GB1601235A

GB1601235A - Method and apparatus for detecting ground shorts in electrical systems

Info

Publication number: GB1601235A
Application number: GB5678/78A
Authority: GB
Original assignee: BBC Brown Boveri AG Switzerland; BBC Brown Boveri France SA
Current assignee: BBC Brown Boveri AG Switzerland; BBC Brown Boveri France SA
Priority date: 1977-03-01
Filing date: 1978-02-13
Publication date: 1981-10-28
Also published as: DE2712975A1; SE7801944L; DE2712975C2; FI780550A; FI67977B; CH615536A5; FI67977C; SE424031B

Abstract

In earth fault protection for a generator in unit connection by measuring the voltage between the star point of the generator and earth, the star point additionally being loaded with an operating-frequency displacement voltage, the problem is that the displacement voltage has to have high amplitudes for the earth fault current detection because of the operating-frequency interference voltages which necessitate that the sensitivity is set comparatively low. To obtain a useful current (Iv) allocated to the displacement voltage (Uv), which is representative of an earth fault, from interference currents (Is) of any type within certain limits and also of operating frequency, the vector of the displacement voltage (Uv) is switched to and fro between two positions (Uv1, Uv2) in the sense of a phase oscillation. The corresponding useful currents (Iv1, Iv2) can be detected with a large signal/noise ratio by phase-selective demodulation or rectification by means of a multiplier (11), which is supplied with the measurement current and a signal corresponding to the phase modulated displacement voltage (Uv). <IMAGE>

Description

(54) METHOD AND APPARATUS FOR DETECTING GROUND SHORTS IN ELECTRICAL SYSTEMS (71) We, BBC BROWN, BOVERI & COMPANY LIMITED, of Baden, Switzerland, a Swiss Company, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to a new and improved method of, and apparatus for, detecting ground faults or ground shorts in electrical systems isolated from ground or having a high ohmic connection with ground.

It is already known in the art to use a test signal, sometimes referred to as a biasing voltage, for detecting a ground fault, namely a ground short, and to derive therefrom a response or measuring signal indicative of the ground short. The selection of the test signal, namely a voltage Uv, governs the measuring principle and therefore also the reliability of the protective device. Up to the present there have been employed primarily two different test signal techniques: (a) The test voltage Uv constitutes a voltage (50 Hz or 60 Hz) at the operating or network frequency. Due to the prevailing disturbances which exist at the network frequency across the ground resistor the amplitude of the test voltage Uv is selected to amount to 15% of the amplitude of the phase voltage.The advantage of this method resides in the inexpensive manner in which it is possible to produce the test voltage Uv by tapping it off the alternating-current network. However, it also has drawbacks, especially the large amplitude of the test voltage which thus requires heavy and expensive transformers.

(b) The voltage Uv is a voltage having a frequency deviating from the operating or network frequency. In order to maintain the influence of the generator-ground-capacitance small, the frequency is selected to amount to between 10 Hz and 20 Hz. This method has the advantage that the measurement is extensively independent of disturbance magnitudes, so that the amplitude of the voltage Uv can be selected to be very low. However, a drawback of such a technique is that generation of the voltage Uv requires an expensive amplifier. If contrary to expectations the selected frequency appears at the generator-null path, then there occurs a false response of the relay. To avoid this the low-frequency of the voltage Uv is additionally coded as a function of time (e.g. permitted to pass for a number of periods, and then blocked for a number of periods).Due to its coding false relay responses are avoided, but again there is present the disadvantage that the relay will become blocked upon the occurrence of a disturbance voltage at the frequency of the voltage Uv i.e. in the case of a fault becomes non-functional.

According to the invention there is provided a method of detecting ground shorts in electrical systems isolated from ground or having a high ohmic connection with ground, comprising the steps of: producing a phase modulated alternating signal to form an alternating test signal; delivering the alternating test signal to the electrical system; and deriving from the electrical system a response signal corresponding to the alternating test signal and indicative of a ground short in the system.

