Power Quality - A brief guide

POWER QUALITY

As the development of non-linear AC electrical technology has progressed, more efficient, energy friendly solutions have been implemented across the globe such as variable frequency drives, Power inverters and rectifiers, DC power sources such as PV, EV charging and battery storage, switch mode power supplies, LED lighting and many more. These technologies, if not correctly designed can impose unwanted power quality irregularities on the electrical network.

Inadequate Power Quality in electrical systems is a common occurrence and can present significant risk to plant and equipment including:

-???????Overheating of Transformers, Motors, Cables and Drives

-???????Inadvertent thermal tripping of overload relays

-???????Fluctuations in communication and control systems

-???????Undue stress on insulation resulting in early failure of equipment

Some Power Quality categories include:

Voltage Dip

Voltage Dip?(IEC) or Voltage Sag (IEEE) is defined in the IEC 61000-4-30 standard as the decrease in the RMS voltage level to 1% - 90% (10% - 90% for IEEE) of nominal. The duration of a voltage dip is between 10 ms and 1 minute. The depth of a voltage dip is defined as the difference between the minimum rms voltage during the voltage dip and the declared voltage. Voltage changes which do not reduce the supply voltage to less than 90% of the declared voltage?are not considered to be dips.

Voltage dip can occur with sudden large change of loads such as motor start-ups, switching operations or short circuit events. Voltage dips are a common power quality event. In recent years, it is required that electrical equipment has the ability to ride through brief voltage sag events. The requirement depends on the sag characteristics and the application.

Voltage Swell

Voltage swells are the opposite of voltage dips and they are defined as a momentary increase in RMS voltage of 10% or more above equipment recommended voltage range for a period of 1/2 cycle to 1 min, as defined in the?IEC 614000-4-30 standard.

Voltage swells are usually related to system fault conditions. Swells can also occur when a large load is turned off or with phase imbalance.

Transient Voltage

Voltage Transients are defined as short duration surges of voltage and are the result of the sudden release of energy previously stored or induced by other means. This is commonly caused by the switching of large inductive loads such as motors, transformers and generators, ground faults or lightning strikes.

Harmonic Content

Harmonics can occur because of distortion of the sinusoidal waveforms of both current and voltage. ?Total harmonic distortion (THD) is used to quantify harmonics in a power system and is defined as the ratio of the equivalent RMS voltage or current of all the harmonic frequencies from the 2nd harmonic onwards over the RMS voltage or current of the fundamental frequency (in this case 50Hz). THD can be calculated as:

Harmonics can be minimised by reducing the non-linear loads connected to the system, in the case that these loads cannot be reduced or eliminated then harmonic filters can be specified and installed. These filters primarily come in two types, passive and active filters.

A passive harmonic filter may contain a series/shunt capacitor/inductor network and a series inductor or transformer. This type of filter often is added to an electrical system as a peripheral to a drive system. It must be tuned to the individual drive. Multiple drives require multiple filters.

Active harmonic filters are sometimes called active power line conditioners. Rather than block or shunt harmonic currents, active filters attempt to condition them. Active harmonic filters monitor and sense harmonic currents electronically and generate corresponding waveforms to counter the original harmonic currents. The generated waveform is injected back into the electrical supply to cancel the harmonic current generated by the load.

IEEE Std 519 ‘Recommended practices and requirements for harmonic control in electrical power systems ‘recommends acceptable values for harmonic distortion. The recommended limit on voltage harmonics is set at 5% for THD and 3% for any single harmonic. These limits are suggestions and purely voluntary.

HOW TO IMPROVE PQ

Power Quality should be considered during the design and concept of all electrical systems operating within high-performing facilities. Electrical Studies and analysis such as Harmonic, Transient, motor starting and Power Factor should all be considered in order to ensure a sound design.

A detailed analysis of a systems power quality can identify troublesome areas and highlight opportunities for improvement to maximise energy savings, fault mitigation, grid code compliance and dramatically increase system reliability and efficiency. A Power Quality Study involves the measurement and analysis of harmonic distortions, Power Factor, Voltage sags or surges, transients, and other power quality irregularities that can potentially cause major disruptions to plant and equipment.