There are a number of devices available to control harmonic distortion. They can be as simple as a capacitor bank or a line reactor, or as complex as an active filter.
A simple mitigation action such as adding, resizing, or relocating a shunt capacitor bank can effectively modify an unfavorable system frequency response, and thus bring the harmonic distortion to an acceptable level.
Similarly, a reactor can perform the same function by detuning the system off harmful resonances. The effectiveness of such simple solutions in controlling harmonic distortion should be explored prior to considering a more complex device. The following material first discusses the effectiveness of a simple in line reactor or choke or zig-zag transformers, in mitigating harmonic distortion. Then, two general classes of harmonic filters, i.e., passive and active filters. The former are based on passive elements, while the latter are based on power electronic devices.
Device # 1. In-line Reactors or Chokes:
A simple, but often successful, method to control harmonic distortion generated by adjustable-speed drives involves a relatively small reactor, or choke, inserted at the line input side of the drive. This is particularly effective for PWM-type drives.
The inductance slows the rate at which the capacitor on the dc bus can be charged and forces the drive to draw current over a longer time period. The net effect is a lower-magnitude current with much less harmonic content while still delivering the same energy.
A typical 3 percent input choke can reduce the harmonic current distortion for a PWM-type drive from approximately 80 to 40 percent. Additional harmonic reduction is rather limited when the choke size is increased beyond 3 percent. The choke size is computed on the drive kVA base.
Figure 7.8 compares the effectiveness of a 3 percent choke in reducing harmonic current distortion to the condition without a choke for various ASD sizes (ASD sizes are normalized to the service transformer kVA).
Representative waveforms for each end of the range are shown. The larger waveform is without the choke. As is clear from Fig. 7.8, a substantial improvement is achieved by inserting a choke in the ASD line. The current THD drops from the 80 to 120 percent range down to approximately 40 percent. Better reduction is obtained when the size of the ASD is significantly smaller than the service transformer. When the size of the ASD is 5 percent of the transformer, the current THD drops from 125 to 40 percent.
It is also important to note that there are other advantages of the choke in ASD applications. The effect of slowing the dc capacitor charging rate also makes the choke very effective in blocking some high-frequency transients. This helps avoid nuisance drive tripping during capacitor energization operations on the utility system. Isolation transformers can provide the same benefit as a choke but may be more costly. However, isolation transformers with multiple drives offer the advantage of creating effective 12-pulse operation.
A 12-pulse configuration can be achieved by supplying one drive through a delta-wye connected transformer, and another drive through a delta-delta connected transformer. Figure 7.9 shows the current waveforms for two separate six-pulse ASDs. When the two waveforms are added together on the primary, the resulting waveform injected onto the utility system has much lower distortion, primarily because the fifth and seventh harmonics are cancelled out. These two harmonics are responsible for most of the distortion for six-pulse drives.
Device # 2. Zig-Zag Transformers:
Zig-zag transformers are often applied in commercial facilities to control zero-sequence harmonic components. A zig-zag transformer acts like a filter to the zero-sequence current by offering a low-impedance path to neutral. This reduces the amount of current that flows in the neutral back toward the supply by providing a shorter path for the current. To be effective, the transformer must be located near the load on the circuit that is being protected.
The two most important problems in commercial facilities are overloaded neutral conductors and transformer heating. Both of these problems can be solved with proper zig-zag transformer placement. Some new commercial buildings include zig-zag transformers on the 480/208 V supply transformer secondary’s to prevent transformer overheating.
A zig-zag transformer located at the supply transformer secondary does not provide any benefit for neutral conductors supplying the loads. Typical results with a zig-zag transformer show that it can shunt about 50 percent of the third-harmonic current away from the main circuit neutral conductors. Thus, the zig-zag transformer can almost always reduce neutral currents due to zero-sequence harmonics to acceptable levels. The largest zero-sequence harmonic will nearly always be the third harmonic in office buildings with many computers and related equipment.
Zig-zag transformers are an excellent choice for existing facilities where neutral conductor problems and possible transformer heating are concerns, assuming that there is a convenient place to install the transformer between the neutral circuit of concern and the actual loads. In new facilities, it may be better to simply design the circuits with sufficient current-carrying capacity in the neutrals and with higher-capacity transformers.
Device # 3. Passive Filters:
Passive filters are inductance, capacitance, and resistance elements configured and tuned to control harmonics. They are commonly used and are relatively inexpensive compared with other means for eliminating harmonic distortion. However, they have the disadvantage of potentially interacting adversely with the power system, and it is important to check all possible system interactions when they are designed. They are employed either to shunt the harmonic currents off the line or to block their flow between parts of the system by tuning the elements to create a resonance at a selected frequency.
