25-06-2013, 12:31 PM
POWER FACTOR CORRECTION AND HARMONIC FILTERING
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INTRODUCTION
Electric power quality, in general, means how pure is the quality of power being used, without any abnormalities, which cause disturbances in it. Although power is generated in the pure form, electrical consumers or loads distort it progressively due to the manner in which it is consumed. In the past major loading of supply system was made of large slowly changing electrical loads like motor, transformer, which were linear in nature that means the current consumed was directly proportional to the supply voltage. The current waveforms being sinusoidal with only power quality problem being that of power factor and the last 10 to 15 years, the gradual and never ending prolification of computers, micro-processor system and power electronics in all kinds of industrial, commercial and domestic facilities have completely changed the nature and profile of supply loading.
In electrical plants the loads draw from the network electric power (active) as power supply source (e.g. personal computers, printers, diagnostic equipment, etc.) or convert it into another form of energy (e.g. electrical lamps or stoves) or into mechanical output (e.g. electrical motors). To get this, it is often necessary that the load exchanges with the network (with net null consumption) the reactive energy, mainly of inductive type. This energy, even if not immediately converted into other forms, contributes to increase the total power flowing through in the electrical network, from the generators, all along the conductors, to the users. To smooth such negative effect, the power factor correction of the electrical plants is carried out.
The power factor correction obtained by using capacitor banks to generate locally the reactive energy necessary for the transfer of electrical useful power, allows a better and more rational technical-economical management of the plants. Moreover, the present spreading of direct current users, such as electronic circuits and electric drives, involve the generation of current harmonics which are injected into the network, with the consequent pollution and distortion of the waveforms on other connected loads. Therefore, the use of harmonic filters, both of passive as well as of active type, contributes to improve the overall power-quality of the network, carrying out also power factor correction at the network frequency,when such filters are properly sized.
WHAT IS POWER FACTOR?
Power Factor Definition: Power factor is the ratio between the KW and the KVA drawn by an electrical load.
Power factor = Cos Φ
CosΦ = kW/kVA
Where the KW is the actual load power and the KVA is the apparent load power. It is a measure of how effectively the current is being converted into useful work output and more particularly is a good indicator of the effect of the load current on the efficiency of the supply system. As with any equipment, an electrical system handles its job to some degree of Efficiency ranging from poor to excellent. The measure of electrical efficiency is known as Power Factor. All current will cause losses in the supply and distribution system. A load with a power factor of 1.0 result in the most efficient loading of the supply and a load with a PF of 0.5 will result in much higher losses in the supply system. A poor power factor can be the result of either a significant phase difference between the voltage and current at the load terminals, or it can be due to a high harmonic content or distorted/discontinuous current wave form. Poor load current phase angle is generally the result of an inductive load such as an induction motor, power transformer, lighting ballasts, welder or induction furnace.
GENERALITIES ON POWER FACTOR CORRECTION
The analogy has been used to describe poor power factor including the following:
Horse Pulling Cart
A cart on a railway track is being towed by a horse that is off to the side of the railway track
(Refer Figure 4.1). The pull directly between the horse and cart is the apparent power (kVA - apparent power). The effective work by the horse is the cart moving down the track, or the real power (kilowatts (kW) - real power). The pull at right angle to the track does no effective work.
(Kilo volt amperes reactive (KVAR) - the reactive power). The horse would ideally pull the cart directly down the railway track so the apparent power equals the real power, thus minimizing wasted energy.
Centralized power factor correction:
The profile of loads connected during the day has a primary importance for the choice of the most convenient type of power factor correction. For installations with many loads, where not all the loads function simultaneously and/or some loads are connected for just a few hours a day, it is evident that the solution of distributed power factor correction becomes too onerous since many of the installed capacitors stay idle for a long time. Therefore the use of one compensation system only located at the origin of the installation allows a remarkable reduction of the total power of the installed capacitors. In centralized power factor correction automatic assemblies are normally used (see below automatic power factor correction) with banks divided into steps, installed directly in the main distribution boards; the use of a permanently connected bank is possible only if the absorption of reactive energy is quite constant all day long.
