Understanding the most common power quality problems

With the growing need for more base- and critical-load supply, many countries and industries struggle to manage their energy usage efficiently, despite the continuous availability of electrical power. This article provides detailed insights on the most common power quality problems, their likely effects and potential solutions.

We live in a fast and constantly evolving society where responsiveness is vital. With this comes availability which, in turn, brings reliability. Our technological world is heavily dependent on these three factors. Can we imagine the effectiveness of an economy in the event of poor power availability? How would the economy function with an unreliable power grid which is subjected to interruptions or disturbances?

The practice of ensuring the quality of power, that is power quality, is a way to minimise such power problems affecting today’s technological equipment, whether sensitive or heavy industrial. These problems reflected at the transmission level are typically caused by weather conditions, including lightning storms along with equipment failure and major switching operations. Similarly, the problems are observed at the load level from heavy start-up loads, a faulty distribution network and electrical noise.

Power quality disturbance is commonly defined as any change in power, voltage, current or frequency that interferes with the normal operation of the electrical equipment. This encompasses numerous types of disturbances. As a result, the IEEE Standard 1159-2009 attempts to define them to clear the confusion created with the mixed use of terminology and applications. Using the correct terms effectively can make a big difference in the decision-making process in selecting the appropriate solution.

This article explains seven categories of power quality disturbances as defined by the IEEE.

Transients

Transients are probably the most damaging type of power disturbance, often categorised as impulsive or oscillatory. Impulsive transients are fast and sudden high, fast peak events that raise voltage and/or current levels from steady state, either positively or negatively, for less than 50 ns. Impulse transients are caused by electrostatic discharge (ESD), lightning, poor grounding, inductive load switching and utility fault clearing. The consequences of a network subjected to impulse transients range from loss or corruption of data to physical damage of equipment. As such, adequate protection is critical for sensitive electronic equipment.

Surge protective devices (SPD) and transient voltage surge suppressors (TVSS) are commonly used in the industry to protect equipment against impulsive transients. ESD is a common effect that, although it may not be dangerous for humans, can destroy sensitive electronic or computer systems. In data centres or electronic manufacturing facilities, for example, the environmental conditions are controlled to adjust the moisture levels to within 40-55% with the aim to reduce the potential for ESD.

Oscillatory transients are sudden changes which occur in the steady-state condition of voltage and/or current, oscillating at the natural system frequency. This is opposed to impulsive transient which introduces short duration peaks and are non-oscillatory. Oscillatory transients normally decay to zero within a cycle, which is relatively slow compared to impulsive transients. These phenomena are caused by the switching in and out inductive or capacitive loads, such as a motor or capacitor bank respectively. They can be disruptive to electronic equipment and can cause overvoltage tripping in variable speed drives (VSD) due to the slow transient causing a rise in the DC link voltage. One solution to this problem is the introduction of series line reactors or DC chokes to dampen the oscillatory transients.

Sag/undervoltage

Sag is a phenomenon observed due to voltage drop, for a given frequency, for 0.5 to 50 cycles. On the contrary, an interruption, which can be instantaneous, momentary, temporary or sustained depending on its duration, is defined as a complete loss of supply voltage or load current. Sags are typically caused by system faults or switching on loads with heavy start-up currents. For instance, a motor can draw up to six times its rated current during start-up, creating a large and sudden electrical load that the system needs to supply. As a result, it draws a large inrush current from the network leading to a voltage drop to the rest of the circuit affecting equipment on the same bus.

Dedicated circuits for large loads may be the most effective solution for performance but are not necessarily practical or cost effective. As alternatives, other solutions such as reduced-voltage starters, solid-state soft starters and VSD may be viable options depending on the application. This is because the speed of the motor is varied according to the load to reduce the large inrush upon start-up. Sags can go undetected until equipment damage, data corruption or process and equipment malfunctions are noticed.

An undervoltage is defined as a 90% reduction in voltage magnitude for more than one minute (ie, >50 cycles). It is the long-term result of problems that lead to sags initially. Due to the sustained duration of reduced voltage, undervoltages can create overheating in motors and lead to failure of non-linear loads. Consequently, there are fewer risks of undervoltages if a system is protected against sags.

Swell/overvoltage

The effect of swell is the opposite of sag. It is a result of an increase in AC voltage, for a given frequency, for 0.5 to 50 cycles. Swells can be caused by high-impedance neutral connections, sudden load reductions and a single-phase fault on a three-phase system. The effects of a swell can be data errors or corruption, lights flickering, degradation of electrical contacts and insulation and damage to sensitive electronics. Similarly, swells can be hard to identify, but installing a UPS or powerline conditioner could provide a solution, by allowing visibility of these events.

An overvoltage can be seen as an extended swell. This can occur in networks where the tap settings on the supply transformer may need to be readjusted due to the load reduction for seasonality reasons. Some equipment may be able to operate on an overvoltage as long as their maximum limit and duration are not exceeded. Some common detrimental effects are the extra heat generated, electrical stress on equipment and nuisance-tripping downstream.

Waveform distortion

DC offset: is the effect seen when a direct current (DC) is induced into an AC system. This is often due to failure of rectifiers within the AC to DC conversion stage. Consequently, the DC component adds unwanted current to devices that may already be operating at their full load. This will lead to overheating and saturation of the transformers which prevent the unit from operating correctly to deliver its full power to the load. As a collateral effect, this creates further instability to electronic equipment. The solution to avoid such damage is to replace the faulty equipment.

