Page 4 - Energize Issue 1 2023

COVER STORY

Talking transformer testing

Information from WearCheck

In the cover story of this edition, WearCheck’s transformer consultant, Corné Dames,
discusses the merits of oil analysis statistics in boosting transformer maintenance.
This article has been shortened for space. Click here to read the full article.

ondition monitoring is the calculated from international standards for mineral oils, and indicate the critical values
frequent collecting, measuring, stipulated in the various standards.
Crecording and analysis of the
relevant data regarding an asset’s The scope of oil analysis, interpretation of the data, and
operation. By interpreting the data critical values
correctly, one understands the asset’s Transformers are divided into classes according to their kV ratings. It is up to the reliability
condition, and this can result in or asset manager to use the guidelines for larger equipment, thereby implementing
extended maintenance-free periods. shorter increment oil analysis and electrical tests. Some of the analysis has been identified
There are key parameters that can as to routine type tests. Still, there is an extensive range of tests that can assist in
give us a more complete picture of the identifying specific problem criteria within the system, which might not be clear through
status of the transformer and guide us the typical day-to-day analysis usually performed.
regarding the actions needed to ensure
the continued reliability of the asset. CATEGORY TYPE OF EQUIPMENT
Category O Power transformers/reactors with a nominal system voltage of 400 kV and above
What the data tells us Category A Power transformers/reactors with a nominal system voltage above 170 kV and
• Whether the unit’s condition has below 400 kV. Also power transformers of any rated voltage where continuity
changed since the last maintenance of supply is vital and similar equipment for special applications operating under
period onerous conditions.
• If it can be operated safely Category B Power transformers/reactors with a nominal system voltage above 72,5 kV and up
• Any signs of deterioration to and including 170 kV (other than those in Category A)
• If the unit can be safely loaded Category C Power transformers/reactors for MV/LV application e.g. nominal system voltages
above its nameplate rating for a up to and including 72,5 kV and traction transformers (other than those in
Category A). Oil-filled circuit breakers with a nominal system voltage exceeding
particular period 72,5 kV. Oil-filled switches, a.c. metal-enclosed switchgear and control gear with a
• Whether any particular actions nominal system voltage greater than or equal to 16 kV
should be implemented now Category D Instrument/protection transformers with a nominal system voltage above 170 kV
• How long we can use the unit Category E Instrument/protection transformers with a nominal system voltage up to and
before we need to consider including 170 kV
replacement
• If recurring problems have been Category F Diverter tanks of on-load tap-changers, including combined selector/diverter
tanks.
identified
Category G Oil-filled circuit breakers with a nominal system voltage up to and including 72,5 kV
Oil-filled switches, a.c. metal-enclosed switchgear and control gear with a nominal
Effective condition system voltage less than 16 kV
monitoring outline NOTE 1 Separated selector tanks of on-load tap-changers belong to the same category as the
It is vitally important to identify clear associated transformer
goals as part of your strategy. By NOTE 2 Oil-impregnated paper bushings and other hermetically sealed equipment may be placed
stipulating the outcome and what you in Category D or E if a routine monitoring programme is desired. The manufacturer’s instructions
want to accomplish, it will be easier to should be referred to.
identify the required parameters. NOTE 3 Regardless of size or voltage, a risk assessment may justify condition-monitoring techniques
Health indexing of assets is usually appropriate to a higher classification.
helpful to get a clearer picture of NOTE 4 For practical and economical reasons, some electrical utilities may decide that their small
the transformer’s condition. Test transformers up to 1 MVA and 36 kV are not included in this classification. Routine monitoring
parameters carry a numerical value may not be considered economical for this type of equipment. Where a monitoring programme is
required for these transformers, the guidelines in category C should be adequate.
which add to the total Health Index
Value. These parameter weights are Table 1: Rating classes

energize | Issue 1 2023 | 4