Spot testing of transformer by TECHIMP – Altanova group

In order to evaluate the health and life of  electrical equipment in service, utilities prefer periodic spot testing. The spot testing is performed online on the energised equipment without taking any shutdown or without removing the asset from the live network. Online testing at short and regular interval of time also helps in having the record of any new PD phenomenon which might come into picture during these intervals. This kind of testing without taking shutdown proves to be economically viable also helps in having a record of the trend of any partial discharge occurring in the insulation with time, which is helpful in analysing the health of the insulation system thereby preventing the sudden system failure.

The online testing was performed on a380kV/110kV power transformer of the National Grid in Saudi Arabia, corresponding to its cable termination. Low magnitude of internal partial discharge was detected within the measurement sensitivity

The online testing was performed by using TECHIMP – ALTANOVA GROUP make AQUILA; a portable PD equipment for spot testing as well as temporary monitoring, and its highly sensitive HFCT (high frequency current transformer) sensor with very high transfer impedance of 19mV/mA, clamped on the cable grounding sheath or bonding cables. In case, if there were no bonding cables available, another option was to use the FMC (flexible magnetic coupler) sensor, which can be placed directly on the power cables near termination.

Sufficient data were acquired during the online testing so as to do a detailed analysis of the acquired data which lead to accurate and right conclusion. This can be helpful in deciding the recommended or suggested actions necessary to keep the system working efficiently without any interruption.

For each acquisition, a PRPD (Phase Resolved Partial Discharge) pattern and T-F map (patented time-frequency map obtained from the Fast fourier transform of the PRPD Pattern),  wave forms were recorded for the post-acquisition detailed analysis of the data. Number of pulses, repetition rate, applied voltage, etc. can  be obtained from the recorded data.

The advantage of patented T-F map technology of TECHIMP-ALTANOVA GROUP is that it is able to diagnose the insulation defects at an early stage. This will help in the optimissation of the energy productivity and maintenance and increasing the reliability and life cycle of the electrical assets. With no interruption of energy required and easy to install the sensors, this equipment is a one-stop solution for Portable PD testing and monitoring of all electrical assets, like transformers, cables, switchgears, etc.

During the analysis of the acquired data, it was observed that there was internal PD phenomenon of 12 mV occurring in the termination corresponding to blue phase having a frequency of about 6-8 MHz.

The PRPD pattern, T-F Map and sub-patterns are depicted in the below pictures obtained from the PD Processing software. The black phenomenon in the T-F corresponds to noise in the electrical system while the red phenomenon corresponds to internal PD occurring in the cable insulation close to transformer termination. In this way, TECHIMP offers to segregate PD from noise and other discharge based on the PRPD and patented T-F mapping.

After analysing the data and seeing the amplitude, type of PD, frequency, repetition rate, etc., it was recommended that the transformer can be continued to be operational and its termination should be tested again within one year to know the trend of the PD phenomenon occurring at present and how it changes in one year and also to detect any new PD phenomenon occurring in this time interval. An increasing trend of the PD can be dangerous and needs to be monitored more frequently to prevent any abrupt system failure.

ALTANOVA GROUP (ISA and TECHIMP) has their portable PD testing and monitoring equipment, AQUILA and MicroAQUILA. With a sampling speed of 100MS/s and 3 PD channels and 1 synchronisation channel, they can be used for spot testing and temporary monitoring. This equipment is proving to be a successful and effective solution for utilities for online testing and monitoring of electrical assets in service.

For more details:
e-mail: info.asia@altanova-group.com