GB2606005A

GB2606005A - Detecting an open pen conductor

Info

Publication number: GB2606005A
Application number: GB2105728.6A
Authority: GB
Inventors: Mennell Tom
Original assignee: Schneider Electric Ltd
Current assignee: Schneider Electric Ltd
Priority date: 2021-04-22
Filing date: 2021-04-22
Publication date: 2022-10-26
Also published as: GB202105728D0

Abstract

A consumer assembly for a three-phase power supply, comprising: three live conductors (L1, L2, L3) and a protective earth and neutral, PEN, conductor, wherein the PEN conductor separates into a neutral conductor and a protective earth conductor; an open delta transformer comprising three primary windings 41 and three secondary windings 43, wherein each primary winding is connected between the neutral or PEN conductor and a respective one of the live conductors and the secondary windings are connected in series; and a voltage sensor connected to the secondary windings, wherein the voltage sensor is configured to measure the neutral displacement voltage output from the secondary windings. At least one of the consumer assembly, utility service unit and utility substation comprise a switching device configured to disconnect the conductors from the power supply if the neutral displacement voltage exceeds a predetermined safety limit.

Description

DETECTING AN OPEN PEN CONDUCTOR

BACKGROUND

The present specification relates to apparatus for detecting a faulty or open protective earth neutral (PEN) conductor. In particular, the present specification provides a system for detecting an open PEN conductor and isolating the conductors from the incoming power supply in the event that a fault is detected.

Public electricity utilities provide the majority of the connections to their consumers at low-voltage. The conductors (usually cables) making the connection usually terminate into a service intake unit which may be separate unit adjacent to the consumers low-voltage assembly or an integral part of it. Tarif metering for the supply can take place upstream at die utility's substation, or it may be part of the service intake unit in the consumers premises.

In the case of an outdoor low-voltage assembly, the service intake unit is likely to be part of a low-voltage assembly that is shared between the utility and the consumer. Locating the service intake unit in close proximity to, or providing it as an integral part of, the consumers low-voltage assembly leads to a number of issues that need to be addressed in order to conform with Regulations and standards, and ensure a safe working environment for all personnel in the vicinity of the low-voltage assembly.

Occasionally, the utility will provide a three-phase, neutral and separate earth facility. This system is not addressed by the present specification. More frequently, the low-voltage electricity supply is provided using a three-phase and combined neutral and earth connection system, or a three-phase and neutral connection system. When the system using a combined neutral and earth connection is provided by the utility it is usually converted to a three-phase, neutral and separate protective earth at the point of connection to the consumers installation.

There are effectively three types of 'earths' that must be considered. These are as follows; a. The combined neutral and earth conductor used by many utilities in the supply to a consumer. Usually this is referred to as a Protective Earth Neutral (PEN) conductor. The PEN conductor is usually connected to an earth (ground) within or close to the substation supplying the installation and at the point of connection to the consumer.

b. The Protective Earth (PE) used to connect any exposed conductive parts associated with die consumers equipment to the neutral earth point of the supply in the customer's installation.

c. Earth (mother earth) -the general mass of ground around the installation, which may or may not be composed generally of reasonably conductive material and may or may not include conductive material such as metal pipes and metal structures.

There is a need for a reliable and cost-effective system to detect an open or faulty PEN conductor upstream of the customer's installation, that ensures safety standards are met and 10 hazards are reduced.

SUMMARY

Aspects of the present disclosure are set out in the accompanying independent and dependent claims. Combinations of features from the dependent claims may be combined with features of the independent claims as appropriate and not merely as explicitly set out in the claims.

According to an aspect of the present disclosure, there is provided a consumer assembly for a three-phase power supply, comprising three live conductors and a protective earth and neutral. PEN, conductor, wherein the PEN conductor separates into a neutral conductor and a protective earth conductor, an open delta transformer comprising three primary windings and three secondary windings, wherein each primary winding is connected between a respective one of the live conductors and either the PEN conductor or the neutral conductor, and the secondary windings are connected in series, and a voltage sensor connected to the secondary windings, wherein the voltage sensor is configured to measure the neutral displacement voltage output from the secondary windings.