The alternating test signal is preferably in the form of an alternating test voltage. The modulated alternating test signal preferably is at the network frequency. Modulation of the phase of the alternating signal may be carried out by continuously switching the phase of the alternating signal between two phase positions.According to another aspect of the invention there is provided apparatus for detecting ground shorts in an electrical system comprising:a transformer having two primary windings and a secondary winding for supplying an alternating test voltage; said secondary winding serving to apply the test voltage to the electrical system; a source for producing the test voltage; said source having a pair of terminals; said source being connected at one terminal with a junction point of both primary windings of the transformer; two electronic switch means having inputs and outputs; said source being connected at its other terminal in parallel with the inputs of said two electronic switch means; a respective output of each of said electronic switch means being connected with the other terminal of a respective one of said primary windings; an oscillator delivering an output signal for alternately opening said electronic switch means; an insulation transformer having a primary winding and a secondary winding; a resistor connected in parallel with the primary winding of said insulation transformer: said secondary winding of said insulation transformer delivering a response voltage corresponding to the test voltage and indicative of the ground short; a multiplier having a first input and a second input; said response voltage of the insulation transformer being delivered to the first input of said multiplier; means for applying a voltage constituting a reference signal to the second input of the multiplier; said multiplier having an output carrying an output signal; a filter for carrying out an integration operation; the output signal of the multiplier being delivered to the filter.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings wherein: Figure I is a circuit diagram of an arrangement according to the present invention, suitable for the practice of the method aspects thereof, and serving for detecting ground shorts in electrical systems; Figure 2 graphically illustrates the two phase positions of a test voltage Uv; and Figure 3 is a circuit diagram showing the connection of the phase oscillation element with the generator-transformer connected in block circuit.

In Figure 1 there is illustrated the functional principle of a phase oscillation element for producing a phase modulated alternating signal, constituting a test signal, here in the form of a test voltage Uv. The phase oscillation element comprises a phase modulation circuit embodying a source 4 from which there is derived the test voltage Uv. The source 4 may be assumed to be operating at the network frequency, although, as mentioned, other frequencies can be employed. Using a source 4 at the operating frequency constitutes a convenient means for producing the test voltage Uv. The test signal i.e.. the voltage signal Uv is coupled into the electrical system i.e.. the object 1 to be protected, here assumed to be the network, and ground by means of a resistor 2 and a transformer 3. The transformer 3 has two primary windings 5 and 6 and a secondary winding 3a.The secondary winding 3a is connected between ground and the object 1 to be protected by means of the resistor 2. On the other hand, the two primary windings 5 and 6 can be connected with the source 4 by means of two electronic switches 7 and 8 here shown in the form of triacs. The one terminal or pole 4a of the source 4 is connected with the common node or junction of both primary windings 5 and 6 of the transformer 3, whereas the other terminal 4b of the source 4 is connected via the triacs 7 and 8 with the free end or other connection point or terminal 5a and 6a of such primary windings 5 and 6 respectively.Both of the electronic switches 7 and 8 i.e. the triacs, are alternately opened at their control gates or electrodes, generally indicated by reference characters 7a and Sn respectively, by means of the output signals of an oscillator 9. The oscillator 9 alternately opens the triacs 7 and 8 at a desired predetermined switching frequency (1/tl), wherein t1 advantageously constitutes a predetermined triggering or firing time of the triacs and a remaining safety factor, as will be explained by way of example more fully herinafter; whereby the voltage Uv has its phase shifted by the switching frequency between 0 and 1800 i.e., the phase positions are out of phase by 1800 with respect to one another.Reference character or symbol Inv constitutes an inverter or reversing means for obtaining the correct control signal phases.

