FIELD OF THE INVENTION

The invention relates to fault locating systems and methods. Particularly, to systems and methods to find and locate fault in ungrounded direct current (DC) circuits, enabling isolation of the grounded circuit.

BACKGROUND OF THE INVENTION

In electric power distribution stations, ungrounded direct current circuits comprising of one or more feeder cables are used in control systems to oversee and protect the electrical installations. Whenever there is a fault, grounded systems typically trip circuit breakers to shut down the electrical system and prevent serious damages, but ungrounded systems will not trip and shut the system but raise an alarm while still being connected to the load. But if two or more ground faults occur, then there is a possibility that a ground circuit may be formed and cause serious damage to electric installations or disconnect the electric distribution system form the load. Therefore it becomes essential to locate the fault sooner and rectify them.

Fault finding devices in the field of the invention are generally used in any electric substation which has a DC battery associated with its charger, DC bus bar, positive feeders, negative feeders and its sub feeders. The positive and negative connection of the battery are floating that is they are not connected to the ground anywhere in the system. If any of the positive and negative connections of the battery comes in contact with the earth then it is termed as the positive or negative fault as the case may be.
Each of the feeders in this DC system feeds a particular circuit breaker or any of the components in the substation from the control room. Each of this connection is termed as feeders.

The DC control system has a positive and negative terminal. This positive and negative terminal is brought to the DC distribution box which has a positive and negative bus bar. Number of connections termed as feeders are connected to this bus bar. Connections from positive and negative bus bars are termed as positive and negative feeders respectively. These feeders feed positive and negative supply to various equipment in the station up to point of connection. These connections can be up to great length in certain big stations and the number of feeders can be 65 or more.

The positive and negative connections emanating from positive and negative feeders can become faulty and touch the ground anywhere in the station. This causes leakage current to flow to ground. If both positive and negative become fault they can result in shortening of battery and draining of battery. So upon occurrence of fault it is necessary to address the issue and remove the fault to prevent damage to the station as all the controls in the station are operated from the station battery. It is also found that on occasion of fault there have been many dangerous mal-function in the station.

Previously it has been a difficult task to detect and remove faults in the DC system. Many systems would remain with the fault undetected. Some prior art apparatuses existed which were unsuccessful in detection and pin pointing of faults as they were affected by the value of existing capacitance on each feeder. The value of the capacitance increased with the length of the feeders. In modern times with huge stations up to 5km radius and long control cables the existing equipments became redundant as the capacitance in the cables causes fault like current to be produced at the test frequency of these instruments indicating all feeders to be faulty.

Hitherto known methods to locate ground fault are laborious and time consuming. So a new approach was necessary to find faults in DC feeder cables in electric sub-stations. There is also a need to eliminate of balance for the capacitance existing on the feeders being tested. Therefore, there is a requirement in the field to overcome the problems of the prior art.

It is an object of the invention to detect faults up to a resistance of 400 kilo ohms on a floating DC system in power stations.

It is another object of the invention to precisely and accurately locate the fault without jeopardizing the installations.

It is another object of the invention to develop a portable and non-intrusive system capable of detecting the ground faults in lessor time.

SUMMARY OF THE INVENTION

To meet the objects of the invention it is herein disclosed, a system for detecting faults in feeder cables in an ungrounded direct current power control system comprising of a low frequency electric signal generator for generating low frequency electric signal and sending it across the feeder cables of the direct current power control systems; one or more non-intrusive probes positioned around the feeder cables at one or more locations for detecting the low frequency signal sent across the feeder cable; a signal receiver for receiving the signal from the probe and detecting the fault and the location in the feeder cable, wherein the signal is detected by the probe only when there is a fault in the feeder cable; and a display means for displaying the fault and location of fault.

