Power Quality

Power Quality: Face the Problems Now to Avoid Them Later (as Much as Possible)

For more than a decade, end-users and electric utilities have become more and more concerned about the quality of electric power. While power quality includes many different issues not necessarily new, the increasing development and use of new load equipment force us to face the problem in the early design phase of a project. These new types of loads, such as variable speed drives and microprocessor-based controls, are not only more sensitive to voltage variations than the equipment used in the good old days, but can also produce "electric pollution" on the supply network. Project designers must be aware of all the implications involved in the design and specifications of electrical equipment at an early stage, in order to assure a reliable operating performance during normal or abnormal conditions that will always be present on a power system. They must have a knowledge of the different power system disturbances in order to ascertain the events and causes of equipment failure, as well as to apply mitigation measures, more effectively.

What is the Problem ?
These days, increased productivity is based on a continuous process operation and just-in-time production, which depends on an uninterrupted and reliable power supply, hopefully free of disturbances. This is why the quality of power usually has a direct economic impact on industries. Any incident resulting in the temporary stoppage of the process can represent significant production and raw material losses, because of the need to clean the machines, restart the process in the proper sequence and recalibrate the production line to meet the required product specifications.

As a general rule, in the event of a temporary voltage fluctuation or interruption, automatic restart of the process, as soon as the power supply is restored, can minimize its impact, namely in the cleaning and calibration procedure of the machinery. On the other hand, the automatic restart of a process is not always safe and simple to apply.

Most small and medium-sized industries share their power supply with other subscribers on medium voltage distribution systems (12.5 kV to 34.5 kV), generally made of overhead lines. These lines are exposed to many hazards (lightning, wind, pollution damage to insulators, animals, etc. ), causing voltage disturbances to sensitive loads which can result in a prolonged production stoppage. For a medium-voltage distribution system, the following rules usually apply :

Residential and commercial sectors are much more exposed to short-term disturbances caused by the multitude of electric distribution equipment and the need to trim back trees.
The number of incidents on a system for any category of client increases with the range of the distribution system and the number of clients. This is a direct consequence of the actual trend to increase the distribution voltage level to gain on the power transfer capability and to increase the line length to serve more clients.
Grouping

industrial customers (in industrial parks for example) on specialized lines is the first step to providing a better quality service. A wide variety of electric phenomena falls into the power quality issue, so it is useful to classify the different types of power supply disturbances in different categories. Some disturbances come from the supply network, while others are produced by the load itself.

Long Duration Voltage Variations
Long duration voltage variations include undervoltage or overvoltage lasting more than 1 minute. An undervoltage is a decrease of more than 10 per cent in the supply voltage whereas an overvoltage is an increase of more than 10 per cent. Overvoltage and undervoltage are voltage regulation problems usually solved by better planning of the distribution system. Voltage regulation problems occur when the power system is not strong enough to supply the load properly and there is too much voltage drop in the power system impedance under heavy load.

Solutions to these problems include:

Increase the size of the transformer, reduce the line length, add series capacitors or increase the size of line conductors (to reduce the system impedance).
Add voltage regulators or automatic on-load tap changers (to improve the voltage profile).
Add shunt capacitors or static var compensators, or upgrade the line to the next voltage level (to reduce the line current)

. Permanent voltage variations (per cent), introduced by the switching of a capacitor bank, can be estimated by the capacitor bank capacity (in Mvar) divided by the short circuit MVA of the supply network. When the voltage variation becomes greater than 3 per cent of the system rated voltage, switching the capacitor bank in steps instead of in one block is normally considered.

Utilities generally try to regulate the voltage supplied to customers within ± 5 per cent (up to +6/-13 per cent under some circumstances) by installing transformers with on-load tap changers at the sending end of the line and voltage regulators along the distribution system. Inside these limits, it is the responsibility of the customer to protect these more sensitive loads which require a better voltage regulation to operate properly. Obviously, it is not up to the utility to correct at the distribution level a voltage regulation problem which is inside the customer installations.

Voltage Sags and Interruptions
A voltage sag is a sudden reduction ( > 10 per cent) of the normal supply voltage, for a short period of time (between 8 milliseconds to 60 seconds), caused by short circuits on the power system or the starting of large motors.

An interruption is a complete loss of voltage caused by the opening of a breaker, which is required to eliminate faults on the power system. In 80 per cent of cases, the faults are momentary and result in an interruption lasting only 2 to 10 seconds.

Although the inconveniences of voltage sags and momentary interruptions are negligible for residential and commercial customers, their impact is more significant for industrial customers where it can shut down a production line for a few hours due to loss of process controllers, drop off of motor starters, nuisance tripping of variable speed drives and so on.

When a long-duration interruption ("long" being more than 1 minute) presents a particular risk of permanent damage, such as in furnaces and other machines vulnerable to product solidification, the costs of repair and production losses can easily exceed the cost of investing in an emergency generator.

Transient Overvoltage
A transient overvoltage designates an undesirable phenomenon arising momentarily during a change of states in the power system. A transient overvoltage can take the form of a positive or negative impulse (lightning strike) or a damped oscillatory wave (capacitor bank energization).

Reduction of capacitor-switching transients involves special techniques such as pre-insertion resistors and synchronous closing, which are not widely used in North America because of their additional cost and complexity.

