How to choose a medium voltage VFD: Line side connection and power quality

Nowadays modern VFDs offer large variety of line side configurations. Which one to select? That is the topic of today.

There are two basic groups of VFDs based on their line side connection:

• Diode Front End (DFE)
• Active Front End (AFE)

Diode Front End

DFE, as obvious from its name, consists of a diode rectifier. It allows only one direction of power flow (from the grid through the VFD to the motor), but this is in majority of cases all what is needed. Basic building block of DFE is a 6-pulse diode bridge. However, 6-pulse DFE would produce significant amount of harmonics to the grid (“grid pollution”) which could cause increased voltage distortion and possible malfunction of other equipment. Therefore, at least 12-pulse DFE is normally required. Manufacturers offer higher pulse numbers such as 18-pulse, 24-pulse or 36-pulse as well. The corresponding input isolation transformer is a multi-winding type with phase shifted secondary windings to achieve cancellation of certain harmonic orders. Manufacturers have simple tools to calculate the harmonic distortion and to quickly assess what is the minimum pulse number in order to comply with respective grid codes. In case of multiple VFDs there is a possibility to virtually increase the overall pulse number by phase shifting the individual transformers against each other (more on that topic in separate article). As the pulse number increases the complexity of the input transformer increases as well. DFE rectifier is a cost effective solution with high efficiency (negligible losses in diode bridges) and preferred choice for many applications where regenerative braking is not required. For occasional active braking a braking chopper can be fully sufficient. Applications such as pumps typically do not require any active braking at all and they simply coast down.

Active Front End

AFE inherently offers a 4-quadrant operation meaning that power can flow from the grid to the motor or vice versa. This feature allows regenerative braking and is often used for dynamic applications such as rolling mill drives or some types of test stands. The hardware of the AFE unit is basically the same as the hardware of the inverter unit. There are active switches (IGBT, IGCT, IEGT) with anti-parallel diodes in case of voltage source inverters or phase controlled thyristors in case of load-commutated inverters. With regards to harmonic distortion the AFE drives eliminate certain harmonic orders by appropriate modulation (e.g. pulse pattern). These units are often just 6-pulse or 12-pulse which helps to simplify the topology of input isolation transformer. However, for network considerations we recommend to request harmonic spectrum up to harmonic order of 100 (5 kHz in 50 Hz grid / 6 kHz in 60 Hz grid). The majority of standards still require harmonic compliance up to 50th harmonic. As the VFDs become more powerful and allow higher switching frequencies, they can eliminate the lower harmonic orders almost entirely. However, the harmonic distortion is still there, it is just shifted towards higher frequencies. Some customers already today request harmonic check up to order of 100 and this trend will surely continue. AFE drives allow regenerative braking, but it does not necessarily mean that they are more efficient. Remember that in normal driving mode AFE drive will have roughly 50-80% more losses than a DFE drive (DFE efficiency 98.4%-99.2%, AFE efficiency 97.4%-98.2%, both incl. auxiliary consumption). The motivation for AFE are often the process requirements rather than energy conservation.

Integrated versus external transformer

In the power range up to several MW the VFD might be available both with external as well as integrated transformer. What is the best choice? There is no simple generic answer, but let’s look at pros and cons of both solutions:

Integrated transformer comes as part of the VFD. It is integrated inside a common enclosure. For the user it is a simple 3 cables in/3 cables out system. The cabling between transformer and VFD is already done and the product shall inherently include transformer protection relay including settings, surge protection etc (if not I would very much question the product). This is definitely an advantage for the user. The integrated transformer is a dry type (typically vaccum pressure impregnated technology) with limited range of input voltages (typically from 3.3 kV up to 11 kV, sometimes up to 13. 8 kV or 15 kV). In order to minimize the footprint the transformer is designed in a compact way and operates at high temperature meaning the efficiency is not so high. Cooling is usually air forced (AF) and the losses are blown into the electric room. The demand for re-cooling is therefore larger meaning larger air conditioning system. In some cases the integrated transformer can be water cooled, but typically only in high power range in combination with water cooled VFD.

External transformer provides full flexibility. Both oil or dry type transformers can be used. There is almost no limitation in terms of power or supply voltage. The transformer is typically located outdoors. The electric room only deals with losses from VFD and not from the transformer (roughly 50% less). The cabling between transformer and VFD shall be considered in addition. Also the transformer protection needs extra consideration.

Summary: In the lower power range (few hundred kW up to 1-2 MW) the integrated transformer solution is very popular due to its simplicity. The transformer losses in absolute value are not so large and do make such a burden for air conditioning. VFDs in this power range are often connected to 6/6.6/10/11 kV grids and integrated transformer solutions are available of the shelf. The user does not need to care about internal cabling and transformer protection. To obtain arc resistant design current limiting fuses are likely to be required. It might be worth to compare the lifecycle cost of integrated and external transformer solution, especially considering the cost of re-cooling. In higher power range the integrated transformer solution is less attractive due to amount of losses evacuated into the electric room. The supply voltage is also a limiting factor as larger VFD might be preferably connected to a stronger grid at higher voltage level. In very high power range external transformer is the only solution. Transformer protection shall be addressed.

Direct to line (transformer-less)

AFE drives allow in some cases so called direct to line connection. It essentially means that the VFD is directly connected to the supply grid without the need of input isolation transformer. Although it sounds very attractive few things shall be kept in mind:

Limited range of line voltages

Direct to line configuration is only possible for specific grid voltage, most commonly for 4.16 kV or 6.0-6.9 kV, some models also allow 11 and 13.8 kV. For any other voltage level the transformer is still needed for voltage adaptation.

Need for harmonic filters

Harmonics and need for input filter

AFE drives with an input transformer often do not require any harmonic filter. The harmonics are minimized by the combination of switching pattern and transformer short circuit impedance (decoupling from the grid). In direct to line configuration there is likely an input reactor needed to substitute the transformer impedance. Harmonic filter might be needed as well.

Short circuit limitation

Input transformer inherently provides short circuit limiting capability. In direct to line connection this feature is gone and VFD would see much higher fault current from the grid. Input reactor is often needed. In case of saturable iron core reactor extra caution shall be made.

Direct to line VFD will be exposed to all faults or transients happening in the supply grid, such as transient overvolages from vacuum circuit breakers (otherwise shielded by transformer with electrostatic shield), occurrence of ground faults etc.

Direct to line configuration is certainly as attractive option for certain installations, but above limitations shall be carefully reviewed. Generally we believe that a solution with input isolation transformer is much more robust and flexible despite slightly higher cost and footprint.

Hope you enjoyed reading this article.