WO2022218575A1

WO2022218575A1 - Direct current quenching at a disconnector switch

Info

Publication number: WO2022218575A1
Application number: PCT/EP2022/025153
Authority: WO
Inventors: Abhinav PANDEY; Varsha SABALE
Original assignee: Eaton Intelligent Power Limited
Priority date: 2021-04-17
Filing date: 2022-04-14
Publication date: 2022-10-20

Abstract

A device for direct current quenching at a disconnector switch (10) comprising a first circuitry comprising a serial connection of a capacitor (12), of a parallel arrangement of a diode (14) and a resistor (16), and of a first switch (18), wherein the first circuitry is connected at the capacitor side with a first connection (20) of the disconnector switch (10) and at the first switch side with a reference voltage (22), a second circuitry comprising a second switch (24) connected between the first connection (20) of the disconnector switch (10) and the connection (26) of the first switch (18) and the parallel arrangement of the diode (14) and the resistor (16), a third circuitry comprising a serial connection of a third switch (28) and an inductor (30), wherein the third circuitry is connected at the third switch side with a second connection (21 ) of the disconnector switch (10) and at the inductor side with the connection (32) of the capacitor (12) and the parallel connection of the diode (14) and the resistor (16), a current sensor (34) configured to sense an electric current flowing through the disconnector switch (10), and a control unit (36) for controlling the first, second, and third switch (18, 24, 28) depending on the sensed electric current

Description

DIRECT CURRENT QUENCHING AT A DISCONNECTOR SWITCH

TECHNICAL FIELD

This specification relates to direct current quenching at a disconnector switch and discloses a device, a system, and a method for direct current quenching at a disconnector switch.

BACKGROUND

A disconnector switch such as a circuit breaker provided for switching a high direct current (DC) may suffer from electric arcs occurring between the opening contacts of the switch, which can avoid or at least hinder the switching of the current and can also damage the contacts.

The US patent application US20180331531 discloses a circuit breaker apparatus for a high- or medium-voltage DC network. The circuit breaker apparatus comprises a branch with a mechanical circuit breaker inserted in the network line, and, connected in parallel therewith, firstly a lightning arrestor branch, and secondly a series connection of a first capacitor bank, a make switch, and an inductor. The circuit breaker apparatus includes at least one resistive voltage divider connected to the network voltage and presenting a low voltage stage connected in parallel with the capacitor bank in order to charge the capacitor bank. The make switch acts on the appearance of a fault current to discharge the capacitor bank through the inductor so as to produce a zero crossing in the current flowing in the mechanical circuit breaker branch.

SUMMARY

This specification describes a device, a system, and a method for direct current quenching at a disconnector switch. According to an aspect of this specification, a device for direct current quenching at a disconnector switch comprises a first circuitry comprising a serial connection of a capacitor, of a parallel arrangement of a diode and a resistor, and of a first switch, l wherein the first circuitry is connected at the capacitor side with a first connection of the disconnector switch and at the first switch side with a reference voltage, a second circuitry comprising a second switch connected between the first connection of the disconnector switch and the connection of the first switch and the parallel arrangement of the diode and the resistor, a third circuitry comprising a serial connection of a third switch and an inductor, wherein the third circuitry is connected at the third switch side with a second connection of the disconnector switch and at the inductor side with the connection of the capacitor and the parallel connection of the diode and the resistor, a current sensor configured to sense an electric current flowing through the disconnector switch, and a control unit for controlling the first, second, and third switch depending on the sensed electric current. This device is particularly applicable with DC disconnector switch and supports to achieve a zero crossing of a DC flowing over the disconnector switch particularly in case of a fault. The control unit may be configured to algorithmically control the behavior of the first to third switches of the devices and the “activating” of the first to third circuitries in a predetermined sequence to optimize the quenching of the DC. Particularly, when a current flowing through the disconnector switch is a critical current due to a fault for example of a load connected to the disconnector switch, for example a short circuit in an electric motor, the opening of the disconnector switch to separate the faulty load from a power supply could incur an electric arc, which could damage the disconnector switch, particularly its contacts. The quenching with the device disclosed herein may efficiently support suppression of such an electric arc by causing a zero crossing of the current, which supports the switching off of the current. The device for direct current quenching may be integrated with the switch or may be implemented as a separate device attachable to a disconnector switch.

The current sensor may in an embodiment be implemented by a Hall sensor. A Hall sensor allows to measure a DC flowing through the disconnector switch and is also suitable for an integration in the device close to a disconnector switch.

