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The stable operation and reliable breaking of large generator current are a difficult problem in power system. It can be solved successfully by the parallel interrupters and proper timing sequence with phase-control technology, in which the strategy of breaker’s control is decided by the time of both the first-opening phase and second-opening phase. The precise transfer current’s model can provide the proper timing sequence to break the generator circuit breaker. By analysis of the transfer current’s experiments and data, the real vacuum arc resistance and precise correctional model in the large transfer current’s process are obtained in this paper. The transfer time calculated by the correctional model of transfer current is very close to the actual transfer time. It can provide guidance for planning proper timing sequence and breaking the vacuum generator circuit breaker with the parallel interrupters.

With the rapid development of the power system and the increase of the transmission capacity, it requires safer and more stable environment. The fault current’s breaking capacity and the longevity of high voltage circuit breaker that controls and protects power system are essential to the reliable operation of electric power systems. It can effectively reduce the average arcing time and peak arc current to use synchronous technology [

The rated current of single vacuum circuit breaker is less than 5 ka. And the single breaker is unable to burden the high rated continuous current and break the large short-circuit current [

The parallel vacuum interrupters are used to share the rated large current. When the fault current occurs, main vacuum interrupter (MVI) is opened firstly. The fault current gradually is transferred from MVI to AVI (auxiliary vacuum interrupter). At the same time, the current constantly decays.

At the end of the current-transfer process, the fault current through AVI is removed with phase-control technology [

In the paper [

At the beginning, the large rated current is shared by MVI and AVI as shown in Figure

MVI and AVI share the large current.

When the short-circuit fault occurs, MVI is opened firstly. The large current gradually is transferred from MVI to AVI. The equivalent circuit is shown in Figure

Equivalent circuit when MVI’s contacts are opened.

Short-circuit current of the system [

In (

The impedance of power source can be ignored when it is compared with the interrupter’s resistance and the system capacity is much larger. Considering the constant arc voltage

In (

The emulational value is deviated from the actual value, because arc resistance is assumed to be constant and unchanged with time in the model of transfer current.

In the experiment, two parallel breakers are used to share the large current as shown in Figure

The two parallel breakers.

The two breakers controlled by the controller are opened according to the timing sequence. The current signal of the two breakers is collected by Rogowski coil and the arc voltage is collected by the resistance-type voltage divider that is parallel with the MVI. In Figures

Transfer current of 5.2 ka and voltage.

Transfer current of 9.6 ka and voltage.

The current that transfers from MVI to AVI is prevented by AVI’s reactance. So the moment of breaking MVI needs to avoid the current’s peak and it is better to choose the time in which current’s value is relatively low. In the moment that is close to current’s zero crossing, the success rate of transferring is higher. The enough computation-time of the controller is required to break AVI with the phase-control technology and the result in the moment is more obviously and more representatively compared with all experimental data. So the time of breaking MVI is selected at 8.3 ms in this paper. The waveforms of the oscilloscope are only listed in 5.2 ka and 9.6 ka due to limited space. MVI’s current (curve 1) and the voltage (curve 2) across the interrupter are shown in Figures

In Figures

It is shown how the MVI’s and AVI’s currents change in the first half-cycle when MVI is opened in Figures

MVI’s and AVI’s currents in transfer current of 5.2 ka.

MVI’s and AVI’s currents in transfer current of 9.6 ka.

The arc voltage in the transfer process.

The real vacuum arc resistance is calculated with transfer current and arc voltage as shown in Figures

The vacuum arc resistance in transfer current of 9.6 ka.

The vacuum arc resistance in all transfer currents.

The vacuum arc resistance is close to exponential growth. The arcing time becomes longer with the increase of transfer current. At the beginning of the transfer process, arc resistance grows very slowly at a low level. On the contrary, the arc resistance increases rapidly to a high value when the arc is extinguished.

The mathematical formula of the arc resistance is expressed with (

In (

The arc resistance is related with transfer current and arcing time. The arcing time becomes longer with the increase of the transfer current and the accumulation of heat becomes more and more. In (

The red points are the experimental data and the blue curve is the calculated value by the mathematical model of vacuum arc resistance. The calculated arc resistance is very close to the actual value with the model as shown in Figures

Arc resistance fitting in transfer current of 3.7 ka.

Arc resistance fitting in transfer current of 6.4 ka.

Arc resistance fitting in transfer current of 7.7 ka.

Arc resistance fitting in transfer current of 9.6 ka.

The

The correctional transfer time is calculated with the correctional model of transfer current. The primary transfer time is calculated with the model of transfer current ((

Three kinds of transfer time’s comparison in different currents.

Three kinds of transfer time’s comparison at different time of breaking MVI.

The comparison of the three kinds of transfer time in different current is shown in Figure

The correctional model of transfer current by analyzing experimental data with Matlab can accurately reflect the changing of the real vacuum arc resistance. The correctional model is more accurate and the correctional transfer time is closer to the actual transfer time. It can provide a proper timing sequence for breaking AVI with the phase-control technology.

So the correctional model can provide more accurate transfer time than the primary model for using the phase-control technology. The main contribution of the models can provide guidance for planning proper timing sequence and breaking a vacuum generator circuit breaker with the parallel interrupters.

Because the vacuum arc is affected by many factors, there is much work to research the more accurate vacuum arc’s model.

The mathematical model of vacuum arc resistance and the correctional model of transfer current are established in this paper. By the analysis of experimental data and the results of simulation, the changing of real arc resistance is described and the real vacuum arc resistance is close to exponential growth. At the beginning of the transfer process, the arc resistance’s growth rate is very low. But when the arc is extinguished, the real arc resistance increases rapidly to a high value. The arcing time becomes longer and the arc voltage is higher with the increase of the transfer current. At the same time, the duration of the low arc resistance’s state maintains more time.

Although the mathematical model of vacuum arc resistance and the correctional model of transfer current are obtained under specific experimental condition, each of the important parameters in the models is adjustable according to specific circumstances. The real vacuum arc resistance’s model and the correctional model of transfer current are widely applicable.

Because the transfer time calculated by the correctional model of transfer current is very close to the actual transfer time and the deviation is smaller, the models can provide proper guidance for breaking a large generator current with the phase-control technology.

As future work, there is much work to integrate the robust control algorithm [

The authors declare that there is no conflict of interests regarding the publication of this paper.

This work is supported by the National Natural Science Foundation of China (no. 51277019).