GB2518733A

GB2518733A - Neutral line breakage detection circuit, method and corresponding residual current circuit breaker

Info

Publication number: GB2518733A
Application number: GB1413299.7A
Authority: GB
Inventors: Jia Chen; Shou Qi Xu; Lin Yang
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 2013-09-05
Filing date: 2014-07-28
Publication date: 2015-04-01
Anticipated expiration: 2034-07-28
Also published as: GB2518733B; GB201413299D0; CN104426128B; CN104426128A

Abstract

A neutral line breakage detection circuit 530 comprises DC power supply input terminals DC+, DC(1), DC-(2) corresponding respectively to a phase line input L, neutral line input N and a protective earth input PE, following rectification by rectification circuit 410. A voltage detection circuit 531 is coupled to the input terminals, for detecting the relationship between a first voltage at a first DC power supply negative terminal DC-(1) corresponding to the neutral line input and a reference voltage at a reference point b coupled to a second DC power supply negative terminal DC-(2) corresponding to the protective earth input, which relationship changes when the neutral line breaks. A control circuit 532 is coupled to the voltage detection circuit for driving a trip unit 340 to execute a tripping action when the relationship changes, to break a power supply connection from a power supply line.

Description

Neutral line breakage detection circuit, method and corresponding residual current circuit breaker The present invention relates to a residual current circuit breaker, in particular to a residual current circuit breaker with a neutral line breakage protection function.

Fig. 1 shows by way of example a typical three-phase, four-line power distribution system. As Fig. 1 shows, in a three-phase, four-line power distribution system, the power supply circuit comprises three phase lines Ll, L2 and L3, a neutral line N and a protective earth FE. Both the neutral line N and the protective earth FE are connected to ground. Fig. 1 also shows residual current circuit breakers ROOB1, ROOB2 and RCOB3, coupled singly to different phase lines. Each residual current circuit breaker ROOD has one end coupled to a phase line (e.g. Dl) and another end coupled to the neutral line N. A load (LOAD) is coupled to the ROOD. The residual current circuit breaker ROOB can determine whether current leakage has occurred in the circuit, breaking the power supply connection to the load (LOAD) upon detection of such current leakage. In Fig. 1 the protective earth FE is connected to the respective casings of the ROOD and the LOAD, for the purpose of protection. Under normal operating conditions, as shown by the dotted line in Fig. 1, current flows out of phase line Li for example, and is sent to load LOAD1 via ROOB1; the current which flows through load LOAD1 then flows back through ROOB1 to the neutral line N. The current ioop thus formed can supply electrical energy to load LOAD1, so that it operates normally.

However, if the neutral line breaks in the power distribution system shown in Fig. 1, the loads (LOAD) which were originally coupled singly to different phase lines, become series-connected pairs subject to the line voltage (380 V) between phase lines. For example, as shown by the dotted line in Fig. 2, when the neutral line N is broken, a current from phase line Li for example passes through load LOAL)l, is sent into load LOAD2 along the neutral line N on the power distribution side, and then returns to L2 along the circuit path collpled to phase line L2. Thus, LOAL1 and LOAD2 are connected in series between phase lines Ti and T2 and have to withstand a vol tage as high as 380 V, for example. By the same principle, loads LOAD2 and LOAD3 are connected in series between L2 and L3, while loads LOAD1 and LOAD3 are connected in series between Li and L3. If the difference in impedance between loads LOAN and LOAD2 is very large, the load voltage on the load with the larger impedance may approach the phase voltage 380 V. In this case, in the absence of a corresponding line protection circuit breaker, electrical eguipment on this phase will burn out very quickly.

To solve the problem posed by the broken neutral line in Fig. 2, it has been prcposed in the prior art that an overvoltage detection circuit be incorporated into the residual current circuit breaker, so that a fault is considered to have occurred when a voltage higher than 280 V for example is detected, and a protecting action can then be executed.

