Automation of Electrical Test Procedures for Low-Voltage Cables

Ahmad Nahas School of Electrical Engineering

Victoria University of Technology PO Box 14428 MC, Melbourne 8001,

Australia Email: Ahmad.Nahas@students.vu.edu.au

Akhtar Kalam School of Electrical Engineering

Victoria University of Technology PO Box 14428 MC, Melbourne 8001,

Australia Email: Akhtar.Kalam@ vu.edu.au

ABSTRACT

For several years, different utilities have performed specific electrical test procedures for a low voltage cable system manually. This procedure has proved to be a costly approach, where excessive amount of time is utilised on completing groundwork, including calculations, mundane record keeping tasks and analysis. In addition, the manpower to allow the procedures to continue is doubled. This wasted energy could be used in a more functional approach, where only one officer can precede tests and with a reduced amount of cost and analysis.

This paper investigates a process to automate the electrical tests procedures for low-voltage cables. Three electrical tests are mandated by the AS/NZ standards: high voltage, insulation resistance and conductor resistance.

Thus, the paper will also discuss into two main streams: an overall program controlling all tests, and a high voltage and high current switching device to switch between the appropriate conductor and electrical test. An overall program handles the three electrical tests and communicates with an AS400 database for cable information. A discussion on prototype will also be included to be built for switching between cores and electrical tests which contain a computer communication interface. The design, development and tests were made at Olex, Australia’s and New Zealand’s largest cable manufacturer. More specifically, this system was established for Olex’ Low-Voltage testing facility. 1. INTRODUCTION

Several cable manufacturers have performed specific electrical test procedures for a low-voltage cable system manually. This procedure has proved to be a costly approach, where excessive amount of time is utilised on completing groundwork, including calculations, mundane record keeping tasks and analysis; also, the manpower to allow the procedures to continue are doubled. This wasted energy could be used in a more functional approach, where only one officer can precede tests and with a reduced amount of cost and analysis.

The test equipment will require an upgrade to advance to the latest technologies. 2. ELECTRICAL TESTS FOR LOW-VOLTAGE

CABLES

As per the AS1660.3: Test methods for electric cables, cords and conductors - Electrical tests, three routine electrical tests should be performed on low-voltage cables [1]. High Voltage, Insulation Resistance and Conductor Resistance tests are performed on most but not all low-voltage cables on three different stages of manufacture: core, lay-up or completed stage. Furthermore, the low-voltage cables include XLPE, Rubber, PVC and Instrumentation cables.

2.1. HIGH VOLTAGE TEST

The high voltage test is the application of high voltage AC with power frequency between the cable cores and between the cable cores and earth to demonstrate the integrity of the cable insulation [2]. It is applied on multi-core cables or screened cables. In case of test failure, the voltage applied will decrease to minimum raising the amount of current passing through the cable cores and insulation. The value of the AC voltage applied depends on the voltage rating of the cable as shown in Table-1. Moreover, all low voltage cables require 5 minutes test duration and a maximum voltage of 15kV.

Table 1: High Voltages applied as per AS/NZ1660.3

2.2. INSULATION RESISTANCE TEST

The insulation resistance test is adopted as a useful test for low-voltage cables to reveal the case when a cable with poor quality of insulation still passes the High Voltage test [2]. It is also applied on multi-core cables or

screened cables. For all relevant cables, the IR test is at 500V DC for duration between 1 to 2 minutes. Moreover, the value obtained from the Megohmmeter is in MΩ which is then calculated into MΩ.km by using the following formula:

)()(1 kmLengthMIRIR Actkm ×Ω= . Where L: the length of the cable in kilometres. 2.3. CONDUCTOR RESISTANCE TEST

Basically, the conductor resistance test is applied on the cables cores to check the continuity of the core. The DC resistance of the conductor shall be measured at ambient temperature and corrected for 20°C. The following formula is used for such correction:

)()(kmLength

kCRCR Tactcal

×Ω=

Where kT: the temperature correction factor for resistance at the temperature of t°C. L: the length of the cable in kilometres. 3. PROJECT DESCRIPTION