Automatic power factor correction:
In most installations there is not a constant absorption of reactive power, for example due to working cycles for which machines with different electrical characteristics are used. In such installations there are systems for automatic power factor correction which, thanks to a monitoring varmetric device and a power factor regulator, allow the automatic switching of different capacitor banks, thus following the variations of the absorbed reactive power and keeping constant the power factor of the installation constant.
STATIC VAR COMPENSATOR (SVC)
The Static VAR Compensator (SVC) is a shunt device of the Flexible AC Transmission Systems (FACTS) family using power electronics to control power flow and improve transient stability on power grids. The SVC regulates voltage at its terminals by controlling the amount of reactive power injected into or absorbed from the power system. When system voltage is low, the SVC generates reactive power (SVC capacitive). When system voltage is high, it absorbs reactive power (SVC inductive). The variation of reactive power is performed by switching three-phase capacitor banks and inductor banks connected on the secondary side of a coupling transformer. Each capacitor bank is switched on and off by three thyristor switches (Thyristor Switched Capacitor or TSC). Reactors are either switched on-off (Thyristor Switched Reactor or TSR) or phase-controlled (Thyristor Controlled Reactor or TCR)
STATIC SYNCHRONOUS COMPENSTOR (STATCOM)
Static Synchronous Compensator (STATCOM) is a shunt device of the Flexible AC Transmission Systems (FACTS) family using power electronics to control power flow and improve transient stability on power grids The STATCOM regulates voltage at its terminal by controlling the amount of reactive power injected into or absorbed from the power system. When system voltage is low, the STATCOM generates reactive power (STATCOM capacitive).
When system voltage is high, it absorbs reactive power (STATCOM inductive).Similarly to the SVC the STATCOM can provide instantaneous and continuously variable reactive power in response to grid voltage transients enhancing the grid voltage stability .Installing a STATCOM at one or more suitable points in the network will increase the grid transfer capability through enhanced voltage stability, while maintaining a smooth voltage profile under different network conditions. The STATCOM provides additional versatility in terms of power quality improvement capabilities
WHY WE CHOOSE CAPACITOR AS CORRECTION MEASURE?
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.
How Harmonics Are Generated
Harmonics are generated by single phase loads at 50 or 60 Hz or odd multiples of the fundamental. The most problematic of these harmonics is the triplen harmonic which oscillates at multiples of the third harmonic. Triples can cause extremely high neutral currents and excessive losses to the transformer.
Harmonics are also generated by nonlinear loads. A nonlinear load is a circuit element that draws current in a non-sinusoidal manner. Until recently, most factory loads were primarily linear, with current waveform closely matching the sinusoidal voltage waveform and changing in proportion to the load. More recently, however, factory loads with major nonlinear components have increased dramatically.
How to minimize harmonics
It is well known that harmonic currents are present in modern electrical distribution systems caused from non-linear loads, such as variable frequency drives, lighting and computers. Harmonics can cause variety of problems ranging from poor power factor and motor failures, to overloaded transformers and conductors.
The best way to deal with harmonics problems is through prevention: choosing equipment and installation practices that minimize the level of harmonics in any one circuit or portion of a facility. Many power quality problems, including those resulting from harmonics, occur when new equipment is haphazardly added to older systems. However, even within existing facilities, the problems can often be solved with simple solutions such as fixing poor or nonexistent grounding on individual equipment or the facility as a whole, moving a few loads between branch circuits, or adding additional circuits to help isolate the sensitive equipment from what is causing the harmonic distortion. If the problems cannot be solved by these simple measures, there are two basic choices: to reinforce the distribution system to withstand the harmonics or to install devices to attenuate or remove the harmonics. Reinforcing the distribution system means installing double-size neutral wires or installing separate neutral wires for each phase, and/or installing oversized or K-rated transformers, which allow for more heat dissipation.
There are also harmonic-rated circuit breakers and panels, which are designed to prevent overheating due to harmonics. This option is generally more suited to new facilities, because the costs of retrofitting an existing facility in this way could be significant. Strategies for attenuating harmonics, from cheap to more expensive, include passive harmonic filters, isolation transformers, harmonic mitigating transformers (HMTs), the Harmonic Suppression System (HSS) from Harmonics Ltd., and active filters.