The application of distribution system capacitor banks has long been accepted as a necessary step in the design of distribution feeders. Design considerations often include traditional factors such as voltage support, power factor, and released capacity. However, as customer systems evolve through the use of power electronics, the distribution system design of the future will include power quality as a consideration. The term “power quality” has many different meanings, perhaps as many as attempt to describe its impact on system operation. The electric utility may describe power quality as reliability and quote statistics stating that the system is 99.95% reliable. The equipment manufacturer often defines power quality as the characteristics of the power supply, which may vary drastically for different vendors. However, the customer is the party ultimately affected by power quality related problems and the best definition should include his perspective. Considering each of these factors, the following definition is often used. Power quality problem: “Any power problem manifested in voltage, current, or frequency deviations that results in the failure or misoperation of customer equipment.”
There are many events that can cause a power quality problem. Analysis of these events is often difficult due to the fact that the cause of the event may be related to a switching operation within the facility to a power system fault hundreds of miles away. Here, I’ll describe some of the most common power quality problems associated with the application of distribution system capacitor banks.
The frequent switching of distribution capacitor banks coupled with the increasing application of sensitive customer equipment has led to a heightened awareness of several important events:
- Magnification of capacitor switching transients
- Nuisance tripping of adjustable-speed drives
These concerns have become particularly important as utilities institute higher power-factor penalties, thereby encourag¬ing customers to install power-factor correction capacitors. Non-traditional customer loads, such as adjustable-speed drives, are being applied in increasing numbers due to the improved efficiencies and flexibility that can be achieved. This type of load can be very sensitive to the transient volt¬ages produced during capacitor switching. The most common methods for controlling these transients include the application of switching control (synchronous closing, preinsertion inductors/resistors) and series inductances, often referred to as chokes.
In addition, these loads often generate excessive harmonic currents that can result in unacceptable voltage distortion levels on both the industrial and utility distribution systems. Capacitors and the system short-circuit impedance combine to create resonances that can magnify harmonic levels to well above accepted limits. The most common solution to harmonic problems involves the application of harmonic filters.
Distribution capacitor switching
Capacitor switching is considered to be a normal event on a utility system, and the transients associated with these operations are generally not a problem for utility equip¬ment. However, the transients can be magnified in a customer facility – if the customer has low-voltage, power-factor correction capacitors.
In addition, nuisance tripping of adjustable-speed drives can occur, even if the customer does not have capacitors. Because capacitor voltage cannot change instantaneously, energisation of a capacitor bank results in an immediate drop in system voltage toward zero, followed by a fast voltage recovery (overshoot) and finally an oscillating transient voltage superimposed on the 50-Hz fundamental waveform. The peak voltage magnitude depends on the instantaneous system voltage at the moment of energisation, and can reach 2.0 times the normal system peak voltage (per-unit) under worst-case conditions. The magnitude is usually less than this due to system loads and damping (resistive elements). Typical distribution system overvoltage levels range from 1.1 to 1.6 per unit. Transient frequencies due to utility distribution capacitor switching usually fall in the range of 300 to 1000 Hz. Transient overvoltages are usually not of concern to the utility, since peak magnitudes are just below the level in which utility surge protection, such as arresters, begins to operate.
Because of the relatively low frequency, these transients will pass through step-down transformers to customer loads.
Secondary overvoltages can cause voltage magnification or nuisance tripping of adjustable-speed drives. Power quality symptoms related to distribution capacitor switching include the following: customer equipment damage or failure (due to excessive overvoltage), adjustable-speed drive or other process equipment shutdown (due to DC bus overvoltage), TVSS failure, and computer network problems.
Voltage magnification occurs when the transient oscillation, initiated by the energisation of the distribution capacitor bank, excites a series resonance formed by the low voltage system. The result is a higher overvoltage at the low-voltage bus. The worst magnified transient occurs when the following conditions are met:
The size of the switched capacitor bank is significantly larger (>10) than the low voltage power factor correction bank (i.e., 3 MVAr versus 200 kVAr = 15).
The energising frequency (f 1) is close to the series resonant frequency formed by the stepdown transformer and the power factor correction capacitor bank (f 2) (i.e.,f 1 = 490 Hz and f2 = 670 Hz). There is relatively little damping (resistive) provided by the low voltage load (typical industrial plant configuration – primarily motor load).
Simulations and in-plant measurements have indicated that magnified transients between 2.0 to 4.0 p.u. are possible over a wide range of low-voltage capacitor sizes.
Typically, the transient overvoltages will simply damage low-energy protective devices (MOVs) or cause a nuisance trip of a power electronic device.
However, I’m aware of several cases when complete failure of customer equipment (single device) occurred.
