Sources of Power Quality Problems

Power quality may be affected by a number of issues. Our discussion in this article focuses on various devices and events that lead to problems in power quality.

The common sources of power quality problems include:

Power Electronic Devices

Power electronic devices cause power quality disturbances and they are also susceptible to them. Variable speed drives, for example, are the most common source of power quality problems. All computers contain a power electronic switch mode power supply (SMPS), which is an affordable and convenient method of converting mains supply into low voltage dc without costly transformer windings. However, these supplies are the cause of a significant increase in the level of 3^rd, 5^th and 7^th harmonic voltage distortion.

Variable Speed Drives

Variable speed motor drives or inverters are highly susceptible to voltage dip disturbances and cause specific problems in industrial processes where loss of mechanical synchronism is an issue. The ideal solution to problems of this nature is for planning engineers to install equipment that has a ‘reasonable level’ of susceptibility to voltage dips from the outset. Unfortunately, this doesn’t happen in many cases, due to the following:

• Where optional equipment filters are provided by the manufacturers at the installation stage, many customers opt not to fit them for economic reasons.
• Manufacturers of these drives often don’t publish detailed information about the equipment’s level of susceptibility to voltage dips.

Switch Mode Power Supply (SMPS) including IT Equipment

Most of equipment for office and domestic applications use switch mode power supplies to convert mains to the required dc level. Most of these converters draw a non-linear current from the supply, which is high in 3^rd and 5^th harmonic content.

Since the third harmonic is a “triplen” harmonic, it is of zero order phase sequence and therefore adds in the neutral of a balanced three phase system.

The increased use of IT equipment has led to concern of the increased overloading of neutral conductors and also overheating of transformers.

Many commercial modern buildings which have fluorescent lighting applications employing switch mode power supplies have large neutral conductors to cope with the levels of 3^rd harmonic, which can theoretically reach three times the magnitude of the fundamental.

Arcing Devices

Electric arc furnaces, arc welders and electric discharge lamps which form part of electric arcing devices, are highly non-linear loads the current waveform of which is characterized by an increasingly arc current limited only by the network impedance.

All arcing devices are sources of harmonic distortion; the arcing load can be represented as a relative stable source of voltage harmonics. Arc welders often cause transients in the local network due to the intermittent switching.

The effects of arc furnaces are challenging to mitigate; balancing the phases with other furnaces may not be effective as arc furnaces are operated in various modes leading to phase imbalance. Balancing the phases to have equal harmonic load is an effective way to minimize the level of the ‘triplen’ harmonics, however on a star-star connected transformer connection no cancellation will happen.

Load Switching

Heavy load switching effect on the local network is a rather common problem causing transients to propagate through to the other electrically close equipment. These transients may be of large voltage magnitude, but have very little energy due to their short duration which is normally measured in terms of milliseconds. Electronic devices which may be sensitive to these voltage impulses can have their operation impaired.

The effect of load switching on the voltage is usually encountered in the form of transient activity as demonstrated in the figure below:

This type of transient may occur as a result of switching in heavy single-phase load, the effect seen on the voltage measured nearby. Other equipment can be protected from these switching transients by isolating them from the affecting equipment.

Large Motor Starting

Owing to the dynamic nature of an induction machine it draws a current depending on the mode of operation; during starting this current can be as high as six times the normal rated current. This increased loading on the local network has the effect of causing a voltage dip, the magnitude of which is dependent on the system impedance. It can take several seconds for motors to reach their rated speed; as a result, measures are taken to reduce the level of current drawn during motor starting. These measures are dependent upon the type of motor and drive. Many modern motors employ a sophisticated power electronic drive which in most cases will control the motor’s starting current to a reasonable level. Some less costly motors employ series capacitors or resistors to minimize the starting current, these components are then switched out once the motor’s rated speed has been reached. Autotransformers are used to start some older motors, these have a variable secondary winding which allows the motor stator voltage to be controlled and hence the current drawn from the supply.

Embedded Generation

An increased amount of embedded generation at substation level and below will lead increased fault levels in the feeders. The voltage of embedded generators, when located at some distance from a substation must be controlled to ensure power flow from the high voltage (substation bus) to the lower voltage (embedded generator connection).

Wind turbines voltage fluctuation due to variations in wind speed leads to problems with voltage regulation and therefore potential power quality problems on a local level.

