WO2018117813A1

WO2018117813A1 - System and method for determining quality of power cable insulation using tangent delta measurement

Info

Publication number: WO2018117813A1
Application number: PCT/MY2017/050082
Authority: WO
Inventors: Huzainie Shafi ABD HALIM; Avinash Ashwin Raj RAJA GOPAL; Zairul AIDA; Tashia Marie ANTHONY; Chandan Kumar CHAKRABARTY
Original assignee: Tnb Research Sdn. Bhd.
Priority date: 2016-12-21
Filing date: 2017-12-21
Publication date: 2018-06-28
Also published as: MY190864A

Abstract

The present invention relates to an improved system and method for determining quality of power cable insulation using tangent delta measurement. The system (100) comprises a grounded enclosure (101), a power cable (102), a voltage source (104), a power amplifier (103), a current monitor (105) and an oscilloscope (106). The power cable (102) and the power amplifier (103) is contained in the grounded enclosure (101) and are interconnected to the voltage source (104), the current monitor (105), the oscilloscope (106) and the voltage probe (107) during testing. The present invention utilizes an input signal of low voltage and high frequency generated from and amplified by the voltage source (104) and the power amplifier (103) respectively, as well as short power cable samples for determining the quality of the power cable (102) insulation through tangent delta measurement based on an output signal obtained. The present invention also provides a method for determining quality of power cable insulation through an input signal generated and amplified by the voltage source (104) and the power amplifier (103) respectively.

Description

SYSTEM AND METHOD FOR DETERMINING QUALITY OF POWER CABLE INSULATION USING TANGENT DELTA MEASUREMENT

FIELD OF INVENTION

The present invention relates generally to power cable testing. More particularly, the present invention relates to an improved system and method for determining quality of power cable insulation using tangent delta measurement. BACKGROUND OF THE INVENTION

Power cables are subjected to high current and voltages during transmission of electrical power in a power system. Power cables are commonly insulated with polymeric materials such as cross-linked polyethylene (XLPE) due to its excellent physical, chemical and electrical properties to ensure safe and reliable electrical power transmission. However, insulation deterioration is inevitable during operation of the power system and failure rate of the XLPE power cables increases with time. In order to measure the degree of deterioration of the power cable insulation, tangent delta (Tan δ) test can be conducted.

Tan δ test also known as loss angle or dissipation factor test is a diagnostic test conducted to determine quality of a power cable insulation by measuring the degree of deterioration of the power cable insulation. The tangent (Tan) of the angle delta (δ) is measured in radians to indicate the level of resistance in the power cable insulation to determine the quality of the power cable insulation. An angle value close to zero indicates a perfect cable insulation that is free from defects such as water trees, electrical trees, moisture, air pockets and the likes. An increasing angle values indicates an increase in resistive current through the insulation, i.e. level of insulation contamination in the power cable. In other words, the larger the angle value, the worse the cable quality. Future breakdown of the power cable insulation can be predicted based on the Tan δ measurement, the maintenance schedule of the power cable can be planned and the cable insulation can be restored accordingly before actual flashover of the power cable. The measurement of Tan δ values in medium voltage and high voltage power cables are commonly performed at very low frequency (VLF) of 0.02 Hz and 1 Hz or power frequencies of 50Hz and 60Hz which requires a very long cable length of at least several tens of meters. The use of VLF (radio frequency) and power frequency requires a high voltage source that is typically expensive and bulky in size in order to supply the required output power to the power cable during testing. The conventional system requires a dedicated space to be occupied which makes the entire Tan δ measurement system expensive, complicated and unfeasible for non-routine test application in a laboratory. Furthermore, the tan delta results obtained from the conventional system are also solely for the total length of the cable used in the measurement which are lacking in spatial resolution.

See, for example, U.S. Patent No. 7,705,607 B2 which provides cable test methods, systems and apparatus to facilitate identification and location of defects along a power cable. According to the '602 patent, the methods and systems performs axial tomography so as to allow dielectric loss or dissipation factor (Tan δ), dielectric constant of the insulation, the resistance and inductance of the cable conductor system to be determined at one or more pre-determined points or sections along the cable axis. However, this approach discloses a notably complex system configuration and an extensive testing method in order to measure Tan δ and determine quality of the power cable insulation. Moreover, the system and method requires the use of long power cables for testing and measurement of Tan δ.

