AQ: Variable Frequency Drive Harmonics
For the AC power line, the system (VFD + motor) is a non-linear load whose current include harmonics (frequency components multiples of the power line frequency). The characteristic harmonics generally produced by the rectifier are considered to be of order h = np±1 on the AC side, that is, on the power line (p is the number of pulses of the variable frequency drive and n =1,2,3).
Thus, in the case of a 6 diode (6 pulses) bridge, the most pronounced generated harmonics are the 5th and the 7th ones, whose magnitudes may vary from 10% to 40% of the fundamental component, depending on the power line impedance. In the case of rectifying bridges of 12 pulses (12 diodes), the most harmful harmonics generated are the 11th and the 13th ones. The higher the order of the harmonic, the lower can be considered its magnitude, so higher order harmonics can be filtered more easily. As the majority of VFD manufacturers, Iacdrive produces its low voltage standard variable frequency drives with 6-pulse rectifiers.
The power system harmonic distortion can be quantified by the THD (Total Harmonic Distortion), which is informed by the variable frequency drive manufacturer and is defined as:
THD = √(∑∞h=2 (Ah/A1)2)
Where
Ah are the rms values of the non-fundamental harmonic components
A1 is the rms value of the fundamental component
The waveform above is the input measured current of a 6-pulse PWM variable frequency drive connected to a low impedance power grid.
Normative considerations about the harmonics
The NEMA Application Guide for variable frequency drive systems refers to IEEE Std.519 (1992), which recommends maximum THD levels for power systems ≤ 69 kV as per the tables presented next. This standard defines final installation values, so that each case deserves a particular evaluation. Data like the power line short-circuit impedance, points of common connection (PCC) of variable frequency drive and other loads, among others, influence on the recommended values.
| Voltage harmonics | |
| Even components | 3% |
| Odd components | 3% |
| THDvoltage | 5% |
The maximum harmonic current distortion recommended by IEEE-519 is given in terms of TDD (Total Demand Distortion) and depends on the ratio (ISC / IL), where:
ISC = maximum short-current current at PCC.
IL = maximum demand load current (fundamental frequency component) at PCC.