Storm and Related Environment Damage

Lightning strikes cause transient overvoltages often leading to faults. The local ground potential can be raised by a nearby lightning leading to neutral current flowing to earth via a remote ground; this can have destructive effects on sensitive equipment. Lightning strikes which hit overhead lines often cause flashovers to nearby conductors as the insulators break-down, the strike will thus not only consist of a transient overvoltage but also fault clearing interruptions and dips.

High winds and storm conditions cause widespread disruption to the supply networks. Where disruptions are caused by faults that can be cleared in less than one minute by use of auto-reclosers for instance, the effect on the network is seen as a power quality issue. Long interruptions above one minute are generally seen as reliability or quality of supply problem.

Snow and ice build-up have a severe effect on the reliability of overhead lines, this has of course power quality/quality of supply consequences.

Sea mists in the vicinity of overhead lines can lead to flashover between conductors; insulators must be cleaned on regular basis in these areas to prevent these problems. In hot and humid climates dust and heavy dew, can cause similar flashover problems requiring non-intrusive insulator cleaning methods.

Damage due to wildlife and trees is common issue in rural areas. As with any faults these are potential causes of power quality problems.

Network Equipment & Design

The circuit breakers have the purpose of increasing the security of supply, fuse saving and minimizing outage time. However, the circuit breakers can cause power quality problems due to the way in which they operate. Fast tripping is the use of circuit breakers or line reclosers to trip in a very short period of time under fault conditions, this is one way of saving fuses, but has adverse effect on power quality. As most faults in rural networks are of a transient nature, when such a fault happens, the auto-recloser will trip, after preset time delay the circuit breaker will reclose. If the fault has cleared then the supply is fully restored and no more disruption will occur, if the fault is not cleared, then the breaker will trip again (up to 4 times). It is this repeated reclosing that has effect on power quality. Customers will have their supplies reconnected for each of the instants when the auto-recloser reconnects the supply, this can lead to equipment damage in extreme circumstances. To minimize this problem, these devices are normally built in ‘dead-time’ which allows the fault more time to clear prior to recloser operation. A ‘lock-out time’ can also be set which will stop the unit from reclosing if a certain number of operations have occurred within a preset time window.

When capacitors are switched into a supply, voltage transients take place, due to the interaction of the network inductive elements and the additional capacitance. When power factor correction capacitors are installed at a customer’s site and utility capacitor switching is happening, the effect of the switching transient can be magnified and oscillatory in nature. This ‘voltage magnification’ is a function of the impedance of both the capacitance’s, the network, end user circuit inductance and the capacitor switching ‘frequency’. If the resonance frequency of the end-user circuit and the network resonance in response to the capacitor switching are equivalent, then the maximum voltage magnification will take place. This can amount to double the nominal supply voltage in extreme cases.

Transformer energisation causes large oscillatory in-rush currents that have an adverse effect on power quality each time a transformer is switched in to the network. The energisation of a transformer can cause dynamic overvoltages for up to one second after it is switched in; the in-rush current is highly distorted. Specific problems are encountered when transformers are connected to power factor correction capacitors, the issue can be eradicated by switching the devices in separately. Ferro-resonance can happen in distribution networks, at frequencies below 300 Hz, as the result of transformer in-rush current harmonic components and series connected capacitors resonating with the transformer magnetizing inductance. To prevent this effect, capacitors are de-tuned from known resonance frequencies.

To minimize the impact of faults on a power system and improve power quality, some of the equipment and design used includes:

Current limiting fuses utilized where the fault current is high and have the effect of enhancing the overall power quality by isolating such a fault in a very short time frame. Typically rated in thousands of Amperes, the fuses will isolate a faulted connection in less than half a mains cycle.

Network sectionalizing on a radial distribution network which consists of the use of line reclosers in strategic positions; allow increased probability of continued supply to more critical loads under fault conditions.

Surge arresters are used in areas where lightning strikes are a frequent occurrence. They have non-linear characteristics which allow current surges, induced by lightning to be ‘bled-off’ to earth.

Line shielding is another technique of minimizing the effect of lightning on overhead lines but in storm-prone areas. Most commonly used on transmission lines, the ground conductor is supported above the phase lines thus reducing the possibility of faults caused by lightning strikes.

Related articles:

• Common Terms Used When Describing Power Quality Problems
• Overvoltage Protection Techniques & Equipment
• Surge Suppression in Power Systems
• Protecting Electronics from Voltage Spikes and Surges
• Switchgear Equipment
• 5 Causes of High-Voltage Switchgear Failures
• Switchgear Terminology & IEC Standards