Although there are systems and methods for the same in the prior art, for many practical purposes, there is still considerable room for improvement. A need therefore exists for providing an improved system and method for determining quality of power cable insulation using tangent delta measurement that overcomes the problems or mitigates the disadvantages of the prior art. SUMMARY OF THE INVENTION

The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is not intended to identify key/critical elements of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later. Accordingly, the present invention provides a system for determining quality of power cable insulation.

The system of the present invention comprises a grounded enclosure for containing a power cable and a power amplifier; a voltage source connected to the power amplifier for generating a low voltage and high frequency input signal and an amplified input signal to be directed to the power cable, wherein the input signal from the voltage source is amplified by the power amplifier connected to one end of the power cable; a current monitor interconnected to the power amplifier, power cable and an oscilloscope for measuring and monitoring current flowing to and from the power amplifier and the power cable; and a voltage probe connected to the oscilloscope for observing an output signal from the power cable, wherein the oscilloscope is configured to display the input and output signals for obtaining a phase shift angle between the input and output signals, whereby the phase shift angle is obtained in order to determine the insulation of the power cable.

Preferably, the low voltage and high frequency input signal is a signal having a predetermined amplitude ranging from 1V to 10V peak-to-peak and a frequency of 1 kHz. Preferably, the amplified input signal is a signal having a predetermined amplitude ranging from 1 kV to 10kV peak-to-peak.

Preferably, the voltage source is a function generator. Preferably, the power cable is a power cable having a length ranging from 1 m to 2m.

In accordance with another aspect, the present invention provides a method of determining quality of power cable insulation. The method of the present invention comprises the steps of:

(i) containing a power cable, a power amplifier, and a current monitor in a grounded enclosure;

(ii) providing a voltage source and an oscilloscope;

(iii) connecting the voltage source to the power amplifier;

(iv) connecting the current monitor to the power amplifier, the oscilloscope and the power cable;

(v) providing a voltage probe and connecting the voltage probe to the oscilloscope;

(vi) setting the voltage source for generating a low voltage and high frequency input signal to be directed to the end of the power cable through the power amplifier, wherein the low voltage and high frequency input signal is set to have an amplitude ranging from 1 V to 10V peak-to-peak and a frequency of 1 kHz;

(vii) controlling the power amplifier for amplifying the low voltage and high frequency input signal received from the voltage source, wherein the low voltage and high frequency input signal is set to be amplified to have an amplitude ranging from 1 kV to 10kV peak-to-peak;

(viii) allowing the amplified input signal to flow through the current monitor to the power cable for obtaining an output signal from the power cable;

(ix) contacting the voltage probe at the one end of the power cable to allow the oscilloscope to display the output signal;

(x) obtaining a phase shift angle from the amplified input signal and the output signal through a displacement of phase between the amplified input signal and the output signal; and

(xi) computing a tangent value of the obtained phase shift angle to determine the quality of power cable insulation.

The present invention has been made to overcome the aforementioned drawbacks of the conventional system and method for determining quality of power cable insulation using tangent delta (Tan δ) measurement. It is therefore an advantage of the present invention to provide a simple, inexpensive and compact system and method for performing Tan δ measurement to determine quality of power cable of power cable insulation without the use of expensive, complicated, bulky devices and complex procedures.

The foregoing and other objects, features, aspects and advantages of the present invention will become better understood from a careful reading of a detailed description provided herein below with appropriate reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic configuration diagram illustrating a system for determining quality of power cable insulation according to an embodiment of the present invention;

Fig. 2 is a graph showing Tan δ measurement for power cables of various conditions of performed at a low power frequency of 50Hz.

Fig. 3 is a graph showing Tan δ measurement for power cables of various conditions according to an embodiment of the present invention. It is noted that the drawings may not be to scale. The drawings are intended to depict only typical aspects of the invention, and therefore should not be considered as limiting the scope of the invention. In the drawings, like numberings represent like elements between the drawings. DETAILED DESCRIPTION OF THE INVENTION

