Power Quality

Presentation for Energy Management Conference
by Peter Kratochvil, 31 October, 1997

With the escalating cost of electrical energy and the constant emphasis on operating cost reduction, institutional and industrial as well as commercial facilities, are using a variety of energy management methods to reduce their operating costs.

In the last several years the reliability of some of the energy management measurers, have been questioned not because of their validity, but because the power quality within the facility was adversely impacted by some energy saving methods, creating unexpected expenses and frustration.

Most power quality problems are created within the facility because of the increase in the non-linear load in the electrical distribution system. Equipment which has become common place in most facilities such as; computer power supplies, solid state lighting ballasts, uninterruptible power supplies (UPS), as well as a variety of office equipment such as printers, fax machines and copiers are all examples of non-linear loads.

Non-linear loads generate voltage and current harmonics which can have adverse effects on equipment designed to operate with linear loads (i.e., loads designed to operate on a sinusoidal waveform of 50 or 60Hz).

Transformers bringing power into a facility which as many harmonic generating sources (non-linear loads) are subject to higher heating losses and often have to be derated to accommodate the extra load. In a facility where non-linear loads are the norm we often suggest installing K-rated transformers. Situations where the harmonic content has not been taken into consideration when the transformers were installed often experience failure due to overheating.

Harmonics can also have a detrimental effect on emergency generators, telephones and other electrical equipment. Because harmonics are a form of electrical disturbance, in the fact that the sign wave is no longer sinusoidal, these disturbances often appear to the equipment as over voltages. A non-linear waveform also has multiple crossover points which will confuse computerized equipment that relies on a zero crossover point for reference causing computer lock-up.

Harmonic producing equipment is also prone to crosstalk, a situation where one piece of equipment interferes with another piece of equipment.

When reactive power compensation (in the form of passive power factor improving capacitors) is used with non-linear loads, resonance conditions can occur that may result in even higher levels of harmonic voltage and current distortion thereby causing equipment failure, disruption of power service, and in extreme conditions, fire hazards.

Lighting retrofits using electronic ballasts are probably the most common culprit of energy savings going astray. Electronic ballasts create a fairly high level of harmonics, when these are multiplied by a large quantity of light fixtures, problems are sure to arise. These problems can show themselves in many ways, such as premature failure of the ballasts and bulbs, computerized equipment failure, overheated/overloaded transformers and overloaded neutral conductors.

These effects of non-linear loads are off-setting the anticipated cost savings and in the long run cause more maintenance problems and reduce the reliability of the operation of the facility. It is possible to take energy management measures without jeopardizing the savings or reducing the reliability of the operation, by doing proper planning and taking into consideration the impact of non-linear loads on the total electrical system.

In the past, the electrical environment has absorbed most of these problems but more and more often the electrical system is rapidly being saturated. Canada will soon be joining Europe, the US, and other countries where power quality problems have reached such a magnitude, that standards to responsibly engineer systems that consider the total electrical environment are being proposed. In the US the IEEE 519-1992 guidelines and in Europe the IEC 555 standard have evolved to become a common requirement cited when specifying equipment or writing on newly engineered projects.

The present IEEE 519-1992 document established acceptable levels of harmonics (in voltages and current) that can be introduced into the incoming feeders by commercial and industrial users. The IEEE 519-1992 relies strongly on the definition of the point of common coupling or PCC. The PCC from the utility view point is usually the point where the power comes into the facility or the point of metering. However the guidelines also suggest that within the facility the point of common coupling is the point between the non-linear and the other loads.

This statement is crucial and often overlooked by building managers and supervisor because they interpret the two locations as being the same. Most of the problems with harmonics are due to this misinterpretation.

It is important to recognize the need for mitigation of harmonics at the point where the offending equipment is connected to the power system. This approach will minimize harmonic problems, thereby reducing downtime, maintenance costs and improve the life expectancy and efficiency of the electrical equipment. When there is successful mitigation of the individual load current harmonics, then the total harmonics at the point of common coupling of the utility will in most cases meet or be better than the IEEE guidelines.

The above reasons should justify the wisdom of planning before the implementation of energy saving measures. For example before doing lighting ballast retrofits a proper re-retrofit analysis of the existing power system by a qualified power quality consultant is a good recommendation. A good analysis will determine the level of existing harmonics and possible shortfalls in the rating of the transformers and the distribution system. At this point any cost for upgrades necessary to implement the retrofit can be included into the cost of the retrofit project and the total cost evaluated. In many instances this does not happen and the problem correction becomes expensive to mitigate and difficult to justify as a part of the project cost.

A good analysis of the power system will not only address the harmonics in the system but also look at other power quality issues, such as grounding problems and up to date wiring methods at the same time.

It is also important to recognize that the electrical code in the past has not taken into account the use of non-linear loads and may be incompatible with non-linear loading. This is especially important when dealing with older buildings. It used to be that the electrical code allowed derating of the neutral wire in services over 200Amps by 50% of the current carrying capacity of the line conductor. This used to be quite sufficient with conventional linear loads since the neutral would carry back only the imbalance, with high levels of non-linear loads this neutral wire could now carry as much as 270% of the line current back to the transformer.

The electrical code has now been changed by the wiring on many older buildings has not. Another reason for an analysis of the electrical system especially in older renovated buildings is the grounding. Grounding codes and practices were based on 60Hz frequencies but digital technologies and harmonic creating equipment require grounding for much higher frequencies.

Electromagnetic incompatibility is the reason for all power quality problems and linear and non-linear loading of the electrical distribution system creates incompatibility. Adhering to sound practices of planning energy management projects and looking at the interaction of non-linear loads with conventional loads within the electrical system can prevent unforeseen problems and provide reliable energy management.