Thus, the present disclosure provides an assembly for measuring the neutral displacement 30 voltage using an open delta transformer, to allow a potential fault to be detected.

if the PEN conductor separates into the neutral conductor and the protective earth conductor upstream of (i.e. before) die open delta transformer, then each primary winding is connected between a respective one of the live conductors and the neutral conductor.

If the PEN conductor separates into the neutral conductor and the protective earth conductor downstream of (i.e. after) the open delta transformer, then each primary winding is connected between a respective one of the live conductors and the PEN conductor.

If the PEN conductor is not broken, then the neutral displacement voltage should be zero 10 (within tolerances). However, if the PEN conductor is broken then voltage through each of the live conductors, and hence the primary windings, will not be equal. Consequently, the voltage sensor will detect a non-zero voltage.

Optionally, the predetermined safety limit is indicative of an open or broken PEN conductor.

Optionally, the voltage sensor is configured to output a signal if the neutral displacement voltage exceeds the predeteimined safety limit. The signal may be a trip signal.

The signal may be output via a relay circuit. Optionally, the output signal may provide an 20 instruction to a switching device. The switching device may be provided in the consumer assembly, or upstream from the consumer assembly.

Optionally, the consumer assembly comprises a switching device having a closed position and an open position. In the open position the switching device disconnects or isolates the live conductors and the PEN conductor from the three-phase power supply. The switching device may be configured to move to the open position if the neutral displacement voltage exceeds a predetermined safety limit.

The switching device may be a switch, or a contactor, or a circuit breaker.

Optionally the PEN conductor splits into the neutral conductor and the protective earth conductor downstream of the switching device. It will be appreciated that 'downstream' effectively means 'after'.

The switching device may be a four pole switching device, wherein each pole is connected to a respective one of die live conductors or the PEN conductor. Thus, each pole of the switching device may be configured to isolate one of the three live conductors or the PEN conductor. Advantageously, a four pole switching device is usually more readily available and less expensive than a five pole switching device. I()

Optionally, the PEN conductor splits into the neutral conductor and the protective earth conductor upstream of the switching device. It will be appreciated that 'upstream' effectively means 'before'.

The switching device may be a five pole switching device, wherein each pole is connected to a respective one of the live conductors, protective earth conductor or neutral conductor. Thus, each pole of the switching device may be configured to isolate one of the three live conductors, the neutral conductor or the protective earth conductor.

Optionally, the consumer assembly further comprises an earth leakage sensor. The earth leakage sensor may be positioned downstream of the switching device.

The earth leakage sensor may be connected to each of the live conductors and the neutral conductor.

Optionally, the consumer assembly may be configured for charging a car. However, it will be appreciated that the present invention is not limited to car charging assemblies.

The consumer assembly may be referred to as a low-voltage consumer assembly.

In a second aspect, the present disclosure provides an electrical supply assembly comprising: a consumer assembly as described in the first aspect of the disclosure; and a utility service unit, wherein the utility service unit comprises three live conductors and a PEN conductor each connected between an input terminal for receiving a three-phase power supply and an output terminal for connection to the consumer assembly.

It will be appreciated that the electrical supply assembly of the second aspect of the disclosure may comprise any embodiment or example of the first aspect of the disclosure.

Optionally the utility service unit comprises a switching device having a closed position and an open position, wherein in the open position the switching device disconnects the live conductors and the PEN conductor from the three-phase power supply. The switching device may be configured to move to the open position in response to the neutral displacement voltage measured by the voltage sensor exceeding a predetermined safety limit.

Thus, die switching device may be provided in the utility service unit instead of in the 15 consumer unit. This may allow a utility service provider to isolate a potential fault using their own equipment, rather than relying on a switching device in the consumer' s assembly.

In sonic embodiments, both the utility service unit and the consumer assembly may provide a switching device. This may provide a back-up in case of the failure or malfunction of one 20 of the switching devices.

The predetermined safety limit may be indicative of an open PEN conductor.

Optionally the switching device is a switch or a contactor or a circuit breaker.

Optionally, the PEN conductor in the utility service unit is connected to an earth connection.

Optionally the three live conductors and the PEN conductor in the utility service unit are protected by a double layer of electrical insulation or a reinforced layer of electrical 30 insulation.

