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DOI: 10.18721/ JEST.230113 УДК 621.387.35
В.Г. Кучинский, Н.Н. Сиддхарт
ПАРАМЕТРИЧЕСКОЕ ИССЛЕДОВАНИЕ МОЩНОГО ВЫКЛЮЧАТЕЛЯ ПОСТОЯННОГО ТОКА, ОСНОВАННОГО НА ВАКУУМНЫХ ДУГОГАСИТЕЛЬНЫХ КАМЕРАХ И ПОЛУПРОВОДНИКАХ
Проведено параметрическое исследование мощного выключателя постоянного тока, выполняющего функцию токограничения за счет своего быстродействия. В состав выключателя входят две вакуумные дугогасительные камеры, соединенные последовательно, причем одна из них шунтирована сборкой полупроводников. Принцип действия выключателя основан на том, что при размыкании контактов вакуумной камеры создается альтернативный путь протекания тока через параллельно включенные диоды, что существенно облегчает условия погасания дуги в вакуумной камере. Эти диоды также облегчают условия восстановления электрической прочности второй вакуумной камеры, конденсаторная батарея противотока переключает ток из цепи диодов и второй вакуумной камеры в батарею и варистор. Быстродействующий выключатель позволяет существенно ограничить амплитуду тока короткого замыкания, что особенно важно для цепей с малыми значениями индуктивностей, где большие производные нарастания тока.
ВЫКЛЮЧАТЕЛЬ ПОСТОЯННОГО ТОКА; ВАКУУМНАЯ ДУГОГАСИТЕЛЬНАЯ КАМЕРА; ПОЛУПРОВОДНИК; КОНДЕНСАТОР; ВАРИСТОР.
Ссылка при цитировании:
В.Г. Кучинский, Н.Н. Сиддхарт. Параметрическое исследование мощного выключателя постоянного тока, основанного на вакуумных дугогасительных камерах и полупроводниках // Научно-технические ведомости СПбГПУ. 2017. Т. 23. № 1. С. 134-139. DOI: 10.18721/ JEST.230113
V.G. Kuchinskii, N.N. Siddharth
PARAMETRIC STUDY OF A HIGH POWER DC CIRCUIT BREAKER BASED ON VACUUM INTERRUPTERS (VI) AND SEMICONDUCTORS
Parametric study of a two-stage high-power DC circuit breaker is carried out. It is a current-limiting breaker which is characterized by a fast opening time. The circuit breaker under consideration consists of two vacuum interrupters in series and one of them has the semiconductors in parallel. The principle of operation of this vacuum interrupter is mainly based on providing an alternate path for the load current to flow through parallel-connected diodes so that the contacts of the vacuum interrupter can be opened under minimal arc conditions. The diodes help restore the dielectric strength of the second vacuum interrupter when the counter pulse capacitor switches the current from the diodes and the second vacuum interrupter to the capacitance and varistor. A fast circuit breaker substantially reduces the short circuit current. This is especially important for the circuits with small inductances and therefore high current derivatives. DC BREAKER; VACUUM INTERRUPTER; SEMICONDUCTOR; CAPACITANCE; VARISTOR. Citation:
V.G. Kuchinskii, N.N. Siddharth, Parametric study of a high power DC circuit breaker based on vacuum interrupters (VI) and semiconductors, St. Petersburg polytechnic university journal of engineering sciences and technology, 23 (1) (2017) 134-139, DOI: 10.18721/ JEST.230113
Introduction
The process of commutation in DC circuits differs from AC circuits in the sense that in AC circuits, the breaker can be opened at the instant
when the current is close to zero. But in the case of DC circuits, since this phenomenon is absent alternate methods of opening the circuit breaker need to be devised. Also, the presence of energy storage
elements (inductances) complicates the process of commutation as the energy stored in these elements can further fuel the arc between the contacts of the circuit breaker. Therefore, all of these constraints are taken into account while designing a DC circuit breaker.
DC breakers
In DC circuits, during the short circuit mode, the current increases monotonically up to the steady state level which can be rather higher than a nominal value.
