Automatic voltage control of networks with embedded generation
by V Thornley, Siemens, and N Hiscock, Fundamentals Limited
Published in: Transformers and Substations Handbook: 2014
Chapter 3: Substation automation - page 58
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This article discusses the application of transformer automatic voltage control (AVC) to networks in which generation is embedded, with reference to the application of MicroTAPP voltage control to these systems. The issues addressed are the particular problems that can occur if the application of voltage control does not take into account the presence of generation. It addresses any requirements on a local (ie distributed) level. An overall network solution may make use of a network automation system to set up the voltage control relay (VCR) for the system conditions. This, however, is a network management issue and is not relevant to the voltage control application.
The MicroTAPP scheme, based on the negative-reactance AVC scheme, resolves the measured current of each transformer into load and circulating elements. Figure 3 shows the current seen by an AVC relay (ICT,1) with respect to its phase voltage (VVT). The circulating current (Icirc) is resolved from ICT,1, being the deviation from a set-point of system power factor (pfsys). This element of current is then used to bias the voltage control in order to minimise the circulating current.
Line drop compensation (LDC) corrects for system voltage drops so that customers receive as close to ideal voltage as is possible. The total load on the busbar is calculated by summating transformer currents ICT,1 and ICT,2 and this is used to calculate a bias to apply to the voltage control. These two simple elements together achieve the main aims of voltage control.
Although the actual power factor at a particular time may not be the specified power factor pfsys, as long as the deviation is not large the voltage control will be satisfactory. If the actual power factor varies greatly from the set-point, the effect will be an error in the controlled voltage, as the load current will be considered as circulating current by the TAPP scheme.
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- The MicroTAPP voltage control described resolves the measured current of each transformer into load and circulating elements.