it is difficult to predict the actual voltage levels in the feeder lines in such a case,and lowering the voltage at the substation can have harmful effects in the far end ofthe network. However, the Rv Pwr flow allowed setting allows also negative LDCterms to be taken into equation.The topology changes in the network can cause changes to the equivalentimpedance value of the network. If the change is substantial, the setting groups canbe used to switch between different setting values for Line drop V React and Linedrop V Ris. In practice this means that the boolean-type information from thetopology change is connected to the active setting group change.The use of the LDC equation in the case of parallel transformers is described in theAutomatic voltage regulation of parallel transformers chapter.Reduce Set Voltage (RSV) inputThe system frequency decreases when the active power production in the networkis smaller than its consumption. Either the power supply has to be increased orsome loads have to be shed to restore the power balance.The simplest way to decrease the load is to reduce the voltage level by giving alower band center voltage value to the regulators. For this purpose, OLATCC hasthe setting group parameter Band reduction. The RSV input activation results inreduction. If this input is set to TRUE, a set target voltage value is decreased byBand reduction. If more than one RSV reduction steps are desired, the setting groupchange has to be used where different Band reduction values are supported. Thedecreased value is kept as a target value as long as the RSV input is TRUE.Because the decrease of frequency indicates a need to reduce the load, it ispractical to connect the start signal of an underfrequency function block to the RSVdigital input.It depends on the load characteristics how much the load is reduced as the voltagedrops. For instance, purely resistive loads are proportional to the square of thevoltage, whereas motor drives based on frequency controllers may draw constantpower despite small voltage changes.The status of the RSV input can be read from the RSV input data.9.5.10 Automatic voltage regulation of parallel transformersIt is likely that a circulating current between transformers occurs if two or moretransformers with slightly different ratios are energized in parallel. This is due tothe unbalanced short circuit impedances of the parallel transformers. To avoid suchcurrents, the tap changers of the transformers should be adjusted to achieveequilibrium. If the transformers are assumed identical, the tap (voltage) steps andtap positions should also match. In this case, the Master/Follower principle can beused. However, unequally rated transformers with different tap steps can beconnected in parallel and these configurations can also be managed by the tapchanger control function. For these configurations, the Minimizing CirculatingSection 9 1YHT530004D05 DControl functions736 615 seriesTechnical Manual
