Transformer Testing
Transformer Testing
Transformer Testing
Type tests are tests made on one unit transformer (Prototype) as sample to demonstrate that
they comply with specified requirements not covered by routine tests:
• Temperature rise test (IEC 60076-2).
• Dielectric type tests (IEC 60076-3).
• Vacuum test of tank and radiators
The special tests of transformer are performed depending upon customer’s requirements to
gathering the information which is useful during the operation and maintenance of the
transformer.
• Dielectric special tests Lightning impulse tests.
• Zero-sequence impedance on three-phase transformers.
• Short-circuit capacity test. (MVA sc withstand time)
• Harmonics on the no-load current.
• Power taken by fan and oil-pump motors.
• Determination of sound levels.
• Determination of capacitances between windings and earth, and between windings.
• Determination of transient voltage transfer between windings.
• Tests intended to be repeated in the field to confirm no damage during shipment, for
example frequency response analysis (FRA).
• Tests on accessories e.g. buchholz relay, oil preservation, temperature indicators,
pressure relief devices etc.
(D) Pre commissioning Tests
The Test performed before commissioning the transformer at site is called pre commissioning
test of transformer. These tests are done to assess the condition of transformer after
installation and compare the test results with the factory test reports.
• Insulation resistance test of transformer and cables
• PI (Polarization index) and DAR (dielectric absorption ratio) Test
• DC Winding Resistance including all Taps steps
• Transformer Turns Ratio including all Taps steps
• Polarity Test and Vector Group
• Transformer core ground test
In a 3 phase transformer, it is essential to carry out a vector group test of transformer. Proper
vector grouping in a transformer is an essential criteria for parallel operation of transformers.
There are several internal connections of three-phase transformer are available on the
market. These several connections give various magnitudes and phase of the secondary
voltage; the magnitude can be adjusted for parallel operation by suitable choice of turn ratio,
but the phase divergence cannot be compensated.
So we have to choose a transformer suitable for parallel operation whose phase sequence
and phase divergence are same. All the transformers with the same vector ground have same
phase sequence and phase divergence between primary and secondary.
Before procuring an electrical power transformer, you should ensure the vector group of the
transformer, whether it will be matched with his or her existing system or not. The vector
group test of transformer confirms his or her requirements.
H - XG
HG - X
H-X
HG - X
Every 10 °C increase in temperature the insulation resistance will decrease to half and vice
versa (Resistance of insulation is inversely proportional with temperature unlike conductor)
NB: It is unnecessary to perform insulation resistance test of transformer per phase wise
in three-phase transformer. IR values are taken between the windings collectively as
because all the windings on HV side are internally connected together to form either
star or delta and also all the windings on LV side are internally connected together to
form either star or delta.
Measurements are to be taken as follows:
1. Keep the tap changer in the lowest position and open all HV and LV terminals
2. Then apply three phase 415 V supply on the line terminals for three-phase
transformers and single phase 230 V supply on single phase transformers
3. Measure the supply voltage and current in each phase
4. Now repeat the magnetizing current test of transformer test with keeping tap
changer in normal position
5. Repeat the test while keeping the tap at highest position
Normally, there are two similar higher readings on two outer limb phases on transformer core
and one lower reading on the center limb phase, in the case of three phase transformers.
An agreement to within 30% of the measured exciting current with the previous test is usually
considered satisfactory. If the measured exciting current value is 50 times higher than the
value measured during factory test, there is a likelihood of a fault in the winding which needs
further analysis.
The frequency of the applied voltage should be double of power frequency too. Here also if no
failure of insulation, the test is successful.
In addition to dielectric tests of transformers, there are other types of test for checking
insulation of transformer, such as lightning impulse test, switching impulse test and partial
discharge test.
Induced Voltage Test of Transformer
The induced voltage test of the transformer is intended to check the inter-turn and line end
insulation as well as main insulation to earth and between windings-