A very important feature of the breaker is the possibility to limit the maximum value of the short circuit current. To execute such a function, the breaker has to be as fast as possible. The current limiting breakers are characterized by a minimal value of opening time usually up to 5 ms.
Fig. 1 illustrates the difference between current limiting and non-current limiting breakers.
The current diagrams for current limiting breaker (/') and for the breaker without current limiting function (i") are shown in Fig. 1. The first interval, to is the time which is needed for the current to reach the limiting level, this depends on the circuit parameters and not on the breaker; tv — is the breaker opening time and it is an important breaker parameter. The current tends to increase for a short period of time until t2, since the voltage appearing on the arc is still less than the source voltage. Time interval t = t0 + t1 + t2 is the time for current limiting. During this time interval, the di/dt (current derivative) is very high and prolongation of it leads to a high increase in current. The values of the t1 + t2 time interval strongly depends on the breaker design. Therefore, by limiting it, the short circuit current can be limited.
Time interval ty is the time duration for arc shut down during which the current monotonically decreases.
Two stage vacuum valve breaker
The DC circuit breaker considered herewith is a two stage breaker. The first stage is one (or several) mechanical vacuum interrupters (VI) which under normal operating conditions carries the current. The second stage is a controlled semiconductor assemblage is connected in parallel to the mechanical valve and this carries the short circuit current for a short period, this is done for the current to bypass the mechanical valve through the semiconductor as-
Fig. 1. Breaker current and voltage behavior:
i' and u' — for current limiting breaker; i" and u" — for non-current limiting breakers
semblage. A counter pulse capacitor (CC) is used to mitigate the current through the semiconductor which is eventually transferred to and damped by a varistor [1—3].
Fig. 2 represents the circuit scheme with two vacuum interrupters QS1 and QS2 which are connected in series. A diode set VD2 is connected in parallel to QS2. The counter pulse capacitance along with the discharger FV1 and diode VD1 together are connected in parallel to both the vacuum interrupters and a varistor is connected across the branch of the counter pulse capacitance.
The vacuum valves QS1 and QS2 can be connected in parallel or in series. The reason for having the vacuum valves in series as opposed to them being in parallel is to reduce the total switching time by opening the contacts of both the vacuum valves simultaneously instead of one after another.
The command to open the contacts of both the vacuum valves, QS1 and QS2 are given simultaneously. Under the influence of the arc voltage drop on QS2 and voltage drop on its inductance, the current from QS2 switches into VD2.
After the current through QS2 reduces to zero and after QS2 regains its dielectric strength, the counter pulse capacitance (C1) is introduced into the circuit by means of igniting the discharger FV1. A choke, L connected in series with C1 is used to limit the value of the current derivative through the semiconductors. As the capacitance discharges, the current through VD2 and QS1 is mitigated and now the current begins to flow through the counter pulse capacitance thereby recharging it. When the current through QS1 reduces to zero, the voltage remaining on the C1 is applied to VD2 to keep it in the off state, so that it is ensured that QS1 regains its dielectric
=w=
VD2
Cl
FVl
L
-ппгг-
VDl
RUl
Fig. 2. Scheme of the with two vacuum interrupter: QS1, QS2 — vacuum interrupter; VDl, VD2 — diode sets; C1 — counter pulse capacitor; L — choke; FV1 — discharger; RU1— varistor
strength. The diode, VD2 serves two purposes, one to provide a path for the current to bypass QS2 so that its contacts can be opened and another to ensure that a high voltage does not appear across QS1 until the moment the voltage across C1 changes polarity. When the voltage of the counter pulse capacitance Clchanges polarity and reaches the operating level of the varistor, the current now flows through the varistor. The operating voltage level of the varistor is approximately two times the initial voltage on the C1 which is close to the voltage of the main DC source. Due to the varistor voltage, the current in the circuit subsequently damps to zero.