The assembly may comprise a first housing for enclosing the utility service unit and a second housing for enclosing die consumer assembly. Optionally, the first housing and the second housing may be attached together.

hi other embodiments, the utility service unit and the consumer assembly may be enclosed in a single integral housing.

The electrical assembly may further comprise a utility substation configured to output a three-phase power supply to the input terminal of the utility service unit.

The utility substation may comprise a switching device having a closed position and an open position, wherein in the open position the switching device disconnects the utility service unit from the three-phase power supply. The switching device may be configured to move to the open position in response to the neutral displacement voltage measured by the voltage sensor exceeding a or the predetermined safety limit Thus, the switching device may be provided in the utility substation instead of in the utility service unit or the consumer assembly. This may more conveniently allow a utility service provider to isolate a potential fault using their own equipment at a central location, rather 20 than relying on a switching device in the consumer's assembly.

In some embodiments, more than one of the utility substation, the utility service unit and the consumer assembly may provide a switching device. This may provide at least one back-up in case of the failure or malfunction of one of the switching devices.

Optionally, the switching device is a switch or a circuit breaker or a contactor.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of this disclosure will be described hereinafter, by way of example only, with reference to the accompanying drawings in which like reference signs relate to like elements and in which: Figure I shows an electrical supply system according to an embodiment of this disclosure; Figure 2 shows an electrical supply system according to another embodiment of this

disclosure; and

Figure 3 shows an open delta transformer according to an embodiment of this disclosure.

DETAILED DESCRIPTION

Embodiments of this disclosure are described in the following with reference to the accompanying drawings.

Figure 1 is a schematic diagram showing an embodiment of an electrical supply system 10 according to an embodiment of this disclosure. The electrical supply system 10 comprises a utility substation 20, a utility service unit 30 and a consumer assembly 40.

The electrical supply system 10 is a low-voltage or secondary supply system configured to carry a three-phase and neutral power supply from distribution transformers to the electricity meters of end customers. The utility substation 20 is a medium voltage/low voltage (MV/LV) substation comprising a step-down transformer 22 configured to output a low-voltage three-phase and neutral power supply. The low-voltage neutral conductor is usually earthed 24 at or close to the substation 20. Thus, the utility substation 20 may output a three-phase and combined neutral and earth power supply.

hi this embodiment, a switching device 26 is provided in the utility substation 20. The switching device 26 has a closed configuration, in which the three-phase power supply is transmitted to the utility service unit 30, and an open configuration in which the live conductors are isolated or disconnected from the incoming three-phase power supply. Thus, when switching device 26 is open, no power is supplied to the utility service unit 30 or the consumer assembly 40. The switching device 26 may be a three pole switching device (as shown in Figure 1), or a four pole switching device. The switching device 26 may be a switch, a contactor. or a circuit breaker.

In other embodiments, no switching device may be provided in the utility substation 20. In some embodiments, the switching device 26 may be replaced by three fuses, one for each live conductor. The fuses would provide overload and short circuit protection but would not provide isolation in the event of an open neutral or open PEN.

The low-voltage three-phase power supply is transmitted from the utility substation to the input terminal of the utility service unit 30. The utility service unit 30 comprises three live conductors Li, L2, L3 and a protective earth and neutral, PEN, conductor. A tariff meter 32 may be provided for customer billing purposes.

The equipment in the utility service unit 30, including the conductors, may be protected by double or reinforced electrical insulation 60. This is particularly important when the utility substation 20 provides a three-phase and neutral supply, but no earthing facility is provided for the consumer's assembly 40.

The PEN conductor is connected (as shown at point 36) to extraneous conductive parts, such as the housing enclosing the utility service unit 30. Optionally, the PEN conductor is earthed 38 at the utility service unit 30. Three fuses 34 are provided, wherein each fuse 34 is connected to a respective one of the live conductors Li, L2, L3. The fuses 34 are provided for overload and short circuit protection. In other embodiments, the fuses 34 may be replaced by a switching device (see Figure 2).

