by contactors, it should be ensured that they are discharged before switching-on to avoid even higher transient currents and welding of the contacts in case of adverse phase angles.
A harmonic component in the supply voltage leads to increased current consumption by the capacitors and results in additional heating of the current carrying circuits. To prevent any undesired temperature rise, the rated operational current of the contactors, load switches and circuit breakers shall be higher than the capacitor rated current. Generally this should only be 70 … 75 % of the rated current of the circuit breaker.
Taking into account the mentioned facts, the switchgear should be dimensioned so that
- it does not weld at the high making currents and
- that no unacceptable temperature rise occurs during continuous duty.
i) Switching-on single capacitors
If a capacitor with a specific capacity is connected to the power supply, then the making current is largely determined by the transformer size and by the network impedance to the capacitors, i.e. from the prospective short-circuit current at the installation site of the capacitor.
The loading of the switchgear increases as
The loading of the switchgear increases as
- the capacitance of the capacitors increases,
- as the rated power of the supplying transformer increases and hence its short-circuit impedance decreases,
- ƒdecreasing impedance of the connecting lines.
ii) Switching of long, screened lines
Long screened lines have comparatively large capacitances and therefore create high transient current loads during switching. Typical applications are variable frequency drives. The peak currents to be expected should be taken into account when selecting switchgear to the same extent as for the switching of single capacitors.
iii) Switching capacitors of central compensation units
If individual capacitors of capacitor banks are switched – for example in reactive power control units - especially adverse conditions occur at closing of the switchgear contacts as the capacitors already connected to the power supply represent an additional source of energy.
The inrush current is limited by the impedance of the circuit (conductors, capacitor inductance, inductances between the individual capacitor branches).
The loading of the switchgear is therefore determined by
The inrush current is limited by the impedance of the circuit (conductors, capacitor inductance, inductances between the individual capacitor branches).
The loading of the switchgear is therefore determined by
- the power ratio of the switched capacitors to those already connected to the power supply and
- the impedance of the individual circuit branches
For avoiding welding of the switching contacts of the contactors the switchable capacitance can e.g. be increased, by additional inductances in the capacitor branches (e.g. a few winding turns of the connecting wires).
With special capacitor-contactors or capacitor-contactor-combinations that connect capacitances to the power supply via pre-charging resistances, a very high switchable capacitance at a minimum of interference with the supplying network can be achieved, as the making currents are specifically limited by the resistances and strongly reduced.
With special capacitor-contactors or capacitor-contactor-combinations that connect capacitances to the power supply via pre-charging resistances, a very high switchable capacitance at a minimum of interference with the supplying network can be achieved, as the making currents are specifically limited by the resistances and strongly reduced.
The main means for the generation of reactive power are:
• synchronous alternators;
• synchronous compensators (SC);
• static var compensators (SVC);
• synchronous alternators;
• synchronous compensators (SC);
• static var compensators (SVC);
c) Reactive power using synchronous alternators
Synchronous alternators are the main machines used for the generation of electrical energy. They are intended to supply electrical power to the final loads through transmission and distribution systems. Besides, without going into technical details, by acting on the excitation of alternators, it is possible to vary the value of the generated voltage and consequently to regulate the injections of reactive power into the network, so that the voltage profiles of the system can be improved and the losses due to joule effect along the lines can be reduced.
d) Reactive power generation using synchronous compensators
They are synchronous motors running no-load in synchronism with the network and having the only function to absorb the reactive power in excess (underexcited operation) or to supply the missing one (overexcited operation).
 |
| under-excited synchronous compensator |
 |
| over-excited synchronous compensator |
E : e.m.f. induced in the stator phases
V : phase voltage imposed by the network to the alternator terminals
I : stator current
V : phase voltage imposed by the network to the alternator terminals
I : stator current
Xs : stator reactance
These devices are used mainly in definite nodes of the power transmission and sub-transmission network for the regulation of voltages and of reactive power flows. The use of synchronous compensators in power distribution networks is not favourable from an economic point of view because of their high installation and maintenance costs.
e) Static var compensators
 |
| Static var compensators |
The considerable development of power electronics is encouraging the replacement of synchronous compensators with static systems for the control of the reactive power such as for example TSC (thyristor switched capacitors) and TCR (thyristor controlled reactors). These are an electronic version of the reactive power compensation systems based on electromechanical components in which, however, the switching of the various capacitors is not carried out through the opening and closing of suitable contactors, but through the control carried out by couples of antiparallel tyristors
TSC allow a step-by-step control of the reactive power delivered by groups of capacitors, whereas with TCR a continuous control of the reactive power drawn by the inductors is possible. By coupling a TSC with a TCR it is possible to obtain a continuous modulated regulation of the delivered/drawn reactive power. From the point of view of applications, these devices are used above all in high and very high voltage networks
Advantages of reactive power compensation
1) Improves system power factor
2) Reduces network losses
3) Avoid penalty charges from utilities for excessive consumption of reactive power
4) Reduces cost and generates higher revenue for the customer
5) Increases system capacity and saves cost on new installations
6) Improves voltage regulation in the network
7) Increases power availability
References: Low-Voltage Switchgear and Controlgear Allen Bradley ,Power factor correction and harmonic filtering in electrical plants ABB
Post a Comment