Fig. 3 shows by way of example a structural block diagram of a residual current circuit breaker with overvoltage protection. Ihe residual current circuit breaker in Fig. 3 comprises a rectification circuit 310, a residual current detection circuit 320, an overvoltage detection circuit 330 and a trip unit 340. Specifically, as Fig. 3 shows, a phase line L (which may represent any one of phase lines Li -L3) and the neutral line N are inputted to the rectification circuit 310 for rectification. The DC outputs (DCI, DC-) of the rectification circuit 310 are sent into the overvoltage detection circuit 330. The overvoltage detection circuit 330 determines whether the input voltage after rectification is higher than a predetermined threshold, and iiff it is higher than the threshoid, drives the trip unit 340 to execute a tripping action, i.e. opens switches on lines L and N to break the power supply connection. The residual current detection circuit 320 on the one hand obtains electrical energy from the DC outputs, and on the other detects a sensing signal from a current transformer, to determine whether current leakage Tias occurred. If current leakage has occurred, then the residual current detection circuit 320 drives the trip unit 340 to execute a tripping action, and similarly breaks the connections on L and N. The circuit shown in Fig. 3 can perform a protecting action upon detecting that the voltage has exceeded the threshold. However, if the phase voltage increases after the neutral line breaks, and is higher than the rated voltage without exceeding the threshold of the svervoltage detection circuit, then the load equipment must withstand this overvoltage for a long period of time. This may cause damage to the load equipment (electrical equipment) One object of the present invention is to provide a neutral line breakage detection circuit, to enable neutral line breakage to be discovered more promptly and a corresponding protecting action to be executed. Another object of the present invention is to ensure that the neutral line breakage detection circuit has a simple structure, is easy to implement and has a low cost. Yet another object of the present invention is to provide a residual current circuit breaker with the above neutral line breakage protection function.

To achieve the above object, the present invention presents a neutral line breakage detection circuit coupled to a rectification circuit and a trip unit, the neutral line breakage detection circuit comprising a DC power supply input terminal, comprising a DC power supply positive terminal, a first DC power supply negative terminal and a second DC power supply negative terminal, and used for receiving an input from a rectification circuit, wherein the DC power suppiy positive terminal corresponds to a phase line input of the rectification circuit, the first DC power supply negative terminal corresponds to a neutral line input of the rectification circuit, and the second DC power supply negative terminal corresponds tc a protective earth input of the rectification circuit; a vcltage detection circuit coupled to an output terminal of the rectification circuit, for detecting the relationship between a first voltage at the first DC power supply negative terminal and a reference voltage at a reference point, wherein the reference point is coupled tc the seccnd DC power supply negative terminal corresponding to the protective earth input, and the voltage detection circuit is designed in such a way that the relationship between the first voltage and the reference voltage changes when the neutral line breaks; a control circuit coupled to the voltage detection circuit, for driving a trip unit to execute a tripping action when the relationship between the first voltage and the reference voltage changes, to break a power supply connection from a power supply line.

According to another embodiment, the voltage detection circuit is coupled between the DC power supply positive terminal and the second DC power supply negative terminal; and the voltage detection circuit comprises a series-connected voltage division circuit in which a voltage division point is shorted to the first DC power supply negative terminal. Preferably, the voltage detection circuit comprises a first resistor and second impedance element connected in series, the second impedance element having one end coupled to the first resistor and another end coupled to the reference point.

Even more preferably, the second impedance element comprises a resistor. Optionally, the second impedance element comprises a first diode, the anode of the first diode being coupled to The reference point, and the cathode thereof being coupled to the first DC power supply negative terminal. Preferably, the second impedance element fnrther comprises a capacitor connected In parallel with the first diode.

In another embodiment of the present invention, the control circuit comprises: a determination circuit, fcr determining whether the output of the voltage detecticn circuit is greater than a predetermined threshold; and a drive circuit, for issuing a trip command when the output of the voltage detection circuit is greater than a predetermined threshold, to drive the trip unit to execute a tripping action. Preferably, the determination circuit comprises a Zener diode, the cathode thereof being coupled to the first DC power supply negative terminal, and the anode thereof being coupled to the drive circuit, the reverse breakdown voltage of the Zener diode being the predetermined threshold. Preferably, the drive circuit comprises a second diode and a load, wherein the anode of the second diode is coupled to the anode of the Zener diode, and the load is coupled between the cathode of the second diode and the reference point.