Nuisance tripping of ASDs
Nuisance tripping refers to the undesired shutdown of an adjustable-speed drive (or other power electronic process device) – due to the transient overvoltage on the DC bus. Very often, this overvoltage is caused by distribution capacitor bank energisation. Considering the fact that many distribution banks are time clock controlled, it is easy to see how this event can occur on a regular basis, thereby causing numerous process interruptions for the plant.
The nuisance tripping event comprises an overvoltage trip due to a DC bus over voltage on voltage-source inverter drives [Pulse-Width Modulated (PWM)]. Typically, for the protection of the DC capacitor and invertor components, the DC bus voltage is monitored and the drive tripped when it exceeds a preset level. This level is typically around 760 V (for 480 V applications), which is only 117% of the nominal DC voltage. The potential for nuisance tripping primarily depends on the switched capacitor bank size, overvoltage controls for the switched bank, the DC bus capacitor size, and the inductance between the 2 capacitors. Importantly, nuisance tripping can occur even if the customer does not have power factor correction capacitors.
Solution to capacitor switching problems
Customer power quality problems, caused by distribution system capacitor switching, can be controlled using a number of different methods. The first step is identifying the problem, then the utility and customer need to work together to determine the best engineering and cost effective solution possible. Possible solutions include the following:
The capacitor energising transient can be controlled using preinsertion resistors/inductors or synchronous closing control (technology available for distribution system applications)
High-energy MOV arresters can be applied to the low-voltage system. The arresters should limit the overvoltage to approximately 1.8 p.u. The energy rating of the arrester must be evaluated (several 1000 joules)
Harmonic filters can be used for power factor correction. The tuned filter changes the response of the circuit (f 2) and usually reduces the overvoltage level seen at the 480¬V bus. More protection can be achieved–by placing MOVs across the capacitors.
Series inductors (chokes) can be installed on the drives to reduce the probability of nuisance tripping. Chokes for this application are commercially available and a size of 3% (of drive rating) is usually sufficient. Isolation transformers with similar ratings (impedance) will also provide protection.
A fundamental objective of electric utility operations is to supply each electric customer with a fairly constant sinusoidal voltage. The voltage signal at any point within the power system is ideally a constant sinusoidal signal that repeats at a rate of precisely 50 times per second, or 50 Hz. Although not perfect, the voltage signal produced by power system generators approximates a perfect sinusoid with a rather high degree of accuracy. Almost all load equipment connected to the electric power system has been designed to operate from a sinusoidal voltage source.
Some load equipment, however, don’t draw a sinusoidal current from a sinusoidal voltage source. This equipment is said to be nonlinear; that is, the relationship between voltage and current at every instant of time is not constant. Because power systems are voltage-regulated, current drawn by any load doesn’t affect neighboring devices – since it is voltage, not current, that they share. Non-sinusoidal currents by themselves are not a concern to parallely connected loads.
Sources of harmonics
The 3 classifications of harmonic sources – saturable devices, arcing devices, and power electronics – all present non-linear voltage/current characteristics to the power system. Arcing and saturable devices are passive, and the non-linearities are a result of the physical characteristics of the electric arc and of the iron core. In power electronics equipment, semiconductor device switching that occurs within 1 cycle of the power system fundamental frequency, is responsible for the non-linear characteristic. For the most part, these devices inject harmonic current components into the system. The level of harmonic voltage distortion developed is a function of both the system impedance and the amount of current injected.
Harmonic analysis methodology
Effective solution of harmonic distortion problems requires a comprehensive approach that includes site surveys, harmonic measurements, and computer simulations. The follow¬ing is a general procedure for harmonic analysis. Preliminary assessment: Simple calculations can be used to determine the system resonant frequencies. Existence of resonances (high or low impedances) near characteristic harmonic frequencies of loads, which have been identified as harmonic sources are an early indication of potential trouble.
This simple relationship provides an excellent 1st check on whether or not harmonics are likely to be a problem. Almost all harmonic distortion problems occur – when this parallel resonance moves close to the fifth or seventh harmonic (h = 5 or 7), since these are typically the largest harmonic current components in customer nonlinear loads (like ASDs).
- Harmonic measurements: The purpose of measure¬ments is to characterise the behaviour of harmonic sources, and to provide preliminary data on the severity of the distortion problem. Measurement of data is extremely valuable for validating detailed computer models and hand calculations. A preliminary indication of harmonic problems can be obtained from new meters that indicate the crest factor of the waveform or from instruments that provide information pertaining to the ratio of the total RMS to the fundamental RMS. Measurements on the distribution system are often more difficult, due to transducer requirements, than in-plant measurements. Existing metering class CTs and PTs may be used to obtain harmonic data.
- Computer simulation, calculations: Once a representation of the significant components in the power system has been developed and verified as accurate by comparison to measurement data, a wide range of conditions can be investigated. System configurations that create resonances can be identified, and alternative configurations can be examined.
