Power quality refers to maintaining the near sinusoidal waveform of power distribution bus voltages and currents at rated magnitude and frequency. Power quality describes the extent of variation of the voltage, current, harmonics, and frequency of the power system. The poor power quality effects industrial competitiveness drastically and thereby impedes our efforts to turn into a global supplier.
Power quality parameters
The ideal supply voltage is pure sinusoidal voltage with nominal frequency and nominal amplitude. Synchronisation of the voltage frequency and phase allows electrical systems to function in their intended manner without significant loss of performance or life.
The term quality is used to describe electric power that drives an electrical load and the load’s ability to function properly. Without the proper power, an electrical device or load may malfunction, fail permanently or not operate at all.
The quality of electrical power may be described as a set of values of parameters such as
- Harmonics current and voltage unbalance
- Rapid voltage changes i.e. transient
- Voltage dips and short-term overvoltage
- Voltage interruptions(VI-short term interruptions)
- phase shifting and reactive power
A Harmonic is a signal wave whose frequency is an integral multiple of the frequency of some reference signal or wave. Harmonics are currents or voltages whose frequency lies above the 50/60 Hz mains frequency and are many times this mains frequency.
Total harmonic distortion
The total harmonic distortion of a signal is a measurement of the harmonic distortion present and is defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental frequency.
Any deviation in voltage and current waveform from perfect sinusoidal, in terms of magnitude or phase shift, is termed as unbalance. The compatibility level for the degree of unbalance of the voltage in stationary operation caused by all mains loads is defined as ≤ 2%. Related to individual load system the resultant degree of unbalance is limited to 0.7 %.
Transients are pulsed electrical phenomena, which exist for just a short period of time. These are usually high frequency, steep signals in the form of transient oscillations. High transient overvoltage and high dv/dt ratios, can lead to insulation damage and the destruction of systems and machines.
Voltage dips and interruptions
A voltage drop is a sudden lowering of the effective voltage value to a value of between 90% and 1% of the stipulated nominal value, followed by the immediate reinstatement of this voltage.
A voltage sag is a short duration reduction and root mean square voltage which can be caused by a short circuit, overload or starting of electric motors.
A voltage swell is a root mean square increase in AC voltage at power frequency from half of a cycle to a few second durations.
Flicker refers to the subjective impression of light density changes or an impression of unsteadiness of visual perceptions, caused by luminous stimuli with temporal fluctuations of the light density for the spectral distribution. Voltage variation caused by individual devices are permissible if the resultant flicker disturbance factor is not greater than 1. The long-term flicker disturbance factor averaged of 12 values must not exceed a value of 0.65.
Phase shifting and reactive power
Reactive power is required in order to generate electromagnetic fields in a machine such as three-phase Motors, Transformers, welding systems, etc. With power factor correction, the proportion of inductive reactive power in the network reduces the reactive power of the power capacitor of the power factor correction system (PFC).
Why should we care about power quality
The issue of electric power quality is gaining importance because of several reasons
- The society is becoming increasingly dependent on the electrical supply. A small power outage has a great economic impact on the industrial consumers. A longer interruption harms practically all operations of a modern society.
- New equipment is more sensitive to power quality variations.
- The advent of new power electronic equipment, such as variable speed drives and switch mode power supplies, has brought new disturbances into the supply system.
- Reduction of Transformer life due to heating.
- Reduced fundamental frequency system capacity.