How to Wire a 3-Phase Differential
When you are installing a 3 phase electrical system, you need to wire the neutral wire correctly to ensure that it will be able to carry current to the supply when necessary. This will be the only way for the system to stay in balance.
Wireing a 3-Phase Differential
The next step in wiring a differential is to connect the phases of the system to each of the transformer bushings. This is done by using a cable or conduit. The order of which phases are connected to each bushing is called the phase-to-bushing connection, and it is important that this is set correctly.
Understanding the phasor relationships of each winding is also a critical step in wireing a differential. This is achieved by performing a voltammetric analysis of the high and low side phase currents of each winding. This will help you understand the actual phase-angle relationship between the high and low side phasors, which will be used for setting internal phase-angle compensation in the relay.
Derivation of the phase-angle relationship is a simple process that does not require a great deal of time or effort. However, it is extremely important to perform this process before any attempt to set the relay.
This will ensure that the internal phase-angle compensation settings are set accurately, and it will also avoid any misoperations in the event of a fault developing on one of the transformer bushings.
Once you have completed this process, you can then move on to the final step in wiring a differential: commissioning. This is an incredibly important step in any new electrical installation and is often overlooked. It is the best chance to verify that all of your relay settings have been formulated correctly and are functioning properly before you start using them in a real world environment.
It is also a good idea to do a power in versus power out calculation to make sure that all of the CTs are tapped with the same value. If any CTs have a higher tap than the relay settings intended, this can result in a relay that will trip prematurely in the event of a fault.
When a transformer has a ground fault, the voltage at its star point shifts to a lower voltage. This means that it will be more difficult for the load to balance and draw the same amount of current on each line. This is why the circuit breaker must be closed when the neutral wire breaks, otherwise this can cause a very dangerous situation to occur.
Similarly, in an unbalanced three-phase circuit, the line currents will be different. This can lead to a load that is not drawing the same amount of current on each line, or it could have a different phase angle. This is a common problem with all three-phase installations.