Each of the live conductors Li, L2, L3 and the PEN conductor in the utility service unit 30 has an output telininal for connection to the low-voltage consumer assembly 40. The consumer assembly 40 comprises three live conductors L 1, L2, L3 and a PEN conductor configured to be connected to the respective conductors in the utility service unit 30. In some embodiments, the consumer assembly 40 may be provided in the same housing as the utility service unit 30. In other embodiments, the utility service unit 30 and the consumer assembly 40 may be enclosed in separate housings.

The present disclosure provides a system in the consumer assembly 40 for detecting if the 5 PEN conductor is broken (i.e. detecting an open PEN conductor). If the load on each of the three live conductors downstream of the break in the PEN conductor is not identical, the neutral voltage will be displaced. As the neutral and separate PE conductors of the consumer assembly are connected together at the point of connection to the electricity supply network, the displaced neutral voltage applied to the PE conductors can result in any exposed 10 conductive parts associated with electrically energised equipment within the installation being substantially the same potential (voltage), relative to that of mother earth, as the displaced neutral.

In situations where electrically energised equipment that contains exposed conductive parts is not well connected to mother earth (i.e. the ground surrounding the energised equipment), or there are adjacent conductive parts that are close to, or connected by some form of metal structure, such as metal pipes, to the PEN conductor, the neutral displacement voltage will appear between exposed conductive parts of the electrically energised equipment and mother earth. Thus, this can present an electric shock hazard.

A broken PEN conductor is particularly hazardous in electric vehicle (e.g. car) charging applications. Cars are usually charged outdoors, where earth continuity between the consumers PE conductor and the area/ground where charging takes place is often limited The car tyres usually insulate the car from mother earth, resulting in the car being at a voltage above the mother earth equal to the neutral displacement voltage. A person or animal touching the car and standing on the ground can be subjected to the neutral displacement voltage. This is a significant safety risk.

In the present disclosure, the neutral displacement voltage is measured using an open delta 30 transformer 42 and a voltage sensor 44. The open delta transformer 42 is shown in more detail in Figure 3 (described below). A switching device 46 is provided downstream of the open delta transformer 42. The switching device 46 is connected, or in communication with, die voltage sensor 44 (e.g. by one or more relay circuits).

hi the embodiment shown in Figure 1, the switching device is a four pole switch. A first pole 5 of the switch 46 is connected to the first live conductor Li, a second pole of the switch 46 is connected to the second live conductor L2, a third pole of the switch 46 is connected to the third live conductor L3 and the fourth pole is connected to the PEN conductor. It may be beneficial to use a four pole switching device 46, as they are often more readily available and less expensive than five pole switching devices. It will be appreciated that any suitable 10 type of switching device may be provided, such as a switch, circuit breaker, or contactor.

If the voltage sensor 44 measures a neutral displacement voltage that exceeds a predetermined safety limit, which may be indicative of an open PEN conductor, then a trip signal is transmitted to the switch 46. The trip signal (or open neutral trip signal) causes the switch 46 to move to the open or disconnected position, isolating the conductors from the incoming power supply.

If the voltage sensor 44 measures a neutral displacement voltage that is below the predetermined safety limit, then the switch 46 may remain in the closed or connected 20 position.

In the embodiment shown in Figure 1, immediately downstream of the switching device 46 the PEN conductor separates into a neutral conductor and a protective earth, PE, conductor.

An earth leakage sensor 48 may be provided. The earth leakage sensor 48 is connected to the neutral conductor and to the three live conductors. lithe earth leakage sensor 48 detects that the stray earth current (e.g. on the metal housing of the consumer assembly 40) exceeds a predeteimined safety limit, then a trip signal is sent to the switching device 46. The trip signal (earth leakage trip signal) causes the switching device 46 to move to the open position to disconnect the consumer assembly 40 from the incoming power supply. Thus, the circuit is broken if a potentially dangerous earth leakage current is detected.

If the utility substation 20 provides only three-phase and neutral (no earthing facility), the connections up to the incoming terminals of the switching device 46 in the consumer assembly should be protected by supplementary or double insulation 60, as described above. hi addition, the consumer must provide their own earth electrode as a means of earthing for the electrical installation. The switching device 46 must then also provide earth leakage protection that is sufficiently sensitive to provide protection against electric shock, as described above. The requirement for supplementary insulation and earth leakage protection, or equal, is mandated in BS 7671, Requirements for Electrical Installations.