Also disclosed herein is a method of detecting fault in feeder cables in an ungrounded direct current power control system comprises the steps of generating a low frequency electric signal in a low frequency electric signal generator and sending it across the feeder cables of the direct current power control systems; detecting the low frequency signal sent across the feeder cable by a non-intrusive probe positioned around the feeder cables; and receiving the signal from the probe in a signal receiver and detecting the fault and the location of fault, wherein the signal is detected by the probe only when there is a fault in the feeder cable.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig 1 is a schematic diagram showing the embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention and its various embodiments is better understood by reading the description along with the accompanying drawings which appear herein for purpose of illustration only and does not limit the invention in any way

In this method a very low frequency sine wave test signal is applied between the positive and the earth of the DC circuit system. The frequency is so slow as to process any wrong fault circuits from being produced in the system. The frequency is such that only a fault in the system can generate a current which can be tested or identified.

Referring to Fig. 1, a sine wave test signal of known frequency is generated in VLF generator (2) and is applied across the positive and the earth points in the DC distribution Board. This causes a circulating fault current on a particular feeders (103,104) on presence of a fault. The current is traced by sensitive probes (8) and this current flows up to the point of fault so that fault is detected, pinpointed and isolated.
The used frequency should cater to any station, small or big. The frequency should be such that it causes a circulation fault current to flow only on occurrence of a fault and should not cause circulating current due to its length and capacitance. Frequency in the range of 1Hz to 0.1 Hz is preferably suitable enough to overcome the capacitance effect of the station and is independent of the length of the cable. Faults up to 400 kilo ohms could easily be detected on these cables. Without using controls such as capacitance balancing adopted by other instruments it is possible to locate faults in a non-intrusive method with non-intrusive probes which are put across each cable to determine the existence of fault. The test signal is connected to the positive of the battery (1), as it is AC signal, it flows through the battery (1) and flows even to the negative terminal of the battery there by the test signal is fed to all the positive (103) and the negative feeders (105) in the station, and all the faults in both the positive (103) and negative feeders (105) are detected.

In accordance with one aspect of the present invention, there is provided a system for locating ground faults in a normally ungrounded DC circuit having a positive and a negative supply bus. The system may have one or more signal generators (2), non-intrusive probes (8) around the feeders (103, 105) and a receiver unit (102) comprising at least one signal processing unit (10) to analyse the signals and display the fault in terms of signal frequency in the signal frequency display (14).

The signal generator (2) generates very low frequency signals which are sent to the DC circuit under test. As the frequency is very low the effect of capacitance causing wrong fault currents is eliminated. Only faults can cause current to flow and be detected. Because of this very low frequency it is possible to locate faults up to 400 kilo ohms. The receiver unit (102) is fitted with high grade 50 hertz filters (9) which eliminate any 50 cycle noise present on the cable being tested.

The fault is detected both on the positive as well as negative feeders of the system. The capacitance present on these feeders does not affect the detection of fault.

By making the frequency very low it is possible to eliminate all the existing 50 hertz on the system to a high degree.

The method is non-intrusive, portable, easy to use, and does not jeopardize system integrity by introducing additional current into the system.

For a healthy system without any faults, the positive voltage will be equal to the negative voltage. This will be equal to half the station battery voltage. In case of a positive fault the positive voltage reduces and correspondingly the negative voltage increases and the total voltage will be equal to the station battery voltage. Similarly, in case of a negative fault, the negative voltage decreases, the positive voltage increases and the total voltage is equal to the station battery voltage.

The system of one of the embodiment of the invention comprises of a transmitter (101) unit to transmit the test signals generated in a VLF generator (2) across the feeders (103, 105) in the station, a non- intrusive probe (8) positioned around the feeders (103, 105) at one or more locations on the feeders for sensing the return signal and a receiver unit (102) for identifying the fault and the location of the fault by further analysis of the return signal.

