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Functional descriptions

In order for a transformer to be able to provide constant voltage to the power grid at all times when it has different loads, it needs a regulating device. In this case, the TAPCON® voltage regulator constantly measures the voltage value and compares it to the configured desired value. If the current value is outside of a defined bandwidth over a defined delay time, then a switching command is sent to the on-load tap-changer. The on-load tap-changer adjusts the winding ratio using appropriate switching, thereby bringing the voltage back into the bandwidth. A switching command is issued without going through a delay time if an undervoltage or overvoltage limit is passed.




Transmitting energy over electrical lines always involves losses. The voltage at the load drops due to resistive and inductive voltage losses along the line. However, the TAPCON® voltage regulator increases the voltage at the transformer by the necessary amount, while taking the line impedance and the current load current into consideration, in order to keep the voltage at the specified level. This causes the voltage at the load to be constant and kept at the same specified level.

Limit Volt is a separate monitoring device that ensures the voltage regulator is functioning error-free. Here, Limit Volt monitors whether the voltage is outside the undervoltage or overvoltage limit and blocks control commands to the tap changer in the event of an error. This means an additional measure of security for providing constant voltage.

Up to 16 transformers of the same power class can be operated in parallel using TAPCON® voltage regulators. A TAPCON® is located on each transformer involved in the process. The regulators communicate via a CAN bus connection. Two principles can be applied during parallel operation. The first of these is tap synchronization for transformers with the same output variables. Here, one voltage regulator is set as the reference regulator (master), which the regulators running in parallel (followers) follow when the tap position is changed. The second principle is circulating reactive current minimization. Here, the the voltage as well as the reactive currents of each transformer are measured and compared to the load current. Circulating reactive current is minimized by appropriately switching the on-load tap-changer involved and thus the transformers run in parallel. In the last method, it is possible to let transformers with comparable power classes run in parallel.
Compliance with the following general conditions is recommended for operating transformers in parallel:

  • Identical rated voltage
  • Ratio of transformer output (<3:1)
  • Maximum deviation of the short-circuit voltages (US) for the transformers switched in parallel ≤10 %
  • Same number of switching groups





Using this function, the TAPCON® voltage regulator records the topology of the switchgear with its disconnectors and circuit breakers. It also detects transformers connected in parallel in the process, without using additional equipment. This puts the TAPCON® in a position to take over configuring the switchgear and adjusting the control response for the present switching state. As part of the topology function, one of the TAPCONs involved takes over controlling the operation of the remaining TAPCON voltage regulators in parallel in addition to regulating the associated transformer. The parallel operating methods, tap synchronization and circulating reactive current minimization, are optionally available for the topology function.


The regulating function and monitoring function (such as Limit Volt) are integrated in one device for another TAPCON voltage regulator for cross-monitoring. In addition to regulation by the assigned transformer, two TAPCON voltage regulators use a second measuring circuit to monitor each other to ensure the other is staying within the voltage limit values. Furthermore, both TAPCON voltage regulators communicate connected via a CAN bus connection.

Parallel bank operation is regulation for what is known as a transformer bank consisting of three single-phase transformers with three separate motor-drive units and on-load tap-changers. Thus, for this application, one sole TAPCON® voltage regulator is sufficient to ensure control for the three motor drive units. The load voltage for a bank of transformers is regulated in the same way as described for voltage regulation.



TAPCON® voltage regulators are in a position to take over regulating three-winding transformers (one high-voltage winding, two low-voltage windings). One set of control parameters is required per secondary winding in each case in the process. During operation, the voltage of the secondary winding where the largest current flows is always regulated. The second secondary winding is monitored for undervoltage and overvoltage in the process.




It is possible to implement regulating functions individualized to a customer's needs as needed. Almost no limits are set on the complexity for these. A project for phase shift regulation that has already been implemented provides an example of this. In the project, a TAPCON® 260 regulates the specific power distribution.