Turning attention to Figure 2 the two phase positons of the test voltage Uv have been designated by reference characters Uvl and Uv2. In the case of an insulation fault at the protected object 1, a current Iv (Iv,. Iv2 in Figure 2) which is derived from the voltage Uv flows through the resistor 2 and at that location produces a voltage drop defining a response voltage which is applied to the primary winding 12a of a transformer 12. This response voltage in the form of a voltage drop, is delivered by means of the secondary winding 12b of the transformer 12 (Figure 1) to an input x of a multiplier 11. At the other input y of the multiplier 11 there appears the voltage Uv as a reference signal, and which may be directly supplied by the source 4. However, in order to avoid the effects of disturbance currents by the impedances of the transformer 3, it is possible to also separately synchronously generate the reference signal. This is so for the circuit arrangement of Figure 1, where the reference signal generator together with the oscillator is shown in a single block 9.

The measurement current flowing through the resistor 2 is composed of a useful signal (response signal) Iv and an undefined disturbance signal Is. The disturbance signal Is can be of random frequency, phase position and within certain limits also amplitude. The product at the output of the multiplier 11 can be expressed by the following: B = Uv x (Iv + Is) = Uv . Iv + Uv . Is.

The part Uvis, when integrated over a random long time tg, equals null. Therefore, there is present at the resistor 2 the possibility of distinguishing between the response signal, here the useful current Iv, the amplitude of which is great only in the case of a ground short, and all possible disturbance currents Is. Integration is accomplished at the filter 13. The integration result i.e. the processed signal then can be beneficially employed to activate a suitable protective device, such as a relay, to shutdown the object 1 to be protected.

Tests carried out at a prototype circuit have found the following values to be advantageous by way of example: Uv = 2% of the amplitude of the network phase voltage.

t, = 60 ms, wherein the triacs 7 and 8 are only fired for 45 ms, the remaining time 15 ms serves as a security or safety factor in order to avoid simultaneous firing of both triacs by the disturbance currents. The predetermined switching frequency amounts to 1/tl as previously explained.

13 = filter of the 12 order ( 50 dB).

Now in Figure 3 there is shown the connection of the phase oscillation element incorporating the transformer 3 by means of its secondary winding 3a with a generator 30 and transformer 14 in block circuit, as generally indicated by reference character 10. The test signal e.g. the test voltage Uv, is coupled to the generator 30 to be protected by means of the star wound transformer 14. In particular, the test signal generator here encompassing the secondary side 3a of the transformer 3, is connected to the open delta winding 14a of the transformer 14. The amplitude of the injection voltage Uv amounts to approximately 2% of the amplitude of the phase voltage.When a ground short occurs, for instance at the location 15, the load impedance of the secondary side 14b of the voltage transformer 14 i.e. the input impedance of the generator 30 drops to such a low value that the useful current (response signal) Iv flows across the resistor 2 and can be measured at that location. All of the disturbance currents flowing through the resistor 2 induing those caused by the ground short, are filtered by the nature of the measuring method.In order to maintain within limits the circuit expenditure of the multiplier and the subsequently arranged filter, it is possible starting with a value of the fault current If (Figure 4) flowing across the ground short and amounting to 0.5. Ifmax (maximum of the fault current) to switch to a simple measurement at the network frequency in the manner of the heretofore mentioned standard 95% - stator ground short protection.

The proposed solution of the invention employing the phase varying test voltage Uv combines the advantages of both of the above-mentioned, conventional methods (a) and (b). The voltage Uv advantageously may be at the operating or network frequency and can be easily generated; this is also the case for the method (a). On the other hand, the amplitude of the voltage Uv, coinciding with the method (b), can be made relatively small.

Reference is directed throughout to copending Applications 7683/78 and 5679/78. Serial No's 1601236, 1601237.

WHAT WE CLAIM IS: 1. A method of detecting ground shorts in electrical systems isolated from ground or having a high ohmic connection with ground, comprising the steps of: producing a phase modulated alternating signal to form an alternating test signal; delivering the alternating test signal to the electrical system; and deriving from the electrical system a response signal corresponding to the alternating test signal and indicative of a ground short in the system.

2. A method as claimed in claim 1 wherein the phase modulated alternating signal is in the form of a phase modulated alternating voltage.