In embodiments, the second switch may be implemented by an insulated-gate bipolar transistor (IGBT). An IGBT is a solid-state switch and does thus not incur electric arcs. Thus, it may avoid any damages due to electric arcs in the second circuitry when the capacitor discharges. In further embodiments, the control unit may be implemented by one or more processors or a logic circuit, particularly a programmable logic circuit. Thus, the control unit can be configured by programming, and so the device can be adapted to different operating conditions such as for application with different disconnector switches and/or different currents.

According to an aspect of this specification, a system for direct current quenching at a disconnector switch comprises a device as disclosed herein and a disconnector switch, wherein the control unit of the device is configured to operate the switches of the device as follows: under normal operation of the disconnector switch and when the sensed electric current is within a predefined range, operating the first circuitry with the first switch being closed and the second and third circuitries with the second and third switches being open; when a fault current is detected by a deviation of the sensed current from the predefined range, operating for a predefined time the first and third circuitries with the first and third switches being open and the second circuitry with the second switch being closed, and after the predefined time has passed, operating the first and second circuitries with the first and second switches being open and the third circuitry with the third switch being closed. Such a system may be for example applied in the switch of high DCs.

According to yet further aspect of this specification, a method of operating a device for direct current quenching at a disconnector switch as disclosed herein is provided, wherein the method operates the switches of the device as follows: under normal operation of the disconnector switch and when the sensed electric current is within a predefined range, operating the first circuitry with the first switch being closed and the second and third circuitries with the second and third switches being open; when a fault current is detected by a deviation of the sensed current from the predefined range, operating for a predefined time the first and third circuitries with the first and third switches being open and the second circuitry with the second switch being closed, and after the predefined time has passed, operating the first and second circuitries with the first and second switches being open and the third circuitry with the third switch being closed. The method may be for example implemented as a firmware or part of a firmware of the device for DC quenching, for example being implemented as a function of a microcontroller firmware for embodying the control unit.

In an embodiment, the predefined time may be determined according to

with Vo being the voltage at the disconnector switch, lcr a critical load current, Iq a load current or fault current, R the resistance of the resistor, C the capacitance of the capacitor, and L the inductance of the inductor. By discharging the capacitor for this predefined time, it may particularly be ensured that the current flowing through the disconnector switch is below a critical current.

In embodiments, the predefined time may be selected not to exceed

with Vo being the voltage at the disconnector switch, Iq a load current or fault current, R the resistance of the resistor, C the capacitance of the capacitor, and L the inductance of the inductor. This limit for the predefined time may ensure that a zero crossing can still be achieved with the charge stored in the capacitor.

The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features and advantages will be apparent from the description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

Fig. 1 shows a circuit diagram of an example of the device for direct current quenching and a disconnector switch; Fig. 2 shows a circuit diagram of an example of the device for direct current quenching and a disconnector switch and the current flow according to different steps of the operation of the device;

Fig. 3A-3C show example traces of the capacitor voltage and current and the current transient during different steps of the operation of the device; and Fig. 4 shows a diagram with examples of discharge times of capacitors for various load currents.

DETAILED DESCRIPTION

In the following, functionally similar or identical elements may have the same reference numerals. Absolute values are shown below by way of example only and should not be construed as limiting.

Fig. 1 shows a DC disconnector switch 10 (in the figure, an equivalent of the switch 10 is shown as resistor) and a device for DC quenching at the switch 10, which comprises a first, second and third circuitry as described below in detail. The disconnector switch 10 connects a power supply E2 with a load R5. The first circuitry comprises a serial connection of a capacitor 12, of a parallel arrangement of a diode 14 and a resistor 16, and of a first switch 18. A resistor R2 for limiting the diode 14 current is also provided. The first circuitry is connected at the capacitor 12 side with a first connection 20 of the disconnector switch 10 and at the first switch 18 side with a reference voltage 22, for example a ground potential. The first circuitry particularly serves to charge the capacitor 12 during normal operation of the disconnector switch 10, i.e. when it is closed and current flows through it within a predefined operation range.

The second circuitry comprises a second switch 24, particularly an IGBT, connected between the first connection 20 of the disconnector switch 10 and the connection 26 of the first switch 18 and the parallel arrangement of the diode 14 and the resistor 16. The second circuitry particularly serves for a controlled discharge of the capacitor 12 to reduce the capacitor voltage to reduce the current flowing through the disconnector switch 10 as will be described later in detail.