Frequency scan simulations (impedance versus frequency characteristic) identify system configurations that can cause harmonic problems due to resonance conditions – and harmonic distortion simulations are completed to evaluate the effectiveness of harmonic filters or other harmonic reduction techniques.
Solution Development: Harmonic voltage levels determined through both simulation and measurement are evaluated against recommended limits. If harmonic voltage distortion levels are not within acceptable limits, the frequency response characteristics of the facility or system can be altered by changing capacitor sizes and/or locations, or by installing harmonic filters.
Impact of harmonics on power factor
The traditional method of power-factor correction, both on the power system and within customer facilities, has been the application of shunt capacitor banks. This is based on the fact that most loads on the system draw a lagging current (partially inductive) at the fundamental frequency. Capacitors draw a leading current at the fundamental frequency and, therefore, can compensate for the current drawn by inductive loads. These characteristics of leading and lagging current are based on the assumption that loads on the system have linear voltage–current characteristics, and that harmonic distortion of the voltage and current is not significant. With these assumptions, the power factor is equal to the Displacement Power Factor (DPF). Calculation of the displacement power factor is completed using the traditional power factor triangle.
Harmonic distortion in the voltage and/or current caused by non-linear loads on the system changes the way power factor must be calculated. True Power Factor (TPF) is defined as the ratio of real power to the total volt-amperes in the circuit. This is a measure of the efficiency with which the real power is being used. Since capacitors only provide reactive power (VARs) at the fundamental frequency, they cannot correct true power factor – when there are harmonics present. In fact, capacitors can make true power factor worse by creating resonance conditions, which magnify the harmonic distortion in the voltage and current.
Displacement power factor is still very important to most industrial customers – because utility billing for power factor penalties is almost universally based on displacement power factor.
Solutions to harmonic problems
Problems with harmonics often show up at capacitor banks first. The main reason for this is that capacitors form the resonant circuit that magnifies harmonic current levels causing high voltage distortion levels. The highest voltage distortion in the resonant circuit occurs at the capacitor bank. This results in high capacitor currents at harmonic frequencies and overheating due to excessive RMS current. This is one common failure mode. Fuse blowing can also occur due to the high harmonic currents in the capacitor bank.
The RMS current through a capacitor can be increased substantially by harmonics even when little voltage distortion exists – because of the low capacitor impedance at harmonic frequencies. If voltage distortion is significant, insulation failure due to excessive peak voltage values can occur, since the peak value of the voltage can be as high as the arithmetic sum of all the individual harmonic voltages.
In the event that distortion levels are not within acceptable limits, the frequency response characteristics of the system can be altered by changing capacitor sizes or locations, by changing source characteristics, or by designing harmonic filters. Filters are the most common solution, because a filter can provide reactive power support at the fundamental frequency – and a low impedance path for one or more harmonic current components to flow. The filter components must be specifically designed to withstand the harmonic components along with the fundamental frequency voltages and currents. A common method for filter design includes the following.
- Apply one single-tuned shunt filter first, designed for the lowest generated harmonic (typically 5th). In general, it is advisable to use capacitors with a higher voltage rating than the system
- Determine the voltage distortion level at the bus. The commonly applied limit of 5% (Total Harmonic Distortion – THD) was introduced in IEEE Standard 519-1981
- If required, determine if the harmonic current levels
- Vary the filter elements according to the specified tolerances & check its effectiveness
- Check the frequency response characteristic to verify that the newly created parallel resonance is not close to a generated harmonic frequency (i.e., 7th harmonic filter may create a new 5th harmonic resonance).
- IEEE Standard 519-1992 is a standard that addresses the need for limiting the harmonic current a customer injects onto the utility system. It also protects the customer by specifying maximum harmonic voltage distortion levels that utilities can supply. The standard should be used for guidance in the design of power systems with nonlinear loads.
- Customer power quality problems can be controlled using a number of different methods. The first step is identifying the problem. Then the utility and customer need to determine the best engineering and cost effective solution possible.
- Devices being applied to the power system these days are more susceptible to power quality variations than equipment applied in the past.
- The increasing emphasis on overall power system efficiency is causing a continued growth in the application of shunt capacitor banks. This is occurring within customer facilities, as well as on the power system.
- Magnification of capacitor switching transients may be the most important concern due to the fact that the transient overvoltages can be very high – and the energy levels associated with these transients can cause equipment failure.
- The possibility of nuisance tripping of adjustable-speed drives, due to utility capacitor switching, can be greatly reduced with the application of an input choke. Typically, a value of 3% is sufficient. Harmonic distortion levels may be reduced with the application of harmonic filters. Harmonic standards, like IEEE Standard 519-1992, should be used as a benchmark for the evaluation of the filter’s performance.
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