Power Quality = Voltage Quality

Power quality, like quality in other goods and services, is difficult to quantify. There is no single accepted definition of quality power. There are standards for voltage and other technical criteria that may be measured, but the ultimate measure of power quality is determined by the performance and productivity of end-user equipment. If the electric power is inadequate for those needs, then the quality is lacking.

1. Definition: Power Quality - Power is the rate of delivery of energy and is proportional to the product of the voltage and current. It would be difficult to define the quality of this quantity in any meaningful manner. The power supply system can only control the quality of the voltage; it has no control over the currents that particular loads might draw. Therefore, the standards in the power quality area are devoted to maintaining the supply voltage within certain limits. 1

Alternating current power systems are designed to operate at a sinusoidal voltage of a given frequency (typically 50 or 60 Hz) and magnitude. Any significant deviation in the magnitude, frequency, or purity of waveform is a potential power quality problem.

Of course, there is always a close relationship between voltage and current in any practical power system. Although the generators may provide a near-perfect sine-wave voltage, the current passing through the impedance of the system can cause a variety of disturbances to the voltage. For example,

1. The current resulting from a short circuit causes the voltage to sag, or disappear completely, as the case may be.
2. Currents from lightning strokes passing through the power system cause high impulse voltages that frequently flash over insulation and lead to other phenomena, such as short circuits.
3. Distorted currents from harmonic-producing loads also distort the voltage as they pass through the system impedance. Thus a distorted voltage is presented to other end users. Therefore, while it is the voltage with which we are ultimately concerned, we must address phenomena in the current to understand the basis of many power quality problems.

Power Quality Concerns

There are four major reasons for the growing concern of the term power quality.
1. Load equipment is more sensitive to power quality variations than equipment applied in the past. Many new load devices contain microprocessor-based controls and power electronics devices that are sensitive to may types of disturbances.
2. The increasing emphasis on overall power system efficiency has resulted in a continued growth in the application of devices such as high-efficiency, adjustable-speed motor drives and shunt capacitors for power factor correction to reduce losses. This is resulting in increasing harmonic levels on power systems and has many people concerned about the future impact on system capabilities.
3. Increased awareness of power quality issues by the end users. Utility customers are becoming better informed about such issues as interruptions, sags, and switching transients and are challenging the utilities to improve the quality of power delivered.
4. Many things are now interconnected in a network. Integrated processes mean that the failure of any component has much more important consequences.

The main impetus behind these reasons is increased productivity for utility customers. Manufacturers want faster, more productive, more efficient machinery. Utilities encourage this effort because it helps their customers become more profitable and also helps defer large investments in substations and power generation by using more efficient load equipment. Interestingly, the equipment installed to increase the productivity is also often the equipment that suffers the most from common power disruptions. And the equipment is sometimes the source of additional power quality problems.

2. Definition: Power Quality Problem - Any power problem manifested in voltage, current, or frequency deviations that results in failure or inoperativeness of customer equipment. There are many misunderstandings regarding the causes of power quality problems. Surveys have recorded the opinions of where is the source of most power quality problems? The resulting opinions were collected from electric utility employees and their customers. One common theme arises most common: the utility and the customer perspectives are much different. While both tended to blame about two-thirds (60-66%) of the events on natural phenomena (lightning), the customers think that the utility is at fault much more frequently than utility personnel. Utility personnel opinion indicate that the utility is at fault 1% of the time where the customer opinion indicate the utility is at fault 17% of the time. The utility personnel opinion indicate the customer is at fault 25% of the time and the customer opinion indicated the customer is a fault 12% of the time.

   When there is a power problem with a piece of equipment, end users may be quick to complain to the utility of an outage or glitch that has caused the problem. However, the utility records may indicate no abnormal events on the feed to the customer. It must be realized that there are many events resulting in customer problems that never show up in the utility statistics.

   One example is capacitor switching, which is quite common and normal on the utility system, but can cause transient over voltages that disrupt electronics and machinery. Another is a momentary fault elsewhere in the system that causes the voltage to sag briefly at the customer in question. This might cause an adjustable-speed drive to trip off, but the utility will have no indication that anything is amiss on the feeder unless it has a power quality monitor installed.

   In addition to real power quality problems, there are also perceived power quality problems that may actually be related to hardware, software, or control system malfunctions. Electronic components can degrade over time due to repeated transient voltages and may eventually fail due to a relatively low-magnitude event. Thus, it is sometimes difficult to associate a failure with a specific cause. Control software may not have anticipated a particular occurrence.

   The electric utility is concerned about power quality issues as well as meeting customer expectations and maintaining customer confidence. With today's movement toward competition between utilities, it is more important than ever to prevent loss of a disgruntled customer to a competing power supplier which would have a very significant impact financially on the utility.

   Load equipment suppliers generally find themselves in a very competitive market with most customers buying on lowest cost. Thus, there is a general disincentive to add features to the equipment to withstand common disturbances unless the customer specifies otherwise. Many manufacturers are also unaware of the types of disturbances that can occur on power systems. There is a need for education on this subject by vendors, suppliers and utilities to all parties.

   The primary responsibility for correcting inadequacies in load equipment ultimately lies with the end user who must purchase and operate it. Specifications must include power performance criteria. Since many end users are also unaware of the pitfalls, one useful service that utilities, load equipment suppliers and knowledgeable vendors can provide is dissemination of information on power quality and the requirements of load equipment to properly operate in the real world.

Additional information and site survey information can be obtained by contacting

POC: Mike Helms mike@lightningmike.com