3. A method as claimed in claim 1, wherein the phase modulation comprises the steps of: continuously switching the phase of an alternating signal between two phase positions.

4. A method as claimed in claim 3, wherein the continuous phase switching of the alternating signal is in the form of phase oscillation.

5. A method as claimed in claim 1, wherein: the phase modulated alternating signal has a frequency which is at the network frequency.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

The measurement current flowing through the resistor 2 is composed of a useful signal (response signal) Iv and an undefined disturbance signal Is. The disturbance signal Is can be of random frequency, phase position and within certain limits also amplitude. The product at the output of the multiplier 11 can be expressed by the following: B = Uv x (Iv + Is) = Uv . Iv + Uv . Is.

The part Uvis, when integrated over a random long time tg, equals null. Therefore, there is present at the resistor 2 the possibility of distinguishing between the response signal, here the useful current Iv, the amplitude of which is great only in the case of a ground short, and all possible disturbance currents Is. Integration is accomplished at the filter 13. The integration result i.e. the processed signal then can be beneficially employed to activate a suitable protective device, such as a relay, to shutdown the object 1 to be protected.

Tests carried out at a prototype circuit have found the following values to be advantageous by way of example: Uv = 2% of the amplitude of the network phase voltage.

t, = 60 ms, wherein the triacs 7 and 8 are only fired for 45 ms, the remaining time 15 ms serves as a security or safety factor in order to avoid simultaneous firing of both triacs by the disturbance currents. The predetermined switching frequency amounts to 1/tl as previously explained.

13 = filter of the 12 order ( 50 dB).

Now in Figure 3 there is shown the connection of the phase oscillation element incorporating the transformer 3 by means of its secondary winding 3a with a generator 30 and transformer 14 in block circuit, as generally indicated by reference character 10. The test signal e.g. the test voltage Uv, is coupled to the generator 30 to be protected by means of the star wound transformer 14. In particular, the test signal generator here encompassing the secondary side 3a of the transformer 3, is connected to the open delta winding 14a of the transformer 14. The amplitude of the injection voltage Uv amounts to approximately 2% of the amplitude of the phase voltage.When a ground short occurs, for instance at the location 15, the load impedance of the secondary side 14b of the voltage transformer 14 i.e. the input impedance of the generator 30 drops to such a low value that the useful current (response signal) Iv flows across the resistor 2 and can be measured at that location. All of the disturbance currents flowing through the resistor 2 induing those caused by the ground short, are filtered by the nature of the measuring method.In order to maintain within limits the circuit expenditure of the multiplier and the subsequently arranged filter, it is possible starting with a value of the fault current If (Figure 4) flowing across the ground short and amounting to 0.5. Ifmax (maximum of the fault current) to switch to a simple measurement at the network frequency in the manner of the heretofore mentioned standard 95% - stator ground short protection.

The proposed solution of the invention employing the phase varying test voltage Uv combines the advantages of both of the above-mentioned, conventional methods (a) and (b). The voltage Uv advantageously may be at the operating or network frequency and can be easily generated; this is also the case for the method (a). On the other hand, the amplitude of the voltage Uv, coinciding with the method (b), can be made relatively small.

Reference is directed throughout to copending Applications 7683/78 and 5679/78. Serial No's 1601236, 1601237.

WHAT WE CLAIM IS: 1. A method of detecting ground shorts in electrical systems isolated from ground or having a high ohmic connection with ground, comprising the steps of: producing a phase modulated alternating signal to form an alternating test signal; delivering the alternating test signal to the electrical system; and deriving from the electrical system a response signal corresponding to the alternating test signal and indicative of a ground short in the system.
2. A method as claimed in claim 1 wherein the phase modulated alternating signal is in the form of a phase modulated alternating voltage.
3. A method as claimed in claim 1, wherein the phase modulation comprises the steps of: continuously switching the phase of an alternating signal between two phase positions.
4. A method as claimed in claim 3, wherein the continuous phase switching of the alternating signal is in the form of phase oscillation.
5. A method as claimed in claim 1, wherein: the phase modulated alternating signal has a frequency which is at the network frequency.
6. A method as claimed in claim 1, further including the steps of:

