Controlled Vacuum 6 (10) kV Circuit Breaker Model
Controlled Vacuum 6 (10) kV Circuit Breaker Model
Controlled Vacuum 6 (10) kV Circuit Breaker Model
Dmitry Pavlyuchenko
Head of Power Supply Systems Department
Novosibirsk State Technical University
Novosibirsk, Russia
d_pavluc@mail.ru
Dmitry E. Shevtsov
Head of Power Supply Systems Department
Novosibirsk State Technical University
Novosibirsk, Russia
dmitriy_shevtsov@mail.ru
Abstract— At the present time, a level of switching overvoltages can be reduced without using special limiting devices, namely by implementing the technology of controlled switching into a circuit breaker. To investigate switching transients, the paper presents the simulation model for a controlled (synchronous) vacuum circuit breaker. The model involves features and parameters of vacuum circuit breakers that were also investigated in the paper.
Keywords— controlled switching, model of controlled circuit breaker, inrush currents, overvoltages, circuit breaker parameters
I. Introduction
As is known, switchings in electrical networks induce transients representing energy redistribution between inductive and capacitive network elements. Such transient processes are not influenced by the type of a circuit breaker. In this case, overvoltage levels and amplitudes of inrush currents depend on the instant of contact switching.
In recent decades, vacuum circuit breakers (VCB) have been preferred as medium voltage protective switching equipment. However, VCB switchings may induce significant high-frequency overvoltages. It is mainly caused by high arc extinguishing capability of vacuum circuit breakers [1]. Overlapping of negative features of circuit breakers and unfavourable switching instants leads to higher overvoltage levels and has a negative impact on high voltage insulation.
To minimize overvoltages and inrush currents at VCB switchings, the technology of controlled switching is of great importance. The main idea of controlled switching is closing or opening an electrical network at optimal instants [2].
The paper attempts to develop the simulation model of a circuit breaker where the principles of controlled switching can be realized and the main parameters of real vacuum circuit breakers can be considered.
II. Investigations of Controlled Vacuum Circuit Breaker Parameters
The main VCB parameters for adequate switching transient simulation are the following: the value of chopping current before the natural zero crossing, the characteristic and recovery rate of dielectric strength in a contact gap, and the maximum rate of interrupting current. Moreover, controlled switching devices shall comply with strict requirements on mechanical characteristics, namely the stability of closing time and opening time.
It is rather difficult to determine these parameters for the specified circuit breaker for evaluation of induced switching overvoltages and inrush currents. It can be explained by the fact that manufacturers do not generally estimate these parameters or estimate only for their own application. Therefore, at present, determination of required VCB parameters is possible only by independent experimental investigations or by analysis of professional literature.
The paper presents the results of investigations of parameters for the controlled vacuum circuit breaker EX-BBC SMARTIC 6(10)-20/1000 [1]. Experimental investigations were carried out at the laboratory of Power Engineering Institute of National Research Tomsk Polytechnic University.
Current chopping is a sharp current drop at circuit breaker opening from some value to zero. Due to a small time of flow and a high decreasing rate, overvoltages can be generated. Current chopping is typical for circuit breakers with any arc-extinguishing medium (i.e. live-tank oil, vacuum, SF6, etc) and depends on the contact material. In vacuum circuit breakers the reason for current chopping is unstable arcing at small currents in contact metal vapours [3].
Fig. 1 illustrates the diagram of the experimental unit. As a power source, a 50 Hz AC power supply is used. Load consists of resistors.
Fig. 1. Diagram of the experimental unit.
During investigations of the controlled VCB, values of chopping currents in three poles at opening of 100A current of 50Hz frequency were obtained.
Processing of experimental data was done with regard to random errors. To identify outliers in the samples the Smirnov – Grubbs criterion was used. In this case, there was an assumption of the normal distribution for the samples.
Testing of the hypotheses on the distribution law was performed using the Cramér – von Mises – Smirnov test (nω2) and Pearson chi-squared test (χ2). The hypothesis of the normal distribution for values of a chopping current was made for testing. The results are given in Table I.
TABLE I. Results of testing for normality
|
|
Cramér–von Mises–Smirnov test |
Pearson test |
||||
|
(Nω2)А |
(Nω2)В |
(Nω2)С |
(Χ2)А |
(Χ2)В |
(Χ2)С |
|
|
Test statistic |
0.432 |
0.157 |
0.216 |
10.32 |
4.649 |
5.623 |
|
Critical values at the significance level of q = 0.05 |
0.465 |
0.465 |
0.465 |
11.07 |
11.07 |
5.991 |
Critical values for the Pearson test for phase A and phase B are determined at nine intervals of experimental data, while for phase C – at five intervals.
