The usefulness of a potential transformer lies in its ability to provide accurate readings to the meter and the protective relay. If there is drifting of the ratio, aging of insulation or incorrectly marked terminals, this results in erroneous readings by all the other instruments using the same readings. It means that the protection relay can fail to operate at a crucial moment. For this reason, understanding how to test a potential transformer properly is essential knowledge for all the people who deal with commissioning, maintaining, and troubleshooting switchgear and substations. This article explains the procedure in detail starting from safety preparations till carrying out tests of the transformers.
The basic tests that need to be performed for the assessment of a potential transformer include insulation resistance test, winding resistance test, polarity test, voltage ratio test, and burden and accuracy tests for metering transformers. Having performed these tests according to the safety procedures and in the proper order, one can prove the transformer is safe to be put into operation and the readings can be relied upon.
What Is a Potential Transformer?
Potential transformer (PT), often known as voltage transformer (VT), refers to a type of instrument transformer that reduces a high voltage of the system to a low and standard value of the secondary voltage so that common relays and meters are able to read it easily and safely. Standard PT may be defined further as it brings down a primary voltage such as 4.16 kV or 13.8 kV to a typical secondary voltage of 120 volts, having a constant relation between the two. Since a PT is designed in such a way that it will not impose any significant load to the circuit under measurement, it utilizes smaller core and requires lower VA rating, being in this regard different from a power transformer. Together with current transformer, PT represents one of the two main types of instrument transformers used for metering and monitoring in power systems.
Knowing what a PT does explains the reason why testing is very important in its case. A PT is not supposed to supply electricity, but rather to provide a scaled version of the voltage of the system being measured. Thus, any test performed is basically used for one purpose only and that is to check whether the PT can be relied upon to give correct values of the voltage.
Why Testing a Potential Transformer Matters
It is the tests conducted before putting a unit into operation and subsequently over time that help identify problems that would not have been noticed until there were failures. The tests performed before commissioning the unit show possible damages that occurred during transportation, establish whether terminal markings are correct and confirm that the connections are properly aligned. Continuous testing helps identify the gradual deterioration of insulation caused by moisture, heat, and the aging process. Due to the fact that instrument transformer testing is often neglected, strict adherence to the testing regime can be regarded as one of the least expensive types of insurance for a substation operator.
Safety Precautions Before You Test a Potential Transformer
Every test described below assumes the unit is fully isolated. Working on a live potential transformer is extremely dangerous, and rushing the preparation is the most common cause of accidents. Before you begin to test a potential transformer, take the following steps.
- First stop the operation of the PT, and ensure that it is completely isolated from the power.
Then discharge any stored energy and ground before you touch the terminals.
Disconnect the wiring of the secondary circuit so that the meters and relays connected to it do not receive test voltage.
Never short-circuit the potential transformer’s secondary circuit. (This is contrary to the rule for current transformers: their secondary circuit must not be open. This is a very common mistake.)
Always put on suitable personal protective equipment before doing any tests and make sure that the area is clear.
Finally, while testing insulation, remember to discharge the instrument as well as the winding in order to make it safe.
Tools and Equipment You Need
A complete potential transformer testing kit is modest but specific. For testing purposes, it becomes imperative to have at your disposal a megger, operating at the required voltage for testing purposes, a transformer turns ratio set or a primary injection kit to perform the ratio test, either a low-resistance ohmmeter or a voltmeter-ammeter to find the winding resistance, a suitable DC source equipped with a sensitive analogue meter for performing polarity test and a multimeter. Finally, it is equally important to make sure that unit nameplate data and factory test report are available as well, since all tests are analyzed based on a comparison with those original readings.
How to carry out a potential transformer test, in steps
The procedures given below are listed in logical order for the commissioning process. Always starts by making a visual check and an insulation test before proceeding to any kind of ratio or injection tests.