switching to a normal network frequency measurement with 95% protection of the ground short when a fault current flows through the ground short and which fault current has at least a value corresponding to approximately one-half of the maximum fault current.
7. A method as claimed in claim 1, wherein the phase modulation comprises the steps of: switching the phase of the alternating test signal between two phase positions which are opposite in phase to one another.
8. A method as claimed in claim 7, including the steps of: performing phase switching of the alternating signal during switching periods amounting to about 60 ms.
9. A method as claimed in claim 8, wherein: each of the switching periods encompasses a triggering time of about 45 ms and a remaining time constituting a safety factor.
10. A method as claimed in claim 1, wherein: the amplitude of the phase modulated alternating test voltage amounts to about 2% of the amplitude of the network voltage.
11. A method as claimed in claim 2, wherein: the electrical system is a stator of a generator.
12. Apparatus for detecting ground shorts in an electrical system comprising: a transformer having two primary windings and a secondary winding for supplying an alternating test voltage; said secondary winding serving to apply the test voltage to the electrical system; a source for producing the test voltage; said source having a pair of terminals; said source being connected at one terminal with a junction point of both primary windings of the transformer; two electronic switch means having inputs and outputs; said source being connected at its other terminal in parallel with the inputs of said two electronic switch means; a respective output of each of said electronic switch means being connected with the other terminal of a respective one of said primary windings; an oscillator delivering an output signal for alternately opening said electronic switch means;; an insulation transformer having a primary winding and a secondary winding; a resistor connected in parallel with the primary winding of said insulation transformer; and secondary winding of said insulation transformer delivering a response voltage corresponding to the test voltage and indicative of the ground short; a multiplier having a first input and a second input; said response voltage of the insulation transformer being delivered to the first input of said multiplier; means for applying a voltage constituting a reference signal to the second input of the multiplier; said multiplier having an output carrying an output signal; a filter for carrying out an integration operation; the output signal of the multiplier being delivered to the filter.
13. Apparatus as claimed in claim 12, wherein: said source operates at the network frequency of the electrical system.
14. Apparatus as claimed in claim 12, wherein: said voltage applying means delivers a reference voltage substantially equal to the network frequency of the electrical system.
15. Apparatus as claimed in claim 12, wherein: said voltage applying means delivers said test voltage as said reference voltage at the second input of the multiplier.
16. Apparatus as claimed in claim 12, wherein: said voltage applying means supplies a separate signal which is generated substantially in synchronism with the test voltage and which appears as said reference signal at the second input of the multiplier.
17. Apparatus as claimed in claim 12, wherein: said electronic switch means comprise triacs.
18. Apparatus as claimed in claim 17, wherein: the amplitude of said test voltage amounts to about 2% of the amplitude of network voltage.
19. Apparatus as claimed in claim 18, wherein: the triacs have switching periods amounting to about 60 ms.
20. Apparatus as claimed in claim 17, wherein: the triacs have switching periods amounting to about 60 ms; said triacs being fired for about 45 ms and the remainder of the time serving as a safety time in order to prevent simultaneous firing of both triacs by disturbance currents.
21. Apparatus as defined in claim 20, wherein; in the presence of a fault current flowing across a ground short starting from a predetermined value which approximately corresponds to one-half of the maximum fault current, there is switched to a normal operating frequency measurement with 95% ground short protection of the electrical system.
22. Apparatus as claimed in claim 12, wherein: said electrical system comprises a stator winding of a generator and which is to be essentially completely protected against ground shorts.
23. A method of detecting ground shorts in electrical systems as claimed in any one of claims 1 to 11 and substantially as hereinbefore described.
24. Apparatus for detecting ground shorts in an electrical system substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.