Testing of Converter Transformers
This post discusses some aspects associated with testing of converter transformers. Most tests performed on converter duty (VFD) transformers are same as those specified for power transformers. However, there are also few differences which are highlighted in corresponding sections of this article. The aim is to explain the purpose of selected tests, describe the test connection and the sequence of testing and to indicate the criteria to pass the test. Finally, some hints and practical experience is shared.
Remark:
‘Converter transformer’ is an IEC term used for power transformers “which are intended for integration within semiconductor converter plants”. It is an equivalent to the term ‘VFD transformers’ commonly used in our other posts. ‘Converter transformer’ is a broader term that includes transformers supplying not just VFDs, but also other applications such as STATCOMs, grid interties, wind converters, shore-to-ship type of SFC, various test benches etc.
Introduction
Transformer factory testing is an essential milestone in the execution of a transformer project. Purpose of testing is mainly to check the quality of the product and to verify the design. In the IEC world the key standard defining the tests is IEC 60076 for power transformers. Additional aspects related to converter operation are governed by IEC 61378-1. In this article, we will have a brief look at some of the tests typically performed during factory acceptance testing (FAT).
Classification of tests
A. Tests throughout the lifecycle of transformer
– Internal tests during manufacturing process
– Factory acceptance tests (⇒ subject of this article)
– Incoming tests on site
– Site acceptance tests
– Tests for diagnostic purposes (condition assessment)
– Post-mortem tests at the end of life
These tests have various nature. Most of them are electrical tests, followed by mechanical, thermal, acoustic or chemical tests etc.
This post focuses on tests conducted during FAT. Such tests are often witnessed by customer and it is therefore useful to have a basic understanding and be able to follow them when attending the test program.
B. Routine, type and special tests
IEC 60076 distinguishes three categories of tests: routine, type and special tests.
Routine tests are tests to which each individual transformer is subjected. Purpose of these tests is a quality control performed by the transformer manufacturer.
Type tests are performed on a transformer that is a prototype or a representative of other transformers. Such test is not repeated on other products of the same type. Aim of this test is to demonstrate that the transformer complies with specified requirements. This category is called ‘Design tests’ according to IEEE.
Special test is each test other than routine or type test. Such test is usually performed on request and shall be explicitly agreed between the purchaser and the manufacturer.
Selected routine tests
In this section selected routine tests are described. Purpose and definition of the test are briefly explained. Special considerations related to converter transformer design are highlighted, if applicable.
1. Measurement of short circuit impedance and load loss
i. Purpose of the test
Short circuit impedance and transformer losses belong to guaranteed parameters of transformers. Therefore, these values are measured as part of routine tests according to IEC and IEEE standards.
Short circuit impedance is an important parameter of a power transformer as it impacts the short circuit current level, voltage drop of a transformer or a reactive power consumption. In VFD applications the transformer short circuit impedance may be essential part of system design.
ii. Measurement
One winding is supplied while the other winding(s) is shorted. Currents and voltages are measured by CTs and VTs and the signals are connected to a power analyzer. The test infrastructure must be able to provide relative high power supply. Required apparent power can be estimated based on the design short-circuit impedance and rated transformer power.
iii. Considerations for converter transformers
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iv. Evaluation
The test is successful if the measured values of short circuit impedance and load loss correspond to the values guaranteed by the manufacturer considering the applicable tolerances.
Selected type tests
In this section selected type tests are described. Purpose and definition of the test are briefly explained. Special considerations related to converter transformer design are highlighted, if applicable.
1. Temperature rise test
Transformer temperature rise test (also called ‘heat run test’) is a type test that provides information about transformer design, particularly the effectiveness of cooling system. Our focus here is mainly on liquid-immersed transformers.
i. Purpose of the test
The test purpose is to verify the thermal design of the transformer, specifically the tempe-rature rise of the insulation and windings and the effectiveness of cooling. Lifetime of a transformer is strongly linked with operating temperatures and therefore the control of temperature rise is very important.
ii. Definitions
Temperature rise
Temperature rise (expressed in K) is the difference of temperature of the specific part and the temperature of cooling medium.
Kelvin (K) is used as unit presenting the relative temperature rise.
Celsius (°C) is used to describe absolute temperature of the parts.
For liquid immersed transformers two values of temperature rise are measured:
– Temperature rise of top oil
– Average winding temperature rise
Temperature rise limits
Limits for temperature rise at continuous rated power are specified in IEC 60076-2.
iii. Considerations for converter transformers
Converter transformers are often subjected to non-sinusoidal currents and associated harmonic losses. These losses must be considered in the temperature rise test.
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iv. Data processing
Certain temperatures can be measured directly while other values need to be calculated (e.g. by extrapolation of measured points).
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v. Evaluation
Temperature rise test is successful if the temperature rise limits are not exceeded.
Temperature rise test of liquid immersed transformers may be combined with a dissolved gas analysis where samples of liquid are taken before and after temperature rise test.
2. Lightning impulse test
Lightning impulse (LI) test is a type test for transformers with Um ≤ 72.5 kV. For Um > 72.5 kV this test becomes a routine test.
i. Purpose of the test
LI test belongs to the group of dielectric tests. The purpose of the test is to verify the insulation system, particularly the ability to withstand transient overvoltages such as a lightning strike.
ii. Waveshape
The waveform of a lightning impulse is normalized. It is characterized with three main parameters:
- Front time T1: 1.2 μs +/-30%
- Half value time T2: 50 μs
- Overshoot β: < 5%
iii. Test equipment and measurement procedure
Now let’s discuss the test infrastructure with focus on the impulse generator and the measurement system.
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iv. Considerations for converter transformers
Converter transformers, particularly multi-winding VFD transformers, require additional considerations when it comes to lightning impulse test.
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v. Data processing
Lightning impulse test is a fast transient test. Data processing is therefore a key point for proper execution of the test and evaluation of test results.
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vi. Evaluation
The test is successful if no collapse of voltage appears during the test. There should be no significant differences in the voltage and current waveforms of the individual impulses. The software is usually comparing the currents by overlapping the recorded waveforms.
References
[1] VFD transformers – entire series, Link to articles