Shunt Passive Filters:
The most common type of passive filter is the single- tuned “notch” filter. This is the most economical type and is frequently sufficient for the application. The notch filter is series-tuned to present a low impedance to a particular harmonic current and is connected in shunt with the power system. Thus, harmonic currents are diverted from their normal flow path on the line through the filter.
Notch filters can provide power factor correction in addition to harmonic suppression. In fact, power factor correction capacitors may be used to make notch filters.
An example of a common 480-V filter arrangement is illustrated in Fig. 7.11. The figure shows a delta-connected low-voltage capacitor bank converted into a filter by adding an inductance in series with the phases. In this case, the notch harmonic h notch is related to the fundamental frequency reactances by-
Note that in this case is the reactance of one leg of the delta rather than the equivalent line-to-neutral capacitive reactance. If phase-to-phase voltage and three- phase kvar are used to compute XC, the factor 3 would be omitted.
One important side effect of this type of filter is that it creates a sharp parallel resonance point at a frequency below the notch frequency [Fig. 7.11(c)]. This resonant frequency must be safely away from any significant harmonic or other frequency component that may be produced by the load. Filters are commonly tuned slightly lower than the harmonic to be filtered to provide a margin of safety in case there is some change in system parameters that would raise the notch frequency.
If they were tuned exactly to the harmonic, changes in either capacitance or inductance with temperature or failure might shift the parallel resonance higher into the harmonic being filtered. This could present a situation worse than one without a filter because the resonance is generally very sharp. To avoid problems with this resonance, filters are added to the system starting with the lowest significant harmonic found in the system.
For example, installing a seventh-harmonic filter usually requires that a fifth-harmonic filter also be installed. The new parallel resonance with a seventh- harmonic filter alone is often very near the fifth, which is generally disastrous. The filter configuration of [Fig. 7.11(a)] does not admit zero-sequence currents because the capacitor is delta-connected, which makes it ineffective for filtering zero-sequence triplen harmonics. Because 480-V capacitors are usually delta-configured, other solutions must be employed when it becomes necessary to control zero-sequence third harmonic currents in many industrial and commercial building facilities.
In contrast, capacitors on utility distribution systems are more commonly wye- connected. This gives the option of controlling the zero-sequence triplen harmonics simply by changing the neutral connection.
Placing a reactor in the neutral of a capacitor is a common way to force the bank to filter only zero-sequence harmonics. This technique is often employed to eliminate telephone interference. A tapped reactor is installed in the neutral and the tap adjusted to minimize the telephone.
Passive filters should always be placed on a bus where the short-circuit reactance XSC can be expected to remain constant. While the notch frequency will remain fixed, the parallel resonance will move with system impedance. For example, the parallel resonant frequency for running with standby generation by itself is likely to be much lower than when interconnected with the utility because the generator impedance is much higher than the utility impedance. This could magnify a harmonic that is normally insignificant. Thus, filters are often removed for operation with standby generation.
Also, filters must be designed with the capacity of the bus in mind. The temptation is to size the current-carrying capability based solely on the load that is producing the harmonic. However, a small amount of background voltage distortion on a very strong bus may impose excessive duty on the filter.
Series Passive Filters:
Unlike a notch filter which is connected in shunt with the power system, a series passive filter is connected in series with the load. The inductance and capacitance are connected in parallel and are tuned to provide a high impedance at a selected harmonic frequency. The high impedance then blocks the flow of harmonic currents at the tuned frequency only. At fundamental frequency, the filter would be designed to yield a low impedance, thereby allowing the fundamental current to follow with only minor additional impedance and losses.
Series filters are used to block a single harmonic current (such as the third harmonic) and are especially useful in a single-phase circuit where it is not possible to take advantage of zero-sequence characteristics. The use of the series filters is limited in blocking multiple harmonic currents. Each harmonic current requires a series filter tuned to that harmonic.
This arrangement can create significant losses at the fundamental frequency. Furthermore, like other series components in power systems, a series filter must be designed to carry a full rated load current and must have an overcurrent protection scheme. Thus, series filters are much less commonly applied than shunt filters.
Low-Pass Broadband Filters:
Multiple stages of both series and shunt filters are often required in practical applications. For example, in shunt filter applications, a filter for blocking a seventh-harmonic frequency would typically require two stages of shunt filters, the seventh-harmonic filter itself and the lower fifth-harmonic filter.
Similarly, in series filter applications, each frequency requires a series filter of its own; thus, multiple stages of filters are needed to block multiple frequencies. In numerous power system conditions, harmonics can appear not only in a single frequency but can spread over a wide range of frequencies.
A six-pulse converter generates characteristic harmonics of 5th, 7th, 11th, 13th, etc. Electronic power converters can essentially generate time-varying inter-harmonics covering a wide range of frequencies.
Designing a shunt or series filter to eliminate or reduce these widespread and time-varying harmonics would be very difficult using shunt filters. Therefore, an alternative harmonic filter must be devised.
