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The Main Factors Affecting Power Supply Quality

February 17, 2020 by Ikimi .O

What is power quality, and why is it essential? This article will provide a basic understanding of power quality, how poor power quality can impact electrical loads and some ways it can be improved.

What is Power Quality?

Power quality is a term that describes how accurately the electrical power that is supplied to appliances or equipment correlates to an ideal supply. Power quality issues are typically caused by deviations in the voltage, current, and frequency of a power system from ideal values.

Why Does Power Quality Matter?

Poor power quality can affect electrical loads connected to a power system in the following ways:

• It can incur higher energy costs

• It can require more frequent repairs due to equipment failures

• It can lead to unstable equipment operation (potentially a full malfunction)

The power quality in any given system can be described by several key metrics, which include voltage and frequency values, power factor, electrical harmonics, and output waveform.

A first-person view of a user inspecting an arbitrary waveform generator. Image used courtesy of U.S. Air Force.

Power Factor

The power quality of an AC power supply is the ratio of the real power dissipated and the product of the current and voltage (apparent power) within the circuit. Mathematically, we can express the power factor in a system as:

PF = W/VA (where W = apparent power and VA = volt-ampere product or apparent power).

The power factor of a practical AC circuit is usually between 1 and 0. The closer the power factor is to zero, the more it is considered ‘poor’ and therefore indicates an inefficient circuit. On the other hand, a power factor closer to 1 (ideal PF) is considered ‘good’ and therefore indicates a more efficient power system.

Industrial machinery, such as induction motors can cause a decrease in the power factor. When the power factor is lower than expected, it can lead to an overcurrent to the electrical loads in the system and corresponding voltage drops in the conductors. A higher power factor value indicates that a system has good load-carrying capacity.

Output Waveform

Output waveform analysis is one of the simplest ways to determine the quality of a power supply. Using a suitable test instrument like an oscilloscope, analyser, or waveform generator, an engineer can sample a given power supply over regular periods to observe its waveshape.

Usually, good power quality will display a smooth voltage curve waveform when measured. Electrical transients, overcurrent, and overvoltage conditions are factors that can distort power output waveforms.

A close-up of a circuit breaker. Circuit breakers provide overcurrent protection in power systems.

Factors that Affect Power Quality

Harmonics, overvoltage, and overcurrent are some of the underlying causes of poor power quality. Let’s take a look at these issues in closer detail.

Electrical Harmonics

In power systems, the term ‘electrical harmonics’ refers to voltage or current in multiples of the fundamental frequency. Harmonics are usually generated by non-linear loads (loads having impedances that vary with the applied voltage). Some examples are switched-mode power supplies and programmable logic controllers (or PLCs).

Harmonics can produce current and voltage distortions that can affect sensitive components in electrical equipment. In power systems, harmonics cause equipment malfunction, winding eddy current in transformers, and capacitor bank failures.

Voltage Transients

A voltage transient (aka voltage spike) is a short-duration surge of electricity in a power system, which is usually caused by a sudden release of induced or stored energy, e.g., from lightning strikes, AC inductive loads, and DC relays.

Depending on the causative factor(s), voltage transients can occur randomly or several times a day. Transient voltages can damage electronic components, cause insulation breakdown, flashover, and equipment malfunction.

Overcurrent

An overcurrent condition is a common power problem that occurs when the current (amperage) in an electrical circuit is greater than its maximum rated current.

Overcurrent is usually caused by the excess flow of current in a circuit due to short circuits—which can be caused by the bridging of terminals, grounding failure, or arcing.

Overcurrent can be prevented by using circuit protection devices, such as fuses or circuit breakers, to open the circuit during a fault. These devices can help to prevent damage to power electronics and current-carrying conductors.

Two energy auditors inspect the power of a factory’s electrical equipment. Image used courtesy of Corner Stone Group

How Power Quality Can Be Improved

Line reactors, harmonic trap filters, and broadband filters can help to reduce harmonics in power systems. All three are inductive components that are capable of absorbing harmonic distortion.

Transient voltages can be minimised using voltage clamping or surge protection devices. These devices are usually connected to the power supply in parallel to the electrical load that is meant to be protected.

When an overvoltage occurs, the surge protection device will either conduct the excess voltage to the earth (aka common mode) or distribute the overvoltage between the live conductors (aka differential mode).

The next two subsections cover more on improving power quality.

Monitoring and Analysis

The most effective way to improve power supply quality in a given locality is to first obtain data about the various loads connected to the system.

There are various tools that can be used to achieve power quality monitoring and analysis. One such device is called a power analyser: a special test and monitoring instrument used to record and examine the supply to, not only record incidents and failures, but reveal the behavior of critical energy criteria, such as output voltage, current, and frequency.

Electrical engineers use such results to troubleshoot power issues and define appropriate measures to improve power quality.

Power Systems Protection

Circuit breakers are automatic switching devices designed to open a circuit during overcurrent conditions: this is to prevent excess electricity from flowing through to damage any equipment.

Circuit breakers are designed to trip automatically but must be manually reset after a trip. A fuse, on the other hand, blows to open a circuit during an overcurrent. However, unlike circuit breakers, fuses are one-time protection devices that are not reusable.

The Overall Importance of Power Quality

Equipment failures, higher utility bills, and downtime are just some of the effects of poor power quality. Understanding why power quality, and using such techniques as those mentioned above, is essential. Ultimately, taking stock of the factors that affect power quality can help electrical engineers to design more stable and robust power systems.