Fundamentals of power quality and solutions

By admin May 15, 2013 9:36 am IST

Hemant Mehta, founder partner of Neelkanth Power Solutions and Neel Controls highlights the power quality issues related to supplyHemant Mehta, Founder Partner of Neelkanth Power Solutions
Power quality (PQ) related issues are of most concern nowadays since these cost failure in production, breakdown of machines or major faults in utility and loads. The electric utility’s primary goal is to meet the power demand of its customers at all times and under all conditions. But as the electrical demand grows in size and complexity, modifications and additions to existing electric power networks have become very necessary. The measuring and monitoring of electric power have become even more critical because of down time associated with equipment breakdown and material failures.
Any electrical or electronics instruments or machines has to be designed to withstand some voltage fluctuations and power disturbances for smooth operations. But when voltage fluctuations or disturbances are beyond the safe operating range of instruments, erratic behaviour, malfunctioning, or damages may observe during operation.
When the input supply is higher than normal voltage, the equipment will consume more power than needed and also in many cases life is reduced – both increasing the cost of ownership and where the input supply is lower than the required, the machine will not give optimum output. In either case losses are obvious.
The widespread use of electronic equipment, such as CNC machines, information technology equipment, medical equipment, textile machineries, and power electronics such as variable frequency drives (VFD), programmable logic controllers (PLC), energy-efficient lighting, 6-pulse or 12-pulse rectifiers, led to a complete change of electric loads nature. These loads are simultaneously the major causers and the major victims of power quality problems. Due to their non-linearity characteristics, all these loads cause disturbances in the power supply system.
Maintaining correct incoming supply voltage to all these equipment is necessary to avert malfunctions and to increase the MTBF (mean time between failures) of the equipment. A few power quality issues related to supply are discussed here.
Supply voltage • Under voltage: If the input supply is less than 10 per cent of nominal supply for one minute, condition can be considered as under voltage• Over voltage: If the input supply is higher than 10 per cent of nominal supply for one minute, condition can be considered as over voltage.
In both the cases source can be utility or facility and severity may be too high to malfunction or damage to the machine or equipment.
Power = Voltage x Current x Time
Where current is decided by load and it is fixed for optimum results, time cannot be changed, so only voltage is the parameter to decide power quality which is controllable by Servo Controlled Voltage Stabiliser, and hence it is very essential for all sophisticated loads.Electrical noise / distortionSwitching of electrical utilities in industrial and commercial environment generates spikes up to 6,000 volts and high frequency power line noises. The spikes can cause the failure of thyristors, IGBT, semiconductors, micro-processors, or other active electronic device. The high frequency noises can interfere with digital-electronic equipment causing data corruption, erratic behaviour, loss of memory etc. Electrical noises are generated due to switching of electrical utilities like Capacitors, switch gears; Thyristor or IGBT switching, inductive loads like big motors, compressors, overhead cranes, elevators, and presses etc.
Large inductance and current change during switching of the system produces larger electrical noises.  Switching equipment like Invertors, converters, and SMPS etc. generate electrical noises due to switching of thyristors, IGBT, and relays etc. Welding systems pollutes earthing systems, adds notches and high frequency noises in the wave form, generating power quality anomalies. Lightening, precipitation of static charges and electrical discharges in the atmosphere are the natural cause of generation of various electrical noises.
Electrical noises are observed over a wide band of frequency ranging from 1 KHz to 100 MHz and above. In magnitude observed to be as high as 4,000 to 6,000 volts on 3-phase supply system. All types of electrical noises, predominantly common mode noises can be eliminated by Ultra Isolation Transformers. Since it isolates primary and secondary or separates neutral to ground bond on the secondary side, can be used to create separately derived source to combat current loops.

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