WO2016193529A1

WO2016193529A1 - Method for earth fault protection for a three-phase electrical network

Info

Publication number: WO2016193529A1
Application number: PCT/FI2016/050158
Authority: WO
Inventors: Sakari Kauppinen; Risto Pitkänen
Original assignee: Jyväskylän Energia Oy
Priority date: 2015-06-03
Filing date: 2016-03-14
Publication date: 2016-12-08
Also published as: CN107735690A; CN107735690B; EP3304105A4; FI126434B; FI20155423A; EP3304105A1

Abstract

The invention relates to a method in the earth fault protection of a three-phase electrical network. In the method the total current (10) is determined continuously in a measurement point (17) of the electrical network (10). In the method an earth fault (16) is detected, after which the necessary measures are performed. In the method the return current (Ip) caused by the earth fault is determined, which is eliminated from the total current (10). In addition, by elimination from the total current (10) the earth fault current (Is) is determined, from which, on the basis of the location of the earth fault (16) the earthing voltage (Urn) is determined, on the basis of which the earth fault (16) is detected and the necessary measures performed.

Description

METHOD IN THE EARTH FAULT PROTECTION OF A THREE-PHASE ELECTRICAL NETWORK

The invention relates to a method in the earth fault protection of a three-phase electrical network, in which method the total current is determined continuously in a measurement point of the electrical network, and in the method an earth fault is detected, after which the necessary measures are performed. Various variables are measured comprehensively from an electrical network at sub and switching stations. In a generally used three-phase electrical network, the currents of all the phases and their total current can be measured. Similarly, the voltages of all the phases can be measured, as can the earth fault voltage. In the detection of an earth fault, mainly only the said total current and the earth fault voltage are used. There are various methods for detecting an earth fault. A method that is particularly suitable for determining the distance of an earth fault is disclosed in EP patent number 2402774.

An earth fault is generally defined as being an insulation fault between a current conductor and the earth. The fault current appearing in an insulation fault can be calculated, if the properties of the electrical network are known. Compared to a normal situation, in an earth fault the phase voltages of the phases of the electrical network relative to the earth increase. At the same time, the total current of the different phases deviates from zero. This deviating part of the current travels through the insulation fault to the earth, forming an earth fault current.

The detection of an earth fault, based on measurement and computation, operates in principle as designed only in a certain specific basic state of the electrical network. The electrical network and its environment, and circumstances in general, change, in which case the properties of the electrical network and especially the earth fault currents increase. Particularly the use of ground cables increases the earth fault current, as the cable acts as a capacitor. For example, in a resonant network the fault current is measured using the resistive part of the total current, which itself is too imprecise to reliably detect an earth fault. In addition, the deviations of the currents going to earthing from the calculated values increase, which causes errors in the calculation. Thus, the detection of an earth fault using the prior art acts too slowly, or even erroneously. Thus the earth fault remains unnoticed, so that risk locations, in which the earthing voltages are considerable, form in the area of the electrical network. Thus increasingly extensive device damage, and in the worst event even cases of death, can be associated with earth fault events. On the other hand, erroneous detections cause unnecessary breaks in the power supply.

The invention is intended to create a new type of method for the earth fault protection of a three-phase electrical network, by means of which an earth fault can be detect more reliably and faster than previously, as well as more comprehensively than previously. The characteristic features of the method according to the present invention are stated in the accompanying Claims. In the invention, the known computation is exploited in a new and surprising manner, which is additionally combined with a surprising phenomenon. Earth faults can then be detected quickly and reliably and the risks and damage they cause can be reduced. At the same time, detection according to the method remains accurate despite changes in the electrical network or varying operating conditions.

In the following, the invention is described in detail with reference to the drawings showing one embodiment of the invention, in which Figure 1 shows schematically an earth fault in a three-phase electrical network,

Figure 2 shows diagrammatically a case of an earth fault.

