VFD transformers: Overvoltage protection

Overvoltage protection of transformers

This is the continuation of [1] that made an introduction into protection of VFD transformers. The focus of this article is on overvoltage protection. Particular attention is paid to very fast transient overvoltages caused by switching of vacuum circuit breakers. Keep reading to learn about the root cause of such overvoltages and strategies to minimize the risk of failure.

Need for overvoltage protection

Overvoltage protection is an essential part of overall protection concept. Every transformer shall be protected against overvoltages. The protection element is either directly mounted on the transformer or installed upstream in the grid. General rule is that the closer to the transformer terminals the better.

Main sources of overvoltages can be classified into two groups: atmospheric and switching overvoltages.

Atmospheric overvoltages

Processes in the atmosphere, i.e. atmospheric overvoltages due to lightning strikes:

– direct strikes

– indirect → induced overvoltages

Switching overvoltages

Overvoltages caused by opening of circuit breakers, particularly severe in case of vacuum circuit breakers (VCB). The currents are interrupted very fast (current chopping) resulting in high-frequency switching overvoltages. Opening or closing a VCB can cause a prestrike or re-ignition, creating high frequency currents. VCB is able to interrupt abruptly causing virtual current chopping at levels near the peak currents.

Traditional overvoltage protection elements

1. Surge arresters

Most common overvoltage protection uses surge arresters [7]. These elements have non-linear impedance as function of applied voltage. In normal conditions the impedance of surge arrester is very high so that it conducts just a negligible current. When the voltage increases above certain value (knee point) the impedance drops dramatically. And this is the principle of sure arrester. In case of high overvoltage the surge arrester creates a low-impedance path to the surge energy and limits the magnitude of overvoltage.

Surge arresters can be installed phase to ground or phase to phase. Best is the combination of both, i.e. phase to ground and phase to phase. Standard connection then requires 6 surge arresters. There is also an economical connection using only 4 surge arresters (see Figure 6).

2. RC snubbers

RC snubber is another means of overvoltage protection. Sometimes it is also called RC network. The principle is used for transformers as well as DOL motors. Integration of RC snubbers is slightly more complex than surge arresters. It requires basic information of the supplying grid and associated small network study.

So what overvoltage protection shall be used? Surge arresters or RC snubbers? It depends on system parameters, such as input voltage level and connection between circuit breaker and transformer, type of circuit breaker, grid characteristic etc. Generally we can say that most efficient overvoltage protection is the combination of both RC snubbers and surge arresters. The question is not only what type of protection element to select, but also what rating it shall have and how the connection shall look like.

Besides surge arresters and RC snubbers there are few other options, such as e.g. smart chokes. However, their use is rather rare and they are not covered in this article.

Below figures show just a principle connection. For details on dimensioning/selection and installation of surge protection refer to corresponding guidelines, such as [8].

There are multiple “smart connections” of surge arresters and RC snubbers for enhanced protection. As those are intellectual property of the manufacturers, we don’t show them here. Ask the manufacturer or system integrator for application guidelines.

Note that surge arresters and RC snubbers shall always be connected as close as possible to the transformer terminals.

Lightning and switching overvoltages

The very fast transients associated with opening of a circuit breaker became actual after the vacuum circuit breakers started to be widely used at medium voltage. The old types of breakers (bulk-oil and minimum-oil circuit breaker, air-blast circuit breaker) were interrupting the current at or close to zero-crossing. Such interruption resulted in less severe transients. As seen in figure below the normalized switching impulse test has much slower rise time than lightning impulse (250 μs vs 1.2 μs).

However, modern VCB changed the picture completely. VCB is the most common type of circuit breaker accounting up to 80% of all installations. The vacuum chamber allows rapid interruption of current causing very fast transient overvoltages as side effect. The fast-impulse test waveform should be the one covering overvoltages caused by VCB. The rise time of voltage is extremely short (3…100 ns) –> extremely high dv/dt! Such fast transient affects the voltage distribution across the windings and is much more demanding for the insulation system.

>> Note that transformer that successfully passed the lightning impulse test in the factory may not withstand the fast transient overvoltages caused by VCB in field operation! <<

What overvoltage protection shall I use to protect my transformer against fast overvoltage transients?

The answer depends on the supply voltage level. The higher the input voltage the more careful the user shall be. While voltage classes from 4.16 kV all the way up to 10/11 kV seem relatively unproblematic, higher voltage levels (e.g. 20 kV or 33 kV) deserve some more attention. A combination of surge arresters and RC snubbers is a good practice. Surge arresters limit the voltage peaks to a defined value, but the voltage remains very steep (high dv/dt). In contrary, RC snubbers help to reduce the dv/dt while the voltage peaks are not so uniquely defined. Therefore, a combination of those two is a good match.

To illustrate the behavior we first show a comparison of VCB switching transients in a 20 kV system.

BIL stands for Basic Impulse Level. It is a term defined in ANSI/IEEE, but popularly used also elsewhere. In IEC world the closest equivalent is the lightning impulse (LI) withstand voltage. BIL level of the transformer under test is depicted by black dashed lines.

What you see summarized in above figure is actually a result of several years of research with extensive testing in ABB high-voltage laboratory in Västeras, Sweden costing millions of dollars. Transient voltage protection (TVP, formerly known as TVTR) is ABB patented technology utilizing winding varistors [9]. The solution is very effective in suppressing the very fast transient overvoltages. TVP does not require any network study and is therefore easy to apply. Another advantage is its compactness. Note that TVP availability may not cover all special multi-winding VFD transformers. However, there are other means of effective switching overvoltage protection. When the rules are followed, the likelihood of a transformer failure is minimized to lowest possible level. Consult the VFD or transformer manufacturer for recommendations and best practice.

Understanding the very fast transient overvoltage phenomena

Deep understanding of very fast transient switching overvoltages requires a multi-disciplinary approach and solid know-how on system components and their interaction. ABB recognized some white spots regarding very fast transient overvoltages associated with variable frequency drive systems. The response of the company was to organize several expert exchanges from the factories as well as corporate research centers. Following parties were involved:

– Medium voltage AC drives

– Dry type transformers

– Vacuum circuit breakers

– Corporate research centers (Sweden, Switzerland)

The collaboration was very effective and fruitful. The phenomenon was well understood and internal guidelines were defined to set best practice and maximize the availability and reliability of drive systems. It is a great example of inter-disciplinary approach for sustainable problem solving.

The number of transformer failures due to very fast transient switching overvoltages dropped dramatically. That is a proof that defined measures are efficient. It is a huge advantage of using a manufacturer with vast system know-how not limited to just a product delivery.