For this article, we will discuss the harmonic distortion created by the input stage of a VFD and how KEB harmonic filters can be implemented to reduce these harmonics.
Variable frequency drives (VFD) used in industrial applications provide many benefits including energy savings, better process control, extended component lifetime and increased machine safety when utilizing functional safety features.
Variable frequency drives consist of three sub-systems. The diode bridge converter, DC bus, and IGBT output inverter. The diode bridge input rectifier typically consists of a full-wave 6-pulse rectifier which rectifies the incoming AC voltage into a DC voltage. The DC bus system utilizes DC capacitors to help smooth the rectified AC voltage and provide voltage storage for the system. The IGBT output system utilizes the DC bus voltage and creates the variable frequency and voltage output to the motor using a pulse width modulated (PWM) control.
Any VFD which uses a 6-pulse rectifier on the input stage may be one source of current and voltage harmonics on the main line (Other sources of main line harmonics include: Arc furnaces, any power supply using a static power converter, inverters for distributed generation). This is due to the non-linear load produced by the 6-pulse rectifier. A load connected to an AC input voltage is a linear load if the current draw is in the same form as the voltage (resistive, inductor or capacitive load for example). A non-linear load is then one that draws current that is not in the same form as the as the AC voltage waveform – non-sinusoidal.
In the VFD system, once the DC bus capacitors are charged, input current to the capacitors will flow only when the incoming AC voltage is greater than the DC bus voltage. This is near the top of the arc of sine wave voltage waveform. As the voltage of the sine wave drops below the DC bus level, the current will stop flowing as shown in the diagram below. This non-linear capacitor current results in a current pulse on the main incoming voltage line.
Typical industrial systems have both linear and non-linear loads. The non-linear loads cause the main line voltage and current waveforms to become distorted. This introduces harmonics into the waveform. The harmonics will be at an integer multiplier of the fundamental frequency. For example, if the input voltage is 60hz, harmonics may result with frequencies of 120hz, 180hz, etc. The harmonics produced by a static power converter rectifier stage can be determined by:
h = n x p ± 1
h = harmonic
n = 1 to ∞
p = number of pulses
Therefore, with a 6-pulse full wave rectifier, we can expect to see harmonics at the 5th, 7th, 11th, 13th… orders. The amplitude of each harmonic typically diminishes as the harmonic value increases. As shown in the diagram below, the distorted waveform is made up of the fundamental (work producing) component and of the harmonic components (non-work producing) components.
Harmonics on the main line can have the following effects:
- Cause interference with communication circuits
- Transformer overheating
- Nuisance circuit breaker trip
- Neutral overloading
- Capacitor bank failure
How much harmonic content on the main line is too much? To assist with this question, a standard for harmonic levels was developed by the Institute of Electrical and Electronics Engineers (IEEE). The current standard for acceptable harmonic content is IEEE519-2014.
The main components of this standard define the acceptable levels of current and voltage harmonics based on the short circuit current rating (SCCR) of the incoming power distribution system. The amount of distortion that a system can tolerate is dependent on the input impedance level of the voltage distribution system. By determining acceptable harmonic levels based on the system SCCR, the standard takes into account the distribution system impedance. Lower impedance or stiffer distribution networks can tolerate a higher level of harmonics. Where higher impedance or softer distribution networks can tolerate a lower level of harmonic content.
The IEEE519-2014 standard also defines the point of common coupling (PCC) which is where the harmonic levels are to be measured. This provides a standard measurement location for the harmonic content of the facility to be measured. Since most industrial environments consist of linear and non-linear loads, even if an individual system produces some harmonic content, the overall effect at the PCC may be negligible. The IEEE519-2014 standard provides some clarification on the PCC over earlier versions of the standard:
Point of common coupling (PCC). IEEE519-2014 defines the PCC as:
…the PCC is usually taken as the point in the power system closest to the user where the system owner or operator could offer service to another user. Frequently for service to industrial users (i.e., manufacturing plants) via a dedicated service transformer, the PCC is at the HV side of the transformer. For commercial users (office parks, shopping malls, etc.) supplied through a common service transformer, the PCC is commonly at the LV side of the service transformer.
The IEEE519-2014 standard provides guidelines for the allowable harmonic content on a voltage distribution system. Standards are provided for both voltage and current harmonic levels.
KEB has developed a line of harmonic filters designed to be used on the input of a 6-pulse full wave rectifier. The standard KEB harmonic filter line is designed to reduce the total harmonic distortion (THD) of both current and voltage on the main input line to a maximum of 8%. This level is valid for an Isc /IL ≥ 20.
As an example of the effectiveness of the KEB harmonic filter, the following waveforms show the advantages of using a KEB harmonic filter.
Voltage and current at the main line – no line choke
Without a line choke high current peaks (5 to 10 times the rms value) occur resulting in a high ripple current in the DC bus of the motor control. This also means higher ripple voltage on the DC bus which can lead to torque ripple or reduced torque output from the motor.
Voltage and current with line choke impedance = 3% 480V/60Hz (4% 400V/50Hz)
With inductance (line choke) the peak amplitude of the current is reduced to a range of 3 to 5 times the rms value. This means lower ripple current in the DC bus of the motor control. This reduces heating in the capacitors and effectively extends the lifetime by a factor of two.
Voltage and current with the harmonic filter THDI < 8%
With the harmonic filter, the line-side current is sinusoidal. The peak amplitude of the current is 1.4 times the rms value. The output of the filter provides a nearly ideal rectangular voltage and current waveform into the motor control resulting in diode conduction of almost 180 degrees. The voltage on the DC bus is practically flat and the DC bus capacitors see minimum ripple current. This results in a further extension of life to 3+ times the nominal. Furthermore the average voltage value of the DC bus does not sag much in comparison to the first two cases. The result is more motor torque at higher speeds.
The KEB harmonic filters utilize a patented design which incorporates the inductor windings on a common core. This common core design provides the following advantages over competitor harmonic filters:
- High damping of main line voltage overshoot
- Minimal oscillations due to rapid load changes
- Increased lifetime of intermediate DC bus capacitors due the trapezoidal output of the voltage waveform (less DC bus ripple).
- Negligible DC bus voltage drop at full load
- Parallel operation of several VFD’s from a single harmonic filter
- Low capacitive reactive power at idle or low loads
KEB harmonic filters utilize three-phase capacitors in delta configuration. The three-phase capacitor design increases the symmetry and helps to protect the VFD from voltage surges in the event of a phase failure. In the event of a capacitor failure, all three phases are disconnected simultaneously. A phase failure on a competitor’s harmonic filters using single phase capacitors can result in a severe voltage imbalance and cause damage to the VFD input stage.
Additional KEB harmonic filter design highlights:
- KEB Harmonic filters are available specifically for applications with a 480VAC, 60hz mains or 400VAC (380), 50Hz mains.
- KEB Harmonic filters are available in sizes ranging from 23A to 400Amp rated input current.
- KEB Harmonic filters are UL rated
- All KEB harmonic filters incorporate a circuit protection device on the capacitor banks of the harmonic filter. The capacitor banks are mounted independent of the inductor core assembly to allow for increased mounting flexibility.
- KEB harmonic filters utilize lug style power connections.
- Capacitor contactor option available to disconnect the capacitor bank when the machine is idle.
- KEB Harmonic filters can be equipped with a line sync module for use with the KEB R6 line regenerative unit.
Please contact KEB today to speak with an application engineer.