Four ways to bolster the short-circuit withstand capability of HV transformers

Short-circuit withstand capability

There are many methods applied to bolster the short circuit withstand capability of the transformers: improvement of the materials, reforming of the design and good maintenance in the operation process and so on.

However, this article deals with methods for the transformers which are in use and those which are hard to modify or expensive to modify.

Contents:

1. Installation of neutral reactors
2. Installation of current limitation series reactors
3. Installation of fast switches with high capacity
4. Installation of controllable Fault Current Limiters
5. The comparison
6. Notes

1. Installation of neutral reactors

Usually, the probability of power systems encountering single phase short circuit accidents is much higher than the probability of power systems encountering three-phase short circuit accidents.

The single phase short circuit current is affected by the positive sequence impedance and zero sequence impedance. One effective way to change the zero sequence impedance is changing the earthing methods of transformer neutral points, or installing the neutral grounding reactor.

Neutral grounding reactors are used for low-impedance grounding of the neutral point of three-phase networks in order to limit the fault current in the event of a phase-to-ground short circuit (fault current will be limited to the level of the phase-to-phase short circuit current).

One reactor terminal is connected to the neutral of the network and the other terminal is grounded.

During normal operation of the power system the current flow through the reactor is almost zero, since it is only driven by the imbalance of the three-phase network.

The ordinary installation place of the neutral reactor is in the compensation equipment called HV shunt reactor.

In China, people often use the star connection for the HV shunt reactor, and then add a reactor in series at the neutral point of the star connection.

The functions of reactors here are to compensate the phase to phase capacitor and grounding capacitor, speed up the termination of the secondary arc current and to make it easier to adopt the single-phase reclosers.

One application example happened in 2004, in Ningbo City, Zhejiang Province,China. Engineers installed little reactors which are 15 Ω as the neutral earthing reactor for a 500kV transformer in Lanting Substation. The short circuit current decreased.

The details are shown in Table 1:

Table 1 – The short circuit current influenced by neutral reactors in Lanting Substation

When the neutral points connect with reactors, the zero sequence impedance will change.

The grounding short circuit current of double phases may be larger than that of the single phase. So it’s necessary to check both the single phase short circuit and double phases short circuit after the installation of a neutral reactor.

Go back to contents ↑

2. Installation of current limitation series reactors

The series reactor is a high-voltage electrical apparatus designed to limit the current of a short circuit and maintain adequate voltage on the buses of distribution switchgear during a short circuit in a network.

It consists of an inductance coil. Such reactors are also used to compensate reactive power in order to improve the transmission capacity of power lines.

The use of reactors is a traditional and commonly used method for the limitation of the short circuit current. The reactors are usually installed at the areas where the short-line-fault may happen and connect in series in the circuits which require a limit to the short circuit current.

The disadvantage is that the reactor will increase the losses of power. It may influence the stability of the power system.

The current limitation series reactor is usually applied at outlets of the LV side, and can also be applied at the 35 kV medium side in 220 kV transformer.

This method is suitable for both three phase short circuit and single phase short circuit.

The example is ABB’s project for the Metro Grid. They build a series reactor which transfers the power from Sydney South to Haymarket Substation in Australia. It’s the biggest reactor made by ABB.

Go back to contents ↑

3. Installation of fast switches with high capacity

The representatives of fast switches with high capacity are Is-limiter (made by ABB), Pyristor (made by Ferraz) and C-Lip (made by G&W).

It has several advantages in the technical field:

1. Fast cut-off ability (less than 2 ms);
2. Fast limitation of the large short circuit current: Is-limiter is capable of detecting and limiting an short circuit current at the first rise, i.e. in less than 1ms
3. Less occupied space
4. Easy to install and maintain

It is a fault current limiting device that uses chemical charges and current-limiting fuses to interrupt the fault current within the first quarter to half cycle (i.e. before the first peak).

In a typical Is-limiter design, the device is composed of two current paths connected together in parallel. One path is an element rated for the full load current (which can have high continuous current ratings, e.g. 3000 A), and the other path provides the current limiting function via a current-limiting fuse (which typically has a continuous current rating of <300 A at 15 kV).

Working principle

The work principle can be described as follows: when the short circuit happens, the current transformer module detects the signal and transfers it to the control module. Then the control module is triggered and turns the disconnector on to ‘move’ the short circuit current into the fuse module at the instant.

Here, the current is cut off.

The interesting thing and special feature is that the device uses the explosive to get the rapid cut-off ability. It can limit both short circuit currents of single phase and three-phase.

The working process can be described as follows:

Go back to contents ↑

4. Installation of controllable Fault Current Limiters

Fault Current Limiter (FCL) is also called Short Circuit Current Limiter (SCCL). There are several different types: Superconducting Fault Current Limiter (SFCL) and the controllable Fault Current Limiter which is based on electronic technology and so on.

The shortcomings of a superconducting fault current limiter are:

1. The working environment is quite harsh: the high-temperature superconductor needs liquid nitrogen (N2). The critical temperature is 77 K (about -196 ℃). And the low-temperature superconductor needs liquid nitrogen liquid helium (He). The critical temperature is 4 K (about 269.15 ℃ ). Once the working temperature is over the critical temperature, the SFCL will not able to keep the superconductor character.
2. The technique is not mature enough. In China, there are only two prototypes running. The number of SFCLs which are running in Switzerland, Germany, Great Britain and USA is less than 20.

So it’s not suitable to be applied for modifying the transformers that are running now.

However, the controllable Fault Current Limiter based on electronic technology is more mature than SFCL.

As an example, see the figure below:

The controllable FCL shown is the series type. It consists of the capacitor (C), the inductance (L) and a bypass switch (K).

Normally, the switch doesn’t work and it’s open. L and C work on the series resonance condition. The impedance could be regarded as zero, compared with the total impedance. So the influence of FCL could be acceptable.

The advantages are:

1. No influence on the protection solution being used
2. No influence on the stability of the current
3. Less occupied space. The weaknesses are high cost and the maintenance.

Go back to contents ↑

5. The comparison

The table below is the comparison of 4 methods described above:

Table 2 – The reinforcement methods of transformers’ short circuit withstand capability

Go back to contents ↑

6. Notes

In the practical work, the risk of three-phase short circuit accidents at the MV side of 500 kV and 220 kV transformers is quite little.

The main risk is the single phase accident. Bolstering short circuit withstand capability of the single phase or the limitation of the short circuit current at the single phase can reduce the number of transformer short circuit accidents significantly.

So the best selection is to install the neutral reactor. If there does exist the need for three-phase protection, we can install the fast switch or FCL.

For the LV side accident, people can opt to install current limitation series reactors, or fast switches if the space is limited.

Go back to contents ↑

References //

1. Transformer Short Circuit Current Calculation and Solutions by Ling Song
2. Electric Power Substations Engineering by John D. McDonald