Fundamentals of power quality and solutions

May 15, 2013 9:36 am

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.
The Ultra Isolation Transformers are available in different levels of noise attenuation capabilities. The most commonly used are 100 and 120 dB. The coupling capacitance between primary and secondary is direct 1:1 relationship with dB levels. Some of the graphs herewith indicate different types of electrical noises observed in typical engineering industries and successfully eliminated.Noise is a high frequency distortion of the voltage waveform. Caused by disturbances on the utility system or by equipment such as welders, switchgear and transmitters, noise can frequently go unnoticed. Frequent or high levels of noise can cause equipment malfunction, overheating and premature wear.
Power conditionerAbove we have seen the problems of large noise, disturbances or spikes in supply. Large amount of CNC breakdowns are attributed to fluctuations in voltage and various types of electrical noises like spikes, surges, H.F. noises, ground noises, etc. Electrical noises are generated due to switching action of breakers, heavy, machinery, capacitor switching, induction hardening, cranes, welding etc. The use of invertors, DC drives, SMPS, further pollutes the electrical environment, causing severe power quality disturbances to various CNC machines, installed in the plant. Advanced CNC machines demand stringent standards of power quality for their smooth and trouble-free operations.
Power conditioner is the combination of voltage stabiliser and ultra isolation transformer in which stabiliser section immediately corrects the input fluctuations to safe stable output voltage and ultra isolation transformer eliminates the electrical noise and surges
Power factorThe power factor of an AC electric power system is defined as the ratio of the real power flowing to the load over the apparent power in the circuit, real power is the capacity of the circuit for performing work in a particular time. Apparent power is the product of the current and voltage of the circuit. Due to energy stored in the load and returned to the source, or due to a non-linear load that distorts the wave shape of the current drawn from the source, the apparent power will be greater than the real power.In any AC system the current, and therefore the power, is made up of a number of components based on the nature of the load consuming the power. These are resistive, inductive and capacitive components. In the case of a purely resistive load, for example, electrical resistance heating, incandescent lighting, etc., the current and the voltage are in phase that is the current follows the voltage. Whereas, in the case of inductive loads, the current is out of phase with the voltage and it lags behind the voltage. Except for a few purely resistive loads and synchronous motors , most of the equipment and appliances in the present day consumer installation are inductive in nature, for example, inductive motors of all types, welding machines, electric arc and induction furnaces, choke coils and magnetic systems, transformers and regulators, etc. In the case of a capacitive load the current and voltage are again out of phase but now the current leads the voltage. The most common capacitive loads are the capacitors installed for the correction of power factor of the load.
In an electric power system, a load with a low power factor draws more current than a load with a high power factor for the same amount of useful power transferred. The higher currents increase the energy lost in the distribution system, and require larger wires and other equipment. Because of the costs of larger equipment and wasted energy, electrical utilities will usually charge a higher cost to industrial or commercial customers where there is a low power factor.
Linear loads with low power factor (such as induction motors) can be corrected with a passive network of capacitors or inductors. Non-linear loads, such as rectifiers, distort the current drawn from the system. In such cases, active or passive power factor correction may be used to counteract the distortion and raise the power factor. The devices for correction of the power factor may be at a central substation, spread out over a distribution system, or built into power-consuming equipment
Problems due to poor power factor• Extra losses in transmission and distribution network ( transformer / cables / switchgears) • Over loading the supply system• Increase in maximum demand• Poor voltage regulation / higher voltage drops in the system• Supply network instability• Higher electricity cost due to levy of penalties / loss of incentives.
As the power factor drops, the system becomes less efficient. A drop from 1.0 to 0.9 results in 15 per cent more current is required for the same load. A power factor of 0.7 requires approximately 43 per cent more current; and a power factor of 0.5 requires approximately 100 per cent (twice as much) to handle the same load. The objective, therefore, should be to reduce the reactive power drawn from the supply by improving the power factor.
Real time power factor correction system (RTPFC) is designed to compensate the reactive power on real time basis for extremely rapid acquisition of the power factor within the range of 40 – 60 msec. Connection and disconnection of the capacitor to and from the network occurs at zero crossing. This smooth connection avoids transient effects like waveform distortions, generation of switching spikes etc., typically created by electromechanically switched contactor based system. The response time of this is much higher than electromechanically switched systems.
Lighting energy: Energy consumption for lighting in industries is approximately 5 -15 per cent and 15 – 40 per cent in commercial area like malls, IT centres. Due to higher voltage commonly seen in industries, energy consume by lighting loads are very high and also frequency of failure of lamps or routine maintenance is also very high. Most industrial and commercial energy users are aware of energy savings in lighting systems. Manufacturers are also aggressively marketing their products these days and help the users to take a decision on lighting energy. Some of the solutions like lighting energy saving products based on voltage reduction, type of lamps to be used, found very reliable on energy saving aspects.
However, in some cases it is necessary to change the lightings type or lighting feeder system in order to achieve the desired energy savings. It is important to understand that efficient lamps alone would not ensure efficient lighting systems. Three primary considerations to ensure energy efficiency in lighting systems are:• Selection of the most efficient light source possible in order to minimise power costs and energy consumption• Matching the proper lamp type to the intended work task or aesthetic application, consistent with colour, brightness control and other requirements• Safe operating voltage to reduce energy consumption. Powerlux is a comprehensive light energy management and saving system which is compatible with lighting loads of all kinds. Powerlux reduces losses, wastage and over lighting conditions. Using the software, facility operator can locally or remotely access Powerlux and manage any of the elements listed below:• Schedule or manually implement automatic energy reduction levels and on/off switching• Participate in peak shaving / Load shedding / utility energy requirements without disruptions of work• View power usage and energy consumption.
With Powerlux we can save the energy up to 30-40 per cent depending on type of lighting loads and supply voltage.