Will your High Voltage systems respond adequately to a network fault!

Is the maintenance of your HV systems compliant with network safety regulations and your insurance company requirements. Your Asset Management Program should consider the safety performance and reliability of your High Voltage Transmission & Distribution systems. A critical part of the reliability of every large facility is the predictive and preventive maintenance of the HV Transmission & distribution network system i.e. HV Switchgear, Transformers, Protection Systems, CT’s & VT’s, Earthing, LV Circuit Breakers, ACO’s and Standby Generators.
Are your critical HV assets forgotten about or does your facility plan in adequate time at the correct frequency to carry out predictive maintenance (PdM) or preventive maintenance (PM) to ensure their safety and reliability.

For many facilities when carrying out their criticality analysis on their plant this is a single point of failure due to the fact that there is often little or no redundancy built in. At many facilities their HV Equipment is indeed included as an integral part of their maintenance strategy and key to the reliability of their production and manufacturing regimes. However for others it’s hidden behind locked doors out of site and out of mind out in the substation or at the back of the premises and not maintained until the day that the system requires to react instantaneously to a fault on the HV system or power network and it fails catastrophically.

Photo above shows a recent Maintenance Event carried out by H&MV Engineering at a 70MW EFW Plant near Liverpool, UK. This facility carries out regular HV equipment predictive & preventive maintenance. The preventive maintenance here included the provision of a Senior Authorised Person (SAP) to carry out HV Switching & Operations to isolate and check for dead before maintenance could commence. The scope of work entailed functional and electrical testing of all 33 kV HV Switchgear in the substation compound, NER, 11/33 kV and 33/6.6 kV Transformer testing and maintenance, with oil sampling and Protection Relay System testing. A transformer Condition Assessment Test (TCA) was carried out on the 33kV/11kV 63MVA Transformer. This involves carrying out a suite of electrical tests by one of H&MV’s Commissioners using state-of-the-art Omicron Test Equipment.

Tests included:

    1. Turns ratio and Exciting current
      • Detectable features:
        • Shorted winding sections
        • Core Faults
    2. Winding resistance (Dynamic and Static)
      • Detectable features:
        • Static Resistance measurement
        • Bad internal contacts (diverter switch and tap selector switch contacts, con­nection clamps
        • Short circuits between conductors of a winding
        • Damaged conductor
      • Dynamic Resistance measurement
        • Bad, bouncing contacts in the OLTC
        • Checks if the OLTC is operating with­out interruption
    3. Short circuit impedance
      •  Detectable features:
        • Shorted winding sections
    4. Tan Delta (Winding and Bushings)
      • Detectable features:
        • Change in geometry due to mechani­cal damage or high currents
        • Breakdowns between grading layers of condenser bushings
        • Water in solid insulation/ Ageing
        • Contamination of insulation liquids
        • Discharge due to bad contacts in bushing test tap
    5. Moisture Determination in liquid and solid insulation (Dirana)
      • Detectable features:
        • Measures the water content in solid insulation
        • Determines the oil conductivity and geometry data
        • Frequency response analysis
        • Detectable features:
        • Powerful and sensitive method for evaluating mechanical integrity of core, windings and clamping structures within power transformers
        • Geometrical changes in this network cause deviations of frequency response

Regular HV Equipment maintenance involves a predictive and a preventive element, or what we at H&MV Engineering like to call “Non–Intrusive and Intrusive Maintenance”.

Intrusive Maintenance: The preventive element – which is carried out on equipment that is shutdown, dead and isolated typically involves a comprehensive set of inspection and maintenance tests of HV primary and secondary Switchgear, transformers and protection relays. For Switchgear it involves racking out the circuit breakers, cleaning, inspecting and lubricating the mechanical elements of the switchgear, functionally testing the equipment, HV terminations inspected and carrying out a series of electrical tests such as a Ductor test on Busbars, Ductor tests of circuit breaker contacts, insulation resistance tests of circuit breaker contacts and primary injection tests of the CT’s and VT’s. The basic checks performed during transformer maintenance includes HV/LV termination inspected, transformer oil level checked and topped up, temperature and pressure alarms and trips tested, cooling fan correct operation, silica gel checked/replaced and carry out Ductor tests on terminations. The DC Tripping unit is inspected and maintained and tested for correct operation and Protection Relays are tested including secondary three phase current injection, prove correct operation of relay as per relevant IEC standards and prove circuit breaker trips via the simulation of the protection relay. We also carry out full transformer condition assessments depending on customer requirements or manufacturer recommendations.

