XLPE CABLE COMMISSIONING TEST PROCEDURE FOLLOWING IEC STANDARD
XLPE Insulated Power Cable Commissioning Test Procedure as per IEC Standard
After completion the installation work of underground power transmission cable line, commissioning test must be carryout just before energize the plant. This article demonstrate the practical commissioning test procedurethe practical commissioning test procedure for extra high voltage (EHV) and high voltage (HV) underground power cable.
Following commissioning test should be carryout step by step:
· Phasing Verification;
· 10kV DC Sheath Integrity Test;
· Cable Insulation Resistance Measurement (IR);
· Capacitance Measurement;
Phasing Verification Test
· Test Purpose: To ensure the continuity and phasing of the cable conductor from terminal to terminal
· Test Equipment: Megger Tester (BM21 or Equivalent)
· Requirement: There are not any discontinuities of the cable conductor.
Conductor continuity and phasing test will be performed after completion of all splices and terminations installation.
Phasing tests will be carried out by using main conductor as shown in Figure, where the phase undergoing the test is grounded while the other two phases of cable are opened.
The cable conductor shall be tested using a minimum of DC 1kV per phase as below figure
10kV DC Sheath Test
· Test Purpose: DC 10kV test shall be performed to confirm the soundness and integrity of the cable sheath.
· Standard: According to clause 14.1 of IEC 62067
· Test Equipment: DC Voltage Tester (Baur PGK 80 or Hi-Leakester or Equivalent)
· Requirement: No breakdown shall be occurred.
During testing period, all necessary safety measures have to be taken into consideration. Special note must be taken of exposed metalwork that will be at test potential during the procedure e.g. cable termination bell housings.
The IR’s of each cable sheath (between Al sheath and ground) will be measured at 5kV for 1minute. The shields of cables not being tested will be grounded during IR measurement.
The tests will be applied at the shield grounding link-box at one end of the circuit. At the link-box at the far end of the circuit from the testing location the links and/or SVL’s will be removed. The link-box will be closed and locked and a notice warning of testing in progress posted. If exposed metalwork that will be hazardous during testing exists appropriate warning signs and a watchman must be posted to prevent accidental contact by any person.
At the testing end link-box the links and / or SVL’s will be removed. The two-phase cables not under test will be grounded by jumper leads. The 10kV DC test set will be prepared and connected to the bonding cable terminal of the first phase to be tested. Check that the test set is grounded. The voltage will be raised gradually but within 5-10 seconds to 10kV
The stop-watch will be started. Occasional adjustment of the applied voltage may be necessary to avoid exceeding 10kV. At about 55 seconds, the leakage current will be noted. At 60 seconds the applied voltage setting will be quickly reduced to zero. The HV source will be switched off to apply the test set integral grounding to the tested cable. The cable will be discharged automatically as indicated by the voltmeter.
Before handling the test connection at the link-box a ground will be applied manually to ensure that complete discharge has been achieved.
The test connection will be transferred to next phase to be tested and the foregoing repeated.
The IR measurements of the cable sheaths will be repeated after the 10kv tests and the obtained values noted.
Cable Insulation Resistance Test
· Test Purpose: To ensure the soundness of the cable insulation, prior to performing the high voltage tests on the cable and after high voltage test, cable insulation resistance by Mega ohmmeter shall be measured by applying DC 5kV for 1 minute between cable conductor and the metallic sheath under test with the adjacent cable conductors and sheaths grounded as shown in Figure.
· Test Equipment: Megger (BM21 or NIEAF Mega ohmmeter or Equivalent)
· Requirement: Measured cable insulation resistance should be higher than calculated value and put on record.
This test shall be performed after completion of all splices/terminations installation with all cable conductors and metallic sheaths of the test end grounded except cable conductor under test as shown in Figure.
All cable conductors and sheaths at the remote end shall be free.
Minimum cable insulation resistance shall be determined by the Equation except D and d; D is outer diameter of the cable insulation and d is inner diameter of the cable insulation.
R: Calculated Insulation Resistance in [MΩ]
ρ: Volume Resistivity of the Cable Jacket Insulation at 20℃, 1014 for XLPE or PE in[Ω-m]
D: Outer Diameter of the Cable Jacket in [mm]
d: Inner Diameter of the Cable Jacket in [mm]
L: Length of the Cable phase under test in [m]
Capacitance Measurement Test
· Test Purpose: The purpose of this test is to compare the measured capacitance with the capacitance calculated value. Capacitance is measured in order to provide a basis for the calculation of energized cable charging currents as required for protection relay setting calculations.
· Standard: IEC 62067 Section 10.10
· Test Instrument: Fluke 187 or NIEAF Tera Ohm 5kV or Equivalent
· Requirement: Corrected cable capacitance shall not exceed 0.227 μF/km specified
The connections to the cable to be measured must be affected as shown on below diagram. The Test Instrument line terminal will be connected to the phase conductor and the outer terminal to be connected to the metallic sheath of the same phase. Figure shows the arrangement of Cable Capacitance Measurement
Calculate the test result with the formula as follows:
Where Cx is the measured capacitance of one phase of the cable circuit in μF, C is the calculated capacitance of one phase of the cable circuit per kilometer in [μF/km] and L is the route length in km.
High Voltage AC Test (On Soak)
· Test Purpose: This test will be carried out to ensure that installed cables have no defects.
· Standard: IEC 62067
· Test Equipment: Substation System Voltage
A.C voltage test shall be applied for 24 hours with the normal operating voltage of the system without load.
AC voltage test of the insulation
The a.c test voltage to be applied shall be subject to agreement between the purchaser and the contractor. The waveform shall be substantially sinusoidal and the frequency shall be between 20Hz and 300Hz. The voltage shall be applied for 1 h, either with a voltage according to table 10 or with 1,7 U0 depending on Practical operational conditions.
Alternatively, a voltage of Uo may be applied for 24 h.
NOTE- for installations which have been in use, lower voltages and/or shorter durations may be used. Values should be negotiated, taking into account the age, environment, history of breakdowns, and of carrying out the test.