According to another aspect of the present invention, the present invention also presents a residual current circuit breaker, comprising: a phase line input terminal, connected to a phase line of a power supply line, to receive electrical energy; a neutral line input terminal, connected to a neutral line of the power supply line; a protective earth input terminal, connected to a protective earth; a rectification circuit, coupled to the phase line input terminal, the neutral line input terminal and the protective earth input terminal, used for outputting a rectified DC power supply, and an output terminal thereof comprising a DC power supply forward terminal corresponding to the phase line input terminal, a first DC power supply negative terminal corresponding to the neutral line input terminal, and a second DC power supply negative terminal corresponding to the protective earth input terminal; a residual current detection circuit, coupled to the rectification circuit, and used for detecting whether a residual current is present; a trip unit, coupled to the rectification circuit, and triggered to trip by the residual current detection circuit when the residual current detection circuit detects the presence of a residual current; and the neutral line breakage detection ci rcui t descri bed above, coupled to the rectification circuit and used for driving the trip unit to execute a tripping action upon detecting breakage of the neutral line in the power supply line.

According to another aspect of the present invention, the present invention also presents a method for detecting neutral line breakage, comprising: detecting a first voltage at a first DC power supply negative terminal corresponding to a neutral line after rectification; acquiring a relationship between the first voltage and a reference voltage at a reference point when the neutral line is normal, wherein the reference point is coupled to a second DC power supply negative terminal corresponding to a protective earth; monitoring the relationship between the first voltage and the reference voltage; upon detecting a change in the relationship between the first voltage and the reference voltage, driving a trip unit to execute a tripping action.

When the neutral line breakage detection circuit and residual current circuit breaker with a neutral line breakage protection function presented in the present invention are adopted, a neutral line breakage can be discovered as soon as it occurs, and a trip action can be perforned immediately. Thus, there is no possibility whatsoever of the level of the neutral point drifting, so there is no possibility of the phase voltage increasing due to drifting of the neutral point, and the LOAD (electrical equipment) will not face a situation where it is operating at overvoltage due to neutral line breakage.

If an existing circuit breaker with overvoltage protection is used, if the phase voltage increases when the neutral line breaks to a point where it is greater than the rated voltage but does not exceed the threshold of the overvoltage protector, electrical eguipment will have to withstand this overvoltage for a long period of time, and may suffer damage as a consequence. Moreover, sinoe the neutral line breakage detection method presented in the present invention enables immediate tripping wTnen the neutral line breaks, there is virtually no possibility of system maintenance personnel being electrocuted.

Furthermore, the solution presented in the present invention has fewer components and a lower cost than existing overvoltage detection solutions. In addition, in the solution presented in the present invention, a series-connected resistor and diode circuit is used to detect neutral line breakage; the use of a diode not only can ensure accurate detection of breakage, but also has no negative impact on downstream circuitry. Moreover, in a preferred embodiment, a capacitor which charges is connected in parallel with the diode, enabling a judgment to be made about whether neutral line breakage has occurred by detecting whether the capacitor has been charged beyond a threshold when the neutral line breaks.

The use of a capacitor avoids unnecessary false alarms, increasing the reliability of the entire residual current circuit breaker.

The accompanying drawings listed below are merely intended to illustrate and explain the present invention schematically, and by no means define the scope thereof.

Fig. 1 shows the connection relationships among different residual current circuit breakers and loads in an existing three-phase, four-line power distribution system in a normal operating state; Fig. 2 shows the connection relationships among different residual current circuit breakers and loads when the neutral line has broken in an existing three-phase, four-line power distribution system; FIg. 3 shows a schematic diagram of a residual current circuit breaker with overvoltage protection

functionality in the prior art;

Fig. 4 shows a structural schematic diagram of a residual current circuit breaker with neutral line breakage protection according to an embodiment cf the present invention; Figs. 5A -5B are structural schematic diagrams of a neutral line breakage detection circuit according to an embodiment of the present invention; Figs. 6A -60 are structural schematic diagrams of neutral line breakage denection circuits acccrding to embodiments of the present invention.

