When retrofitting an existing system or specifying a step-down transformer for a new system, it is critical to understand the voltage and current demands placed on the step-down transformer – and the overall system – when introducing a voltage regulator. The voltage regulator is often necessary to properly mitigate variation in the system voltage (in this example +10 to -25% voltage). In the previous examples, the step-down transformer was rated at 1000kVA 13.8 kV to 480 with a high-voltage winding designed to carry 41.8 amperes and a low-voltage winding to carry 1202.8 amperes. In Figure 1, this was a perfectly acceptable arrangement based on the assumption that the source voltage was near desired nominal (in this case, 13.8kV). In Figure 3, however, the input voltage was 10% higher than the step-down transformer rating. In Figure 4, the input voltage was 25% low, requiring the step-down transformer’s input and output currents to be 33% higher than nominal.
Typically, a transformer can operate reasonably well without long-term damage at 10% over voltage, if we ignore core saturation, higher-than-designed core losses, and high audible noise). A transformer cannot operate, however, for prolonged periods of time with current overloads as high as 33%. In the event of a prolonged overload of 33% or more, there are two acceptable (though not optimal) options:
- When faced with a voltage sag, limit the load served so that the current requirement does not exceed the step-down transformer rating.
- Replace the step-down transformer with a unit that can carry the additional current. In the case described previously, the replacement transformer would need to be rated at 1333 kVA.
Transformers are typically specified to ensure that the load they carry will be only 50-75% of their ratings. In our example, the 1000kVA step-down transformer may only be expected to service a 500kVA load. As such, even when the regulator corrects for voltage sags (as shown in Figure 4), the currents passing through the step-down transformer might not exceed its rating. One can often “get away” with this arrangement due to the design margin built into the system. As the system load grows over time, however, the demands on the step-down transformer increase, and a situation that once was acceptable due to over-design is a hidden problem waiting to emerge as a transformer failure.