The open PEN detection system provided in the present disclosure satisfies the requirements laid out in BS 7671.

The consumer assembly 40 also provides outgoing circuits 70 to consumer loads. Although two outgoing circuits are shown in Figure 1 for simplicity, it will be appreciated that the low-voltage consumer assembly 40 typically includes more than two outgoing circuits 70. Each outgoing circuit 70 is connected to the live conductors and the neutral conductor by a respective switch 52, 54. At the outgoing circuit 70 the protective earth (PE) conductor is connected to extraneous conductive parts, such as the enclosure if it is made of conducting material.

Another embodiment of the network or electrical supply assembly 10 is shown in Figure 2. The features that are common between Figures 1 and 2 have the same reference numerals.

hi comparison to the embodiment shown in Figure 1, the utility service unit. 30 in Figure 2 comprises a switching device 134, rather than three fuses 34. The switching device 134 has a closed position, in which the three-phase power supply is transmitted to the consumer assembly 40, and an open position in which the live conductors and PEN conductor are isolated or disconnected from the incoming three-phase power supply. The switching device 134 is a four pole switching device. The switching device 134 may be a switch, a contactor, or a circuit breaker. In some embodiments, the switching device 134 may he provided in addition to, or instead of, the switching devices 26, 146 in the utility substation 20 and consumer assembly 40. In other embodiments, no switching device may be provided in the utility service unit.

In Figure 2 the switching device 146 in the consumer assembly 40 is a five pole switching device. This is because the PEN conductor separates into the neutral conductor and the PE conductor upstream of the switching device 146, rather than downstream of the switching device. Each pole of the switching device 146 is connected to one of: the three live conductors Li. L2, L3, the neutral conductor or the PE conductor.

Figure 3 shows an embodiment of the open delta transformer 42 used in the consumer assembly 40 shown in Figures 1 and 2. The open delta transformer 42 comprises three plimary windings 41 and three secondary windings 43. Each of the primary windings 41 is connected between a respective one of the live conductors Ll. L2 or L3 and either the neutral or PEN conductor. Although Figures 1 and 2 show the open delta transformer 42 connected to the live conductors and the PEN conductor, it will be appreciated that the PEN conductor (in the consumer assembly 40) may separate into the neutral conductor and the PE conductor upstream of the open delta transformer 42.

The three secondary windings 43 are connected in series. The voltage sensor 44 (not shown in Figure 3) is configured to measure the voltage between terminals A and B (i.e. the output induced voltage from the secondary windings 43). This is the neutral displacement voltage. This system does not create or require a virtual or artificial neutral to measure the neutral displacement voltage.

If the PEN conductor is not faulty or open upstream of the consumer assembly 40, then the voltage through each of the live conductors LI, L2, L3 relative to the neutral or PEN conductor should have the same magnitude with a relative phase difference of 120°. As such, the induced voltage through the secondary windings 43 should sum to zero (within tolerances). Thus, the voltage measured by the voltage sensor 44 should he below the predetermined safety limit.

If the PEN conductor is faulty or open upstream of the consumer assembly 40, the voltage through each of the live conductors LI, L2, L3 will not have die same magnitude relative to the neutral or PEN conductor in the consumer assembly 40. This means that the induced voltage through the secondary windings 43 will not sum to zero. Thus, the neutral displacement voltage measured by the voltage sensor 44 will exceed the predetermined safety limit. A trip signal will then he output to one or more of the switching devices 26, 134, 46, 146 causing the switching device to open, thereby isolating the faulty PEN conductor. This safety measure should prevent any exposed conductive parts in the consumer assembly 40, such as the housing, presenting an electronic shock risk. I()

Accordingly, there has been described a consumer assembly for a three-phase power supply, comprising: three live conductors and a protective earth and neutral, PEN, conductor, wherein the PEN conductor separates into a neutral conductor and a protective earth conductor; an open delta transformer comprising three primary windings and three secondary windings, wherein each primary winding is connected between the neutral conductor or PEN conductor and a respective one of the live conductors and the secondary windings are connected in series; and a voltage sensor connected to the secondary windings, wherein the voltage sensor is configured to measure the neutral displacement voltage output from the secondary windings. At least one of the consumer assembly, utility service unit and utility substation comprise a switching device configured to disconnect the conductors from the power supply if the neutral displacement voltage exceeds a predetermined safety limit.