List of labels in the accompanying drawings Li, L2, L3 phase lines; N neutral line; FE protective earth; RCCB residual current circuit breaker; LOAD load (electrical eguipment) 310 rectification circuit; 320 residual current detection circuit; 330 overvoltage detection circuit; 340 trip unit; 410 rectification circuit (including protective earth FE) 430 neutral line breakage detection circuit; 530 neutral line breakage detection circuit; 531 voltage detection circuit; 532 control circuit; 632 control circuit.

Fig. 4 shows schematically a residual current circuit breaker 400 with neutral line breakage protection functionality according to an embodiment of the present invention. Unlike Fig. 3, the residual current circuit breaker in Fig. 4 has three input terminals (L, N and FE) and circuit 430 in Fig. 4 is a neutral line breakage detection circuit, not the overvoltage detection circuit in Fig. 3. Furthermore, elements in Fig. 4 which are the same as in Fig. 3 have the same labels as in Fig. 3, and the specific functions thereof are not repeated here.

As Fig. 4 shows, in addition to the phase line T and the neutral line N which are the same as in Fig. 3, the input terminals of the residual current circuit breaker 400 also include a prorective earth FE. Thus, the reotification oircuit 410 also differs somewhat from Fig. 3. The reotification oirouit 410 has three input terminals, namely the phase line input 5, the neutral line input N and the protective earth input FE. The rectification circuit 410 outputs a DC power supply after rectifying the AC inputs. The rectification circuit 410 has three DC power supply output terminals, namely a DC power supply positive terminal DC+ corresponding to the phase line input L, a first DC power supply negative terminal DC-(1) corresponding to the neutral line input N, and a second DC power supply negative terminal DC-(2) corresponding to the protective earth input FE. Here, when the neutral line is operating normally, the ground potentials of the first and second DC power supply negative terminals are substantially the same, and the current of the residual current circuit breaker 400 flows baok to DC-(1) whioh serves as ground, and thereby flows baok to the neutral line N. When the neutral line is broken, the neutral line input N has an open circuit, and the residual current circuit breaker 400 takes DC-(2), which corresponds to the protective earth FE, as ground.

The neutral line detection circuit 430 is coupled to the three DC terminals of the rectification circuit 410, and used for determining whether neutral line breakage has occurred, and for issuing a trip command to the trip unit 340 upon determining that neutral line breakage has occurred, to promptly open the switches on lines S and N. -10 -In the embodiment shown in Fig. 4, the residuai current detection circuit 320 is connected as a icad of the neutral line detection circuit 430. In other applications, the residual current detection circuit 320 could be connected in parallel with the neutral line detection circuit 430.

Figs. hA and 55 show by way of example the specific structure of a neutral line detection circuit 530 according to an embodiment of the present invention. In general terms, as Fig. 5 shows, the neutral line detection circuit 530 comprises a voltage detection circuit 531 and a control circuit 532. The voltage detection circuit 531 is coupled to the three DC terminals of the rectification circuit 410, and used for detecting the relationship between the voltage at the first DC power supply negative terminal DC-(1), i.e. voltage V at point a, and a reference voltage Vb at a reference point b. The reference point b is coupled to the second DC power supply negative terminal DC-(2), which corresponds to the protective earth input FE. Here, the voltage detection circuit 531 is designed in such a way that the relationship between voltage Vd and reference voltage V. will change if the neutral line breaks. The control circuit 532 is coupled to the voltage detection circuit, and issues a trip command to the trip unit 340 when the relationship between V and reference voltage Vb changes, so that a tripping action is executed, and the switches on power supply lines L and N are opened.