The third circuitry comprises a serial connection of a third switch 28 and an inductor 30, wherein the third circuitry is connected at the third switch 28 side with a second connection 21 of the disconnector switch 10 and at the inductor 30 side with the connection 32 of the capacitor 12 and the parallel connection of the diode 14 and the resistor 16. The third circuitry particularly serves to generate an oscillating current flow through the disconnector switch 10 so that a zero crossing occurs and electric arcs can be avoided when opening of the contacts of the disconnector switch 10.

The current flowing through the disconnector switch 10 is sensed with a current sensor 34, for example a Hall sensor. The current sensor 34 is switch in the load side connection of the disconnector switch 10 with the load R5, thus, sensing the current flow in the load R5 and being able to quickly detect a fault of the load R5, for example a short circuit of the load, which would result in a high load current or fault current Iq flowing through the disconnector switch 10, particularly exceeding a critical load current (CLC) lcr. The CLC may be defined as current under which the risk of starting a fire within an electrical device is high, the life cycle of an electrical device may be reduced due to contacts degradations, the electrical function of a device cannot be guaranteed for breaking and isolation of nominal current.

The switches 18, 24 and 28 are controlled by a control unit 36. The control unit 36 may be implemented by one or more processors, particularly a microcontroller, or a (programable) logic circuit. The control unit 36 is configured to control the switches 18, 24, 28 depending on the electric current sensed with the current sensor 34 and, thus, receives as input current measurements from the sensor 34.

Next, the operation of the system comprising the disconnector switch 10 and the first, second and third circuitries forming the quenching circuitry under control of the control unit implementing the following algorithm is described with reference to Fig. 2 showing a simplified circuit diagram of a DC switch connecting a Photovoltaic (PV) power supply delivery DC to a load over the DC switch. The DC load current is sensed by a current sensor (not shown in Fig. 2). Under normal operation, i.e. when the DC load current is within a predefined range, switch S1 is closed (Step 1) while the other switches S2 and S3 are open, and the capacitor C is charged via the path capacitor C - parallel connection of diode D and resistor R - closed switch S1 - ground GND. When a faulty DC load current is sensed, switch S1 is opened and switch S2 closed (Step 2) so that the capacitor C can discharge via the resistor R and the switch S2 switch S3 remains open). After a predefined time, switch S2 is opened and switch S3 is closed (Step 3), while switch S1 remains open. Thus, an oscillating, particularly sinusoidal current is established over the DC switch in the path S3 - L - C- DC switch, which may comprise zero crossings and allow to open the DC switch and to interrupt the DC load current flow with a reduced risk of forming of electric arcs between the opened contacts of the DC switch.

The predefined time may be determined according to

with Vo being the voltage at the DC switch, lcr the critical load current, Iq the load current or fault current, R the resistance of the resistor 16 (resistance R3 in Fig. 1), C the capacitance of the capacitor 12, and L the inductance of the inductor 30. The current Iq is the actual current flowing through the disconnector switch 10 (Fig. 1) and can be measured with the current sensor 34. The current lcr is a property of the disconnector switch 10 based on its construction. When the current Iq reaches the current lcr, an opening of the contacts of the disconnector switch 10 may be undesirable, particularly such a current would probably incur arcing between the opening contacts, which may be hard to eliminate. If the contacts are opened when Iq = lcr then this may lead to generation of an arc, which can eventually not be automatically removed from the chamber of the switch 10 and may damage the switch assembly and particularly its contacts. Thus, Iq = lcr is never desired. During the predefined time span, the discharging of the capacitor 12 may introduce an oscillatory behavior in the current flowing through the disconnector switch 10 and at the same time may ensure a zero crossing of the current flowing through the switch 10 in such as a way that it does not encounter the lcr value while moving from a positive to a negative value. The faulty DC load current or load current or fault current is represented by the current Iq, which is the current to be quenched. The predefined time span may be stored in the control unit as a parameter. Thus, the control unit may perform step 2 for the predefined time and thereafter proceed with step 3. The predefined time may be also selected not to exceed

Vo being the voltage at the disconnector switch, Iq a load current or fault current, R the resistance of the resistor 16 (resistance R3 in Fig. 1), C the capacitance of the capacitor 12, and L the inductance of the inductor 30. This upper limit of the predefined time may ensure that the discharge time of the capacitor 12 is limited and the capacitor charge is still sufficient to allow to generate an oscillating current over the inductor 30 and the capacitor 12 high enough to allow a zero crossing of the faulty DC load current flowing through the DC switch.