5 Figure 1 shows schematically a three-phase electrical network 10. At the substation 11, for example the 20 kV voltage of the high tension network is converted to the 220 V voltage of the low tension network. Figure 1 shows only one conductor 12 and its three phases 13, 14, and 15. In this case, there is an 10 earth fault 16 in one phase 15. The loads connected to the phases are not shown.

The invention relates to a method in the earth fault protection of a three-phase electrical network. In the method, the total

15 current 10 is determined continuously in a measurement point 17 of the electrical network 10. The total current is also referred to as the zero current, because in a normal situation the sum of the currents of the phases is close to zero. For example, the substation's electronics measures the variables of

20 the electrical network and calculates the desired values from them. In the method, an earth fault 16 is detected, for example on the basis of the aforesaid calculation, after which the necessary measures are performed. According to the invention, the return current Ip caused by the earth fault 16 is

25 determined, and is eliminated from the total current 10. The share of the return current and its error causing effect can then be removed. In this way, the earth fault current can be sufficiently precisely determined. In the example of Figure 1, the return current Ip is caused when the current of one phase

30 15 goes to earth through the earth fault 16 and then through the earth back to the two other phases 13 and 14 (arrow shown with a broken line) . Further, by elimination from the total current 10, the earth fault current Is is determined, from which, on the basis of the location of the earth fault 16, the

35 earthing voltage Urn is determined, on the basis of which the necessary measures are performed. In other words, the earth fault protection method is based on the earth fault current and from that in turn on the earthing voltage. By determining in this way, the real earthing voltage can sufficiently precisely determined, on the basis of which the earth fault protection can be defined in a new way. At the same time, the limit values set for the contact voltage can be taken into account. The necessary measures can then be taken sufficiently quickly and reliably, thus significantly improving device and personal safety .

According to the example of Figure 1, when the earth fault 16 is in a single phase 15, the return current Ip is determined from the two other phases 13 and 14. The properties of the conductor 12 subject to the earth fault 16 can then be taken into account. The return current Ip can be defined as the total current of the sum zero circuit of the two phases 13 and 14. More generally, in the measurement point 17 the return currents are measured or otherwise determined, which case specifically are added to or subtracted from the total current, taking into account the compensation degree of the electrical network and the earth fault current produced by the relevant voltage source. Vector calculation is used in the definition. In practice, the return current is defined as the sum of the currents of the zero circuit.

Preferably, the elimination uses the zero voltage U0 defined in the measurement point 17 as well as the defined total current 10, from which the earth fault current Is is determined through the return current Ip. The real earth fault current is then reached and from it the earthing voltage. Part of this is the determining of the location of the earth fault, for which it is possible to use, for example, the aforementioned EP patent number 2402774. More specifically, the location of the earth fault 16 is determined on the basis of the residual voltage of the measurement point 17 and the total current 10. In the method according to the invention, the total current 10 and the return current Ip are determined from one and the same conductor 12. This avoids the complicated calculations that would be required if the other conductor outputs were included in the determining.

The actual measures can vary. Generally, the determined earthing voltage Urn is compared to the permitted contact voltage duration ratio, when, if the earthing voltage and/or the duration is exceeded, the switching off of the electrical network 10 is performed. Inversely, the contact voltage duration ratio can be programmed into the protection devices, such as the protection relay 24 shown schematically in Figure 1, when an accurate and sufficiently fast earth fault protection will be achieved. Thus, the earthing current and voltage that have previously not been taken into account are now taken into account in the method when detecting an earth fault. At the same time, a possible contact voltage can be defined and the accident risk thus reduced. Figure 1 shows only the breakers arranged in the phases of the protection relay 24, by which the electrical network 10 can be switched off. The other connections of the protection relay to the electrical network are not shown. In practice, a known locating method and the related calculation are utilized in the calculation of the earthing voltage. The parameters of the electrical network required in the method can then also be calculated. More specifically, the faulty phase's share of the voltage can be determined by calculation. By taking this voltage share into account, the real earthing voltage can then be determined, which is exploited in the manner according to the invention.