In specific terms, in the example shown in Fig. 5, the voltage detection circuit 531 is designed to include a resistor R1 and an impedance element 52 which are connected in series; this series-connected circuit is coupled between the DC power supply positive terminal DC+ and the second DC power supply negative terminal DC-(2) The point of coupling, a, between the resistor 51 and the impedance element 52 is in turn coupled to the first DC power supply negative output terminal DC-(1) . The -1l -reference point b is coupled to the second DC power supply negative terminal DC-(2) Fig. 5A shows the direction of current flow when the neutral line is operating normally, as shown by the dotted arrow in the figure. when the neutral line is operating normally, the voltage at point a is low, and owing to the direction of cijrrent flow, the voltage Va at point a is less than the voltage Vb at point b. The output Va of the voltage detection circuit 531 is a LOW, to indicate normal operation of the neutral line. Fig. 53 shows the direction of current flow when the neutral line is broken. When the neutral line is broken, the first DC power supply negative terminal DC-(1) is floating, and current flows through the series-connected circuit 531 into the second DC power supply negative terminal DC-(2), which corresponds to the protective earth input PE. At this time, the voltage Va at point a is greater than the voltage Vb at point b. The voltage detection circuit 531 outputs a HTGH signal, to indicate that the neutral line has broken. The control circuit 532 determines whether it is necessary to issue a trip command to the trip unit 340 according to whether the level outputted by the voltage detection circuit is HIGH or LOW.

Figs. 6A -6C show three specific current detection circuits. In these three figures, the control circuits 632 are the same. The control circuit 632 comprises a determination circuit and a drive circuit. The determination circuit, for example, determines whether the relationship between voltage Va and reference voltage Vh has changed, i.e. whether the output level of The voltage detection circuit is higher than a predetermined threshold. The drive circuit is used for issuing a trip command to the trip unit 340 when the relationship between the first voltage Va and the reference voltage Vb changes, for example when the output level of the voltage detection circuit is higher than a predetermined threshold.

Specifically, in Fig. 6, the control circuit 632 comprises -12 -a Zener diode D2, the cathode thereof being coupled to the output terminal of the voltage detection circuit, i.e. point a, and the anode thereof being coupled to a load RT, via a diode D3. The reverse breakdown voltage of the Zener diode is the predetermined threshold. The load R may for example be the residual current detection circuit 320 sTiown in Fig. 4.

As Fig. 6A shows, the current detection circuit may comprise a first resistor Rl and a second resistor R2 connected in series between DC+ and DC-(2) . In Fig. 6A, when the neutral line is operating normally, the voltage at point a is low, current flows from point b to point a, and voltage Va is less than voltage Th. The voltage detection circuit 531 outputs a LOW signal, to indicate that the neutral line is operating normally. Thus, D2 in the control circuit is switched off, and an ourput signal 5Lip of the control circuit is invalid LOW. When the neutral line is broken, DC-(1) is floating; current flows through series-connected resistors R1 and R2, and then into DC-(2) . At this time, the voltage Va at point a is larger than the voltage Vb at point b. The voltage detection circuit 531 outputs a HIGH signal, to indicate that the neutral line has broken. Tf the level at point a is larger than the reverse breakdown voltage of 52, the Zener diode D2 will conduct, so that the output signal 3trp of the control circuit 632 is valid HIGH.

In Fig. 6B, the current detection circuit may comprise a first resistor Ri and a diode Dl connected in series between DC-I-and DC-(2) . The anode of Dl is coupled to a reference point b; the cathode of Dl is coupled to DC-(l), i.e. point a. In Fig. 65, the control circuit 632 is the same as in Fig. 6A. When the neutral line is operating normally, the voltage at point a is low, the diode Dl conducts, and a return current flows from point b to point a. Due to the voltage drop arising from conduction through the diode Dl, the voltage Va is less than the voltage Vb. Therefore, the voltage detection -13 -circuit 531 outputs a LOW signal, to indicate normal operation of the neutral line. When the neutral line is broken, DC-(l) is floating, the resistor Ri and diode Dl are connected in series between DC+ and DC-(2), and Dl is switched off, so that the voltage Va at point a is larger than the voltage Vb at point b. The current detection circuit 531 outputs a RTGH signal, to indicate that the neutral line has broken.

