Network operating practice for efficient connection of distributed generation

The voltage regulating transformer

The voltage regulating transformer or Under Load Tap Changer (ULTC) performs the role of stepping up or down the voltage between Medium Voltage (33kV < MV ≤ 132kV) circuits and High Voltage (HV ≥ 132kV) circuits or between MV and Low Voltage (LV < 33kV) circuits or directly between LV and HV circuits.

In addition to this it is able to vary the exact transformer ratio by small steps as detailed further in this section. The actions are determined by the Automatic Voltage Control relay (AVC). In this way it is able to manipulate the voltage of buses connected either directly or indirectly to the ULTC.

In a similar way, the autotransformer can be used for voltage control between buses of similar nominal voltage.

Location

There are two roles the Under Load Tap Changer (ULTC) can perform. The first is to step down the voltage at the grid supply points (GSPs). Here the AVC acts to control the voltage of the lower voltage bus. The second is to step up the voltage from generating plant.

The AVC then acts to keep the voltage of the higher voltage bus within limits.

In general then, the Automatic Voltage Control relay (AVC) seeks to control the voltage of the bus on the side closer to the consumer. None of the examples in this study will contain transformers connecting buses of similar nominal voltage.

Operation

The tap-changing ability of the ULTC is provided by a mechanism that connects more or less of either the low voltage (LV) or high voltage (HV) winding. The voltage on the LV winding is defined as:

V_LV = V_HV x N_LV / N_HV

where:

• N is the number of turns on each winding.
• N_HV/N_LV defines the turns ratio

The transformers in the network examined are of the type that changes the number of turns of the high voltage winding that are connected. In this case, connecting less of the high voltage winding, lowers the tap ratio and thus raises the voltage on the low voltage side.

As a product of the construction of a two winding transformer, reducing the number of turns connected on the high voltage side is termed tapping up. A tap up raises the LV voltage and a tap down lowers the LV voltage.

In this study, the lowest number of connected turns is termed tap position 1, the highest number is termed tap position n where n is the number of taps.

As a consequence of that definition, the term “tap up” refers to increasing the tap position and consequently increasing the number of turns on the HV winding. The term “tap down” refers to decreasing the tap position and consequently decreasing the number of turns on the HV winding.

Note, however, that this is contrary to the practice of numbering the taps according to their physical arrangement in relation to the ground. Harker reports that the taps should be numbered 1 for the most turns connected and N for the least number of turns connected.