The Scope of Power Quality

Published by CEA Technologies Inc. (CEATI), POWER QUALITY Energy Efficiency Reference Guide, Chapter 1 – The Scope of Power Quality.

1.1 Definition of Power Quality

The Institute of Electrical and Electronic Engineers (IEEE) defines power quality as:

“The concept of powering and grounding electronic equipment in a manner that is suitable to the operation of that equipment and compatible with the premise wiring system and other connected equipment.” 1

Making sure that power and equipment are suitable for each other also means that there must be compatibility between the electrical system and the equipment it powers. Th ere should also be compatibility between devices that share the electrical distribution space. This concept is called Electromagnetic Compatibility (“EMC”) and is defined as:

“the ability of an equipment or system to function satisfactorily in its electromagnetic environment without introducing intolerable electromagnetic disturbances to anything in that environment.” 2

The best measure of power quality is the ability of electrical equipment to operate in a satisfactory manner, given proper care and maintenance and without adversely aff ecting the operation of other electrical equipment connected to the system.

1 – IEEE-Std 1100-1999, IEEE Recommended Practice for Powering and Grounding Electronic Equipment, New York, IEEE 1999.
2 – A definition from the IEC at

1.2 Voltage

The voltage produced by utility electricity generators has a sinusoidal waveform with a frequency of 60 Hz in North America and 50 Hz in many other parts of the world. This frequency is called the fundamental frequency.

Figure 1: Pure Sinusoidal AC Voltage Waveform

Any variation to the voltage waveform, in magnitude or in frequency, is called a power line deviation. However, not all power line deviations result in disturbances that can cause problems with the operation of electrical equipment.

1.2.1 Voltage Limits

Excessive or reduced voltage can cause wear or damage to an electrical device. In order to provide standardization, recommended voltage variation limits at service entrance points are specified by the electrical distributor or local utility. An example of typical voltage limits is shown in the table below.

Table: Typical Voltage Limits

In addition to system limits, Electrical Codes specify voltage drop constraints; for instance:

(1) The voltage drop in an installation shall:

  • Be based upon the calculated demand load of the feeder or branch circuit.
  • Not exceed 5% from the supply side of the consumer’s service (or equivalent) to the point of utilization.
  • Not exceed 3% in a feeder or branch circuit.

(2) The demand load on a branch circuit shall be the connected load, if known, otherwise 80% of the rating of the overload or over-current devices protecting the branch circuit, whichever is smaller.3

3 – Check with your local Authority Having Jurisdiction for rules in your area.

For voltages between 1000 V and 50 000 V, the maximum allowable variation is typically ±6% at the service entrance. There are no comparable limits for the utilization point. These voltage ranges exclude fault and temporary heavy load conditions. An example of a temporary heavy load condition is the startup of a motor. Since motors draw more current when they start than when they are running at their operating speed, a voltage sag may be produced during the initial startup.

Figure 2: RMS Voltage and Current Produced when Starting a Motor

(Reproduced with Permission of Basic Measuring Instruments, from “Handbook of Power Signatures”, A. McEachern, 1988)

It is not technically feasible for a utility to deliver power that is free of disturbances at all times. If a disturbance-free voltage waveform is required for the proper operation of an electrical product, mitigation techniques should be employed at the point of utilization.

1.3 Why Knowledge of Power Quality is Important

Owning or managing a concentration of electronic, control or life-safety devices requires a familiarity with the importance of electrical power quality.

Power quality difficulties can produce significant problems in situations that include:

  • Important business applications (banking, inventory control, process control)
  • Critical industrial processes (programmable process controls, safety systems, monitoring devices)
  • Essential public services (paramedics, hospitals, police, air traffic control)

Power quality problems in an electrical system can also quite frequently be indicative of safety issues that may need immediate corrective action. Th is is especially true in the case of wiring, grounding and bonding errors.

Your electrical load should be designed to be compatible with your electrical system. Performance measures and operating guidelines for electrical equipment compatibility are available from professional standards, regulatory agency policies and utility procedures.

1.4 Major Factors Contributing to Power Quality Issues

The three major factors contributing to the problems associated with power quality are:

Use of Sensitive Electronic Loads

The electric utility system is designed to provide reliable, efficient, bulk power that is suitable for the very large majority of electrical equipment. However, devices like computers and digital controllers have been widely adopted by electrical end-users. Some of these devices can be susceptible to power line disturbances or interactions with other nearby equipment

The Proximity of Disturbance-Producing Equipment

Higher power loads that produce disturbances – equipment using solid state switching semiconductors, arc furnaces, welders and electric variable speed drives – may cause local power quality problems for sensitive loads.