A low-pass broadband filter is an ideal application to block multiple or widespread harmonic frequencies. Current with frequency components below the filter cutoff frequency can pass; however, current with frequency components above the cutoff frequency is filtered out. Since this type of low-pass filter is typically designed to achieve a low cutoff frequency, it is then called a low-pass broadband filter.
In distribution system applications, the effect of low-pass broadband filters can be obtained by installing a capacitor bank on the low-voltage side of a transformer. The size of the capacitor bank would have to be so selected to provide the desired cutoff frequency when combined with the transformer leakage inductance and the system impedance.
It is then capable of preventing harmonics above the cutoff frequency from penetrating the high-voltage side of the transformer. Since the cutoff frequency can be sometimes quite low, the size of the capacitor bank may be fairly large. This will result in a significant voltage rise. Should the voltage remain high, a voltage regulator or transformer load tap changer (LTC) must be used to lower the voltage to an acceptable level.
In a substation serving multiple feeders, a line reactor and voltage regulator can be installed at the beginning of the feeder to isolate the portion of the system subject to high voltage. This arrangement will allow voltage levels at other feeders to be maintained at normal values.
The combination of the transformer leakage inductance, the line reactor, the voltage regulator, and the capacitor bank yields the desired cutoff frequency. In industrial system applications, commercial low-pass broadband filters have been used to prevent harmonics produced by nonlinear loads from entering the ac system.
A line reactor installed in series with the main ac line is used to provide an electrical separation between the ac system and the nonlinear load. A capacitor bank is installed in shunt to form a low-pass broadband filter configuration. Since the presence of the capacitor bank increases the voltage at the input of the ASD, a buck transformer is needed to bring the voltage at the line reactor output down to a level where the voltage at the capacitor is acceptable. The optimum performance of a low-pass broadband filter in ASD applications is obtained when there is no series inductor between the filter capacitor banks and the ASD dc bus capacitor.
Any impedance in between reduces the charging capability of the dc bus capacitor since it is charged from the filter capacitor. The cutoff frequency for a low- pass broadband filter for ASD applications is typically designed at a low harmonic frequency, such as at 100 to 200 Hz on a 60-Hz system. With this low tuning frequency, the filter is unlikely to excite any undesired resonance with the rest of the system and can filter out much of the harmonic currents.
In ASD applications the filter can generally reduce the overall current harmonic distortion from the 90 to 100 percent range down to the 9 to 12 percent range under rated load conditions. This performance is certainly much better than a simple ac line choke, which only reduces the overall current distortion down to the 30 to 40 percent range. However, the cost of an ac line choke is less than a low-pass broadband filter.
C filters are an alternative to low-pass broadband filters in reducing multiple harmonic frequencies simultaneously in industrial and utility systems. They can attenuate a wide range of steady-state and time-varying harmonic and inter-harmonic frequencies generated by electronic converters, induction furnaces, cycloconverters, and the like.
The configuration of a C filter is nearly identical to that of the second-order high-pass filter. The main distinction between the two configurations is that the C filter possesses an auxiliary capacitor Ca in series with the inductor Lm.
The auxiliary capacitor Ca is sized in such a way that its capacitive reactance cancels out Lm at the fundamental frequency, bypassing the damping resistance R. For this reason, the losses associated with R are practically eliminated, allowing a C filter to be tuned to a low frequency.
The impedance frequency response of a C filter is also essentially identical to that of a second-order high-pass filter. At high-order harmonic frequencies, the reactance of Ca is small, while that of Lm is large. Therefore, the impedance of the series Lm and Ca branch is dominated by the reactance of Lm. The high-frequency responses of the C filter and second-order high-pass filters are similar.
Figure 7.17 shows an equivalent circuit for deriving filter components R, Ca, and Lm. The short-circuit reactance is denoted as XS.
Filter components can be computed as follows:
Device # 4. Active Filters:
Active filters are relatively new types of devices for eliminating harmonics. They are based on sophisticated power electronics and are much more expensive than passive filters. However, they have the distinct advantage that they do not resonate with the system. Active filters can work independently of the system impedance characteristics.
Thus, they can be used in very difficult circumstances where passive filters cannot operate successfully because of parallel resonance problems. They can also address more than one harmonic at a time and combat other power quality problems such as flicker. They are particularly useful for large, distorting loads fed from relatively weak points on the power system. The basic idea is to replace the portion of the sine wave that is missing in the current in a nonlinear load.
An electronic control monitors the line voltage and/or current, switching the power electronics very precisely to track the load current or voltage and force it to be sinusoidal. As shown, there are two fundamental approaches: one that uses an inductor to store current to be injected into the system at the appropriate instant and one that uses a capacitor. Therefore, while the load current is distorted to the extent demanded by the nonlinear load, the current seen by the system is much more sinusoidal.
Active filters can typically be programmed to correct for the power factor as well as harmonics.