1. Visual and Physical Inspection
Initially, the unit should be inspected for defects such as damaged bushings, oil leaks, corrosion, loose terminals, and any indication of impact damage during transportation. Make sure the nameplate ratio, accuracy class, and rated burden coincide with the measures received at the installation site and check carefully the identification of the primary terminals (P1 and P2) and secondary terminals.
2. Insulation Resistance Test
Insulation resistance testing involves using a megger to check the insulation between windings and between windings and earth. The secondary circuit should have been disconnected and neutral earthing removed. Then measurements are to be made in three configurations: the unit; primary and earth; primary and secondary; secondary and earth. It is common in the field to use voltage in the range of 5 kV for a primary to earth measurement, 2.5 kV for primary to secondary measurement and anything up to 1000 volts for a secondary to earth measurement, although values must always be determined correctly in regard to the rated measure of the unit.The minimum acceptable reading is usually considered to be at least 1 million ohms for each kV of rated voltage, with the measurement not being significantly below the factory value. One way of gathering a broader view of the general state of insulation will be the polarization index calculation. This is a ratio between 60-second and 15-second readings (or 10-minute and 1-minute readings in larger devices). The result that exceeds 1.3 means acceptable insulation, while a lower value signals water presence or aging, which requires additional investigation before turning the device on.
3. Winding Resistance Test
This test is used to measure the direct current (DC) resistance of windings and find out whether there are problems with the windings, like broken wires, loose joints, or short circuits. The operation includes passing DC current through the winding and measuring the drop in voltage. After that, it is necessary to calculate resistance and compare it with the factory testing result. If the result deviates from the reference value considerably, or you see significant discrepancies between phases, then you should conclude that something is wrong with the windings.
4. Polarity Test
It is crucial to have the right polarity for the secondary voltage what makes the polarity test necessary. The method verifies the relation between primary and secondary terminals which is especially necessary when the device lacks markings. This process applies the DC voltage to primary terminals P1 and P2, while the sensitive analog voltmeter observes the readings from the secondary. It is enough to check the direction of bending in order to know about the polarity.
5. Voltage Ratio Test
The voltage ratio test is the most significant electrical test with regard to any potential transformer. It shows whether the transformer gives out the expected voltage. In order to perform such a test, the source voltage is applied to the primary winding terminals and the secondary voltage is measured. The relations ratio is compared with average values given by a manufacturer.
6. Burden and Accuracy Test
The accuracy of the transformer is claimed to be valid when burdened is within the rated value. The correlation between burden and accuracy has been established, therefore, these aspects are usually checked in tandem. It is important to know how actual burden differs from rated values and whether the transformer has maintained accuracy characteristics with this actual burden.
How to Interpret the Test Results
The majority of PT tests are assessed via comparison and not through a certain global passing score. Insulation values need to go beyond the threshold of around 1 megohm for every kilovolt, and what is of even greater importance is that they haven’t decreased much from the factory/previous date values with a polarization index of over 1.3. The winding resistance must be approximately the same with the factory data as well. The ratio must stay within the limits set by the accuracy class and the polarity must be checked against the terminal markings. In case of any single result being out of the ordinary, it is better to compare it to the differences across the phases and previous records before condemning the device as the result can be affected by fluctuations in temperature and conditions of the test.
Common Faults Found When Testing a Potential Transformer
Certain problems turn up again and again during potential transformer testing. Low insulation resistance is the most frequent and usually points to moisture ingress or aged insulation. A ratio outside tolerance suggests shorted turns or a manufacturing or wiring issue. An unexpectedly high or open winding resistance indicates a broken conductor or loose internal connection. Reversed or incorrectly marked polarity, if undetected, causes protection schemes to misoperate. Physical damage to the core or bushings, often from rough transport, may show up as a combination of these electrical symptoms. Catching any of these before energization is exactly why the tests are worth the time.