Vo e expression

sqrt

Th Iq is valid for one particular Iq value. The control unit may store the predefined time span data for various different Iq values, and based on the current sensed by the current sensor, the control unit may select one value from stored predefined time spans, which corresponds to the sensed Iq current value. Fig. 4 shows the minimum discharge time of a capacitor for various combinations of the load current Iq and critical current lcr. The critical current lcr can be different for different disconnector switches so that one of the Iq & lcr combinations can be selected to take care of several possibilities of Iq & lcr combinations.

As described above, the predefined time t should thus be within the limit as described by the following inequality:

Vo

RC In sqrt

Icr+Iq

Vo

RCln sqrt If the predefined time t is less than Icr+Iq

, then the capacitor may not be discharged to a sufficient level and after a zero crossing (positive to negative) the oscillating, particularly sinusoidal current lcr flowing through the disconnector switch may encounter an lcr value, and if the switch contacts are then opened at this very instant then it may be harmful for the disconnector switch and its contacts. Ideally, an opening of the contacts of the disconnector switch is desired at exactly a zero crossing and after some time the contacts will open. The above limit for the predefined time t defines a buffer, which may ensure that an opening of the contacts of the disconnector switch might be less harmful as possible.

If the predefined time t is greater than ^{RC ln L} [ V ^l£J ^{S£ ‘}i^rt ©^L/J., then the capacitor may get discharged beyond a desired limit and a zero crossing of the resultant oscillating current may not be achieved In the following, some background information is given for the critical load current lcr, which is particularly relevant when the disconnector switch is applied in DC application such as with photovoltaic (PV) panels. When the herein described device for direct current quenching at a disconnector switch is applied with a PV panel, the critical load current lcr is the current appearing in the PV panel when sun irradiation is low and the PV panel generates an electrical power at a lower current than a rated current of the system comprising the PV panel and the disconnector switch.

In general, for industrial applications, the contact system and the arc quenching mechanism of a disconnector switch is designed for its intended rated current and higher fault currents only. The switch performance at currents less than its rated current is usually not checked assuming it will perform well since it is a low severity condition. In industrial application, it is rare to be seen that a disconnector switch is getting used/ operated at lower currents consistently. However, in case of PV panels the current of the power getting generated can’t be controlled. And that current flows through the complete set up. Also, a decision about the selection of a disconnector switch is made considering a maximum worst case power generation/current flowing through the PV panel so that the switch is capable to carry that current continuously without any malfunctioning. However, it may be observed that in PV applications, switching of the device at lower currents is common and frequent. And clearing/ quenching such lower currents in the same contact system/ quenching mechanism is difficult. An arc may remain for a longer time in the contact system and hence may erode the contacts and may reduce the switch life. Some consequences of not being able to correctly break a critical load current may be the risk of starting a fire within the electrical device, an accelerated life cycle of electrical device due to degradation of electrical contacts, and the electrical function of the device may not be guaranteed for breaking and isolation of nominal current. Therefore, particularly when the disconnector switch with the herein disclosed device for direct current quenching is applied in DC applications such as with PV panels, the critical load current lcr is a fixed value and regarded as a property of the disconnector switch, which is particularly determined by the overall electrical and mechanical construction of the disconnector switch. A lower current flowing through the disconnector switch may in these applications be more critical than a higher current since electrodynamic forces may not be high enough with a lower current to push an arc for example in an arc extinction chamber.

Figs. 3A and 3B show example traces of the simulated capacitor voltage (Fig. 3A) and current (Fig. 3B) over a time span of 400 ms: charging: 0 - 300 ms (S1 ON/closed; S3 & S2(IGBT) OFF/open); discharging to optimum voltage level: 300ms - 380 ms (S2 (IGBT) ON(closed; S1 & S3 OFF(open); LC Oscillations: 380ms -

400ms (S3 ON/closed; S1 & S2(IGBT) OFF(open). Fig. 3B shows a trace of the simulated current transient through the disconnector switch (at an example load current of 200 Amp): LC oscillation current superimposed with the load current.

Fig. 4 shows example times of discharge of the capacitor for various load currents. The discharge time of the capacitor depends on following factors:

1. Critical current of the DC disconnector switch

2. Power Supply Voltage, for example Grid Voltage

3. Inductance of the LC tank circuit (decided by peak load current to be quenched)

4. Capacitance of the LC tank circuit (decided by peak load current to be quenched) 5. Load Current flowing through the breaker at the time of opening

Some advantages of the herein disclosed devices, systems, and methods are in the following briefly summarized: opening of a DC disconnector switch at exact zero crossing is very difficult to achieve because of the inherent time delay associated with the tripping & actuation, and the situation may become worse if the current flowing through the switch is a critical current at the time of opening of DC disconnector switch, so based on the critical current of the DC disconnector switch - an algorithm is proposed with which a capacitor can be discharged in such a way that the current flowing through the DC disconnector switch while opening its contacts will be less than critical current and there may be zero crossing of the current at the same time. To protect the DC disconnector switch or breaker from over-voltage, an MOV (Metal Oxide Varistor) can be employed across the breaker.