Source of Supply

Increasing energy costs, price volatility and electricity related reliability issues are expected to continue for the foreseeable future. Businesses, institutions and consumers are becoming more demanding and expect a more reliable and robust electrical supply, particularly with the installation of diverse electrical devices. Compatibility issues may become more complex as new energy sources and programs, which may be sources of power quality problems, become part of the supply solution. These include distributed generation, renewable energy solutions, and demand response programs.

Utilities are regulated and responsible for the delivery of energy to the service entrance, i.e., the utility meter. The supply must be within published and approved tolerances as approved by the regulator. Power quality issues on the “customer side of the meter” are the responsibility of the customer. It is important therefore, to understand the source of power quality problems, and then address viable solutions.

1.5 Supply vs. End Use Issues

Many studies and surveys have attempted to define the percentage of power quality problems that occur as a result of anomalies inside a facility and how many are due to problems that arise on the utility grid. While the numbers do not always agree, the preponderance of data suggests that most power quality issues originate within a facility; however, there can be an interactive effect between facilities on the system.

Does this matter? After all, 100% of the issues that can cause power quality problems in your facility will cause problems no matter where they originate. If the majority of power quality issues can be controlled in your own facility, then most issues can be addressed at lower cost and with greater certainty. Understanding how your key operational processes can be protected will lead to cost savings.

Utilities base their operational quality on the number of minutes of uninterrupted service that are delivered to a customer. Th e requirements are specific, public and approved by the regulator as part of their rate application (often referred to as the ‘Distributors Handbook’).

While some issues affecting the reliability of the utility grid – such as lightning or animal caused outages – do lead to power quality problems at a customer’s facilities, the utility cannot control these problems with 100% certainty. Utilities can provide guidance to end users with power quality problems but ultimately these key principles apply:

  • Most PQ issues are end-user issues
  • Most supply issues are related to utility reliability
1.6 Countering the Top 5 PQ Myths

1) Old Guidelines are NOT the Best Guidelines

Guidelines like the Computer Business Equipment Manufacturers Association Curve (CBEMA, now called the ITIC Curve) and the Federal Information Processing Standards Pub94 (FIPS Pub94) are still frequently cited as being modern power quality guidelines.

The ITIC curve is a generic guideline for characterizing how electronic loads typically respond to power disturbances, while FIPS Pub 94 was a standard for powering large main-frame computers.

Contrary to popular belief, the ITIC curve is not used by equipment or power supply designers, and was actually never intended for design purposes. As for the FIPS Pub 94, it was last released in 1983, was never revised, and ultimately was withdrawn as a U.S. government standards publication in November 1997. While some of the information in FIPS Pub 94 is still relevant, most of it is not and should therefore not be referenced without expert assistance.

2) Power Factor Correction DOES NOT Solve All Power Quality Problems

Power factor correction reduces utility demand charges for apparent power (measured as kVA, when it is metered) and lowers magnetizing current to the service entrance. It is not directly related to the solution of power quality problems. There are however many cases where improperly maintained capacitor banks, old PF correction schemes or poorly designed units have caused significant power quality interactions in buildings.

The best advice for power factor correction is the same as the advice for solving power quality issues; properly understand your problem first. A common solution to power factor problems is to install capacitors; however, the optimum solution can only be found when the root causes for the power factor problems are properly diagnosed. Simply installing capacitors can often magnify problems or introduce new power quality problems to a facility.

Power factor correction is an important part of reducing electrical costs and assisting the utility in providing a more efficient electrical system. If power factor correction is not well designed and maintained, other power quality problems may occur. The electrical system of any facility is not static. Proper monitoring and compatible design will lead to peak efficiency and good power quality.

3) Small Neutral to Ground Voltages DO NOT Indicate a Power Quality Problem

Some people confuse the term “common mode noise” with the measurement of a voltage between the neutral and ground wires of their power plug. A small voltage between neutral to ground on a working circuit indicates normal impedance in the wire carrying the neutral current back to the source. In most situations, passive “line isolation” devices and “line conditioners” are not necessary to deal with Neutral to Ground voltages.

4) Low Earth Resistance is NOT MANDATORY for Electronic Devices

Many control and measurement device manufacturers recommend independent or isolated grounding rods or systems in order to provide a “low reference earth resistance”. Such recommendations are often contrary to Electrical Codes and do not make operational sense. Bear in mind that a solid connection to earth is not needed for advanced avionics or nautical electronics!

5) Uninterruptible Power Supplies (UPS) DO NOT Provide Complete Power Quality Protection

Not all UPS technologies are the same and not all UPS technologies provide the same level of power quality protection. In fact, many lower priced UPS systems do not provide any power quality improvement or conditioning at all; they are merely back-up power devices. If you require power quality protection like voltage regulation or surge protection from your UPS, then make sure that the technology is built in to the device.