Switching virtual test
Main technical parameters of the circuit components:
1. Circuit inductance and resistance are calculated from the nominal values of source voltage and maximum prospective short circuit current. Tests are done for two values of source voltages, 5 kV and 10 kV, while the maximum value short circuit current in both the cases is taken to be 200 kA. The time constant in the short circuit mode is 6ms. From the aforementioned values, the values of circuit resistances are 0,05 Q and 0,025 Q for 10 kVand 5 kV respectively and the values of circuit inductances are 0,3 mH and 0,15 mH for 10 kV and 5 kV respectively.
2. The inductance of the vacuum interrupter is fixed as 0,4 |iH. The dynamic resistance of the arc in the vacuum interrupter is 1,5 mQ. The cathode arc voltage drop is 15 V. The vacuum interrupter voltage recovery time is taken to be 0,1 ms and the vacuum
interrupter opening time is 0,5 ms. It can be reached by help of the special electro dynamic drive system.
3. The parameters of the diode set (VD2) in parallel to the vacuum interrupter are:
The inductance is 0,1 |iH, VD2 threshold voltage — 4 V, VD2 slope resistance — 0,15 mQ. Permissible value of the current derivative in the VD2 — 1 kA/ ^s.
4. The counter pulse capacitor (C1) which is initially charged is provided to supply a current in the opposite direction to the fault current so as to mitigate it.
C1 parameters are as follows:
Initial voltage — 10 kV (for the source of 10 kV) and 5 kV (for the source of 5kV).
The initial value of the C1 capacitance was taken as 0,4 mF, but it may be changed to obtain the required time interval to recover the interrupter voltage strength (0,1 ms). Permissible overvoltage of the C1 = = 20 kV (for the source of 10 kV) and 10 kV (for the source of 5 kV).
The dynamic resistance of the FV1 switch is 0,1 mQ.
Results and observations
Switching test was carried out for two values of source voltages 10 kV and 5 kV.
The following are the results obtained for a source of 10 kV.
The behavior of the load current can be seen in Fig. 3.
Fig. 4 represents the voltage across the counter pulse capacitor.
I(QS2) is the current through the VI connected in parallel across the VD2. I(VD2) is the current through the diode set. I(C1) is the current through the counter pulse capacitance.
0,00 0,80 1,60 2,40 3,20 Time, ms
Fig. 3. Load current vs. time
Voltage, kV
15,00
7,50
0,00
-7,50
15,00
0,00
0,80 1,60 2,40 3,20
Fig. 4. Current pulse capacitor voltage vs. time
Time, ms
Fig. 5. Breaker current vs. time: I(VB1) — vacuum interrupter QS1 current; I(DS) — diode set VD2 current; I(CC) — current pulse capacitor current; I(RU) — varistor RU1 current
The typical operation of the DC breaker under consideration can be divided into three stages. The first stage begins when the command to open the contacts of both the vacuum interrupters (QS1 and QS2) is given. As the breaker opening time is specified to be 0,5 ms, the contacts are opened after this duration and an arc appears between the contacts. Under the influence of the arc voltage drop, the current in QS2 start to reduce and switches to the diode set
(VD2). The current through QS2 completely drops to zero at t2 this is the end of the first stage and after a period of 0,1 ms which is the time necessary for the vacuum interrupter to regain its dielectric strength, the discharger (FV1) is ignited so as to introduce the counter pulse capacitor (C1) into the circuit which is the beginning of the second stage at t3. The C1 is initially charged and when connected, it discharges by supplying a current in the direction opposite to
the load current through VD2 and QS1. When this current completely drops to zero at t4, the remaining voltage on the C1 kept applied to VD2 in order to keep it in the switch off state and to ensure that there is no voltage applied to QS1 in order to enable it to regain its dielectric strength. It is necessary that the back voltage is kept applied across the VD2 for not less than for 0,1 ms.
Thus, for a period from to to t4, the load current is completely transferred from the branch ofthe vacuum interrupters QS1 and QS2 to the counter-pulse capacitance branch. Now the current entirely flows through the C1 branch and thus forming a series RLC circuit. The end of the second stage is attained when the voltage on the C1 changes its polarity and increases up to the level of the DC source voltage and even further. So the load current reaches its maximum value and begins to decrease thereafter.
