VFD transformers: Introduction

After reading this blog post you should understand the purpose of a VFD transformer and the differences between distribution/power transformer and a VFD transformer. Let’s start with the definition of a VFD transformer: This term refers to the input isolation transformer that supplies the rectifier section of VFD (passive or active rectifier). Other names of VFD transformers are:

– converter transformers [1]

– rectifier duty transformers

– ASD transformer

– VSD transformers

The term ‘converter transformer’ has a bit broader meaning. The load is not necessarily only a VFD, but generally a converter (static frequency converter for a grid inter-tie, converters for HVDC transmission, high power rectifiers for aluminum smelters etc). In this series we focus specifically on VFD transformers , i.e. main transformers in electric power drive systems.

Remark:

Note that the standards sometimes denote the semiconductors as valves. Have you ever wondered why? This has a historical reason since the first rectifiers were based on mercury arc valves before the era of modern power electronics started. For instance IEC uses the term ‘valve’ to the class of rectifying and switching devices which includes solid -state silicon based semiconductors as well as vacuum, gas and vapour based tubes [2]. After this explanation you will no more be surprised reading about semiconductor valves or seeing individual thyristors inside a rectifier bridge marked as V1, V2,… When talking about valve side of converter transformer it refers to windings connected to the converter. So much to a small historical excursion.

Purpose of VFD transformers

There is more than just one:

Voltage adaptation from feeding grid to the rectifier input level

Typical rectifier input voltage of medium voltage VFD is in the range 1 – 2 kV while the grid voltage is generally different (e.g. 4.16 kV, 6.0 – 6.9 kV, 10-11 kV, 13.8 kV etc up to 132 kV or even higher).

Galvanic isolation of the VFD from the grid

Galvanic isolation is relevant for easier protection. It allows the VFD to operate despite of a single phase ground fault in the feeding network. This would not be possible in transformer-less configuration.

Limitation of short circuit current

The transformer inherently restricts the current from the grid in case of a fault. The transformer impedance is carefully specified by VFD manufacturer. It is often a part of the protection concept.

Reduction of harmonics (injected to the grid)

Transformer short circuit impedance reduces the magnitude of harmonics (same principle as e.g. input reactor in low voltage drives). Moreover, appropriate vector group with phase shifted secondary windings help to cancel certain harmonic orders in the spectrum.

Reduction of high frequency disturbances (EMC)

Rapid commutation of semiconductors emits high frequency spectrum. Transformer helps to suppress radio frequency interference in range of kHz up to MHz.

Protection of VFD against fast voltage transients

Severe overvoltages may occur in a power grid. These are either atmospheric overvoltages (e.g. lightning stroke) or switching overvoltages. Transformers with specially designed electrostatic screen minimize the impact on the VFD and help to protect sensitive power electronics.

Is VFD transformer always required? Well, almost always (95% cases). Although there are direct to line (DTL) types of VFD available on the market, i.e. VFDs that are directly connected to the supplying grid without galvanic isolation, vast majority of medium voltage VFDs still require an input isolation transformer (see also [4]). The purpose of such transformer has just been explained. The DTL drives do not need an input transformer for voltage adaptation, but the other functions such as short circuit limitation or harmonic reduction shall still be provided by other means.

Design considerations for VFD transformers

Besides the standard design consideration valid for distribution or power transformers, the VFD transformers shall fulfill additional requirements:

Presence of harmonics [5, 6]

– Effects of non-sinusoidal load currents.

– Additional losses (harmonic heating) and potential risk of hot spots (cooling requirements).

– Equivalent sinusoidal current shall be used for temperature rise test instead of nominal current.

– DC components and its impact on core design (to avoid saturation).

– Reduced flux density in some cases.

Winding connections / transformer vector groups [7]

– Large number of winding connections and configurations available.

– Transformer configuration linked with VFD rectifier topology and harmonic cancellation.

– Multi-winding design with phase shifted secondary windings.

