Power quality is a term that means different things to different people. For me, power quality is a concept that defines efficient electrical energy. This means that if the equipment is operating correctly due to the power it is receiving, the quality of the power is good. But if the equipment is not operating correctly the quality of the power is bad. But having a good power quality is not that much easy. Sometimes unwanted situations occur in the electrical network. When disturbances occur, electrically powered equipment may malfunction, fail prematurely or shut down unexpectedly. For the energy consumer, the economic impact of power disturbances can range from hundreds of dollars to millions of dollars. To avoid these negative effects you should be aware of the major power quality issues.
Major Power Quality Issues
The major power quality problems are:
When used to describe a speciﬁc type of long-duration variation, overvoltage refers to a voltage having a value greater than the nominal voltage for a period of time greater than 1 min. Typical values are 1.1–1.2 pu.
Causes of overvoltage:
- Load switching (e.g. switching off a large load or energizing a capacitor bank).
- Atmospheric changes along the length of the line.
- Incorrect tap settings on transformers.
- Direct lightning.
- The overvoltage result is because either the system is too weak for the desired voltage regulation or voltage controls are inadequate.
Undervoltage refers to a voltage having a value less than the nominal voltage for a period of time greater than 1 min. Typical values are 0.8–0.9 pu.
Causes of undervoltage:
- Overloaded utility transformers.
- Poor wiring.
- Weather conditions.
- Under voltages are the result of switching events that are the opposite of the events that cause overvoltages.
- A load switching ON or a capacitor bank switching off (de-energization) can cause an under-voltage until the voltage is back to within tolerances.
3. Sags (Dips)
A decrease in voltage to a value between 0,1 and 0,9 pu in RMS voltage or current at the power frequency for durations from 0,5 cycle to 1 min. A voltage sag of 10% means that the line voltage is reduced to 10% of the nominal value.
Causes of sags:
- Faults on the transmission or distribution network.
- Arcing fault in the system.
- Faults in the consumer’s installation.
- Switching off the large reactive power source.
- Connection of heavy loads and start-up of large motors.
4. Swells (Surges)
A temporary increase in rms voltage or current of more than 10% of the nominal value at power system frequency, which lasts from 0.5 cycles to 1 min. Typical rms values are 1.1–1.8 pu.
Causes of swells:
- Start/stop heavy loads.
- Transfer of loads from one power source to another
- Badly dimensioned power sources.
- Badly regulated transformers.
- Energization of a large capacitor bank.
Sinusoidal voltages or currents have frequencies that are multiples of the fundamental power frequency. Distorted waveforms can be decomposed into a sum of the fundamental frequency wave and the harmonics caused by nonlinear characteristics of power system devices and loads.
Causes of harmonics:
- Magnetic saturation.
- Arc-discharge devices, such as fluorescent lamps.
- Arc furnaces, welding machines, rectifiers, and DC brush motors.
- Synchronous machines.
- All non-linear loads such as power electronics equipment.
- Switched-mode power supplies.
Unwanted electrical signals produce undesirable effects in the circuits of the control systems in which they occur. Noise is a high-frequency, low-current, low-energy waveform superimposed on the sine wave of the alternating current (AC) mains. The frequency of noise can range from low kilohertz to megahertz region. This low-level interference is typically characterized by a voltage of less than 50 V and an associated current of less than 1 A.
Causes of electrical noise:
- Electromagnetic interferences are provoked by microwaves, computers, and television diffusion.
- Radiation is due to welding machines, arc furnaces, and electronic equipment.
- Improper grounding.
These pertain to or designate a phenomenon or quantity varying between two consecutive steady states during a time interval that is short compared with the timescale of interest. A transient can be a unidirectional impulse of either polarity or a damped oscillatory wave with the ﬁrst peak occurring in either polarity.
Causes of transients:
- Power capacitor energization.
- Cable switching.
- Heavy-duty motor starting.
The complete loss of voltage (below 0.1 pu) on one or more phase conductors for a certain period of time. Momentary interruptions are deﬁned as lasting between 0.5 cycles and 3 s, temporary interruptions have a time span between 3 and 60 s, and sustained interruptions last for a period longer than 60 s.
Causes of blackouts:
- Faults at power stations.
- Extreme weather conditions.
- Damage to the electric transmission lines, substations, or other parts of the distribution system.
- Short circuit, or the overloading of the mains supply.
9. Voltage Flicker
Voltage ﬂuctuations are systematic variations in the envelope or a series of random voltage changes with a magnitude that does not normally exceed the voltage ranges of 0.9–1.1 pu. Such voltage variations are often referred to as ﬂicker. The term ﬂicker is derived from the visible impact of voltage ﬂuctuations on lamps. Among the most common causes of voltage ﬂicker in transmission and distribution systems are arc furnaces.
A periodic transient reduction in the magnitude of the quasi-sinusoidal mains voltage. It lasts less than one half-cycle and usually less than a few milliseconds. Notching is caused mainly by power electronic devices that draw a heavy load current during a small portion of the sine wave. Frequency components associated with notching can, therefore, be very high, and measuring with harmonic analysis equipment may be difﬁcult.
Because of these problems, power quality has become an important area of study in electrical engineering, especially in electric distribution and utilization systems. It has created a great challenge for both electric utilities and manufacturers. Utilities must supply consumers with good quality power for operating their equipment satisfactorily, and manufacturers must develop their electric equipment either to be immune to such disturbances or to override them.