Common Myths Regarding Power Quality and Compensation-4
Performing the Reactive Power Compensation as Central is always the most suitable solution
There are more than one methods for performing reactive power compensation in electrical facilities. Different compensation methods may be implemented depending on facility type, type of the load for implementation, load variation frequency and period. Determining the correct compensation method generally helps to get rid of reactive power fine applied by the distributor company.
First implementation method is the transformer fixed compensation. Distribution transformers located in facilities consume inductive reactive power under unloaded and loaded operating conditions and cause a loss. Although this loss can be neglected under loaded operating condition, it must be considered under unloaded operating condition. Reactive loss rate of a standard distribution transformer manufactured with current conditions is within 1% – 2% range of transformer nominal power. For instance, idle inductive reactive loss of 1000 kVA nominal powered 34,5 kV/04 kV voltage distribution transformer shall be within the range of 10-20 kVAr. Meters subject to invoice are generally at MV side of the transformer therefore this inductive reactive power arising from the transformer should be compensated. This compensation method is called as “transformer fixed compensation”. The capacitor selected depending on the transformer power shall be located at a suitable place under the transformer (generally upper leg of OV main breaker) then compensation shall be realized.
Another compensation method is called as “fixed compensation”. Fixed compensation depends on connecting a capacitor with suitable power to the load to be compensated as parallel to the system, and switching the capacitor with activation-deactivation of the load. This is a very suitable and cheap solution for load groups with less quantities and relatively high power (e.g. high powered MV engines). Technically, as generation of the reactive power at the point nearest to the point of consumption is the most suitable method, it is the most efficient method regarding effectiveness. However, implementation shall be difficult and expensive in facilities with very high quantities and having loads not functioning simultaneously. Such facilities shall also experience difficulties in maintenance and failure detection of compensation products.
The compensation implemented in facilities with higher load varieties and variation speed is called as “central automated compensation system”. In this compensation method, a reactive power control relay that can observe and process variation of information on power withdrawn by loads automatically activates and deactivates a capacitor group consisting of more than one levels. In case the reactive power demand increases, more levels are activated via contactor / thyristor switching module and related groups are deactivated when requirement terminates. Therefore, cosphi value continuously remains at requested level. Today, most of compensation systems implemented in facilities are central automated compensation systems. These systems ensure a design that can provide same reactive power with less expense and also an easier and longer life operation can be maintained through maintenance/failure detection from single point.
Above mentioned compensation systems can be either implemented separately or implemented by combining few of them. The important issue at that point is to correctly determine load type and reactive power requirement in the facility and perform the suitable engineering design according to this requirement.
Harmonics on the System have no effect on non-filtered compensation system
False. Bare capacitors are mostly affected from harmful impacts of harmonics among elements used in electrical facilities.
Capacitor is the linear circuit element used for performing reactive power compensation in electrical facilities for a long time. Capacitor impedance is calculated with the formula given below:
As it can be seen clearly in the capacitor impedance formula, capacitor impedance shall decrease exponentially with the increase of frequency.
Harmonics are components having full times magnitude frequency of 50 Hz as the basic grid frequency. Particularly single harmonics are frequently seen in electrical facilities and cause problems. Harmonic currents with high frequency shall attempt to flow on the capacitor with lowered impedance through increasing frequency when compared with basic frequencies such as 150Hz, 250Hz, 350Hz.
Capacitors are manufactured as durable against excessive current/voltage pulses at the level given in international standards depending on manufacturing technologies. However, harmonic currents flowing on the capacitor may excessively increase the capacitor RMS current and cause permanent damage on the capacitor.
Another problem encountered in non-filtered compensation systems is to increase the existing harmonic deterioration level (THDV% and THDI%) available in the facility. Upon activation of non-filtered compensation system, magnitude of the harmonic current to be flowed between the capacitor and harmonic generators shall increase and indirectly cause increased THDI% and THDV% levels. This event caused by non-filtered compensation systems is a sample of parallel resonance experienced in high frequencies. Utilization of compensation systems equipped with harmonic filter reactors in facilities with harmonic creating loads is very important regarding operating health and reliability.
No current flows over the Neutral line
False. No current flows over the neutral line only in three phased pure sinusoidal balanced systems. However, it is not likely to encounter with such as load characteristic in current electrical facilities, therefore almost all industrial/commercial facilities have current load on the neutral line.
There are two main reasons of current flow over the neutral line:
- Unbalanced load arising from monophase/diphase loads on phases
- Three and three fold harmonics
Three phase devices withdraw balanced current from the grid. Magnitude of the current withdrawn from the grid is equal for three phases, and vector angles among phases are 120 degrees each. However, single phase and two phase loads do not equally load three phases. Single phase loads such as illumination, computer, socket loads are very high particularly in commercial businesses. Single phase devices are connected between one phase and the neutral and naturally complete the circuit over the neutral line. Although the power consumption per device is lower in such devices, excessive number of devices used makes it inevitable to load the neutral line by devices. Non-balance arising from single phase loads and neutral line loading problem can be solved with “balanced load distribution based on phases” that is implemented at the projecting stage. When balanced distribution is made based on phases, non-balance problem can be solved considerably.
Another and more dangerous reason of the current passing over the neutral line for today’s facilities is three and three fold harmonics. Almost all single phase devices used in commercial and industrial businesses generate harmonics. These devices, containing semi-conductors, particularly generate three and three folds harmonics. As phase difference between three and three folds harmonics is full times of 360° (n, as harmonic levels n. Between harmonic components n x 120°) , all of them are on the same phase. That means; if respectively 40A-40A-40A tertiary harmonic current is withdrawn from R-S-T phases in the facility, 3x40A =120A tertiary harmonic current shall flow from the neutral line. This current may sometimes be more than the current flowing over the phase line. This current passing over the Neutral line causes current sourced losses and heating problems and it also induces a current on the neutral line and creates current problems between neutral-earth. The neutral-earth voltage arising due to current flowing over the neutral line can be increased to 10V-15V levels. All sensitive devices, monitoring devices, computers, PLCs in the business get affected from this voltage and faulty works/failures occur. Active filters are used for eliminating the current passing over the neutral line.
According to applicable internal installation directive, (entered into force upon published in 16th June 2014 dated and 25494 numbered Official Gazette, page 37, table 3) neutral line conductor can be selected at half section of the phase conductor in a medium and large scaled commercial business. Harmonics passing over the inadequately selected neutral line considerable load the neutral line and cause mechanical/electrical problems. Therefore, it shall be suitable for facilities with intense single phase loads withdrawing current with harmonics to select the neutral section at the same section with the phase conductor or at a higher section as minimum.
Uğur YAŞA, Güç Kalitesi Müdürü – Aktif Mühendislik