The accuracy of instrument transformers is essential for the reliable operation of relay systems, as well as correct energy metering and billing. For the accuracy of a transformer, upper limits for the error (amplitude and phase) are stipulated in standards for a range of operating points in relation to the input quantity (voltage) and the nominal load. Other specific requirements must be fulfilled depending on the class, such as parameters relating to the transient behaviour of capacitive voltage/potential transformers.
In addition to transmission accuracy, the insulating function of voltage/potential transformers also plays an important role. In order to reliably separate parts carrying high voltage and ground potential, the quality of the transformer insulation must be retained after many years of operation.
Why are voltage/potential transformers checked?
The accuracy of a transformer may deteriorate due to various factors, though this is almost impossible to detect during continuous operation. For this reason, it is important to guarantee its accuracy through testing in order to avoid damage culminating in the failure of the substation and significant material damage.
These factors include the aging of the transformer and external influences, such as overloading or underloading. Other possible causes are design faults (incorrect dimensioning of the transformer core or insulation) and manufacturing defects (short-circuits, insulation defects). Operation beyond the specifications and inﬂuences from grid operation (lightning and switching impulses, over-voltages) can also lead to faults. Leaks and the ingress of moisture or oxygen impair the quality of the insulation and can cause ﬁres or explosions, potentially resulting in personal injury and material damage. In the case of capacitive voltage/ potential transformers, the divider capacities are subject to aging, which has a negative impact on accuracy.
When should you check voltage/ potential transformers?
Over the service life of a voltage/potential transformer, various tests are useful for ensuring safety, quality, and operation that fall in line with the specifications.
Incorrect dimensioning and manufacturing processes involving a high degree of manual work can lead to faults. Tests performed during the production process enable the condition and performance data to be determined at various stages of production. As a result, defective or imprecise devices can be identified at an early stage and the efficiency of the process is enhanced.
Once production is complete, compliance with the specifications stipulated in the standard should be checked by thorough tests. Reference measurements that can be used later for comparison as “fingerprints” are also recommended at this stage.
Any transportation-related damage can be identified before commissioning at the site. During commissioning, suitable tests are carried out to ensure the system has been correctly installed and is functioning correctly in the operating environment.
Tests should be performed as part of a regular maintenance program throughout the service life of voltage/potential transformers. Being aware of the condition of the transformer helps avoid risks to persons and installations as well as expensive faults, failures, and subsequent lengthy service interruptions. For instance, in some cases, such as the conversion of switchgear, the accuracy will need to be verified again using laboratory tests or on-site measurements.
What is the best way to check voltage/ potential transformers?
For the purpose of insulation diagnosis, tests comparable to those on the bushings of power transformers are also performed on voltage/potential transformers, such as power/ dissipation factor measurements, dielectric response and partial discharge measurements.
A test voltage is applied to the primary side of the transformer in order to determine the ratio and polarity. This is done to make sure that it is functioning correctly. Nominal voltage is applied or a model-based method is used to perform an accuracy test.
For many years, it was only possible to carry out accuracy tests using high voltages and heavy equipment. Nowadays, modern test sets are capable of using a model-based testing approach, which enables lightweight and portable measuring instruments to achieve more or less the same level of accuracy. These instruments take an automatic series of measurements and model the voltage/potential transformer on the basis of its equivalent circuit diagram. This method provides detailed test data, such as ratio accuracy, turns ratio, polarity, and excitation curve.