Specialized Engineering Services, Inc.
provides comprehensive High Potential (Hi pot) cable testing. We perform Hi Pot testing for a variety of facilities including industrial power plants, large commercial facilities, and power generation stations. S.E.S. is available 24 hours a day for emergency response situations
The Importance of Cable Testing
Methods of Testing
S.E.S. approaches each cable testing in accordance with NETA MTS procedures. Whether the cable's voltage is High, Medium, or Low, visual and mechanical inspection is the best first course of action to determine the proper tests required. Visual and mechanical inspections consist of but are not limited to:
-
Inspect exposed sections of cables for physical damage and evidence of overheating
-
Inspect compression-applied connectors for correct cable match and indentation
Additional testing can be done after inspection to prevent further damage to the cables. NETA specifies Dielectric Withstand & Baseline diagnostic tests are required for Medium and High-Voltage cables. The specific test methods for Medium and High-Voltage cables are listed below:
-
Dielectric Withstand
-
Direct current (dc) dielectric withstand voltage
-
Very low frequency (VLF) dielectric withstand voltage (VLF Testing)
-
Power frequency (50/60 Hz) dielectric withstand voltage
-
-
Baseline Diagnostic Tests
-
Power factor/ dissipation factor (tan delta)
-
Power frequency (50/60 Hz)
-
Very low frequency (VLF)
-
-
DC insulation resistance
-
Off-line partial discharge
-
Power frequency (50/60 Hz)
-
Very low frequency (VLF)
-
-
After testing is done on any High, medium, or low voltage cable, Based on the test methodology chosen, refer to applicable standards or manufacturer’s literature for acceptable values
The Basic test Voltage for Hi Pot testing is the 2X (Operating Voltage) + 1000 V. The reason for using 1000 V as part of the basic formula is that the insulation in any product can be subjected to normal day-to-day transient over voltages.
Types of Hi Pot Testing
In accordance with ICEA, IEC, IEEE and other power cable consensus standards, testing can be performed by means of direct current, power frequency alternating current, or very low frequency alternating current. These sources may be used to perform insulation-withstand tests, and baseline diagnostic tests such as partial discharge analysis, and power factor or dissipation factor.
-
Insulation Resistance Testing
-
VLF Hi Pot Testing:
Very Low Frequency or VLF testing is non destructive and has become the industry standard for medium voltage conductors. The test in non destructive and requires smaller test equipment making the work for efficient.
-
DC Hi Pot Testing:
DC Hi Pot testing is primarily used for testing laminated cable systems such as paper insulated lead covered cable types. DC Hi Pot testing units are relatively easy to control, provide accurate leakage current data, and are relatively small and lightweight. Additionally, DC test voltage can be set to a much lower value that that of AC testing, allowing a manufacturer to filter testing based on equipment with marginal insulation.
-
AC Hi Pot Testing:
AC Hi pot test is that it can check both voltage polarities, whereas a dc test charges the insulation in only one polarity.
-
Tan Delta Testing:
A very low frequency test voltage is applied across the equipment whose insulation is to be tested. First the normal voltage is applied. If the value of tan delta appears good enough, the applied voltage is raised to 1.5 to 2 times of normal voltage, of the equipment. The tan delta controller unit takes measurement of tan delta values. A loss angle analyser is connected with tan delta measuring unit to compare the tan delta values at normal voltage and higher voltages, and analyse the results.
During test it is essential to apply test voltage at very low frequency.
Failures Due to Insulation Breakdown
Over time, insulation of electrical system components deteriorate due to environmental wear, frequency of use, and lack of proper maintenance. Per the IEEE Gold book, table 36, the percentage of failures due to insulation breakdown per component is shown below.
Component Percentage of Insulation Failure
Transformers 84%
Circuit Breakers 21%
Disconnect Switches 15%
Insulated Switch gear Bus 95%
Bus duct 90%
Cable 89%
Cable Joints (splices) 91%
Cable Terminations 87%
S.E.S. provides Hi Pot testing services on cables to help drastically reduce the percentage of failure, ultimately saving time, money, and the safety of employees.
High Potential (Hi Pot) Testing
The Hi Pot test is a nondestructive test that determines the adequacy of electrical insulation for the normally occurring over voltage transient. This is a high-voltage test that is applied to all devices for a specific time in order to ensure that the insulation is not marginal. Hi Pot tests are helpful in finding nicked or crushed insulation, stray wire strands or braided shielding, conductive or corrosive contaminants around the conductors, terminal spacing problems, tolerance errors in cables, inadequate creepage and clearance distances introduced during the manufacturing process.