Effect of Voltage Variation on Variable Speed Drives
Variable speed drives and their internal power electronic components are very sensitive to transient overvoltage, causing nuisance tripping due to dc bus overvoltage or input fuses blowing. The most effective solution to this problem is to install line chokes (a series inductance of 3 to 5 per cent of the drive kVA rating) at the input of the drive, to isolate it from the power system.

To minimize the harmful effects of nuisance tripping of variable speed drives due to excessive voltage variations, it is preferable, from the start, to provide this equipment with an automatic restart feature which will allow the restarting of motors from a resting position, or better, restarting of decelerating motors. This general rule tries to prevent a simple voltage variation from causing a long-term stoppage of operations. However, in order to obtain the maximum efficiency with this palliative measure, a case-by -case study of each motor must be carried out with regards to its function in the manufacturing sequence.

Still with respect to variable speed drives, in addition to voltage variations, sudden variations in the system voltage phase angle can occur during a fault, a switching of transformers and lines or a load transfer on the supply network. The control logic of drives using thyristors at the input stage interprets such variations as being rapid frequency variations, giving zero crossings different to those normally expected by the control logic. AC drives that use diode bridges at the input stage are immune to this specific problem.

Voltage Unbalance
Voltage unbalance is a condition in which the three-phase voltage differs in amplitude and/or does not have its normal 120 degree phase relationship. It is based on the average value of the three- phase voltage and is calculated by taking the ratio of the maximum voltage deviation from the average over the average value.

Any power system, no matter how well balanced, always has in a steady state mode, a voltage unbalance of 1 per cent to 1.5 per cent of nominal voltage. This is mainly caused by the asymmetry in the geometry of overhead distribution lines, as well as from load unbalance, whether it is from a client having a large unbalanced load or from various single-phase clients connected to the distribution system. Single phase voltage regulators, with phase-to-ground voltage controls, can aggravate the problem of voltage unbalance to downstream customers.

Power utilities and distributors constantly control this important parameter, both for their own generators as well as for their client’s motor load, knowing their limited capacity to operate at full rating even with the slightest voltage unbalance. The NEMA-MG1 standard, which applies to motor load, allows voltage unbalance of no more than 1 per cent. Anything above that requires some attention, as showed by the reduction factor illustrated in the following graph.

A voltage unbalance of 5 per cent is the maximum limit acceptable, above which it is not advisable to run a motor due to the risk of rotor overheating.

Experience indicates that for industrial clients who are supplied by a high voltage system (at 120 kV or more), the voltage unbalance of the supply system is generally maintained below 1 per cent, with 1.5 per cent being the usual maximum expected. However, for smaller industries who are supplied at a medium voltage level, the further they are from the utility power station, the greater the voltage unbalance. The voltage unbalance is generally maintained below 1.75 per cent, with 2.5 per cent being the usual maximum expected, and this despite the 1 per cent limit recommended by the NEMA standard previously mentioned.

Harmonics
Harmonics are sinusoidal current and voltage frequencies that are integral multiples of the normal power system frequency (the normal frequency is 60 Hz and is also called the fundamental). Distorted waveforms can be decomposed into a sum of the fundamental frequency and the harmonics.

Harmonics are caused by any device or load that has a non-linear voltage-current characteristic, such as variable speed drives, electronic rectifiers and power supplies, arc furnaces, etc. While the harmonic producing equipment is a load for the fundamental current, it can be viewed as a source of harmonic current. The level of harmonic current flowing into the system impedance (which varies with frequency) determines the harmonic voltage distortion level. Harmonic current flows through the electrical system, thereby distorting the voltage seen by other equipment. Since the system impedance is usually low (except during resonance), the magnitude of voltage harmonics, and the extent of voltage distortion, are usually lower than that for the corresponding current distortion.

Industrial clients are generally required to compensate their power factor to avoid billing penalties. Most of the time capacitor banks are used for this. However, care must be taken that no critical resonance frequency is introduced and that harmonics present in the system are not excessively amplified (resonance phenomena and harmonic filtering will not be covered by this article). Also, the untuned capacitor bank of a customer can sometimes amplify transient overvoltages caused by the energization of utility capacitor banks.

Adding a capacitor bank in any power system always requires the calculation of resonance frequency and the measurement of harmonic current and voltage before and after installation, to confirm beyond any doubt the expected performance, and to detect any unanticipated problems before they can cause serious damage.

Harmonic current distortion greater than 5 per cent will contribute to the additional heating of a power transformer, so it must be derated for harmonics. Motors can also be affected by harmonic voltage distortions and sometimes need to be derated (this is less of a problem with the new PWM drives since the harmonics applied to the motor are of a higher order).

Experience indicates that harmonics are more of a local problem for the end user than for the utility, especially when compared to voltage sags and interruptions, which represent the most numerous power quality problems.

Whose Problem is it Anyway?
Our experience with over 100 power quality case studies at end-user facilities has shown that utilities and their customers no longer live in two different countries, arbitrarily separated by the utility meter. Those concerned with power quality issues have to consider both sides of the meter.

End users could avoid most power quality problems (approximately 80 per cent) through careful design engineering and equipment specifications. This, in turn, will incite designers to make equipment more compatible with real-life power systems. Finally, with a better understanding of customer needs, utilities are already reviewing their power system designs and operation practices to achieve better power quality.

Jean Tessier and Maurice Brisson are with Breton, Banville and Associates, a consulting engineering firm located near Montreal and specializing in power systems.