The final product is computer-controlled system operating all test equipment in order to perform all electrical tests. The system design is divided into two major components: software and hardware Development. The software development is about creating a program for each electrical test operating the equipment. The three programs are linked together controlled by another subprogram creating different sequencing patterns for various types of cables at different manufacturing stages. Moreover, a user friendly interface for accessing the testing program, database and other different files. After the completion of the software, a micro-processor based device will be designed and built to switch between the cores of the cable against each electrical test. This device will require a computer communication port that could interface with the program written for “talk/listen” procedures. 3.1. SOFTWARE DEVELOPMENT

The software component of the project was developed using Laboratory Virtual Instrument Engineering Workbench (LabVIEW), software created by National Instruments. This is a graphical programming language that uses icons instead of lines of text to create applications [3]. The software contains three major sections as shown in Figure-1. Since this system is designed to operate at Olex’ low-voltage testing department, the automatic testing sequence is similar to the manual testing procedures.

= Figure 1: Subprograms represented in a flowchart.

The Test Sequence program performs the three electrical tests depending on the stage of manufacture and type of the cable under test. Thus, a database of cables and different data had been implemented for this reason.

3.1.1. THE USER INTERFACE

The User Interface is the user friendly and allows the operator to access all different parts of the software. As shown in Figure-2, the main window of the program allows the operator to access two different sections: Administrative section for setup purposes or General section for the operator to login to perform electrical tests.

Figure 2: Main window of User Interface

3.1.2. ELECTRICAL TESTS PROGRAM

Three electrical tests are performed as mentioned earlier. The High Voltage and Insulation Resistance Tests are performed on some cables unlike the Conductor Resistance Test that is performed on all cables. Thus, after all cable drums are settled in the test cage, the High Voltage Test is performed first followed by Insulation Resistance then Conductor Resistance. Therefore, following this specific test sequence, the program will list all cables in the window for each test individually. 3.1.2.1. HIGH VOLTAGE TEST PROGRAM

The High Voltage Test program follows the flowchart in Figure-4. This specific program built was for a high voltage test set by American HV Test and was followed by two different interfaces: Relay and Analog Interfaces, which are responsible for controlling and measuring the voltages and currents from the test set. The two

Software

Test Sequence

High Voltage Test Program

Insulation Resistance Test

Program

Conductor Resistance Test

Program

User Interface

Test Certificate

Quality Reports

Daily Reports

Label

Report and Label Generation

Database Access

Program

interfaces contain a “talk / listen” GPIB, General Purpose Interface Bus. The used to be known IEEE488.1 is now converted into USB, Universal Serial Bus, which is now more popular and practical than the GPIB. The High Voltage test program basically acquires the required test voltage and duration of test from the database at the beginning of the test program. As shown in Figure-3, the High Voltage Test program shows the readings of the voltage in kV and current in Amps acquired from the HV controller during the test. Moreover, it shows a timer counting downwards form the specified duration to zero in minutes and seconds.

Figure 3: High Voltage Test Program Display

Is VoltageIncreasing?

Connection Properties

Duration of test (min)

Test Voltage

Parameters from database, user inputs

or other programs

No

Yes

Initialise

HV OFF

Raise Voltage

Asks to check Connection

to cable

Reached requiredvalue?

No

Keep fixedfor duration

Detected a Short-circuit?

LowerVoltage

Yes

HV OFF

Duration over?

No

NoYes

HV Failed HV Passed

Is VoltageIncreasing?

Connection Properties

Duration of test (min)

Test Voltage

Parameters from database, user inputs

or other programs

No

Yes

InitialiseInitialise

HV OFFHV OFF

Raise VoltageRaise Voltage

Asks to check Connection

to cable

Reached requiredvalue?

No

Keep fixedfor durationKeep fixedfor duration

Detected a Short-circuit?

LowerVoltageLowerVoltage

Yes

HV OFFHV OFF

Duration over?

No

NoYes

HV Failed HV Passed Figure 4: High Voltage Test Program.