1.7 Financial and Life Cycle Costs

The financial and life cycle costs of power quality issues are two fold;

  1. The “hidden cost” of poor power quality. The financial impact of power quality problems is often underestimated or poorly understood because the issues are often reported as maintenance issues or equipment failures. Th e true economic impact is often not evaluated.
  2. The mitigation cost or cost of corrective action to fix the power quality issue. The costs associated with solving or reducing power quality problems can vary from the inexpensive (i.e., checking for loose wiring connections), to the expensive, such as purchasing and installing a large uninterruptible power supply (UPS).

Evaluation of both costs should be included in the decision process to properly assess the value, risk and liquidity of the investment equally with other investments. Organizations use basic financial analysis tools to examine the costs and benefits of their investments. Power quality improvement projects should not be an exception; however, energy problems are often evaluated using only one method, the ‘Simple Payback’. The evaluation methods that can properly include the impact of tax and cost of money are not used, e.g., Life Cycle Costing.

Monetary savings resulting from decreased maintenance, increased reliability, improved efficiency, and lower repair bills reduce overall operating costs. A decrease in costs translates to an increase in profit, which increases the value of the organization.

Regrettably, the energy industry has adopted the Simple Payback as the most common financial method used. Simple Payback is admittedly the easiest, but does not consider important issues. To properly assess a capital improvement project, such as a solution to power quality, Life Cycle Costing can be used. Both methods are described below.

1.7.1 Simple Payback

Simple Payback is calculated by dividing the initial, upfront cost of the project (the ‘first cost’), by the annual savings realized. The result is the number of years it takes for the savings to payback the initial capital cost. For example, if the first cost of a power quality improvement project was $100,000, and the improvements saved $25,000 annually, the project would have a four year payback.

As the name implies, the advantage of the Simple Payback method is that it is simple to use. It is also used as an indicator of both liquidity and risk. Th e cash spent for a project reduces the amount of money available to the rest of the organization (a decrease in liquidity), but that cash is returned in the form of reduced costs and higher net profit (an increase in liquidity). Thus the speed at which the cash can be ‘replaced’ is important in evaluating the investment.

Short payback also implies a project of lesser risk. As a general rule, events in the short-term are more predictable than events in the distant future. When evaluating an investment, cash flow in the distant future carries a higher risk, so shorter payback periods are preferable and more attractive.

A very simple payback analysis may ignore important secondary benefits that result from the investment. Direct savings that may occur outside the immediate payback period, such as utility incentive programs or tax relief, can often be overlooked.

1.7.2 Life Cycle Costing

Proper financial analysis of a project must address more than just ‘first cost’ issues. By taking a very short-term perspective, the Simple Payback method undervalues the total financial benefit to the organization. Cost savings are ongoing, and continue to positively impact the bottom line of the company long after the project has been ‘repaid’.

A full Life Cycle Costing financial analysis is both more realistic, and more powerful. Life Cycle Costing looks at the financial benefits of a project over its entire lifetime. Electrical equipment may not need replacing for 10 years or more, so Life Cycle Costing would consider such things as the longer life of the equipment, maintenance cost savings, and the potential increased cost of replacement parts. In these cases, the time value of money is an important part of the investment analysis. Simply stated, money received in the future is less ‘valuable’ than money received today. When evaluating long-term projects, cash gained in the future must therefore be discounted to reflect its lower value than cash that could be gained today.

1.7.3 The Cost of Power Quality Problem Prevention

The costs associated with power quality prevention need to be included with the acquisition cost of sensitive equipment so that the equipment can be protected from disturbances. Installation costs must also be factored into the purchase of a major electrical product. Th e design and commissioning of data centres is a specific example. The costs that should be considered include:

  • Site preparation (space requirements, air conditioning, etc.)
  • Installation
  • Maintenance
  • Operating costs, considering efficiency for actual operating conditions
  • Parts replacement
  • Availability of service on equipment
  • Consulting advice (if applicable)
  • Mitigating equipment requirements

The cost of purchasing any mitigating equipment must be weighed with the degree of protection required. In a noncritical application, for instance, it would not be necessary to install a UPS system to protect against power interruptions.

Power supply agreements with customers specify the responsibilities of both the supplier and the customers with regard to costs.

For very large electrical devices, even if no power quality problems are experienced within the facility, steps should be taken to minimize the propagation of disturbances which may originate and reflect back into the utility distribution system. Many jurisdictions regulate the compatibility of electrical loads in order to limit power quality interactions.

Published by PQBlog

Electrical Engineer

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