As part of the control of an AC induction motor, the adjustable speed driver receives three-phase AC sinusoidal power and converts it into DC power. In this process, the adjustable speed driver itself generates harmonic currents. Regarding the generation and suppression of harmonics in AC drives, remember: 1) Regardless of whether the power converter is composed of diodes, silicon controlled rectifiers, or insulated-gate bipolar transistors with inertial diodes, they will generate harmonics when switched on and off. 2) Harmonic currents can cause voltage distortion on the power converter's power supply lines. 3) Some types of power converters can generate wire notches, which can also cause voltage distortion. Compared to DC motor drives, AC motor drives have fewer harmonic problems. Poor design can cause many problems for power distribution equipment and other devices connected to the power distribution system. Finding the Root Cause of the Problem As the number of drives in automated systems increases, the misconception that drives are the sole cause of harmonic problems is also growing. In fact, any device that converts AC to DC will generate harmonics, including most low-level factory equipment and office machines (e.g., computer power supplies, telephone chargers, and copiers), and even fluorescent lamp ballasts can cause harmonic distortion. Therefore, it is crucial to analyze all potentially problematic electrical loads before indiscriminately installing filters on every drive in the equipment. In reality, harmonic-related problems caused by AC drives are very rare. Harmonic problems mainly manifest as overheating of transformers and drive feeders because designers haven't considered other harmonic currents, causing fuses to blow prematurely and circuit breakers to trip for the same reason. [align=center] Figure 1: Basic drive current waveform shows approximately 110% distortion. Figure 2: Conductor reactors reduce current distortion by approximately 50%, but cause DC bus voltage attenuation. Harmonic currents on power lines can cause voltage distortion, leading to problems with other devices connected to that line, such as dimming lights or overheating motors. Some devices, such as DC drives or unfiltered inverters, generate notches in addition to harmonics. These voltage notches can easily cause other parts of the equipment to malfunction. This creates the illusion that parts that are actually functioning correctly are causing problems, while the parts that are actually causing problems appear to be working properly. When designing a new system or increasing the number of adjustable drives in an existing power system, designers need to understand how much harmonics each part of the equipment contributes to the system and what the possible effects of these harmonics are. The industry standard IEEE 519-1992, "IEEE Operating Procedures Recommendations and Requirements for Harmonic Control in Power Systems," provides guidance on reducing harmonic-related equipment problems. Part 10 contains two key tables covering "Operating Procedures Recommendations for Individual Users," which are also applicable to most industrial applications. The first table: "Classification and Distortion Limits of Low-Voltage Systems." In factory power supply transformers and at wattmeters, both linear and nonlinear loads will be used together. Table 1 provides guidelines for voltage distortion limits and conductor notch measurements in these cases. For example, if a transformer supplies power only to AC drives, voltage distortion is allowed up to 10% without affecting normal operation. If the line has both motors and DC drives, or both linear and nonlinear loads, then voltage distortion should be kept below 5%. In hospitals and airports, for safety reasons, the maximum voltage distortion should be kept below 3%. To reduce the possibility of abnormalities in other loads connected to the distribution system, the standard also provides limits on the depth and area of ​​conductor notches. In addition to voltage distortion limits, IEEE 519 recommends limits on current distortion as well, as shown in Table 2, “Current Distortion Limits for General Distribution Systems.” The values ​​in the table do not represent the current distortion between the public grid and the terminal instrument, but rather the recommended current distortion values ​​between the public grid and the user, which is precisely the location of the common coupling point as defined by IEEE 519. This standard is also intended as a guideline for the public power grid and users. These restrictions, as a fair approach, allow each user's plant connected to the public grid to receive relatively low-distortion voltage. No user is allowed to cause excessive harmonic currents, otherwise he will cause significant distortion of the voltage supplied to other users. Although the data in IEEE 519 does not imply that it is an equipment standard, many consultants have applied it to current distortion caused by nonlinear loads or adjustable speed drives (ASDs). This usually means that users will spend more money on the filtering specified in IEEE 519. Evaluation of Filtering Suppression Methods If the line current harmonics of an AC drive need to be reduced, several methods for mitigating harmonic distortion are effective. These methods not only reduce harmonics and voltage distortion, but also affect other aspects of the power and drive system in different ways. Here are some things drive system designers need to know. ■ Basic AC Drive—For comparison, a basic AC drive can be considered as a basic AC drive with a three-phase diode bridge rectifier, a DC bus capacitor bank filter, and a three-phase IGBT bridge inverted converter. It does not include a DC choke and AC reactor. Its line current waveform is shown in Figure 3, with approximately 110% current distortion, as shown in the "Basic AC Driver Current" graph. [align=center] Figure 3: For motors below 150 horsepower, active filters reduce current distortion to approximately 8%. Figure 4: Active pre-circuit filters with the best distortion suppression can reduce distortion to 5%. [/align] ■ Line Reactors—; The most convenient and economical way to reduce harmonics in a line is to add a 3% line reactor before each driver or before a three-driver group. This will reduce current distortion in the line by 50%, as shown in the "Line Current Waveform with 3% AC Line