The transmitter (101) unit gives an output of very low frequency sinusoidal signal of required amplitude and very low frequency. The unit may be adapted to be connected to a power source or optionally be operated by a battery (1). This unit has at least the following parts:

a) VLF generator (2): This unit generates the sine wave of required frequency and amplitude.
b) DC blocker (3): This blocks the station battery DC voltage from damaging the VLF generator.
c) Panel mounted voltmeters (not shown in diagram) to detect the direction of faults; that is whether the positive feeders or the negative feeder is at fault. There may be at least two meters: one for positive to earth and another from negative to the earth. These meters read the voltage present between positive to earth and negative to earth when connected to station battery.

Visual indication means like LED indicators may also be used to indicate the transmission of VLF to the station bus.

The non-intrusive current probe (8) is clamped on at various points on various feeders in the station to determine the faulty feeder. The receiver unit (102) is connected to the non-intrusive current probe. The signal sensed by the probe (8) is processed in the receiver by amplifying, filtering and converting to audio visual alarms which are heard and seen on the receiver unit. This is also converted to numeric form and the level of fault may also be indicated by means of a numeric LCD display. It is possible by means of alpha numeric display to determine the frequency of the detected signal and it is possible to correlate that the frequency transmitter is received.

Further tracing the fault on the faulty feeder takes us to the point of fault which is then pinpointed and removed. The receiver unit houses various filters (9) for 50 cycles to remove the noise emanating in the stations. This can be used on various sizes of the cables.

A person skilled in the art will understand that the invention can also be used in permanent installations to identify and locate fault on a continuous basis.

The embodiments of the invention are capable of operating on broad frequency ranges and not limited to the range indicated herein which is disclosed for the purpose of explanation only and does not limit the scope of the invention in any way.

It will be clear to a person skilled in the art that with few appropriate modifications and changes the system and method disclosed herein can be used to identify and locate fault in alternating current (AC) circuits as well.

It will be obvious to a person skilled in the art that with the advance of technology, the basic idea of the invention can be implemented in a plurality of ways. The invention and its embodiments are thus not restricted to the above examples but may vary within the scope of the claims.

Further the above-described embodiments of the present invention are intended to be examples only. Alterations, modifications and variations may be effected to the particular embodiments by those of skill in the art without departing from the scope of the invention, which is defined solely by the claims appended hereto.

We Claim:

1. A system for detecting faults in feeder cables in an ungrounded direct current power control system comprising of:

a. a low frequency electric signal generator for generating low frequency electric signal and sending it across the feeder cables

of the direct current power control systems;

b. one or more non-intrusive probes positioned around the feeder cables at one or more locations for detecting the low frequency signal sent across the feeder cable;

c. a signal receiver for receiving the signal from the probe and detecting the fault and the location in the feeder cable, wherein the signal is detected by the probe only when there is a fault in the feeder cable; and

d. a display means for displaying the fault and location of fault.

2. A system as claimed in claim 1 wherein the said probe is a Hall-effect probe.

3. A system as claimed in claim 1 wherein the said low frequency generated by the low frequency electric signal generator is preferably in the range of 1 Hz- 0.1 Hz.

4. A system as claimed in claim in any of the claims wherein the said display means may be an alpha-numeric display

5. A method of detecting fault in feeder cables in an ungrounded direct current power control system comprises the steps of:

a. generating a low frequency electric signal in a low frequency electric signal generator and sending it across the feeder cables of the direct current power control systems;

b. detecting the low frequency signal sent across the feeder cable by a non-intrusive probe positioned around the feeder cables;
and

c. receiving the signal from the probe in a signal receiver and detecting the fault and the location of fault, wherein the signal is detected by the probe only when there is a fault in the feeder cable.

6. A method as claimed in claim 5 wherein the said probe is a Hall-effect probe.

7. A method as claimed in claim 5 wherein the said low frequency generated by the low frequency electric signal generator is preferably in the range of 1 Hz- 0.1 Hz.

8. A method as claimed in in any of the claims 5, 6 or 7 wherein the said display means may be an alpha-numeric display.

9. A system as substantially as herein described in the complete specification and with reference to accompanying drawings.

10. A method as substantially as herein described in the complete specification and with reference to accompanying drawings.