Using two above mentioned tests, it was found that test statistics do not exceed critical values. It shows that the hypothesis of the normal distribution for chopping currents is accepted for three phases, i.e. for three poles of the circuit breaker.
As a result, statistical characteristics of chopping currents were obtained, including confidence intervals for MIchop and DIchop, given in Table II.
TABLE II. Statistical characteristics of the controlled circuit breaker
|
Parameter |
Phase A |
Phase B |
Phase C |
|
Sample size |
96 |
100 |
100 |
|
ICHOPmin, A |
1.0 |
0.5 |
0.7 |
|
ICHOPmean, A |
1.93 |
2.6 |
2.14 |
|
ICHOPmax, A |
3.7 |
5.0 |
5.8 |
|
MIChop, A, at P=0.95 |
1.780<MIChop<2.06 |
2.45<MIChop<2.80 |
1.99<MIChop<2.29 |
|
D, A2 |
0.41 |
0.77 |
0.56 |
|
DIChop, A2, at P=0.95 |
0.32<DIChop<0.56 |
0.60<DIChop<1.04 |
0.43<DIChop<0.76 |
Differences between chopping currents can be explained by different interrupting currents and different initial velocities of contact moving.
Obtained results show that it is required to carry out experiments with several circuit breakers of the same type and different poles of the same circuit breaker to get adequate averaged values.
Arc reignitions in the arc-extinguishing chamber of a circuit breaker induce dangerous high-frequency overvoltages. Arc reignitions are caused by insufficient dielectric strength of a contact gap [3]. Therefore, the most important parameters characterizing the level of high-frequency overvoltages are the characteristic and the recovery rate of dielectric strength in a contact gap.
The characteristic of dielectric strength at circuit breaker opening can be expressed by the linear function:
udiel(t) = k · t (1)
where k – recovery rate of dielectric strength in a contact gap, kV/ms, t – current instant, ms.
The kind of characteristic (1) is influenced by the following physical process: the increase of dielectric strength in a contact gap is caused by the growth of distance between separating contacts of a circuit breaker. It should be noted that before arc extinction circuit breaker contacts successfully separate to some distance that provide definite dielectric strength after arc extinction.
Analysis of professional literature shows that dielectric strength recovery rate in a contact gap for modern vacuum circuit breakers is equal to k = 20…80 kV/ms [4, 5].
Voltage applied to separating contacts can be calculated as:
uCB(t) = uS(t) – uL(t) (2)
where uCB(t) – voltage between circuit breaker contacts, uS(t) – voltage of a circuit breaker pole from the source side, uL(t) – voltage of a circuit breaker pole from the load side.
The first arc quenching occurs at the instant when instantaneous current becomes lower than chopping current ichop. After current interruption voltage is recovering across circuit breaker contacts. If recovery voltage between contacts (2) exceeds dielectric strength of a contact gap (1), than arc reignition will occur. Further arc extinction is possible at current zero crossing provided that the rate of high-frequency current is not greater than the maximum value of di/dt. The maximum rate of interrupting current for vacuum circuit breakers is in the range of 50-160 A/ms [4].
Controlled switching devices shall comply with strict requirements on stability of closing time and opening time: not more than ± 1-2 ms [6]. In this case, single-pole switching is required.
It should be noted that scattering of closing time and opening time should not exceed permissible values at changes of ambient temperature, power supply fluctuations, and wearing and aging of circuit breaker components.
Investigations on scattering of time of the beginning of contact separation and time of the end of contact closing were performed. The results of measurements of time of the beginning of contact separation are presented in Table III.
Table III shows that scattering of time of the beginning of contact separation for a single pole is not greater than 0.12 ms. Scattering of contact separation time between poles is not greater than 0.64 ms. Time of nonsimultaneous contact separation of a circuit breaker is in the range from 1.62 ms to 2.26 ms.
