Power quality determines electrical supply of constant magnitude and frequency – sinusoidal voltage waveform to consumer devices. Synchronisation of the voltage, frequency and phase allows electrical systems to function in their intended manner without significant loss of performance or life of consumer devices. The Power Quality of a system expresses to which degree a practical supply system resembles the ideal supply system. The term is used to describe electric power that drives an electrical load and the load’s ability to function properly.
What is power quality?
Power Quality = Voltage Quality
P = V I
V-Voltage-consistent controlled by power supply system
I- Current-varied by particular load
Therefore, the standards on the power quality are maintaining the supply voltage within certain limits. Any power problem manifested in voltage, current or frequency deviations results in failure or misoperation of customer equipment. However, it is actually the quality of the voltage that is being addressed in most cases.
Technically, in engineering terms, power is the rate of energy delivery and is proportional to the product of the voltage and current. It would be difficult to define the quality of this quantity in any meaningful manner. The power supply system can only control the quality of the voltage; it has no control over the currents that particular loads might draw. Therefore, the standards in the power quality area are devoted to maintaining the supply voltage within certain limits. AC power systems are designed to operate at a sinusoidal voltage of a given frequency (typically 50 or 60 Hz) and magnitude. Any significant deviation in the waveform magnitude, frequency, or purity is a potential power quality problem. Of course, there is always a close relationship between voltage and current in any practical power system. Although, the generators may provide a near-perfect sine-wave voltage, the current passing through the impedance of the system can cause a variety of disturbances to the voltage. For example:
- The current resulting from a short circuit causes the voltage to sag or disappear completely, as the case may be.
2. Currents from lightning strokes passing through the power system cause high-impulse voltages that frequently flash over insulation and lead to other phenomena, such as short circuits.
3. Distorted currents from harmonic-producing loads also distort the voltage as they pass through the system impedance. Thus, a distorted voltage is presented to other end users.
Figure 1: Instantaneous voltage swell caused by an SLG fault
Therefore, while it is the voltage with which we are ultimately concerned, we must also address phenomena in the current to understand the basis of many power quality problems.
Why are we concerned about power quality?
The ultimate reason that we are interested in power quality is economic value. There are economic impacts on utilities, their customers, and suppliers of load equipment. The quality of power can have a direct economic impact on many industrial consumers. Thus, like the blinking clock in residences, industrial customers are now more acutely aware of minor disturbances in the power system. There is big money associated with these disturbances.
Causes of poor power quality:
- Variationsin the peak or RMS voltage.
2. Swell: When the RMS voltage exceeds the nominal voltage by 10 to 80% for 0.5 cycle to 1 minute, the event is called a ‘swell.’ Refer Figure 1.
3. Dip/Sag: The RMS voltage is below the nominal voltage by 10 to 90% for 0.5 cycle to 1 minute. Refer Figure 2.
4. Flicker: Random or repetitive variations in the RMS voltage between 90 and 110% of nominal can produce a phenomenon known as ‘flicker’ in lighting equipment. Flicker is rapidly visible changes of light level.
5. Spikes/Impulses/Surges: Abrupt, very brief increases in voltage, called ‘spikes’, ‘impulses’, or ‘surges’, generally caused by large inductive loads being turned off, or more severely by lightning.
6. Undervoltage: An undervoltage is a decrease in the RMS AC voltage to less than 90% at the power frequency for a duration longer than 1 min. The term ‘brownout’ is an apt description for voltage drops somewhere between full power (bright lights) and a blackout (no power – no light). A load switching on or a capacitor bank switching off can cause an undervoltage until voltage regulation equipment on the system can bring the voltage back to within tolerances.
7. Overvoltage: An overvoltage is an increase in the Rms AC voltage greater than 110% at the power frequency for a duration longer than 1 min. Overvoltages are usually the result of load switching (e.g., switching off a large load or energising a capacitor bank). The overvoltages result because either the system is too weak for the desired voltage regulation or voltage controls are inadequate. Incorrect tap settings on transformers can also result in system overvoltages.
8. Power Frequency Variations: Power frequency variations are defined as the deviation of the power system fundamental frequency from its specified nominal value (e.g., 50 Hz).The power system frequency is directly related to the rotational speed of the generators supplying the system. There are slight variations in frequency as the dynamic balance between load and generation changes. The size of the frequency shift and its duration depends on the load characteristics and the response of the generation control system to load changes.
9. Waveform Distortion: Waveform distortion is defined as a steady-state deviation from an ideal sine wave of power frequency principally characterised by the spectral content of the deviation. It is usually described as harmonics at lower frequencies (usually less than 3 kHz) and described as Common Mode Distortion or Interharmonics at higher frequencies. All this phenomena potentially lead to inefficient running of installations, system down time and reduced equipment life and consequently high installation running costs. If due to poor power quality the production is stopped, major costs are incurred.
Figure 2
Possible consequences of poor power quality
- Equipment failure or malfunctioning.
2. Unexpected power supply failures (breakers tripping, fuses blowing).
3. Equipment overheating (transformers, induction motors etc.) leading to their lifetime reduction.
4. Increase of system losses.
5. Damage to sensitive equipment (Computers, control systems equipments etc).
6. Communication Interference in case of electronics devices.
7. Increases the need of oversize installations to cope with additional electrical stress, which leads to consequential increase of installation and running costs.
8. Poor power factor that leads to penalty or increases need and cost of installation of power factor correction equipments.
Power Quality Monitoring
Power quality monitoring is the process of gathering, analysing, and interpreting raw measurement data into useful information. The process of gathering data is usually carried out by continuous measurement of voltage and current over an extended period. The process of analysis and interpretation has been traditionally performed manually, but recent advances in signal processing and artificial intelligence fields have made it possible to design and implement intelligent systems to automatically analyse and interpret raw data into useful information with minimum human intervention.
Power quality monitoring programs are often driven by the demand for improving the system wide power quality performance. Many industrial and commercial customers have equipment that is sensitive to power disturbances, and therefore, it is more important to understand the quality of power being provided.
Different types of instruments are used to monitor the power quality parameters like CT, VT, Transducers, digital meters, modern system called ‘smart grid’ etc. Modern systems use sensors called Phasor Measurement Units (PMU) distributed throughout their network to monitor power quality and in some cases respond automatically to them.
Using such smart grid features of rapid sensing and automated self healing of anomalies in the network promises to bring higher quality power and less downtime while simultaneously supporting power from intermittent power sources and distributed generation, which would if unchecked degrade the power quality.
Different types of power quality parameters that should be monitored are given in Table 1.
Methods for power quality problems correction
- Proper designing of the load equipment.
2. Application of passive, active and hybrid harmonic filters.
3. Proper designing of the power supply system.
4. Application of voltage compensators.
5. Use of Uninterruptible Power Supplies (UPSs).
6. Reliability on standby power.
Conclusion
This article has reviewed the importance of good power quality. It has presented power quality costs and solutions to poor power quality. A basic description of power quality has been given together with its quantification through different parameters.
The information from power quality monitoring systems can help improve the efficiency of operating the system and the reliability of customer operations. These are benefits that cannot be ignored. The capabilities and applications for power quality monitors are continually evolving.
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Designed to store energy, both batteries and capacitors can effectively counteract undesirable electrical characteristics like factor lag, power relieving utilities from having to generate additional power.