The attached project contains a QDSL model which models an automatic tap changer for a transformer with time-delayed tap changes. The permissible voltage band is formed using the input variables 'u_set' and 'u_tolerance'. In the example, there are three study cases that show the behaviour of the model. Activate a study case and execute the QDS to see the behaviour in the plots on page "Tap Changing":

1. Test Case - Load Jumps: The power of a load is changed to simulate voltage changes at the MV busbar and see the reaction of the tap changer model.
2. Test Case - Load Jumps Blocking: The same as 1. but an external blocking signal is applied via parameter events
3. Test Case - Voltage Characteristic HV: The voltage of the HV grid changes due to a time characteristic that is used in the external grid.

Behaviour of the model

Delay time

You can choose between a fixed delay time 't_delay' or an adaptive delay time that decreases with voltage deviation and that is determined as a function of the voltage deviation using the equation:

'time_factor' * 'delay_factor' * U-deviation^('delay_exponent')

'U-deviation' is assumed to be a value in percent. The method is selected by setting the input parameter 'delay_adaptive' to 1.

If the time steps of the QDS are greater than the determined delay times of the controller, it is also possible that several stages are run through in one time step.

Fast switching

In the event of larger voltage deviations, fast switching can be activated, which executes tap changes with a specified fast switching time until the tolerance band is reached again. The fast switching is active when 'fast_switch' is set to 1. The parameters 'fast_switch_up', 'fast_switch_down' and 'fast_switch_time' then define the limit values and the delay time of the fast switching.

Current compounding

The voltage setpoint can be increased based on the current if the input variable 'set_adaptive' is set to 1. An increase only occurs when there is an active power flow into the ownstream grid. The setpoint is calculated as a function of the current 'I_act' using the equation

u_set + 'rise_u_set'/100 * (I_act/'rise_In')

'rise_u_set' specifies the increase determined at I_act = 'rise_In'. The increase is limited to 'rise_u_set_max'.

Internal Blocking

In the event of extreme voltage deviations, it is possible to block the automatic tap changer. To do this, the input variable 'block' must be set to 1. The input parameters 'block_up' and 'block_down' are used as limit values.

External Blocking

It is also possible to use an external signal to block the tap changer. For this, the variable 'is_blocked_externally' has to be set to 1, for example by using a parameter event or other QDSL models.