TABLE III. Time of the beginning of contact separation for a circuit breaker, ms
|
№ |
А |
В |
С |
|
1 |
9.60 |
8.50 |
10.2 |
|
2 |
9.56 |
8.58 |
10.4 |
|
3 |
9.56 |
8.44 |
10.4 |
|
4 |
9.56 |
8.48 |
10.6 |
|
5 |
9.56 |
8.52 |
10.7 |
|
6 |
9.48 |
8.52 |
10.6 |
|
7 |
9.48 |
8.48 |
10.6 |
|
8 |
9.56 |
8.52 |
10.7 |
|
9 |
9.48 |
8.52 |
10.7 |
|
10 |
9.50 |
8.50 |
10.6 |
The results of measurements of time of the end of contact closing are presented in Table IV.
TABLE IV. Time of the end of contact closing for a circuit breaker, ms
|
№ |
А |
В |
С |
|
1 |
27.0 |
26.2 |
25.2 |
|
2 |
27.2 |
26.7 |
25.5 |
|
3 |
27.1 |
26.4 |
25.2 |
|
4 |
27.0 |
26.4 |
25.2 |
|
5 |
27.0 |
26.3 |
25.3 |
|
6 |
27.0 |
26.4 |
25.3 |
|
7 |
27.0 |
26.4 |
25.3 |
|
8 |
27.0 |
26.4 |
25.2 |
Table IV shows that scattering of time of the end of contact closing for a single pole is not greater than 0.5 ms. Scattering of contact closing time between poles is not greater than 0.5 ms. Time of nonsimultaneous contact closing of a circuit breaker is in the range from 1.5 ms to 2.0 ms.
Scattering of closing time and opening time complies with requirements imposed to controlled switching devices. Time of nonsimultaneous contact closing or contact opening of circuit breaker poles can be reduced by using special switching algorithms.
Investigations of closing time and opening time for circuit breakers with expired switching lifetime were also carried out. Stability of operating times of a circuit breaker did not change.
III. Development of the Controlled Vacuum Circuit Breaker Model
The model of the controlled vacuum circuit breaker was developed in Matlab/Simulink which allows modeling complex power systems using simulation and structural modeling [7].
Under real conditions, circuit breaker operation is different from ideal operating conditions when contacts are closed quickly and contact opening occurs at zero crossing of industrial frequency current. Modern circuit breakers may have the following processes:
- current chopping at interruption before zero crossing;
- arc reignitions due to insufficient dielectric strength of a contact gap just after the arc extinction;
- virtual current chopping in adjacent phases because of arc reignitions in the first interrupted phase;
- prestrikes with contact gap reduction during circuit breaker closing.
Moreover, circuit breakers which can use the technology of controlled switching should have a possibility of separated switching for circuit breaker poles.
Based on above-mentioned information, the model of the controlled vacuum circuit breaker was developed (Fig. 2) with the following parameters:
- chopping current of the circuit breaker is 3-5 A;
- dielectric strength of a contact gap is described by a linear function and equal to 50 kV/ms;
- maximum rate of interrupting current is 50 A/ms;
- switching of different phases is possible at various instants.
The model of the controlled vacuum circuit breaker (Fig. 2) is developed for three poles of the circuit breaker using blocks of simulation modeling. Nonsimultaneous switching of circuit breaker poles is performed by Step1 ... Step3 blocks where the instant of switching is set for each pole. After sending a command for opening, circuit breaker contacts simulated by IdealSwitch1 ... IdealSwitch3 blocks are opened. Processes of arc ignition and arc extinction are modeled by IdealSwitch4 ... IdealSwitch6 blocks.
Current chopping occurs in all interrupted poles after the first arc extinction. Current is measured by Current1 ... Current3 blocks, than it is compared with the specified current value. If this current is less than or equal to the specified value , a command for arc extinction between contacts is sent by EnabledSubsystem7 ... EnabledSubsystem9 blocks. After that, only virtual current chopping may occur.
Arc reignitions may appear in three phases of the circuit breaker in the case of insufficient dielectric strength of a contact gap. Dielectric strength starts rising after the beginning of contact opening for each pole which is simulated by Integrator4 ... Integrator6 blocks.
Fig. 2. Model of the controlled vacuum circuit breaker
Electric arc is extinguished at current zero if the specified rate of current (when arc extinction is possible) is not exceeded. Blocks EnabledSubsystem13 ... EnabledSubsystem15 generate a command for extinction of are reignitions. Processes of arc ignition and arc extinction may repeat several times depending on the parameters of the interrupted circuit.
The presented simulation model can be used for development of models for controlled circuit breakers of other rated voltages.