It is an object of the present invention to provide an improved system for determining quality of power cable insulation in an enclosed testing environment such as laboratories and research center with a simple yet efficient equipment and configuration. It is another object of the present invention to omit the use of expensive and bulky high voltage source to provide high voltage and high frequency input signal during testing and measurement of Tan δ of a power cable insulation. It is yet another object of the present invention to prevent the use of long power cable to perform Tan δ measurement effectively. Figure 1 shows a system 100 for determining quality of power cable insulation. According to the present invention, the system 100 includes a grounded enclosure 101 , a power cable 102, a power amplifier 103, a voltage source 104, a current monitor 105, an oscilloscope 106 and a voltage probe 107. The grounded enclosure 101 is configured to contain the power cable 102 having a predetermined length and the power amplifier 103 in order to prevent contact with dangerous high voltage during testing of the power cable 102. The power cable 102 may be positioned on a cable stand 109. Preferably, the grounded enclosure 101 is a covered section formed by at least one layer of woven wire mesh screen made of conducting material such as copper or metal grounded to dissipate electric currents and to block external electromagnetic interference. The voltage source 104 and the oscilloscope 106 is configured outside the grounded enclosure 101 and may be positioned on a table 110 for convenience and safety purposes to avoid direct contact to high voltage during testing. The voltage source 104 is preferably a function generator connected to the power amplifier 103 to generate a low voltage and high frequency input signal and an amplified input signal. The amplified input signal having a predetermined amplitude is generated by the power amplifier 103 to be directed to one end of the power cable 102 for Tan δ testing and measurement. Preferably, the amplified input signal from the power amplifier 103 is directed to the power cable 102 through a cable lug 108. The current monitor 105 is interconnected to the power amplifier 103, power cable 102, and the oscilloscope 106 to measure and monitor current flowing to and from the power amplifier 103. The voltage probe 107 is connected to the oscilloscope 106 for observing an output signal from the power cable 102. A phase shift between the input signal and the output signal is obtained from displayed signals displayed on the oscilloscope 106 in order to measure Tan δ and subsequently determine quality of the power cable 102 insulation.

In one preferred embodiment, the input signal is a low voltage and high frequency signal generated by the voltage source 104 to have a voltage ranging from 1V to 10V peak-to-peak and a frequency of 1 kHz. The amplified input signal on the other hand, is a high voltage and high frequency signal amplified from the low voltage and high frequency input signal by the power amplifier 103 to have a voltage ranging from 1 kV to 10kV peak-to-peak with the same frequency of 1 kHz. The power cable 102 is preferably a sample cable constructed to have a predetermined length ranging from 1 m to 2m.

According to another preferred embodiment of the present invention, there is provided a method of determining quality of power cable insulation. The method preferably begins with the step of containing a power cable 102 (of length about 1 m to 2m), a power amplifier 103, and a current monitor 105 in a grounded enclosure 101 by placing and positioning the power cable 102, the power amplifier 103, and the current monitor 105 in the grounded enclosure.

Once the power cable 102, the power amplifier 103, and the current monitor 105 is positioned in the grounded enclosure 101 , the next step is to provide a voltage source 104 and an oscilloscope 106. The voltage source 104 is then connected to the power amplifier 103. Subsequently, the current monitor 105 is connected to the power amplifier 103, the oscilloscope 106 and the one end of the power cable 102. Following that, a voltage probe 107 is provided and connected to the oscilloscope 106.

After the connections between the power cable 102, the power amplifier 103, the voltage source 104, current monitor 105, the oscilloscope 106 and the voltage probe 107 has been made, the next step is to set the voltage source 104 to generate a low voltage and high frequency input signal having an amplitude ranging from 1 V to 10V peak-to-peak with a frequency of 1 kHz to be directed to the power cable 102 through the power amplifier 103. The low voltage and high frequency signal generated from the voltage source 104 is then amplified by controlling the power amplifier 103 to amplify the low voltage and high frequency input signal having an amplitude ranging from 1V to 10V peak-to-peak with a frequency of 1 kHz to an amplified input signal having an amplitude ranging from 1 kV to 10kV peak-to-peak with the same frequency of 1 kHz. The amplified amplitude signal is then directed to the one end of the power cable 102 by allowing the amplified input signal to flow through the current monitor 105 to the power cable 102 in order to obtain an output signal from the one end of the power cable 102. Next step is to contact the voltage probe 107 to the one end of the power cable 102 to allow the oscilloscope 106 to display the output signal on the oscilloscope's 106 monitor. Subsequently, a phase shift angle is obtained from the amplified input signal and the output signal through a displacement of phase between the amplified input signal and the output signal.