Reactor" graph. However, line reactors also have a drawback: they cause a decrease in the DC bus voltage in the AC driver when the motor load increases and the speed accelerates. A 3% line reactor will cause the DC bus voltage to decrease by 3% at full speed and full load. This means that when operating at full speed and full load, the motor cannot receive its rated voltage, leading to rotor sag and increased current, resulting in more heat generation during motor operation. This issue doesn't exist if your speed doesn't exceed 97% of full speed under full load, or if you have high line voltage supply. Similarly, a 5% line reactor will cause a 5% drop in DC bus voltage. ■ DC Choke—A DC choke is placed in the drive between the inverter output and the DC bus capacitor bank. It reduces current distortion to approximately 40% without voltage drop at full speed and load. Some drive suppliers offer DC chokes as an option or standard component of their drives; most drives larger than 5 horsepower have a built-in DC choke. ■ Passive Harmonic Filter—Passive harmonic filters are popular for reducing line harmonic currents and are cost-effective for drives below 150 horsepower. Passive filters act as a storage point for harmonic currents during power inversion. Once the filter provides current, the transformer doesn't need to operate, thus reducing overheating and voltage distortion. These filters can reduce current distortion to 8%, as shown in the figure "Basic AC Drive Line Current with Passive Harmonic Filter". The biggest drawback of passive harmonic filters becomes apparent when the process requires the AC drive to operate at low speed and low load for additional periods. In such cases, the passive filter can cause the transformer to generate a leading power factor current, leading to overheating. This also becomes a problem if the AC drive is powered by a backup generator. To help mitigate this deficiency, some filter suppliers offer a contactor that automatically disconnects the capacitor bank in the filter once the drive stops or operates at low speed. Under full load, although the power factor in the filter is close to 1, it is best to confirm with the supplier before adding 10 or more drives with passive filters to a transformer to ensure that there will be no voltage or current resonance between the filters. ■ Active Harmonic Filters—Active harmonic filters are a great choice for a single drive, especially when several drives are in a motor control center. The filter provides the harmonic current required by all drives and can automatically adjust its operation according to the drive's requirements, typically reducing distortion current to 5%. Active harmonic filters do not cause a leading power factor under no-load conditions. When the drive stops and no more harmonic current is needed, it automatically reduces its output to zero. Active harmonic filters appear to have no technical or systemic flaws. ■ Multi-pulse transformers—Transformers have been used for phase shifting for decades, and 18-pulse transformers and inverters are popular because they easily reduce current distortion to 5%. Multi-phase transformers, as autotransformers and isolation types, are prone to breakdown. Autotransformers are cheaper and much smaller than isolation transformers because they require less space. 12-pulse transformers are no longer popular because they only reduce current distortion by 9-15%. 24-pulse systems and systems with even more pulses are also available, as the improvement in distortion (up to 4.5%) does not justify the additional cost. On the positive side, there is no leading power factor under no-load conditions, and the DC bus voltage remains at a normal level throughout the drive's speed and load range. This option is cost-effective for applications above 150 horsepower. [align=center] Data from standard IEEE 519 shows distortion limits, notch depth limits, and notch area limits in various practical applications. Data from IEEE 519 shows current distortion limits for various Isc/IL sources based on the maximum required load current in the PCC.[/align] ■ Harmonic Suppression Transformer—HMT Introducing specific phase shifts at 0°, 15°, 30°, and 45°, harmonic suppression transformers are useful if multiple drivers can be divided into 2 or 4 groups, with each group having approximately the same total drive power. By using two groups of drivers, one operating at a 0° phase shift and the other at a 30° phase shift, it removes most of the 5th and 7th harmonics, like a 12-pulse system. This conclusion also holds true for transformers with 15° and 45° phase shifts. Four groups of drivers, each placed at each of the four phase shifts, remove most of the 5th, 7th, 11th, and 13th harmonics, like an 18-pulse system. If some of the drivers have stopped working, less harmonics are removed. Voltage distortion is most severe when all drivers are fully loaded. ■ Active Preamplifier Circuits (AFEs) – Users typically purchase them as part of a complete driver system rather than as an upgrade, even if they can be used as standalone modules for several drivers operating on a common DC bus. An AFE consists of a three-phase IGBT bridge rectifier, similar to the inverter section of an AC driver with a 10% lead reactor. During motoring and regeneration operation, it acts as a step-up transformer that controls the DC bus voltage. If the IGBTs are modulated at a 3kHz carrier frequency, the harmonics are equivalent to those in a 50-pulse system. As shown in the figure “Line Current of Active Preamplifier Circuit,” an AFE can reduce current distortion to 5%. However, a small notch filter is needed to minimize notches generated during modulation. Without this notch filter, these line notches can cause operational problems for other devices connected to the same transformer as the AFE. Drive system designers need to clearly understand how each harmonic suppression method affects the power and drive system. It's not just Ithd (Total Harmonic Distortion of Input Current) that is affected. For plant systems, attention should be paid to keeping voltage distortion and line notch within certain ranges to prevent problems with other equipment. For interfaces with the public power grid, attention should be paid to limiting current distortion below recommended values ​​to prevent problems with the grid and other users. This is the purpose of IEEE 519, and also the purpose of system designers.