Then, the program starts rasing the voltage while constantly monitoring the voltage and current readings across the cable cores. It also checks for short circuit during the rise of voltage and during the actual test. The short circuit is determined when the voltage applied decreases while the current increases dramatically. Thus, when short circuit occurs, the program decreases the

voltage as quickly as possible and turns off the High Voltage on the set. If more than one cable were tested together, it will allow the operator to test each cable individually because only one cable might be failing the test. 3.1.2.2. INSULATION RESISTANCE TEST PROGRAM

The Insulation Resistance program interacts with a 5kVolts DC LEM Megohmmeter via RS232. The program reads whether each cable under test require insulation resistance test. This program runs similarly to the high voltage test; however, all cables that require insulation resistance test are tested under 500Volts DC for duration of one or two minutes. This test is performed on only one cable at a time. The steps followed in the Insulation Resistance Test program are shown in Figure-6.

Figure 5: IR Test Program Display

In Figure-5 above is the Insulation Resistance Test program front view demonstrating the readings of the insulation resistance for duration of one minute.

Initialise

Connection Properties

Length (meters)Min. IR

Allowed (MΩ/km)

Parameters from database, user inputs

or other programs

Measure

No

Yes

InsulationResistance

IR > 1GΩ ?

Calculates for 1km

IRCalc(MΩ.km)

No

Yes

IRCalc<IRMin?

Cable fails IR Test

Cable passes IR Test

Exit

YesNo

Is Megger ready?

Initialise

Connection Properties

Length (meters)Min. IR

Allowed (MΩ/km)

Parameters from database, user inputs

or other programs

Measure

No

Yes

InsulationResistance

IR > 1GΩ ?

Calculates for 1km

IRCalc(MΩ.km)

No

Yes

IRCalc<IRMin?

Cable fails IR Test

Cable passes IR Test

Exit

YesNo

Is Megger ready?

Figure 6: Insulation Resistance Test Program.

3.1.2.3. CONDUCTOR RESISTANCE TEST PROGRAM

This program communicates with the CROPICO micro-ohmmeter via RS232 interface. Once the Kelvin clips are

connected to each end of the cable core, the program reads the conductor resistance when the operator starts the measurement and, hence, this measurement is compensated for a 20˚C and for a 1km length. In some cases, multiple cores of the same cable are joined for the same conductor resistance test. The procedures followed in the program are detailed in the flowchart shown in Figure-7.

Initialise

Resistance > 10kΩ ?

Connection Properties

Type of Conductor

Length (meters)Max. CR

Allowed (mΩ/km)

Parameters from database, user inputs

or other programs

Resistance

Measure

Temperature

No

Yes

CompensatedResistance

Resistance > 1kΩ ?

Calculates for 1km

CRCalc (mΩ/km)

No

Yes

CRCalc>CRMAX?

Cable fails CR Test

Cable passes CR Test

Exit

YesNo

Initialise

Resistance > 10kΩ ?

Connection Properties

Type of Conductor

Length (meters)Max. CR

Allowed (mΩ/km)

Parameters from database, user inputs

or other programs

Resistance

Measure

Temperature

No

Yes

CompensatedResistance

Resistance > 1kΩ ?

Calculates for 1km

CRCalc (mΩ/km)

No

Yes

CRCalc>CRMAX?

Cable fails CR Test

Cable passes CR Test

Exit

YesNo

Figure 7: Conductor Resistance Test Program.

3.1.2.4. TEST SEQUENCE PROGRAM

Most important of all, the above three subprograms require a program that operates them in a sequential procedure. This program is responsible for acquiring the necessary information from the database. This information is showed to the operator for confirmation as shown in Figure-8. Hence, all the cable and their cores will distribute under each test. The cables that require High Voltage test will show in the toolbar waiting for the operator to select the cables and pilots for test. Once, the “OK” button is pressed, the program checks if all selected cables require the same high voltage test value; hence, the test will commence. When all the cables showed in the toolbar are tested, the toolbar then shows all the cables that require Insulation Resistance test and that has passed the high voltage test. At the completion of the IR test, the toolbar shows all the cores of all the cables.

