# Voltage regulation with step transformers parallel to busbars (Part 2)

Info Letter No. 10

The following considerations apply to a symmetrical operation of a three-phase network, so that the entire three-phase network can be modelled by a single-system equivalent circuit diagram. It should however be noted that the nominal voltage and the short-circuit voltage at the three-phase transformer are still specified as external conductor voltages. The nominal voltages U10 und U20 indicated on the rating plate (upper and lower voltage) always refer to the off-load operation. The nominal transmission ratio is UN = U10 / U20.

The fundamental considerations in Part 1 for the parallel operation of DC sources can be transferred to the parallel operation of transformers.

#### Requirements for efficient parallel operation

The main tasks for parallel operation are therefore:

• Avoidance/minimization of the circulating current

These tasks can be fulfilled if the following conditions are met:

• same nominal voltages, same transmission ratio (the same off-load voltages)
• same nominal value of the short-circuit voltages
• Ratio of the rated outputs is not greater than 1:2. If the rated output is exceeded, the dependent values of uk  are so far apart that at the parallel connection too great differences occur between the relative loads.

#### Distribution of the load current

If the primary side and secondary side respectively of transformers A and B are placed directly on the same bus bar, both the magnitude and angle of the voltage vector of the two input voltages and output voltages must be the same. It is assumed that the impedance of the connecting lines between A and B is equal to 0! For the voltage drop UF across the windings, the following must therefore apply.

Contact

## News from the product group

### REGSys® - News

Info letter

#### Info Letter No. 16: Hot-spot temperature and service life of transformers

The failure of a transformer not only has major economic consequences for the energy supplier, it can...

Info letter

#### Info Letter No. 13: Parallel operation of step transformers

The rule of thumb that with parallel operation of transformers the ratio of the rated outputs should not...

Info letter

#### Info Letter No. 11: Voltage regulation with step transformers in parallel to busbars (Part 3)

Direct measurement of the circulating current is not possible with the usual measurement...

Info letter

#### Info Letter No. 9: Voltage regulation with step transformers in parallel to busbars (Part 1)

Within a station, the transformers are in close proximity. Therefore, the line impedances between the...

Info letter

#### Info Letter No. 7: Load-dependent voltage change

For the regulation of the voltage ULoad at a predetermined value at the specified load point (e.g. the...

Info letter

#### Info Letter No. 4: Summation of Alternating Currents

For the parallel connection of power sources, the principle of superposition applies, when all resources...

General Knowledge

#### REGSys® Parallel Regulation of Transformers with Tap-Changers

The regulation of transformers with tap changers is an essential subject within the field of power supply and distribution. Today these tasks are accomplished electronically with high regulation quality. Digital regulators, such as the freely programmable REGSys® voltage regulator system, are in use.

Special Publication

#### Safely Regulate the Load Flow

Phase-shifting transformers or shunt regulators are special power transformers. They allow the active power flow (load flow) in alternating current networks to be controlled. During tapping, the ...

Special Publication

#### The REGSys® Triple-Wound Feature

Since then, it has become possible to use the feature for other application such as so-called “energetic recovery” or to implement applications requiring a second voltage (U1, U2) and current (I1, I2) measurement. In these...

Special Publication

#### Regulating the Active and Reactive Power with REGSys®

he voltage regulator REG-D(A) is normally tasked with controlling voltage on the secondary side of a transformer. The main task here is to achieve the best possible balance between voltage stability and the number of taps...