What types of VFD grid connection exist?
How is the VFD connected to the supplying grid? There are several ways. In this post we look at the conventional ones as well as more specific solutions regarding variants of VFD grid connection.
1. Connection through at input isolation transformer
Most common solution in medium voltage is to connect the VFD with the grid via an input isolation transformer [1]. This concept brings many advantages. Let’s name few of them:
- Adaptation to any grid voltage level (full flexibility)
- Galvanic isolation of VFD from the grid
- Inherent limitation of fault current
- Multi-phase supply to match the rectifier with higher pulse number
- Multi-winding design with relative phase displacement to eliminate certain harmonic orders from the spectrum
- Electrostatic shield to minimize high-frequency disturbance (EMC)
The transformer can be either dry type [2] or a liquid immersed type. Installation can be indoor or outdoor. Dry type transformer is mostly installed indoor while liquid immersed transformers are preferably located outdoors. However, other options exist as well. For example, our rental transformer is of dry type technology and can be installed either indoor or outdoor.
Some VFD have the input transformer integrated, i.e. it is an integral part of the VFD (installed inside VFD cabinet). In such case the user does not need to care about the power connections between VFD and transformer. In addition, the transformer protection is already solved as well. it makes the installation easier and faster. Of course, the integrated transformer also comes with couple of restrictions:
- Limited range of input voltages
- Limited maximum power
- Often air cooled → losses dissipated into the electric room
Integrated transformer is either an option to complement an external transformer (e.g. ACS1000 or ACS2000 from ABB portfolio) or as the only choice for certain VFD topologies (cascaded H-bridge such as ABB’s ACS580MV, Siemens Sinamics GH180 etc).
For more details concerning the comparison of integrated and external input transformer refer to our previous post [3].
VFDs utilizing a grid side transformer can be both with diode rectifier or active rectifier [4]. Diode front end (DFE) solution at medium voltage typically consists of a multi-winding transformer and corresponding multi pulse rectifier bridge. The combination of high pulse number and phase shifting input transformer allows to eliminate certain harmonic orders and reduce the remaining ones to a level that usually complies with (inter)national harmonic standards. Active front end (AFE) drives are typically used when bi-directional power flow is required (such as regenerative braking). They frequently allow to control the input power factor which is mostly controlled to unity (1.0) but can be adjusted to a different level, too.
Connection of VFD to the grid through an input isolation transformer offers unmatched flexibility with respect to grid voltage level or robustness to grid faults and disturbances. The solution with external transformer has virtually no limitation with respect to grid voltage or rated power. Integrated transformer is a popular option in the lower power range.
2. Transformerless connection
The other alternative is a transformerless solution. Such variant is much less common then a drive system with an input transformer. Transformerless concept also brings several limitations that need to be respected. But let’s start with the benefits first:
- Reduced capital expenditures
- Compact solution with reduced footprint
- Elimination of transformer losses
- Less cabling / easier installation
The reduced footprint can be beneficial when retrofitting an existing installation with space restrictions (for example replacement of a turbine with electric driver). Nevertheless, transformerless drive system may need some other “inductive component” to at least partly limit the grid fault current and to decouple the VFD from the grid. This is mostly achieved with an input reactor.
From cost perspective the investment saving represent the price difference between an input transformer and input reactor.
As already mentioned, there are several limitations as well:
- Limited to certain range of input voltages (e.g. 4.16 kV, 6.0 – 6.9 kV, 10/11 kV)
- Input reactor required to limit the fault current and comply with harmonic standards
- No multi-pulse diode rectifier possible
- More sensitive to grid disturbances
- VFD cannot operate in case of earth fault in the grid
- Problematic design when used as soft starter or drive with synchronous bypass
Despite these limitations a transformerless concept can be an attractive solution in specific cases (matching grid voltage, major space restrictions, use of bi-directional power flow).
Instead of using the term ‘transformerless’, some manufacturers call this topology Direct-to-Line (ABB) or Direct-to-Drive™ (Rockwell).
Restricted input voltage
The range of input voltages is limited. Supported input voltages are normally similar or same as the output voltages (i.e. transformerless VFD with 6.6 kV inverter output voltage most likely expects ~ 6.6 kV at its input as well). Most ‘transformerless’ VFDs go up to 11 kV as nominal input voltage. Few models are designed for 13.8 kV as well. Higher input voltages are very rare.
This works fine in the lower power range. Nonetheless, at higher power it is common to connect the VFD to a higher grid voltage to improve the harmonic performance and power quality (higher short circuit level → lower harmonics). After all, it depends on the existing distribution grid anyway.
Other drawbacks are mainly linked with grid disturbances such as transients (sudden events) or medium/long term issues (earth fault).
Can we connect the VFD directly to a generator?
Above described considerations apply for a VFD connected directly to the distribution grid. However, there are other particular connections as well. One of them might be a direct connection of a VFD to an asynchronous or synchronous generator. Such setup usually applies to a small isolated system. Examples are marine propulsion using electric drives, pumped storage power plant or a frequency converter within a wind turbine. Thus, it is generally possible providing that right considerations are made for the design of the generator and the frequency converter.
Marine propulsion and wind power have one thing in common: the space and weight in the machine room or turbine nacelle is very limited. Thus, having an isolation transformer between VFD and generator is a luxury that the engineers cannot afford.
For such direct (transformerless) connection an AFE drive is normally used. Thus, the transformerless VFD is more attractive when the application benefits from bi-directional power flow (4-quadrant operation). Otherwise, the cost and loss savings on the input transformer are quickly eaten by a more expensive active rectifier with higher losses.
In a utility grid, VFD with simple 6-pulse diode rectifier would not be accepted due to high harmonic distortion. On the other hand, in island systems such concept is thinkable. One example is mentioned marine case.
Detailed comparison
A detailed comparison of the types of VFD grid connection and their suitability for specific applications is available for premium subscribers.
Conclusion
This blog post provided a short description of the VFD solutions with regards to the grid connection. Main emphasis was put on the two categories: VFDs using an input isolation transformer and VFDs in transformerless configuration. The advantages and drawbacks had been explained. In majority of cases the medium voltage drive system requires an input transformer. The direct (transformerless) solution might be an interesting alternative when several conditions are fulfilled.
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References
[1] VFD transformers: Introduction, https://mb-drive-services.com/vfd-transformers-introduction/
[2] Dry type transformer technology, https://mb-drive-services.com/dry-transformer-technology/
[3] Integrated versus external transformer, https://mb-drive-services.com/integrated-versus-external-transformer/
[4] How to choose a medium voltage VFD: Line side connection and power quality, https://mb-drive-services.com/how-to-choose-mv-vfd-line-conn/
