New challenges with the operation of VFD based on active front end converter in distribution system.

It has been the practice of installing six pulse drive VFD (Diode rectifier) has been very common until recently. This was used to automate and control operations, besides to save energy. But the flip side has been injection of Harmonics into the grid. These harmonics was causing reliability and power quality issues, such as Power factor and poor power quality. Harmonics generated by these VFD and VFD range from 40% to 100% depending on manufacturers of these drives. The dominant frequencies has been the 5th (250Hz), 7th (350Hz), 11th, 13th.....

These harmonics interact with network harmonics and capacitor banks causing high voltages and High current posing problem like electrical system stability, reliability, electrical efficiency, power quality and resilience. They also cause expensive breakdowns and disruption in operations affecting the productivity of the plant. Maintenance of Capacitor banks becomes a challenge with frequent capacitor failures and malfunctioning of PF controller.

To surmount these adverse issues maintenance engineers resort to installation of Active Harmonic filter (AHF), passive harmonic filters, tuned filters, detuned filters for maintaining harmonics levels as stipulated by Individual distribution companies as per IEEE-519.

Active front end converters:

With innovation in switching devices like the SIC, MOSFET and IGBT, and new techniques in switching topology of the latest VFD has undergone dramatic change in design. The current VFD are switching at higher frequencies and the pattern of switching is such that the input voltage waveform and current waveform are sinusoidal. This facilitated in marketing the active front end VFD as a energy efficient drive free of harmonics. But however the other issues and practical problems of these VFD associated with high frequency current harmonics in Power distribution system has been seldom presented or disclosed.

This opens up new challenges in maintaining Power quality in Industries and integration of the high frequency and its impact on the grid.

The problems are compounded by the inverter side high frequency operation, the problem of high voltage and leakage currents. This is a separate subject by itself and will have to be dealt with separately.

Some of the issues are as follows:

#Activefrontend converters employs pulse width modulation (PWM), the input voltage dominant harmonics are pushed to higher frequencies thereby improving the current waveform. Another unique feature is the ability to control Power factor to near unity.

The high frequency used in the VFD rectifier create operates at currents a new frequency spectrum in the input current generating different side effects in the power distribution system.

Switching frequency Interaction:

The operation of the rectifier and the inverter at different switching frequency, the frequency changer creates a input current with a frequency spectrum that presents both dominant harmonics, synchronized at each switching frequency. These high frequency increase the losses and stresses the input reactor.

Interaction with other converters connected at the same Bus:

From the power distribution point of view, the Active frond end rectifier operates as a current source and as such injects high frequency current harmonics into the grid. If the VFD that use diode based rectifier are connected at the same grid, the high frequency current components are pushed into the DC bus. This is due to the fact they offer a low impedance path to the higher frequency components (Due to DC link capacitor) overloading the respective converters. Until the converter shuts down, hopefully by means of the DC link over voltage protection. The Active Front End converter normally generates 23rd to 25th harmonics, this increases the DC voltage.

Problems associated with ground protections:

Normally ground faults are eliminated with zero sequence relays connected through the ring transformers, or to the neutral to ground connection. Incase no ground fault is present in the system, zero sequence currents do not exist. However if an active front end converter is connected to the power distribution system, high frequency currents tend to sink into the ground across parasitic capacitances, which are reflected?in the ring current transformer,?forcing the operation of ground fault relays.

One possibility to avoid this problem is to connect a passive filter tuned to 50Hz at the terminals of the secondary winding of the current transformer. In this way the ripple current generated by the PWM AFE rectifiers circulates through the filter and does not propagate to the protection relay.

Conclusion:

The issues of High frequency current harmonics is similar in solar inverters, with use the same technology in Inverters. This could pose similar problems at injection point of the grid.

Undue care would be necessary on the type of Solar Inverters technology adopted by the manufacturer to ensure high frequency harmonic don’t interact with grid impedances. Issues of harmonic resonance and its disturbances due to amplification of harmonic voltages on grid is common in solar installation. especially if the common bus is rich in harmonics.?

Stay tuned for our upcoming article where we will explore this topic further and provide insights on high frequency harmonics in solar inverters.

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