Mobile high-voltage source for on-site insulation testing on dry-type transformers

A dry-type transformer is a safe, low-maintenance and environmentally-friendly solution suitable for both indoor and outdoor applications where safe and dependable power is important. It is also more compact in size than a liquid-filled transformer, making it especially suited for hard-to-access areas with limited space, such as wind turbines, off-shore platforms, indoor and underground substations, industrial plants and even high-rise buildings.

Assessing insulation condition
The primary concern with all types of transformers, as well as the key indicator of their life expectancy, is the condition of their insulation systems. The structural strength and insulating properties of materials used for these insulation systems degrade over time through normal aging. They can also degrade prematurely due to overheating as well as mechanical and electrical stresses.

A dissolved gas analysis is the method most commonly used for determining the winding insulation condition in liquid-filled transformers, but it is not possible for dry-type transformers, because their coils are cooled with air ventilation.

Compared with other possible diagnostic measurements, including a dissipation/power factor measurement, partial discharge (PD) testing can most effectively detect the presence and location of PD activity and resulting insulation defects in a dry-type transformer. Analysis of the PD measurements can be used to assess the condition of the insulation and to determine the need for repair or replacement before a failure and costly outage occurs.

Performing off-line PD measurements
To determine the insulation condition of a dry-type transformer, a high-voltage source is required to perform off-line PD measurements and AC voltage withstand tests. However, such a high-voltage source is often not conveniently available, especially for field testing. In this case, the induced voltage test can be performed as an alternative to build up the required voltage level.

The induced voltage test requires the transformer core to be energised to induce the test voltage on the high-voltage side. To accomplish this, the external voltage source is connected to the low-voltage side of the transformer to induce the test voltage on the high-voltage side.

Lightweight and portable high-voltage source
OMICRON’s modular and mobile CPC 100 testing system can be used as a high-voltage source for induced voltage tests on dry-type transformers. Configurable matching transformers enable you to achieve different voltage levels on the low-voltage side of a dry-type transformer. Each matching transformer can output a maximum power of 5 kVA. This applies to a frequency range from 15 Hz to 400 Hz.

Multi-functional testing for substations
In addition to serving as a mobile high-voltage source for on-site testing, the CPC 100 can also be used to perform a variety of standard and advanced diagnostic tests for commissioning and maintenance on such primary assets as power transformers, current and voltage transformers, circuit breakers and switchgear, rotating machines, grounding systems, as well as cables and transmission lines. It can thus replace several individual test devices, which also saves you time and provides you with an economical alternative.

When more power is needed
As in the case of dry-type transformers, the power required to energise the test object itself can often exceed the power ratings of only one CPC 100. The number of CPC 100 devices which are needed to energise the transformer and induce the test voltage depend on the core losses and reactive power intake.

This situation can be resolved in that the power output can be easily multiplied to achieve a sufficient high-voltage source for induced voltage testing. This is accomplished by using the Sync function of the CPC 100, which allows you to synchronise up to three CPC 100 devices with matching transformers to create a high-voltage source with up to 15 kVA.

Low disturbance levels
The challenge with the PD measurement on transformers is that PD signals are of low magnitude and getting the basic noise level down in the field to where you can see PD activity is not so simple.

To solve this situation, the CPC 100 comes with a frequency-variable amplifier. This allows you to tune the frequency to bring down the reactive power intake to a minimum so that it does not interfere with PD measurements. OMICRON’s MPD 600 PD measurement and analysis system also features various advanced tools that effectively separate different PD sources from noise for a reliable analysis of PD activity.

Flexible and modular setup
Three CPC 100 units with the CPC Sync function and three matching transformers were used to excite each of the dry-type transformers. The PD measurement was then carried out phase-by-phase with a one-channel MPD 600 system.

Efficient testing procedure
Only two and a half days were required for setup and PD testing on the dry-type transformers. Before starting the test, the PD detection system had to be calibrated. Therefore, a voltage and a charge calibration were performed using the 2kV output of the CPC 100 to obtain the voltage divider factor of the PD measuring system. After everything was set up and the adjustments were made to minimize interference, the measurements could be performed quickly and efficiently.

The PD data was reviewed directly after each measurement and data streams were recorded for reporting. Each of the transformers was in good condition with no noticeable PD activity.

Authored article by:
Moritz Pikisch
Product Manager,OMICRON