Transformers are passive electromechanical components operating on the principle that each electrical field creates a magnetic field. This will isolate two electrical circuits by making the first electrical circuit a non-electrical magnetic field. This second electrical circuit gets the energy from the created magnetic field. Testing a medium voltage transformer ensures that all of its components works properly. Here are the steps to doing it.
Visually inspect the transformer. Overheating happens when the transformer’s windings run at temperatures that are too high, and is a common reason for transformer failure. If you see that the transformer’s exterior has burn marks or is bulging, do not proceed on testing it.
Determine the transformer wiring. A transformer needs to have clear labeling. However, it is best that you get a schematic of the transformer’s manufacture so you will know its limitations. The circuit schematic should be available in the product documentation.
Know the transformer outputs and inputs. The first electrical circuit is connected to the transformer’s primary windings. The voltage supplied to the primary windings should be on both the schematic and the transformer label. The second circuit that gets power from the magnetic field is connected to the transformer’s secondary windings. The voltage coming from the secondary windings has to be on both the schematic and the transformer labeling.
Identify the output filtering. It’s common to attach diodes and capacitors to the transformer’s secondary windings to shade the AC powder coming from the DC power output. This shaping and filtering is not available from the transformer label. This should be shown on the schematic.
Prepare for circuit voltage measurement. Remove the panels and covers as necessary to get access to the circuits containing the transformer. Acquire a digital multimeter so you can take the voltage readings. Digital multimeters are available online and electrical supply stores.
Confirm proper output. Add power input to the circuitry. Use the digital multimeter in AC mode and measure the transformer primary. If it’s less than 80 percent of the expected voltage, the fault might be in the circuity giving the primary power, or the transformer. So, separate the input circuit from the primary. If the input power reaches the expected value, the transformer’s primary may be bad. But if the input power doesn’t reach the expected value, the problem is with the input circuity, not with the transformer.
Measure the secondary output. After determining that there is no shaping or filtering done on the secondary circuitry, use the digital multimeter in AC mode. If there is shaping and filtering, use the DC scale. If there is no expected voltage, either a shaping or filtering component, or the transformer is bad. Separately test the shaping and filtering components. If this doesn’t show problems, the problem lies with the transformer itself.
Testing medium voltage transformers in NJ without being skilled in the tasks at had can lead to injury. Therefore, make sure that you have the proper training or ask for help from an electro mechanical expert. Companies that build custom power transformers withal subject their products to quality testing but it is wise to do your own. When working with a saturable reactor, it is important to be extra careful due to the direct current you may be dealing with.