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4 Main Means For The Generation Of Reactive Power
4 Main Means For The Generation Of Reactive Power

It’s all about reactive power…

The four main means for the generation of reactive power are:

  1. Synchronous alternators
  2. Synchronous compensators (SC)
  3. Static var compensators (SVC) and
  4. Banks of static capacitors

1. 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.

Large 4-Pole Generators - ALSTOM Power
Large 4-Pole Generators designed by ALSTOM Power for maximum reliability, availability and maintainability. The design also takes into account optimum erection, commissioning, testing and plant layout

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2. 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 (under-excited operation) or to supply the missing one (overexcited operation).

Under-excited synchronous compensator
Figure 1 – Under-excited synchronous compensator

Over-excited synchronous compensator
Figure 2 – Over-excited synchronous compensator

Where:

  • E = e.m.f. induced in the stator phases
  • 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.

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3. 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).

TSC (thyristor switched capacitors) and TCR (thyristor controlled reactors)
Figure 3 – 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.

Chenier Static VAR Compensator Substation - 735 kV by ABB / Hydro-Quebec
Chenier Static VAR Compensator Substation – 735 kV by ABB / Hydro-Quebec

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4. Banks of static capacitors

A capacitor is a passive dipole consisting of two conducting surfaces called plates, isolated from one another by a dielectric material.

Bank of static capacitor
Figure 4 – Bank of static capacitor

The system thus obtained is impregnated to prevent the penetration of humidity or of gas pockets which could cause electrical discharges.

The last generation capacitors are dry-type and undergo a specific treatment which improve their electrical characteristics. Using dry-type capacitors there is no risk of pollution because of the incidental leak of the impregnating substance.

According to the geometry of the metal plates, it is possible to have:

  • Plane capacitors;
  • Cylindrical capacitors;
  • Spherical capacitors.
Capacitor types (plane, cylindrical and spherical)
Figure 5 – Capacitor types (plane, cylindrical and spherical)

The 4 main parameters which characterize a capacitor are:

  1. The rated capacitance C – the value obtained from the rated values of power, voltage and frequency of the
    capacitor;
  2. The rated power Qn – the reactive power for which the capacitor has been designed;
  3. The rated voltage Un – the r.m.s. value of the alternating voltage for which the capacitor has been designed;
  4. The rated frequency fn – the frequency for which the capacitor has been designed.

When an alternating voltage is applied across the plates, the capacitor is subjected to charge and discharge cycles, during which it stores reactive energy (capacitor charge) and injects such energy into the circuit to which it is connected (capacitor discharge).

Such energy is given by the following relation:

Capacitor energy formulae

where:

  • C is the capacitance;
  • U is the voltage applied to the terminals of the capacitor.
Because of their capability of storing and delivering energy, capacitors are used as basic element for the realization of power factor correction banks (for all voltage levels) and of static devices for the regulation of reactive power.

In particular, the power factor correction capacitors used for low voltage applications are constituted by single- phase components of metalized polypropylene film and can be of the self-healing type.

In these capacitors, the dielectric part damaged by a discharge is capable of self-restoring; in fact, when such situations occur, the part of the polypropylene film affected by the discharge evaporates due to the thermal effect caused by the discharge itself, thus restoring the damaged part.

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Reference // Power factor correction and harmonic filtering in electrical plants – ABB

About Author //

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Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears.Professional in AutoCAD programming and web-design.Present on

4 Comments


  1. abbas alaeibakhsh
    Sep 14, 2016

    very good


  2. Daniel
    Jul 22, 2016

    gracias!! me ayudo bastante


  3. mukesh
    Dec 09, 2015

    Thanks for giving valuable information.


  4. jackson
    Nov 13, 2015

    Very informative thank you

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