VFDs & Harmonics

What are Harmonics

Harmonics are sinusoidal voltages or currents having frequencies that are integer multiples of the fundamental frequency at which the power system is designed to operate. This means that for a 60-Hz system, the harmonic frequencies are 120 Hz (2nd harmonic), 180 Hz (3rd harmonic) and so on. Harmonics combine with the fundamental voltage or current producing a non-sinusoidal shape, thus, a waveform distortion power quality problem. The non-sinusoidal shape corresponds to the sum of different sine waves with different magnitudes and phase angles, having frequencies that are multiples of the system frequency.

Harmonics that are multiples of 2 (2nd, 4th, 6th, 8th etc.) are not harmful because they cancel out. Because the power supply is 3 phase, the third order harmonics (3rd, 6th, 9th etc.) cancel each other out in each phase. This leaves only the 5th, 7th, 11th, 13th etc. to discuss. The magnitude of the harmonics produced by a VFD is greatest for the lower order harmonics (5th, 7th and 11th) and drops quickly as you move into the higher order harmonics (13th and greater).

Harmonics exists due to the nonlinear characteristics loads and devices on the electrical power system. These devices can be modeled as current sources that inject harmonic currents into the electrical system. Consequently, voltage distortion is created as these currents produce nonlinear voltage drops across the system impedance. Harmonics can cause problems in electrical systems. Higher order harmonics can interfere with sensitive electronics and communications systems, while lower order harmonics can cause overheating of motors, transformers, and conductors. The opportunity for harmonics to be harmful, however, is dependent upon the electrical system in which they are present and whether any harmonic sensitive equipment is located on that same electrical system.

How a VFD Causes Harmonics

By its nature, the VFD is a nonlinear load in the electrical system in the sense that it draws a non-sinusoidal current even if the applied voltage is perfectly sinusoidal. VFDs have an AC to DC rectifier unit with a large DC capacitor to smooth the voltage ripple. The DC bus capacitor draws charging current only when it is discharged into the motor load. The charging current flows into the capacitor through the input rectifier when the instantaneous input voltage is higher than the DC voltage across the bus capacitor. The pulsed current drawn by the DC bus capacitor is rich in harmonics because it is discontinuous. The voltage harmonics generated by VFDs are due to the “Flat-Topping” effect caused by a weak AC source charging the DC bus capacitor without any intervening impedance. The distorted voltage waveform gives rise to voltage harmonics, which is of more importance than current harmonics. This voltage is shared by all loads and it affects all loads connected in an electrical system. Current distortion has a local effect and pertains to only the circuit that is feeding the nonlinear load. Non-sinusoidal currents that draw from the ac source cause undue stress on power delivery equipment that results in poor overall efficiency.

Harmonic Mitigation

Multi-Pulse VFD

There is a minimum of six rectifiers for a three-phase variable frequency drive. There can be more, however. Basically manufacturers offer 12, 18, 24, and 30 pulse VFDs. A standard six-pulse VFD has six rectifiers, a 12-pulse VFD has two sets of six rectifiers, an 18-pulse VFD has three sets of six rectifiers and so on. If the power connected to each set of rectifiers is phase shifted, then some of the harmonics produced by one set of rectifiers will be opposite in polarity from the harmonics produced by the other set of rectifiers. The two wave forms effectively cancel each other out. In order to use phase shifting, a special transformer with multiple secondary windings must be used. Multipulse operation is possible entirely due to the phase shifting transformer. The power electronic devices in the converter are uncontrolled silicon diodes, but as rectifier sections are added, footprint and cost go up.

Passive Harmonic Filters

Passive harmonic filters provide an inexpensive way to mitigate the harmonics created by VFDs and other harmonic sources. Passive filters are best used as individual filters for each small 6-pulse VFD or as common filters connected to a motor control center (MCC) supplying several VFDs. Basically, the passive filter is a series inductor-capacitor resonant circuit that is tuned to a specific frequency and connected in parallel with the VFD. The filter consists of a 3-phase iron-cored inductor in series with a
wye-connected or a delta-connected 3-phase capacitor. Passive filters are useful only in limited conditions, and they have a large footprint and generally use more energy than other alternatives.

Active Front-End VFDs

Active front-end (AFE) VFDs have come into use only recently and are gaining rapid acceptance. Active front end drives (AFE) not only reduces harmonics, but also provides other benefits that can reduce costs for the end user. AFE VFDs are basically 6-pulse VFDs, with the exception that the line converter is not the uncontrolled diode bridge rectifier as in other 6-pulse VFDs but uses controllable power devices, such as IGBTs. IGBTs are devices whose switching is controlled electronically — hence the term “active” front end. The AFE monitors the input current waveform and shapes it to be sinusoidal, reducing total harmonic distortion (THD) to 5 percent or less. (Note that THD is only measured for lower-order harmonics. An LCL filter is necessary to reduce higher-order harmonics caused by the switching frequency of the IGBTs.)

Because the IGBTs are switched at a high frequency, the AFE VFD requires a large inductor-capacitor filter at the line end. The filter is provided integral to the VFD. It increases the footprint of the VFD, but not as much as the phase-shifting transformer of the 18-pulse VFD. Therefore, in general, the AFE VFD has a smaller footprint than an equivalent multi-pulse VFD.

Another benefit of an AFE is their ability to handle regenerative power. The IGBT rectifier design allows four-quadrant operation, meaning the motor can produce either positive or negative torque (as occurs during braking or back driving), during either the forward or reverse motor rotation. When torque and motor rotation are in different directions (i.e. clockwise torque and counterclockwise motor rotation, or vice-versa), the motor acts like a generator, and the power produced by the motor can be fed back into the electrical supply. This eliminates the need for large resistor banks to dissipate the regenerated energy and reduces costs by feeding the energy back to the line, where it can be reused.

Harmonic Mitigation Transformers

Harmonics Mitigating Transformers are special kind of transformers which are used to mitigate harmonics in the connected loads. They are often confused with K Transformers which are a type of transformer that is over sized to supply loads which generate harmonics. A Harmonics Mitigating Transformer, however, works by passively suppressing the harmonics. A Harmonics Analysis is conducted to identify the order of the harmonics generated before choosing the type of Harmonics Mitigating Transformer. Mitigating Transformers are designed like ordinary transformers except that they have specific phase shifts (+15 deg, -15 deg, 30 deg). The primary winding is usually delta while the secondary winding is a zig zag winding. The zig zag winding of the secondary cancels an order of harmonics depending on the phase shift. The harmonics are prevented from reaching the primary and affecting transformer output.