Field of the invention

The invention relates generally to the field of power transformers, and in particular to the protection thereof.

Background of the invention

Polyphase electrical power distribution systems may be of ungrounded type, which provides continuous service even when a ground fault occurs on one phase. In such systems, a single-phase to ground fault causes a voltage-to-ground of the non-faulted phases to rise a great deal. In particular, the overvoltage on the non-faulted phases has a phase-to- ground amplitude equal to the primary side phase-to-phase voltage multiplied with the transformer ratio.

The overvoltage will remain until ac circuit breakers on the primary side are opened, which may take several seconds.

Today, arrester devices are used on the secondary side, and they have to be dimensioned to take care of a very high energy and may further be damaged. In the prior art, several such arrester devices are connected in parallel in order to be able to handle the high energies, which is a costly solution .

Summary of the invention

It is a general object of the invention to provide protection means for a secondary side of a transformer, eliminating or at least alleviating the shortcomings of the prior art. It is a particular object of the present invention to provide means for reducing overvoltages on a transformer secondary side caused by phase-to-ground faults.

These objects, among other, are achieved by a method and by a transformer as claimed in the independent claims.

In accordance with the invention, a method for protecting a transformer is provided. The transformer comprises a primary side and an ungrounded secondary side, wherein the secondary side comprises a neutral. The method comprises the steps of: establishing a single-phase-to-ground fault on the secondary side; and operating a switching device for connecting the neutral to ground. By means of the invention, and in particular by quickly connecting the neutral to ground upon detection of a fault, overvoltages on the secondary side caused by single-phase-to-ground faults can be reduced by a factor of 1/V3. Further, a more cost-efficient solution is provided since the number of arresters can be reduced compared to known solutions. Further yet, by means of the invention, the functioning of existing ac circuit breakers on the primary side may be made more reliable, in that the switching device of the invention may prevent dc-voltages from entering the transformer, whereby the circuit breakers are more reliably opened.

In accordance with an embodiment of the invention, the transformer further comprises an arrester device connected in parallel with the switching device. The method then comprises the further step of connecting the neutral to ground by means of the arrester device during the time interval elapsed between the steps of establishing the single-phase-to-ground fault, and the step of operating the switching device. The arrester device can. be dimensioned accordingly _. , which puts much lower requirements on the arrester device than in prior art solutions having no switching device that quickly connects the neutral to ground. The invention thus provides a more cost-efficient solution.

The invention also relates to a transformer, whereby advantages corresponding to the above are achieved.

Further features and advantages thereof will become clear upon reading the following detailed description and the accompanying drawings .

Brief description of the drawings

Figure 1 illustrates a protection device for a transformer in accordance with the present invention.

Figure 2 illustrates a flow chart over steps of a method in accordance with the present invention.

Detailed description of embodiments of the invention

Figure 1 illustrates a transformer 1 in accordance with the present invention. The transformer 1 comprises a primary side 2 and an ungrounded, secondary side 3. The transformer illustrated in the figure 1 comprises three phases, but it should be realized that any poly-phase transformer having two or more phases may benefit from, the present invention.

The primary side 2 may be Y-connected, also denoted wye- connected or star-connected, or it may be delta-connected.

The secondary side 3 is Y-connected. That is, conceivable transformer 1 connections thus comprise delta--wye and wye- wye .

The primary side 2 is, for example, connected to a power distribution system. The primary side 2 further comprises a number of ac circuit breakers 9, typically one for each phase, for tripping the corresponding phase upon detection of fault, in a known manner.

The secondary side 3 comprises a neutral 4, to which the phases are all connected. The secondary side 3 is connected to a load S, for example a voltage source converter (VSC) . It is noted that the load 3 may be any DC-load requiring an ungrounded transformer secondary side.

In accordance with the invention, the transformer i comprises a switching device 5 connected at one end to the neutral 4 and at. the other end to ground 6. The switching device 5 is thus arranged to connect the neutral 4 to ground 5.

An example of switching device 5 that can be used in the system in accordance with the invention comprises arc protection relays, such as Arc Protection Module, provided by and available from ABB. Another example comprises the use of CapThor, also provided by and available from ABB. Briefly, CapThor is a new protection scheme for series capacitors, replacing conventional forced triggered spark gaps used in various combinations with MOVs (metal oxide varistors) . CapThor is a very fast switch comprising a combination of arc plasma injection followed by a very fast closing mechanical contact, which make it suitable for use with the present invention. The switch has a high closing capability. Further, CapThor requires no electrode adjustments or other particular adaptations for the different capacitor voltages or fault currents for specific projects. Since the CapThor does not require a high electrical field between the electrodes for proper operation, it does not suffer from conventional spark gap design difficulties: electrodes close enough for secure operation, yet separated enough for not to unintentionally spark over. It is realized that yet other fast grounding switching devices could be used.

By mearis of the invention, overvoltages caused by phase-to- grourid faults are reduced. As mentioned in the introductory part, a single-phase-to-ground fault 10 of one of the phases will cause a high overvoltage in the other phases. The overvoltages in the other phases will have a phase-to-ground amplitude equal to the primary side phase-to-phase voltage multiplied with the transformer ratio and these overvoltages will remain until the ac-breaker 9 is opened (tripped).

As the transformer secondary side neutral 4 is connected to ground 6 by the fast switching device 5, the overvoltages on the secondary side 3, caused by the single-phase-to-ground fault. 10, will be reduced to the primary side phase to ground voltage multiplied with the transformer ratio, i.e. the overvoltages will be reduced by a factor of 1/V3.

The single-phase-to-ground fault 10 may be detected in any suitable manner, for example by detecting an increased DC voltage at the load 8.

In accordance with an embodiment of the invention, the transformer 1 comprises an arrester device 7 connected in parallel with the switching device 5. That is, the ar.resr.er device 7 is connected at one end to the neutral 4 and at the other to ground 6. The arrester device 7 may be any known arrester device, although requiring lower protective level than what is used in the prior art, and having only one parallel column.

The arrester device 7 limits the overvoltages up to the moment when the switch contacts of the switching device 5 are closed. When the switching device 5 is closed, the neutral 4 is grounded.

The invention may enable a more reliable tripping of the ac- ci.rcu.it breakers 9 on the primary side 2, by preventing dc- voltages voltages from entering the transformer and thereby making the opening of the ac-circuit breakers 9 more easy.

The invention also provides a method for protecting a transformer 1 as described above. With reference to figure 2, the method 20 comprises the first step of establishing 21 a singie-phaεe-to-ground fault on the secondary side 3. As mentioned earlier, the single-phase-to-ground fault can be established or detected in any suitable manner. The method 20 comprises the second step of operating 22 the switching device 5 for connecting the neutral 4 to ground 6. The switching device 5 is automatically closed upon detection of the fault, typically within a few milliseconds (ms) .

In case the transformer 1 further comprises the arrester device 7, as described earlier, connected in parallel with the switching device 5, the method 20 comprises the further step of connecting the neutral 4 to ground 6 by means of the arrester device 7. The arrester device 7 connects the neutral 4 to ground 6 during the time interval that elapses from the establishment, of the single-phase-to-ground fault to the moment the switching device 5 is closed. The arrester device 7 can be dimensioned so as to handle the energies occurring during the short span of time, typically milliseconds, that elapses before the switching device 5 closes. The arrester device 7 thus needs to be of considerably lower dimensions than prior art safety measures, wherein several arrester devices have to be connected in parallel in order to be able to handle the high energies during the overvoltages. The invention thus provides a more cost-efficient solution.

The method 20 may be applied to any transformer provided that the secondary side thereof is of ungrounded star type.