Types of Potential Transformer and How Testing Differs
Before you test potential transformer units in the field, it helps to know which type you are dealing with, because the construction affects both the test setup and the expected results. There are three primary types of potential transformer in use. The electromagnetic type of transformer is a standard transformer commonly used for power applications. The electromagnetic type requires the same tests of insulation, winding resistance, ratio, and polarity as described earlier in the case of other types of transformers. The capacitor voltage transformer, or CVT, uses capacitive reduction before feeding it into an electromagnetic transformer and has applications in extremely high voltages. The CVT requires not only testing for the electromagnetic transformer part but also the capacitor. The optical type of transformer uses the polarization properties of light and has special tests which do not match any tests described earlier but require specific tests developed by the manufacturer. Therefore, identifying the type of electric transformer is the first step in the testing process.
Where the Potential Transformer Fits in Your Power System
A potential transformer never works alone. It is installed in substations and switchgear alongside the power and distribution transformers that actually carry the load, and its readings protect and monitor that primary equipment. When you are commissioning a substation, the reliability of the whole installation depends as much on the quality of the main transformers and switching equipment as on the accuracy of the instrument transformers.
This is where a dependable equipment supplier makes a difference across the entire installation. ShineGrand Electric manufactures the core distribution and power equipment that a PT is installed to monitor, including oil-immersed units such as the 6-10KV Oil-Immersed Distribution Transformer for medium-voltage networks and the high-voltage 110-132kV Oil-Immersed Distribution Transformer for substation duty. For ground-level and indoor applications there are options such as the Three Phase Pad Mounted Transformer Series and the fire-safe 20-35kV Epoxy Dry-Type Transformer. Because a substation is a system rather than a collection of parts, ShineGrand also supplies the surrounding protection and switching gear, including surge arresters and disconnect switches, so the transformers your instrument transformers monitor are backed by matched, reliable equipment.
Frequently Asked Questions
How often should a potential transformer be tested?
A potential transformer should be tested before it is first energized, as part of commissioning, and then periodically as part of a preventive maintenance program. It should also be tested any time it is suspected of causing metering errors or relay misoperation, or after any event that may have stressed it electrically or mechanically.
What insulation resistance is acceptable for a potential transformer?
As a general rule, insulation resistance should be at least 1 megohm per kilovolt of rated voltage, and it should not be significantly lower than the factory or previous reading. A polarization index above 1.3 indicates the insulation is in acceptable condition. Values well below these thresholds usually indicate moisture or insulation aging.
Can you test a potential transformer without de-energizing it?
No. All standard tests require the unit to be fully de-energized, isolated, and discharged first. Attempting to test a live potential transformer is extremely dangerous and also produces unreliable results.
What is the difference between testing a PT and a CT?
The tests are similar in principle, but the critical safety rule is reversed. When you test a potential transformer, you must never short-circuit its secondary, whereas a current transformer’s secondary must never be left open. Confusing these two rules can damage equipment and endanger personnel.
What causes a potential transformer to fail its ratio test?
A ratio outside tolerance is most often caused by shorted turns within a winding, but it can also result from a manufacturing defect, incorrect internal connections, or damage sustained in transport. A failed ratio test means the unit’s readings cannot be trusted and it should not be placed in service until the cause is resolved.
Which tests should be done at commissioning versus routine maintenance?
At commissioning, run the full set: visual inspection, insulation resistance, winding resistance, polarity, ratio, and burden and accuracy. During routine maintenance, insulation resistance and a ratio check are the most valuable, since they catch the two most common developing problems, aging insulation and drifting accuracy. Any unexpected result should then trigger the remaining tests to pinpoint the cause before the potential transformer is returned to service.
Reliable Substation Equipment From ShineGrand Electric
Accurate instrument transformer readings only matter when the equipment they monitor is dependable. ShineGrand Electric Co., Ltd. designs and manufactures a complete range of power and distribution transformers, along with high-voltage fuses, surge arresters, and disconnect switches, for utility, industrial, and commercial substations worldwide. Whether you are building a new substation, upgrading an existing one, or sourcing reliable transformers to sit at the heart of your protection and metering scheme, explore our full transformer product range or contact the ShineGrand team for expert support tailored to your project.