IV. Verification and Validation of the Controlled Vacuum Circuit Breaker Model
For simulation modeling, the electrical network 6 kV with the unloaded transformer 630 kVA was chosen (Fig. 3). Switching of unloaded transformers may cause high overvoltages and arc reignitions which leads to insulation degradation. Inrush currents may occur at circuit breaker closing that causes relay desensitization and results in higher electrodynamic impacts on transformer windings.
Fig. 3. Model of the electrical network.
The transformer was modeled by the Three-Phase Transformer (Three Windings) block. The model involves a non-linear saturation curve of a magnetic core.
Fig. 4 shows the oscillogram of interruption of the unloaded transformer by three circuit breaker poles simultaneously. In this case, overvoltages reach 4.5·Urated with arc reignitions between circuit breaker contacts. Voltage after the next reignition is recovering with lower rate of rise.
Energization of the unloaded transformer is characterized by high inrush currents which may reach 6·Irated (Fig. 5). Among having the high amplitude, magnetizing current also flows for a long time. Magnetizing inrush current may decay for several seconds.
Obtained overvoltage levels and inrush currents comply with the values given in [8, 9].
Fig. 4. Oscillogram of voltages between phase C contacts of the circuit breaker at traditional interruption of the unloaded transformer.
Fig. 5. Oscillogram of magnetizing inrush currents in the transformer.
In a three-phase electrical network, voltage of each phase crosses zero at different instants. Thus, to minimize switching transients, circuit breaker poles should be switched separately.
When using controlled interruption for an unloaded transformer in the ungrounded network, the first phase should be switched at current zero. Currents of other phases become equal with opposite direction. Then, other circuit breaker poles are interrupted at current zero [2].
The oscillogram of voltages between circuit breaker contacts at controlled interruption of the transformer is shown in Fig. 6. In this case, overvoltages do not exceed 1.7·Urated and reignitions are not observed.
Fig. 6. Oscillogram of voltages between circuit breaker contacts at controlled interruption of the transformer.
Fig. 6 demonstrates that interruption of phase A occurs at the instant of 85 ms at current zero in this phase. Then, after 5 ms, phase B and phase C are switched off simultaneously at current zero.
To suppress magnetizing inrush currents of the transformer during energization, two phases should be switched at the maximum of phase-to-phase voltage. After that, the third phase should be energized at the maximum of phase voltage [2]. Using this switching algorithm, magnetizing inrush currents are prevented. However, overvoltages are still observed as shown in Fig. 7.
Fig. 7. Oscillogram of magnetizing inrush currents at controlled closing of the transformer
Fig. 7 illustrates that closing of phase A and phase B occurs at the instant of 3.3 ms at the maximum of phase-to-phase voltage. Then, phase C is energized at the maximum of phase voltage.
It should be noted that the best switching instants for minimizing inrush currents are usually the worst instants for induced overvoltages, and vice versa.
V. Conclusions
1. Parameters of the controlled vacuum circuit breaker EX-BBC SMARTIC 6(10)-20/1000 were experimentally investigated. Values of chopping currents were obtained for phase A – 1.9 A, phase B – 2.6 A, phase C – 2.1 A which are normally distributed with standard deviations of 0.4 A2 – for phase A, 0.8 A2 – for phase B, 0.6 A2 – for phase C.
2. Mechanical characteristics of the circuit breaker were experimentally investigated. It is shown that closing time of the circuit breaker is not greater than 27.2 ms with the scatter of no more than 0.5 ms. Opening time of the circuit breaker is not greater than 10.7 ms with the scatter of no more than 0.12 ms. It can be argued that the considered circuit breaker can be used as a controlled circuit breaker.
3. Analysis of professional literature for modern vacuum circuit breakers shows that dielectric strength recovery rate in a contact gap is equal to k = 20…80 kV/ms and the maximum rate of interrupting current is in the range of 50…160 A/ms.
4. The mathematical model of the controlled vacuum circuit breaker was developed with regard to features of controlled switching and parameters of real vacuum circuit breakers. The proposed model can be used for investigations of switching transients in 6 (10) kV electrical networks.
5. Using the example of switching of the unloaded transformer, the model of the 6(10) kV controlled vacuum circuit breaker was verified and validated. Obtained results on overvoltages at closing being equal to 4.5·Urated and inrush currents being equal to 6·Irated comply with reference data. The developed model involves single-phase switching of circuit breaker poles.
References
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