Adjustable speed drives (ASDs) can be both a source and a victim of poor power quality.
ASDs as Victim LoadsAlthough ASDs are usually depicted as the culprit in the PQ scenario, there are ways in which they can be a victim load as well.
Capacitor switching transientsHigh-energy (relatively lowfrequency) transients that are characteristic of utility capacitor switching can pass through the service transformer, feeders, and converter front-end of the drive directly to the dc link bus, where it will often cause a dc link overvoltage trip. Input diodes could also be blown out by these transients.
Voltage distortionIf high-voltage distortion shows up as excessive flat-topping, it will prevent dc link capacitors from charging fully and will diminish the ride-through capability of the drive. Thus a voltage sag which would not normally affect a drive will cause the drive to trip on undervoltage.
Improper groundingwill affect the internal control circuits of the drive, with unpredictable results.
ASDs as Culprit LoadsA drive can definitely be a "culprit load" and have a major impact on system PQ. But before we talk of problems, let's put in a good word for the positive effects of drives on PQ. First of all, they offer built-in soft-start capabilities. This means there will be no inrush current and no voltage sag effect on the rest of the system. Secondly, if the drive is of the PWM type, with a diode converter front-end, the Displacement Power Factor is high (commonly > 95 % at rated load) and more or less constant throughout the range. This means that drives can reduce energy usage and correct for DPF at the same time. It's a good thing too, because drives and PF correction capacitors don't mix. Caps are vulnerable to the higher frequency harmonic currents generated by drives, since their impedance decreases as frequency increases.
The type of drive has a major impact on the PQ symptoms, because of the different converter designs (converters or rectifiers turn ac to dc and are the first stage of the drive). There are two major types of converter design.
SCR Convertor with Voltage Source Inverter/Variable Voltage Inverter (VSI/VVI) DrivesCommonly called six-step drives, they use SCRs (Silicon - Controlled Rectifiers) in their converter front-ends (the following discussion applies to CSI, Current Source Inverter drives, which also use SCRs). VSI and CSI drive designs tended to be applied on larger drives (> 100 HP). SCR converters control the dc link voltage by switching on (or "gating") current flow for a portion of the applied sine wave and switching off at the zerocrossing points. Unlike diodes, SCRs require control circuits for gate firing.
For the SCR converter, there are three main issues that affect line-side PQ:
- Commutation notches. SCR switching or commutation is such that there are brief moments when two phases will both be "ON." This causes what is in effect a momentary short circuit that tends to collapse the line voltage. This shows up as "notches" on the voltage waveform. These notches cause both high VTHD and transients. The solution is to place a reactor coil or isolation transformer in series with the drive's front end to clean up both problems.
- Displacement Power Factor declines as drive speed decreases. This is not as serious a problem as it sounds, because the power requirement of the drive-motor-load decreases even more.
- Harmonic currents, typically the 5th and 7th, are generated by VSI drives.
Diode Convertor with Pulse Width Modulation (PWM) DrivesThe other and more common converter design uses diodes and is used in the PWM drive. The diodes require no switching control circuitry. One of the main trends in the industry has been the proliferation of PWM drives, mainly due to the continued development of fast-switching, efficient IGBTs (Insulated Gate Bipolar Transistors) used in the inverter section of the drive (inverters turn dc to ac). For all practical purposes, PWM drives are the industry standard.
For the diode converter, the main PQ issue is harmonics. The actual harmonic orders being generated depend on the number of diodes in the front end. For three-phase conversion, a minimum set of six diodes is required. This "six-pulse" converter will generate 5th and 7th harmonics. If a 12-pulse converter were used, the 11th and 13th harmonics will be generated instead of the 5th and 6th - and, very importantly, for the same load, the amplitude of the 11th and 13th would be considerably less than the 5th and 6th. Therefore, the THD would be less. The vast majority of drives, however, are six-pulse PWM style, which is one reason we see so much 5th harmonic on the system.