The Benefits of Carrying Out Planned Preventative Maintenance (PPM)


As part of our extensive maintenance package, EME Power Systems regularly carries out PPM on customer owned HV assets. The purpose of this is to preserve the condition of the asset and prevent faults from occurring that would result in costly and inconvenient power outages. The following tasks are carried out as part of our PPM service:

  • Visual inspection.
  • Oil sampling.
  • Partial discharge testing.

Visual Inspection:

The visual inspection is a non-intrusive assessment of the asset. As part of this inspection, the fitter will complete a comprehensive checklist on site which is issued to the customer along with advice and guidance on how to rectify any possible issues. Below is a selection of the items included as part of the checklist:

  • Checking the asset for any signs of vandalism (missing earthing for example).
  • Checking the oil level and for any leaks.
  • Checking the sound and temperature of the unit.
  • Ensuring the required safety signage is present.
  • Identifying whether the silica gel breather requires replacing.

Oil Sampling:

During the fitter’s visit and with no interruption to the electrical supply, they will obtain an oil sample via the transformer drain valve. This oil sample is then sent to an independent laboratory which has the following suite of tests carried out on it:

  • Routine (Dielectric strength, acidity, moisture content, appearance).
  • Dissolved gas analysis (DGA) (Checking for the presence of various gases such as Ethane, Ethylene, Hydrogen, Methane).
  • PCB (Polychlorinated Biphenyls).

The oil sample results are a way of determining the condition of the oil and the transformer. If there is a high moisture content and low dielectric strength for example, then we would recommend that an oil change is carried out to prevent a possible fault from occurring. However if there is a high gas content detected, then we would arrange an outage of the transformer to allow an intrusive inspection of the unit to be carried out. We would then identify the issue (loose connections, high resistance joints etc.) and rectify if possible.

Partial Discharge Testing:

Partial discharge testing is the assessing of the health of the electrical insulation inside HV transformers. HV switchgear & HV cable termination boxes. We carry out this assessment by using one of our in house Ultra-Probe and TEV sensor devices. As the asset needs to be live to carry out this check, there is no interruption to the electrical supply.

The sound produced by partial discharge is usually at a frequency too high for the human ear, (ultrasonic). Using an Ultra-Probe is the most effective way to detect PD where there is an air path between the source and the microphone. However as there is often metalwork between the source and the microphone, using a TEV sensor is often the only way of detecting an issue. Partial discharge also produces Transient Earth Voltages (TEV) which escape through openings in the metalwork e.g. gaskets. Therefore TEV methods are particularly effective at finding internal PD (HV cable terminations for example) as PD activity can be detected by placing a TEV sensor on the outside of the switchgear to measure these pulses.

If the presence of PD is detected during an inspection, then an outage will be arranged with the customer to allow an intrusive inspection of the asset to be carried out. If an issue is detected, then a solution will be determined and proposed to the customer. By carrying out these checks, potential faults can be avoided.

PPM Summary:

If you are interested in having your valuable HV assets maintained by ourselves, or maybe have a query relating to the above, please call the office on 01529 410999 where one of our friendly engineers will be happy to discuss your requirements.