Finally, computing a tangent value of the obtained phase shift angle (Θ) to determine the quality of power cable insulation. The tangent value is computed using a Tan delta (δ) formula as given in the equation below:

Tan δ = I—^— I [1] ' Tan Θ ^{L 1} With reference to Figure 2, experimental results are plotted for a sample of a

132kV 400mm² single core power cable having a length of 1 m performed at a low power frequency of 50Hz. The graph shows Tan δ measurement for power cables of three conditions namely good, void and contamination when tested at 5kV and 10kV. It can be seen that the resolution of Tan δ values cannot be clearly discriminated between all cable conditions.

With reference to Figure 3, experimental results are plotted for a sample of a 132kV 400mm² single core power cable having a length of 1 m according to an embodiment of the present invention. The graph shows Tan δ measurement for power cables of four conditions namely good, void, contamination and scotched when tested at 2kV, 6kV and 10kV. It can be seen that the resolution of Tan δ values of contamination, void and good cables can be clearly discriminated between all cables at 6kV and 10kV. The best resolution of Tan δ values can be clearly seen at 6kV. This shows that the present invention is able to discriminate the resolution of Tan δ value better.

The terms "a" and "an," as used herein, are defined as one or more than one. The term "plurality," as used herein, is defined as two or more than two. The term "another," as used herein, is defined as at least a second or more. The terms "including" and/or "having," as used herein, are defined as comprising (i.e., open language). While this invention has been particularly shown and described with reference to the exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the present invention as defined by the appended claims.

Claims

A system (100) for determining quality of power cable insulation, characterized in that, the system (100) comprising:

a grounded enclosure (101) for containing a power cable (102) and a power amplifier (103);

a voltage source (104) connected to the power amplifier (103) for generating a low voltage and high frequency input signal and an amplified input signal to be directed to the power cable (102), wherein the input signal from the voltage source (104) is amplified by the power amplifier (103) connected to one end of the power cable (102);

a current monitor (105) interconnected to the power amplifier (103), power cable (102) and an oscilloscope (106) for measuring and monitoring current flowing to and from the power amplifier (103) and the power cable (102); and

a voltage probe (107) connected to the oscilloscope (106) for observing an output signal from the one end of the power cable (102), wherein the oscilloscope (106) is configured to display the input and output signals for obtaining a phase shift angle between the input and output signals, whereby the phase shift angle is obtained in order to determine the insulation of the power cable (102).

The system according to Claim 1 , wherein the low voltage and high frequency input signal is a signal having a predetermined amplitude ranging from 1V to 10V peak-to-peak and a frequency of 1 kHz.

The system according to Claim 1 , wherein the amplified input signal is a signal having a predetermined amplitude ranging from 1 kV to 10kV peak-to-peak.

The system according to Claim 1 , wherein the power cable (102) is a power cable having a length ranging from 1 m to 2m.

5. The system according to Claim 1 , wherein the voltage source (104) is a function generator. A method of determining quality of power cable insulation, characterized in that, the method comprising the steps of:

containing a power cable (102), a power amplifier (103), and a current monitor (105) in a grounded enclosure (101);

providing a voltage source (104) and an oscilloscope (106); connecting the voltage source (104) to the power amplifier (103); connecting the current monitor (105) to the power amplifier (103), the oscilloscope (106) and the power cable (102);

providing a voltage probe (107) and connecting the voltage probe (107) to the oscilloscope (106);

setting the voltage source (104) for generating a low voltage and high frequency input signal to be directed to the one end of the power cable (102) through the power amplifier (103), wherein the low voltage and high frequency input signal is set to have an amplitude ranging from 1V to 10V peak-to-peak and a frequency of 1 kHz;

controlling the power amplifier (103) for amplifying the low voltage and high frequency input signal received from the voltage source (104), wherein the low voltage and high frequency input signal is set to be amplified to have an amplitude ranging from 1 kV to 10kV peak-to-peak;

allowing the amplified input signal to flow through the current monitor (105) to the power cable (102) for obtaining an output signal from the power cable;

contacting the voltage probe (107) at the one end of the power cable (102) to allow the oscilloscope (106) to display the output signal;

obtaining a phase shift angle from the amplified input signal and the output signal through a displacement of phase between the amplified input signal and the output signal; and

computing a tangent value of the obtained phase shift angle to determine the quality of power cable insulation.