Although particular embodiments of this disclosure have been described, it will be appreciated that many modifications/additions and/or substitutions may be made within the 25 scope of the claims.

Claims

CLAIMSA consumer assembly for a three-phase power supply, comprising: three live conductors; a protective earth and neutral. PEN, conductor, wherein the PEN conductor separates into a neutral conductor and a protective earth conductor; an open delta transformer comptising three primary windings and three secondary windings, wherein each primary winding is connected between a respective one of the live conductors and either the neutral conductor or the PEN conductor, and wherein the secondary windings are connected in series; and a voltage sensor connected to the secondary windings, wherein the voltage sensor is configured to measure the neutral displacement voltage output from the secondary windings.
2. The consumer assembly of claim 1, further comprising a switching device having a closed position and an open position, wherein in the open position the switching device disconnects the live conductors and the PEN conductor from the three-phase power supply, wherein the switching device is configured to move to the open position if the neutral displacement voltage exceeds a predetermined safety limit.
The consumer assembly of claim 2, wherein the switching device is a switch, or a contactor, or a circuit breaker.
4. The consumer assembly of claim 2 or claim 3, wherein the PEN conductor splits into the neutral conductor and the protective earth conductor downstream of the switching device.
5. The consumer assembly of claim 4, wherein the switching device is a four pole switching device, wherein each pole is connected to a respective one of the live conductors or PEN conductor.
6. The consumer assembly of claim 2 or claim 3, wherein the PEN conductor splits into the neutral conductor and the protective earth conductor upstream of the switching device.
7. The consumer assembly of claim 6, wherein the switching device is a five pole switching device, wherein each pole is connected to a respective one of the live conductors, protective earth conductor or neutral conductor.
8. The consumer assembly of any preceding claim, further comprising an earth leakage sensor connected to each of the live conductors and the neutral conductor.
9. The consumer assembly of any preceding claim, wherein the predetermined safety limit is indicative of an open or faulty PEN conductor.
10. An electrical supply assembly comprising: a consumer assembly according to any one of claims 1 to 9; and a utility service unit, wherein the utility service unit comprises three live conductors and a PEN conductor, each connected between an input terminal for receiving a three-phase power supply and an output terminal for connection to the consumer assembly.
11. The electrical supply assembly of claim 10, wherein the utility service unit. further comprises: a switching device having a closed position and an open position, wherein in the open position the switching device disconnects the live conductors and the PEN conductor from the three-phase power supply, wherein the switching device is configured to move to the open position in response to the neutral displacement voltage measured by the voltage sensor exceeding a or the predetermined safety limit
12. The electrical supply assembly of claim 11 herein the predetermined safety limit is indicative of an open PEN conductor.
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19.The electrical supply assembly of claim 11 or claim 12, wherein the switching device is a switch, or a contactor, or a circuit breaker.The electrical supply assembly of any of claims 10 to 13, wherein the PEN conductor in the utility service unit is connected to an earth connection.The electrical supply assembly of any of claims 10 to 14, wherein the three live conductors and the PEN conductor in the utility service unit are protected by a double layer of electrical insulation or a reinforced layer of electrical insulation.The electrical supply assembly of any of claims 10 to 15, further comprising a first housing for enclosing the utility service unit and a second housing for enclosing the consumer assembly.The electrical supply assembly according to any of claims 10 to 16, further comprising: a utility substation configured to output a three-phase power supply to the input terminal of the utility service unit, wherein the utility substation comprises a switching device having a closed position and an open position, wherein in the open position the switching device disconnects the utility service unit from the three-phase power supply, wherein the switching device is configured to move to the open position in response to the neutral displacement voltage measured by the voltage sensor exceeding a or the predetermined safety limit.The electrical supply assembly of claim 17, wherein the predetermined safety limit is indicative of an open PEN conductor.The electrical supply assembly of claim 17 or claim 18, wherein the switching device is a switch, or a contactor, or a circuit breaker.