Figure 2 shows a case of an earth fault. Here, there is a transformer 20 between the high tension network 18 and the low tension network 19. There is protective earthing 25 around the transformer 20. In Figure 2, there is an earth fault 16 in the protectively earthed parts of the transformer 20. In other words, due to the fault there is a connection from the live conductor to the earthed part of the transformer. Here, a metal bodied electrical device 22, in which there is a motor 23, is connected to an earthed socket 21 of the low tension network 19. There is also protective earthing in the electrical connection of the house. In the earth fault situation in question, a rise in voltage from the low tension network arises in the body of the electrical device connected to the protective earthing 20. In this example case, there is an 800 V difference in potential 2xUtp, in which Utp is the contact voltage, between the body of the device and the earth. On the basis of the permitted contact voltage duration ratio the trigger time should be about 0.3 seconds. Generally, in the method according to the invention, the greater the earth fault current, the shorter the switch off time. This simple rule is easy to add as part of the control of the protection relay, so that the definition, according to the invention, of the earth fault current takes account of the change in the electrical network. More specifically, the method takes into account changes taking place in the earth fault current during the operation of the electrical network, so that the definition remains accurate and sensitive.

In the calculation of the earth fault current, two simple procedures, for example, can be used. In the first procedure, the currents of all the phases and the vector quantities of the phase and main voltages are used in order to determine the magnitude of the earth fault current. This works particularly when the earth fault currents are large and the loads are small. In calculating the actual earth fault and in turn the earthing voltage, the zero voltage and zero current of the electrical network as well as the aforementioned voltage quantities of all the phases are used. The earthing voltage arising in the earthing, on the basis of which the requirements of safety regulations can be taken into account, is then determined from the voltage of the faulty phase.

In the second procedure, in addition to the above, the value of the total current corrected with the reactive return current is used to determine the current going to the fault location. In this situation, the resistive part of the return current is very small. Correspondingly, in a two phase earth fault the current can be determined by correcting the calculation with the effective power and reactive power of the loads. Further, the earth faults of the earthing of two or three phases or those taking place through the earth and the earth fault currents caused by them can be calculated. Also a frequently occurring two phase short circuit with an earth contact and a simultaneous earth fault in the third phase can be eliminated with simple short circuit protection, in the operation of which the results obtained from the earthing voltage calculation according to the invention are taken into account. Generally, an earth fault 16 is detected from the zero voltage U0, the earthing voltage Urn, the total current 10, the return current Ip, the earthing current Is, and the fault resistance in the earth fault 16.

A simple resistance based calculation, on the basis of the earthing voltage and the earthing current, can also be obtained for the detection and removal of high resistance faults. The method can be used to ensure and improve, for example, the detection of conductors that have fallen to the ground, as part of the already known admittance based protection. The detection and elimination of high resistance faults can then be improved in electrical network switching and alteration situations and also in changes of the degree of compensation of the earth fault current. A buried cable is like a capacitor and in practice earth fault current charges in the buried cable. For example, in a four kilometre long earth cable there can be a current of as much as about 20 amperes. In a fault situation, such as an earth fault, the current discharges to the earth. In a resonant electrical network the earth fault current is a few amperes and the situation is controlled using blow-out coil. If the electrical network for some reason changes, the blow-out coil is re-excited, which can lead to the earth fault current increasing many times over. This raises the contact voltage in step with it. By means of the method according to the invention the earth fault current and in turn the earth fault voltage can be determined more accurately than previously in each situation and thus the necessary measures can be performed reliably if an earth fault occurs. According to the standard in force, the entire transformer circuit must be earthed. Generally in the transformer circuit, for example, the main transformer, the transformers, and the distribution cabinets are indeed earthed and the various earthings are connected in parallel to each other. The potential is then the same in the entire transformer circuit. For example, in a device connected by an extension cable to the earthed distribution cabinet of Figure 2 the potential can become high relative to the reference earth. The total current can be measured or determined in different ways. For example, at an electrical substation a coil of a current transformer can be set around the phases, by means of which the total current can be measured. Another way is to determine the total current by means of a phase transformer fitted to each phase. Both the voltage and the current are then measured from each phase and the total current is determined from the measurement results. A current transformer is simple, but at small currents it causes an error, which reduces the reliability of the earth fault protection. Phase current transformers demand the calculation power that is found in modern protection relays. By means of phase specific measurement and calculatory definition sensitivity is achieved, for example, for a protection relay.

