*6.1. Idea*

This section will present the algorithm designed for counteracting the aggravation of the reactive power deficit in a voltage stability risk situation. The algorithm can be described in the following manner: if the voltage level in a HV network is lower than the accepted threshold value, *V*HVmin, and its decrease rate exceeds the preset threshold value of . *V*HVmax, then:

• If the coefficient d*Q*/d*V* > 0, then the transformer controller will operate following the constant lower voltage value criterion (*V*MVref = const.), with the simultaneous correction of the preset value to the lowest permissible value of *V*MVref = *V*MVmin. Thus, the controller will reduce the MV side voltage to *V*MVmin, and after exhausting the regulation capacities (the outermost tap) or reaching the preset value, it will maintain the constant voltage ratio.


An exemplary diagram of the operation algorithm of an adaptive HV/MV transformer regulator is shown in Figures 8 and 9.

**Figure 8.** Block diagram of the proposed algorithm of the HV/MV substation transformer controller.

**Figure 9.** Simplified diagram of the network used in simulation studies.

An important parameter that activates the operation of the proposed algorithm is the maximum rate of reduction of the HV side voltage . *V*HVmax. Assuming that the voltage ration value is constant, the voltage decrease rate at either side of the transformer will be identical. If the voltage decrease rate on the primary side is lower than the secondary side voltage change rate caused by the voltage ratio change, then the secondary side voltage will be maintained at a constant level. In that case, the secondary side voltage drop will be compensated for by the change of the voltage ratio. With a view to the above, the maximum voltage decrease rate can be determined from the relationship:

$$
\dot{V}\_{\text{HVmax}} = \frac{-d\upsilon\_{\%}}{t\_{\Sigma}}\tag{6}
$$

while:

$$t\_{\Sigma} = \, \_{\text{op}} + \mathbf{t\_{db}} \tag{7}$$
