POWER QUALITY BASICS: WHAT IS POWER QUALITY?

Monday, March 7, 2011

Power Quality (PQ) has become an interesting issue in the power industry since the 1980s. It has increasingly affected both electric power end-users and electric distribution utilities all over the world. The recent growth of interest in power quality can be explained by these four major reasons:

1.   Electricity consumers are becoming better informed about power quality issues such as voltage fluctuations, interruptions and switching transients. In addition, many governments have revised their policies in order to regulate electric utilities pushing them to improve power quality within the set standards and limits.

2.    Modern load equipment with microprocessor-based controls and power electronic devices are more sensitive to power quality changes than its conventional counterparts.

3.    Emphasis on overall power system efficiency has resulted in growing applications of high-efficiency devices, adjustable-speed motor drives and shunt capacitors for power factor correction and reduction of losses. Consequently, increased harmonic levels on electrical power systems have threatened the operability, reliability and safety of the system.

4.    Many things are now interconnected in the electrical network such as distributed generation (DG) – particularly solar and wind, and the so-called integrated processes. Both have a number of significant power quality issues that must be addressed as part of the interconnection assessment.

The concern for power quality is expected to increase, especially now that it has direct economic impacts to equipment suppliers, utilities and the end-users.

What is Power Quality?

Power Quality (PQ) has several definitions depending on one’s perspective. The term “power quality” originated in 1968 from a U.S. Navy study, after specifications for the power required by electronic equipment, including the use of monitoring equipment presents a good overview of the power quality field. Currently, there is no consensus on the use of the word, but the following terminologies are common to describe power quality:

A.   Power Quality is the concept of powering and grounding sensitive equipment in a manner that is suitable to the operation of that equipment - ANSI/IEEE

B.    The IEC uses the term Electromagnetic Compatibility (EMC), which is not the same as power quality, but there is a strong overlap between the two terms. EMC is the ability of an equipment or system to function satisfactorily in its electromagnetic environment without introducing intolerable electromagnetic disturbances to anything in that environment. However, the IEC has already started a project group on power quality and has adopted the following definition: Set of parameters defining the properties of the power supply as delivered to the user in normal operating conditions in terms of continuity of supply and characteristics of voltage (symmetry, frequency, magnitude, and waveform).

C.    Quality of Power Supply includes a technical part and a non-technical part referred to as the quality of service. The latter deals with the interaction between the utility and its customers (i.e. speed of response to complaints and calls).

Moreover, power quality and voltage quality are being used interchangeably by most literatures. Although electric power is proportional to both voltage and current, in most cases, it is the quality of voltage that is being addressed - as the power supply system can only control the quality of the voltage. Standards in the power quality area are mostly dedicated to maintaining the supply voltage within certain limits. 

Nonetheless, electric current and voltage are strongly related in any practical power system. For example:

1.     Electric current resulting from a short-circuit condition would reduce voltage, or even cause it to drop to zero.

2.   Nonlinear loads produce harmonic currents that can distort the voltage as they pass through the system impedance. Therefore, a distorted voltage is presented to other end users.

3.    Lightning current passing through the electrical power system causes high-impulse voltages that often lead to flash over insulation and to other phenomena such as faults.

Thus, it is also important to address the current quality aside from voltage quality, to understand the basis of many power quality problems.
Ideal Voltage and Current Waveform
Ideal Voltage and Current Waveform
Summing up, Power Quality is the study or description of both voltage and current disturbances. It can be seen as the combination of voltage quality and current quality. As an example, the ideal voltage and current waveform is a pure sinewave of constant magnitude and frequency (typically 50 or 60 Hz). Any deviation from the ideal that exceeds the limits set by the Standards is a power quality issue.

Play Video: What is Power Quality?


References:
Bollen, M. (2000). Understanding Power Quality Problems: Voltage Sags and Interruptions
Dugan, R., McGranaghan, M., Santoso, S., and Beaty, H.W. (2004). Electrical Power Systems Quality (2nd ed.)

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I am an Electrical Engineer with a Masters Degree in Business Administration. My interest is in Power Quality and Protective Relaying. I have been working in an electric distribution utility for about seven years now as a Planning & Design Engineer. I handle PQ studies, power system analysis and capital budgeting for company projects.