Typical intrusive type preventive maintenance tasks include:

  • HV & LV Switchgear Inspection & Testing
  • SF6 Switchgear gas handling equipment and leak detection, disposal and recycling
  • Transformer Inspection & Testing
  • Transformer Condition Assessment
  • Tap Changer Maintenance
  • Busbar Inspection
  • Cable Termination Inspection
  • Cable VLF & Partial Discharge
  • Protection Relay Inspection

Non-Intrusive Maintenance:
The predictive element – which is carried out on live equipment involves carrying out an annual visual inspection of the Substation, Switchrooms and HV Equipment which includes:

  • Equipment Visual Inspection
  • Thermal Imaging (TI)
  • Partial Discharge Testing (PD)
  • Dissolved Gas Analysis (DGA) and Oil Quality Analysis of your transformer oil.
  • Voltage, Amps & Temperature readings
  • Power Quality Analysis

Where access to carry out thermal imaging is provided (Thermal Infrared Windows at the rear of switchgear), this will allow us to see any thermal anomalies on the HV equipment that may indicate a potential problem with a poorly terminated connection, dry joints, overheating busbar or a thermal anomaly on a transformer body indicating a localised heating or cooling issue. Partial Discharge Testing allows us to use ultrasound test equipment to check for partial discharge or corona discharge which are small electrical sparks that occur within the insulation of medium and high voltage electrical equip­ment. These discharges erode insulation and eventually result in insulation failure and total equipment breakdown. Typical equipment that faults can be detected on include;

  • Cables, splices, and terminations
  • Power transformers and bushings
  • Switchgear, Motors and generators.

Oil Sample Analysis:
For oil analysis or dielectric fluid sampling and analysis our standard tests include Dissolved Gas Analysis (DGA) and an Oil Content and Quality evaluation. This is performed on transformer and switchgear insulating fluids in accordance with required specifications to determine the health of the equipment.
Our standard testing of DGA and Oil Content & Quality are inclusive of diagnosis and recommendations and include

  • Indication of an ongoing problem
  • Prevention of a serious occurrence such as explosive or sudden outage
  • Advanced warning of developing faults
  • A determination of the improper use of the apparatus tested
  • Status checks on new and repaired units
  • A means to trend the levels in order to conveniently schedule repairs

The interpretation of data from a DGA can be complex due to the many different characteristics and operating conditions that affect gas formation. Therefore establishing a baseline and performing regular testing and monitoring the content is the best way to determine and control the fault process.
For a more comprehensive set of tests, or further interpretation of a diagnosed fault or health condition, or if a customer’s insurance company requires a more in-depth analysis of large generator transformers, or step down transformers, H&MV provide a whole suite of tests such as:
1. DGA – Dissolved Gas Analysis (IEC60567/ASTM D3612-02)
2. Moisture content – Water (ASTM D 1533)
3. Dielectric strength (Break Down Voltage) IEC60156
4. Neutralisation Number (Acidity) ASTM D664-01
5. Dielectric Dissipation Factor @ 90 Deg C. (DDF, or Power Factor, or Tan Delta) IEC 60247 ASTM D 924 – 04
6. Interfacial Tension (IFT) ASTM D1698-03
7. Furan analysis, 2 Furaldehyde, IEC61198 ASTM D5837-99
8. Resistivity@ 90 Deg C. IEC60247 ASTM D924-04
9. Particle count. IEC 60970
10. Density. ASTM D1298
11. Colour. ASTM D1524
12. PCB (Polychlorinated Biphenyl) IEC61619 ASTM D4059
13. Corrosive Sulphur. IEC62535 ASTM D1275B
14. DBDS (Dibenzyl Disulphide) IEC62697-1
15. Degree of Polymerisation (Insulating paper sample) IEC60450 ASTM D4243
16. Viscosity@40 Deg. C. ISO 3104
17. Sediments and soluble sludge. ASTM D1698-03
18. Metal Passivators. IEC60666 TTAA: BTA: TTA
19. Metals in oil. ASTM D7151
20. Stray gassing. ASTM D 7150

All our PdM and PM HV maintenance events are carried out using highly trained and competent HV Service Engineers and HV Commissioners using Omicron state of-the–art test equipment. If you are interested in hearing more about our services or how we can help you design your asset management program to suit your facility reliability program please contact us at info@hmveng.ie or info@hmveng.co.uk

Technical article written by Martin Whelan, (Asset Manager, H&MV Engineering)