Generally, a protection relay is arranged in the electrical network, which is applied to the definition of the earthing current and the earthing voltage, as well as of the earthing resistance, and further to the other necessary calculations. The capacity of the protection relay can then be exploited. In practice, the functionality of the method according to the invention can be incorporated in the latest protection relays by means of software, making device installation unnecessary. At the same time, the risks due to an earth fault can be reduced beforehand and damage can be reduced.

Claims

1. Method in the earth fault protection of a three-phase electrical network, in which method the total current (10) is determined continuously in a measurement point (17) of the electrical network (10), and in the method an earth fault (16) is detected, after which the necessary measures are performed, character! zed in that in the method the return current (Ip) caused by the earth fault (16) is determined, which is eliminated from the total current (10), and by elimination from the total current (10) the earth fault current (Is) is determined, from which, on the basis of the location of the earth fault (16) the earthing voltage (Urn) is determined, on the basis of which the earth fault (16) is detected and the necessary measures performed.

2. Method according to Claim 1, characterized in that, when the earth fault (16) is in one phase (15), the return current (Ip) is determined from the other two phases (13, 14) .

3. Method according to Claim 2, characterized in that the return current (Ip) is determined as the sum of the currents of the zero circuit of the two phases (13, 14) .

4. Method according to any of Claims 1 - 3, characterized in that in the elimination the zero voltage (U0) determined in the measurement point (17), as well as the determined total current (10) are used, from which the earth fault current (Is) is determined through the return current (Ip) .

5. Method according to any of Claims 1 - 4, characteri zed in that the location of the earth fault (16) is determined on the basis of the residual voltage of the measurement point (17) and the total current (10) .

6. Method according to any of Claims 1 - 4, characterized in that the location of the earth fault (16) is determined using the method according to EP patent number 2402774.

5 7. Method according to any of Claims 1 - 6, characterized in that the total current (10) and the return current (Ip) are determined from one and the same conductor (12) .

8. Method according to any of Claims 1 - 7, characterized

10 in that the determined earthing voltage (Urn) is compared to the permitted contact voltage duration ration and when the earthing voltage and/or the time is exceeded, the electrical network (10) is switched off.

15 9. Method according to Claim 8, characterized in that, as the earth fault current (Is) increases, the switching off time is shortened.

10. Method according to any of Claims 1 - 9, characterized 20 in that the earth fault (16) is detected from the zero voltage (U0), the earthing voltage (Urn), the total current (10), the return current (Ip), the earthing current (Is), and the fault resistance in the earth fault (16) .

25 11. Method according to any of Claims 1 - 10, characterized in that the method is used in the detection of a single phase and/or a dual phase earth fault (16) .

12. Method according to any of Claims 1 - 10, characterized 30 in that the method is used for detecting high resistance faults on the basis of the earthing voltage or the earthing current.

13. Method according to any of Claims 1 - 12, characterized in that the earth fault current (Is) is determined from the

35 currents of all the phases (13, 14 , 15), as well as from the vector quantities of the phase and main voltages.

14. Method according to any of Claims 1 - 13, characterized in that, in determining the earthing current (Is) the value of the total current (10) corrected by the reactive return current is used.

15. Method according to any of Claims 1 - 14, characterized in that a protection relay (24) is arranged in the electrical network (10), to which is applied the definition of the earthing current (Is) and the earthing voltage (Urn) .