The toolbar shows ten cores, pilots and interstitial earths at a time; once the ten cores have finished the conductor resistance test, the rest will ten appear ready for the test. An example is shown in Figure-9, where two different cables are being tested in the same cage.

Figure 8: Windows acquiring and showing information before the test.

Figure 9: Example on the Toolbar during the three tests on two different cables.

3.1.2.5. REPORT GENERATION

For this specific program, up to four different reports are required for the cables under test. There is the Test Certificate for finished cables, Quality Reports for cables under a certain category, Daily Reports for all cables tested for a certain day and the Labels printed for finished cables that are ready to be despatched.

All above reports are generated under different formats such as Microsoft Word and Excel. As for the Label, this is generated by a WebSphere written program called by ftp commands accessing the AS400 database. All reports are saved on a share drive on the network that can be accessed anytime through the Report Access section of the software.

3.2. HARDWARE DEVELOPMENT

The second half of the project is designing and building a microprocessor-based prototype of a high voltage and current switching device. This device contains a digital component to interface with a computer, a high voltage and current relay matrix, and a protection component that activates in case of failure such as a short-circuit in the circuit. Basically, this prototype contains the RS232 interface which communicates directly with the desktop computer through a LabVIEW written program. Furthermore, the overall program, as mentioned above, will be modified to talk with this device sequencing the high voltage and high current relays to switch between the test equipment and the core under test. Figure-10 presents a general description of the switching device.

P2

HV Lead 2

9

10

11

4

12

13

14

15

Red Core

Blue Core

Black Core

White Core

1

2

3

4

6

5

7

7

8

HV Lead 1HV Test Set

Lead 2

Lead 1Megohmmeter

P1

C1

C2

Micro-ohmmeter

White CoreOther end

N/O Relay

N/C Relay

Figure 10: Schematic representation of the device

3.2.1. COMPUTER COMMUNICATION INTERFACE

The computer interface selected for this project was the RS232 interface. The RS232, Recommend Standard number 232, is a serial communication interface with bit rate up to 256kbps and line lengths of 15 meters or less [4]. The construction of the RS232 interface will consist of a PIC18F458 micro-controller and a MAX232, RS232 transceiver. 3.2.2. HIGH VOLTAGE AND CURRENT SWITCHING DEVICE

The number of high voltage relays implemented in the device depends on the maximum number of the cores handled at the same time. It also should contain protection elements, such as fuses or circuit breaker, in case of a failure in the cable especially during high voltage tests. The device should also contains a protection section to prevent the damage of any instruments connected to the switching device in case of a failure or over-current. Unfortunately, due to time constraints, this device could not be built and tested. 4. CONCLUSION

This paper has presented a scenario to provide automation solutions for electrical tests for different types and rating of cables. The concept can be extended to further include Partial Discharge Test where, in this case, it will involve Medium and Extremely High Voltage power cables. The system has proved to be efficient and time saving. All the record keeping and report generation is now being done automatically involving the operators to switch and change cores for testing until the device is ready to be used. 5. ACKNOWLEDGEMENTS

I would like to thank Professor Akhtar Kalam from Victoria University for his wise guidance and support during the time of the project. I would also like to thank Mr. Chris Slater for the opportunity to work on the project at Olex. And I would not forget the generous technical support from Mr. Stephen Paul, IT specialist, and Mr. Colin Ritchie, Electrical test officer.

REFERENCES

[1] AS/NZ 1660.3: Test methods for electric cables, cords and conductors Method 3: Electrical tests

[2] Olex Internal Procedures, “PT-X 214: Operation Procedures for the ET1 (Water Test) Area”, 1992.

[3] “Intuitive Graphical Programming Language for Engineers and Scientists”, http://www.ni.com/labview/whatis/intuitive_graphical.htm, 2006.

[4] “PC Interfaces and Controlling Devices”, http://www.epanorama.net/links/pc_interface.html, 2006.

[5] “Introduction to High Voltage Relays”, http://relays.tycoelectronics.com/kilovac/hvintro/, 2006.