The third and final stage begins immediately after the completion of the second stage when the load current is decreasing. When the capacitor voltage exceeds the operating voltage of the varistor, it leads to a rapid decrement of the varistor resistance and, as a result, a complete shunting of the capacitor bank by the varistor takes place. Full transition of the load current from the capacitor branch to the varistor occurs at the instant t6. Varistor parameters are selected such that the voltage across the capacitor does not exceed 20 kV for a source voltage of 10 kV (and 10 kV for a source voltage of 5 kV). Thus provides both a sufficient level of the load voltage which is necessary for a rapid decrease of the load current,
and protection of the capacitor bank against overvoltages. The varistor subsequently damps the load current to zero which terminates the switching process at t6. The results obtained for a source of 5 kVhave similar characteristics to that of 10 kV.
For successful switching, the value of the counter pulse capacitance should be such that a minimum of 100 (is is ensured between the instants when the current through the VD2-QS1 branch drops to zero and the instant of changing polarity of the C1 voltage. It is found to be 0,55 mF for a source voltage of 10kV and 1.1 mF for a source voltage of 5 kV. Therefore, it is observed that as the source voltage is halved (10 kVto 5 kV), the minimum capacitance value is doubled (0.55mF to 1,1 mF), but the capacitance energy will be halved too. The maximum stored energy in the C1 was found to be 27,5 kJ (for 10 kV) and 13,75 kJ (for 5 kV).The maximum load current was found to be 47,08 kA (for 10 kV) and 45,875 kA (for 5 kV).
Summary and conclusion
The switching off process in the two stage breaker was studied. It was shown that by using vacuum interrupters with a small opening time (0,5 ms), one can significantly reduce the maximum values of the short circuit current from 200 kA up to 48 kA. The diode assemblage connected in parallel to one of the two vacuum interrupters, which are connected in series, allows not to pay attention to the value of the current derivative near the zero point during the current switching off as it is usually important to the vacuum interrupters.
СПИСОК ЛИТЕРАТУРЫ
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3. Кучинский В.Г. Электрические аппараты постоянного тока: Учебное пособие. СПб.: Изд-во СПбГПУ, 2012. 200 с.
СВЕДЕНИЯ ОБ АВТОРАХ
КУЧИНСКИЙ Владимир Георгиевич — доктор технических наук профессор Санкт-Петербургского политехнического университета Петра Великого. 195251, Россия, г. Санкт-Петербург, Политехническая ул., 29. E-mail: vladkuchinsky@mail.ru
СИДДХАРТ Нандакумар Назаре — студент Санкт-Петербургского политехнического университета Петра Великого. 195251, Россия, г. Санкт-Петербург, Политехническая ул., 29. E-mail: nnsiddharth@ gmail.com
REFERENCES
1. Anoshin O.A., Kutler P.P. Sravneniye tekhnicheskikh parametrov vakuumnykh vyklyuchateley razlichnykh firm. Elektrooborudovaniye: ekspluatatsiya i remont. 2007. № 8. S. 29-32. (rus.)
2. Sidorov V.A., Alferov D.F., Alferova E.D. Dielectric Strength of Series-Connected Vacuum Gaps. IEEE Trans.
On Dielecteric and Electrical Insulation. Vol. 13, Issue 1. Feb. 2006. P. 18-25.
3. Kuchinskiy V.G. Elektricheskiye apparaty postoy-annogo toka. Uchebnoye posobiye.SPb.: Izd-vo SPbGPU, 2012. 200 s. (rus.)
AUTHORS
KUCHINSKII Vladimir G. — Peter the Great St. Petersburg Polytechnic University. 29 Politechnicheskaya St., St. Petersburg, 195251, Russia. E-mail: vladkuchinsky@mail.ru
SIDDHARTH Nandakumar N. — Peter the Great St. Petersburg Polytechnic University. 29 Politechnicheskaya St., St. Petersburg, 195251, Russia. E-mail: nnsiddharth@gmail.com
Дата поступления статьи в редакцию: 10.10.2016.
© Санкт-Петербургский политехнический университет Петра Великого, 2017