Claims

1. A device for direct current quenching at a disconnector switch (10) comprising

- a first circuitry comprising a serial connection of a capacitor (12), of a parallel arrangement of a diode (14) and a resistor (16), and of a first switch (18), wherein the first circuitry is connected at the capacitor side with a first connection (20) of the disconnector switch (10) and at the first switch side with a reference voltage (22),

- a second circuitry comprising a second switch (24) connected between the first connection (20) of the disconnector switch (10) and the connection (26) of the first switch (18) and the parallel arrangement of the diode (14) and the resistor (16),

- a third circuitry comprising a serial connection of a third switch (28) and an inductor (30), wherein the third circuitry is connected at the third switch side with a second connection (21 ) of the disconnector switch (10) and at the inductor side with the connection (32) of the capacitor (12) and the parallel connection of the diode (14) and the resistor (16),

- a current sensor (34) configured to sense an electric current flowing through the disconnector switch (10), and

- a control unit (36) for controlling the first, second, and third switch (18, 24, 28) depending on the sensed electric current.

2. The device of claim 1 , wherein the current sensor is implemented by a Hall sensor (34).

3. The device of claim 1 or 2, wherein the second switch is implemented by an insulated-gate bipolar transistor (24).

4. The device of claim 1 , 2 or 3, wherein the control unit (36) is implemented by one or more processors or a logic circuit, particularly a programmable logic circuit.

5. A system for direct current quenching at a disconnector switch (10) comprising a device of any preceding claim and a disconnector switch, wherein the control unit (36) of the device is configured to operate the switches (18, 24, 28) of the device as follows:

- under normal operation of the disconnector switch (10) and when the sensed electric current is within a predefined range, operating the first circuitry with the first switch (18) being closed and the second and third circuitries with the second and third switches (24, 28) being open;

- when a fault current is detected by a deviation of the sensed current from the predefined range, operating for a predefined time the first and third circuitries with the first and third switches (18, 28) being open and the second circuitry with the second switch (24) being closed, and after the predefined time has passed, operating the first and second circuitries with the first and second switches (18, 24) being open and the third circuitry with the third switch (28) being closed.

6. The system of claim 4, wherein the predefined time is determined according to

Vo

RC In Icr+Iq sqrt © - ] with Vo being the voltage at the disconnector switch, lcr a critical load current, Iq a load current or fault current, R the resistance of the resistor (16), C the capacitance of the capacitor (12), and L the inductance of the inductor (30).

7. The system of claim 4 or 5, wherein the predefined time is selected not to exceed

_wjth Vo being the voltage at the disconnector switch, Iq a load current or fault current, R the resistance of the resistor (16), C the capacitance of the capacitor (12), and L the inductance of the inductor (30).

8. A method of operating a device for direct current quenching at a disconnector switch (10) of any of claims 1 to 4, the method operating the switches (18, 24, 28) of the device as follows: - under normal operation of the disconnector switch (10) and when the sensed electric current is within a predefined range, operating the first circuitry with the first switch (18) being closed and the second and third circuitries with the second and third switches (24, 28) being open; when a fault current is detected by a deviation of the sensed current from the predefined range, operating for a predefined time the first and third circuitries with the first and third switches (18, 28) being open and the second circuitry with the second switch (24) being closed, and after the predefined time has passed, operating the first and second circuitries with the first and second switches (18, 24) being open and the third circuitry with the third switch (28) being closed.

9. The method of claim 8, comprising determining the predefined time according to

Vo

RC In sqrt -

Icr+Iq ©] with Vo being the direct current voltage at the disconnector switch, lcr a critical load current, Iq a load current or fault current, R the resistance of the resistor (16), C the capacitance of the capacitor (12), and L the inductance of the inductor (30).

10. The method of claim 8 or 9, comprising selecting the predefined time not to exceed

with Vo being the direct current voltage at the disconnector switch, Iq a load current or fault current, R the resistance of the resistor (16), C the capacitance of the capacitor (12), and L the inductance of the inductor (30).