In Fig. 60, the current detection circuit may comprise a first resistor Ri and a parallel-connected branch, connected in series between DC+ and DC-(2) The parallel-connected branch comprises a diode Di and a capacitor Cl connected in parallel, the anode of Dl being coupled to a reference point b, and the cathode of Dl being coupled to DC-(1), i.e. point a. The capacitor Ci is connected in parallel with the diode Dl. In Fig. 60, the control circuit 632 is the same as in Fig. 6A. When the neutral line is operating normally, the voltage at point a is low, the diode Dl conducts, and a return current flows from b to point a, in a manner similar to that in Fig. 6B.

Due to the voltage drop arising from conduction through the diode Dl, the voltage Va is less than the voltage Vb.

Therefore, the voltage detection circuit 531 outputs a LOW signal Va, to indicate normal operation of the neutral line. When the neutral line is broken, DC-Mi is floating, and the resistor Rl is connected in series with the parallel-connected branch formed by the diode Di and capacitor Cl, between DC+ and DC-(2) . At this time, Dl is switched off, and the capacitor Cl charges, so that the voltage Va at point a is larger than the voltage Vb at point b. Therefore the voltage detection circuit 531 outputs a signal Va of a level which gradually increases.

When the voltage Va on the capacitor Ci exceeds the reverse breakdown voltage of the Zener diode D2, the Zener diode D2 conducts, and thereby outputs a valid HIGH trip command 3Tio* -14 -As stated above, the neutral line breakage detection circuit presented in the present invention actually perforns detection on the neutral line by the following method. The method comprises: Step 1: detecting a first voltage (Va) at a first DC power supply negative terminal DC-(l) corresponding to a neutral line N after rectification; Step 2: acguiring a relationship between the first voltage Va and a reference voltage Vb at a reference point b when the neutral line is normal, wherein the reference point b is coupled to a second DC power supply negative terminal DC-(2) corresponding to a protective earth FE; Step 3: monitoring the relationship between the first voltage Va and the reference voltage Vb; Step 4: upon detecting a change in the relationship between the first voltage Va and the reference voltage Vb, issuing a valid trip command, to drive a trip unit to execute a tripping action.

When the neutral line breakage detection circuit and residual current circuit breaker with a neutral line breakage protection function presented in the present invention are adopted, a neutral line breakage can be discovered as soon as it occurs, and a trip action can be performed immediately. Thus, there is no possibility whatsoever of the level of the neutral point drifting, so there is no possibility of the phase voltage increasing due to drifting of the neiltral point, and the LOAD (electrical equipment) will not face a situation where it is operating at overvoltage due to neutral line breakage.

If an existing circuit breaker with overvoltage protection is used, if the phase voltage increases when the neutral line breaks to a point where it is greater than the rated voltage but does not exceed the threshold of the overvoltage protector, electrical eguipment will have to withstand this overvoltage for a long period of time, and may suffer damage as a consequence. Moreover, since the neutral line breakage detection method presented in the -15 -present invention enables immediate tripping when the neutral line breaks, there is virtually no possibility of system maintenance personnel being electrocuted.

Furthermore, the solution presented in the present invention has fewer components and a lower oost than existing overvoltage detection solutions. In addition, in the solution presented in the present invention, a series-connected resistor and diode circuit is used to detect neutral line breakage; the use of a diode not only can ensure accurate detection of breakage, but also has no negative impact on downstream circuitry. Moreover, in a preferred embodiment, a capacitor which charges is connected in parallel with the diode, enabling a judgment to be made about whether neutral line breakage has occurred by detecting whether the capacitor has been charged beyond a threshold when the neutral line breaks.

It should be understood that although description is carried out herein according to various embodiments, it is by no means the case that each embodiment includes just one independent technical solution. This method of presentation is adopted herein solely for the sake of clarity. Those skilled in the art should consider the Description in its entirety. The technical solutions in the different embodiments may also be suitably combined to form other embodiments capable of being understood by those skilled in the art.

The above embodiments are merely particular illustrative embodiments of the present invention, which are by no means intended to define the scope thereof. Any eguivalent changes, amendments or combinations made by those skilled in the aro without deviating from the concept and principle of the present invention should fall within the scope of protection thereof.