Harmonics: a harmonic is a signal (or wave) whose frequency is an integral multiple of the reference frequency, in electrical terms, the fundamental frequency. Harmonic distortion is the corruption of the fundamental 50 Hz sine wave at multiple integer frequencies. Non-linear loads, such as power electronics typically present in switch-mode power supplies (SMPS), VSD, inverters and UPS are the most common harmonic generators. The symptoms of harmonic distortions include overheated transformers, neutral conductors, tripping of circuit breakers and loss of synchronisation on timing circuits requiring zero crossing trigger. Harmonics can be very damaging to equipment, especially destructive to capacitor banks if not protected by detuning reactors, as well as causing transformers to saturate.

The four most common mitigation methods applicable to reducing or eliminating harmonics in an electrical network today are: AC line reactors or DC chokes; passive filters; active front end rectifiers; active harmonic filters (AHF).

While active harmonic filtering solutions provide some very unique added value, every method has its place and adds its own complexity. Other less-common solutions such as multi-pulse or K-rated transformers exist.

Thanks to advances in technology, new IT tools are now equipped with more efficient SMPS and power factor corrected power supplies operating as linear, non-harmonic loads. These do not contribute to harmonic pollution, but do, however, introduce the challenge of having an overall leading power factor. Fortunately, some active harmonic filtering solutions are capable of correcting this problem.

Interharmonics: these are a type of waveform distortion created as the result of the inter-modulation of the fundamental and harmonic components of the system with any other frequency components. They are introduced by loads such as static frequency converters, induction motors, arcing devices, cyclo-converters and sub-synchronous converter cascades - essentially, all loads that do not pulsate synchronously with the fundamental frequency.

Cycloconverters, used to control large linear motors in rolling mill, cement and mining are the major contributors of interharmonics problems. The result is a supply voltage transformed into an AC voltage of a frequency lower or higher than that of the supply frequency. The impacts of interharmonic frequency components greater than the fundamental frequency are overheating, just like with harmonics, flicker, torsional oscillations, overload of conventional series tuned filters and communications interference.

Passive and active filters are the most common techniques to mitigate interharmonics. Once more, while passive filters are relatively inexpensive compared to active filters, this method is insufficient since the series tuned filter causes a parallel resonance when interharmonics are present and need to cover a wide range of frequencies. As such, passive filters must be equipped with damping resistors to minimise that effect as well as be a multistage filter which add to the complexity.

Notching: this has a similar effect as impulse transient. However, since it is a periodic disturbance caused by electronic devices under normal operation, notching is considered a waveform distortion problem. The consequences of notching may be seen as being less severe than the ones above but can however lead to system halts, data loss and data transmission losses. UPS and filtering solutions are viable solutions to rectify this problem.

Noise: this is typically used to refer to background or stray signals. Noise is unwanted voltage or current signals superimposed on the power system waveforms. Possible sources are power electronic devices, control circuits, arc welders, SMPS and radio transmitters. IT and telecommunications equipment and computer systems are the most susceptible to noise which can cause data errors, equipment malfunction, prolonged equipment failure and affect video data. Noise filters, isolation transformers and proper grounding and shielding techniques are some of the many approaches used to reduce noise.

Voltage fluctuation

Voltage fluctuation is fundamentally different from the rest of the waveform anomalies in that the variation in the voltage waveform occurs in an orderly manner. The series of random voltage changes are relatively small, notably 95 to 105% of the nominal voltage at a low frequency.

Whenever significant current variations are present, voltage fluctuations are introduced. Arc furnaces are the major contributors of this problem on distribution and transmission networks. The effects are flicker and possible damage to sensitive electronic devices. UPS and some dynamic VAR compensation systems have the capabilities to resolve this problem.

Frequency variation

Extensive investment is made to maintain the electricity grid within its frequency tolerance to avoid any possible equipment failure on the customer sites. However, when sites or plants run on standby generators or poor power infrastructure, frequency variation is a common problem especially if the generator is not properly sized. Such a problem mainly affects the motors and sensitive devices that rely on steady regular power cycles over time. Failure to maintain the frequency within its limits can cause a motor to accelerate or run slower to match the frequency of the supply. This could potentially lead to mechanical damage, cause the motor to run inefficiently and lead to overheating as well as degradation of the motor.

Voltage imbalance

Voltage imbalance is regarded as a power quality problem of significant concern at the electricity distribution level. This type of power quality disturbance is caused by an unequal distribution of loads amongst the three-phase power distribution system. Although the voltages are quite well balanced at the generator and transmission levels, the voltages at the utilisation level can become imbalanced. This is due to the unequal system impedances and the unequal distribution of single-phase loads. An excessive level of voltage imbalance can have serious impacts on mains-connected induction motors. The level of current imbalance that is present is several times the level of voltage imbalance.

Due to voltage imbalance, the line currents can lead to excessive losses in the stator and rotor that may cause protection systems to operate causing loss of production. Greater imbalances may cause excessive heat to motor components and the intermittent failure of motor controller. Correcting voltage imbalance involves rebalancing the loads physically. Alternatively, some dynamic VAR compensators have the capabilities to perform load balancing depending on the application.

Power quality solutions critical in all situations

The need for power quality solutions is everywhere, considering the widespread use of critical and sensitive electronics which require both clean and reliable power. Similarly, mission-critical applications such as data centres and hospitals cannot afford to be subjected to even the smallest electrical fluctuations given their dependence on critical power. In industrial applications, downtime and business performance rely on power reliability and availability. Power quality aims to bring together a synergy that ensures clean, reliable and dependable power.

*Larue is the product manager for power factor correction and harmonic filtering solutions. He has been with the company for four years with prior experience in deploying energy management and SCADA systems. He is responsible for the growth and market penetration of the technology. Coming from a power systems background where he has been involved in power generation and developing excitation control systems for microhydroelectric generators, he shares a passion for electrical machines and energy efficiency. He holds a degree in Electrical Engineering from the University of Technology Sydney (UTS), specialising in power system engineering.