It’s all about reactive power…
The four main means for the generation of reactive power are: synchronous alternators, synchronous compensators (SC), static var compensators (SVC) and banks of static capacitors.
- Synchronous alternators
- Synchronous compensators (SC)
- Static var compensators (SVC) and
- Banks of static capacitors
Synchronous alternators are the main machines used for the generation of electrical energy.
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.
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).
- 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
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.
From the point of view of applications, these devices are used above all in high and very high voltage networks.
A capacitor is a passive dipole consisting of two conducting surfaces called plates, isolated from one another by a dielectric material.
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.
The 4 main parameters which characterize a capacitor are:
- The rated capacitance C – the value obtained from the rated values of power, voltage and frequency of the
- The rated power Qn – the reactive power for which the capacitor has been designed;
- The rated voltage Un – the r.m.s. value of the alternating voltage for which the capacitor has been designed;
- 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:
- C is the capacitance;
- U is the voltage applied to the terminals of the capacitor.
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.
Reference // Power factor correction and harmonic filtering in electrical plants – ABB