– Star, delta, extended delta, polygon delta and zig-zag connections.

Increased insulation level

– Converter side windings are exposed to common mode voltage and high dv/dt.

– Requirements for higher insulation levels (e.g. 40 kV or 60 kV BIL for 2 kV winding).

– Considered in higher test voltages (lightning impulse LI/basic insulation level BIL).

Electrostatic shield

– Most VFD manufacturers require electrostatic screen between primary and secondary windings.

– Dual purpose:

1) Protect VFD against transient overvoltages in the grid

2) Minimize EMC interference.

– Specific construction is part of manufacturer’s know-how.

Short circuit impedance [8]

– Impedance value specified by VFD manufacturer. It is typically little higher than impedance of distribution transformer with comparable rating.

– Transformer impedance impacts grid harmonic distortions, limits the short circuit current, but also causes a voltage drop, impacts the power factor etc.

– There is always an optimal range; both too low and too high impedance has negative impact on drive system.

– In multi-winding transformers we shall differentiate the impedance when one secondary winding is shorted and multiple secondary windings are shorted.

– Uniformity of impedances affects the dynamic forces and mechanical stress.

Coupling / mutual impedances [9]

– There is always certain magnetic coupling between the circuits (common mutual impedance).

– Multi-winding transformers shall have as small mutual impedance as possible –> loosely coupled design.

– Aim is to avoid interactions during commutations (self disturbance).

Short circuit considerations

– Fault conditions within the rectifier cause faulty current that may be higher than fault current derived for conventional power transformers.

– Phenomena such as ‘arc-back’ or ‘back fire’ [1] may occur.

Some of these topics will be explained in greater detail in next posts of this series on VFD transformers.

What makes the VFD transformer different from a common distribution or power transformer?

– VFD transformer supplies the VFD and is usually exposed to harmonic currents and increased insulation stress (such as rapid voltage rise shortly called ‘dv/dt’ and common mode voltage). This imposes different thermal dimensioning and increased insulation requirements.

– Flux density of VFD transformer might be lower compared to distribution or power transformer.

– VFD transformer often has multiple secondary windings supplying 12-pulse to 36-pulse rectifier (pulse number higher than 36 is principally possible, but very rare).

– Multiple secondary windings are phase shifted against each other to eliminate selected harmonics and minimize harmonic distortion in the supply network. For 12-pulse rectifier one secondary winding is star connected, the other secondary winding is connected in delta (e.g. Yy0d11). For higher pulse number of rectifier the phase shift can be achieved by using zig zag connection or extended delta connection.

– Electrostatic shield between primary and secondary windings is highly recommended to protect the VFD against fast voltage transients (e.g. switching overvoltages due to breaker operation or atmospheric overvoltages).

– VFD transformer might be exposed to failure modes that do not exist in distribution or power transformer applications. Probability of external short circuit is somewhat higher (failure in rectifier circuit).

– Protection of VFD transformer deserves more attention.

– Requirements on short circuit impedance might be more complex. Typically there is a desired impedance range for a given VFD. Too low or too high impedance negatively impacts the drive system.

– VFD transformer may be exposed to more frequent energization compared to a distribution transformer that is almost never disconnected.

– Depending on VFD type some VFD transformers have to withstand certain amount of DC component.

– VFD transformers might have additional auxiliary winding to supply the own consumption of VFD (cooling fans or pumps, control circuits etc).

VFD transformer can be either dry type or liquid filled (mineral oil or special liquids), same as other transformers. Some VFDs (lower power) use integrated transformers – dry type forced cooled transformers integrated in VFD enclosure.

Summary:

Most VFDs require a dedicated input isolation transformer. This transformer does not just adapt the voltage level but provides several other functions that are very important as well. Due to a different nature a distribution transformer design cannot substitute a VFD transformer. Understanding the requirements on VFD transformer and its correct specification contributes to an increased reliability of variable speed drive system.