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FAQs: What is the difference between SB, PV and PQ Generators?
The choice of generator depends on how the generators on the network react to changes in load. One needs to consider both static load-flows and TMS studies. 
An SB (swing-bus) generator acts as a sink or source of real and reactive power. In versions of PTW older than V8.0, at least one swing bus generator (or utility) is needed on a network to act as either an infinite source of power or as a load if there is excess generation. From version V8.0 onwards this is no longer a requirement for Load Flow (DAPPER) studies but still applies for the TMS module
(Left): Example entry for an SB generator in the PTW Component Editor.  The fields within the "Initial Operating Conditions and Participation Factors" section are unique to the Swing Bus.  Use the drop-down box marked "Schedule:" to change to a different type of generator. 
A PV (regulated) generator produces a fixed amount of real power, and the required amount of reactive power (up to its limit) instantaneously (instantaneous excitation response, fixed governor). A PV generator will keep the voltage fixed at unity, as long as the amount of reactive power needed to do so isn't greater than its limits. 
(Left): Example entry for a PV generator in the PTW Component Editor.   For clarity, only the fields unique to the PV model are shown. 
A PQ (non-regulated) generator produces a constant amount of real and reactive power (fixed governor and exciter, and no inertia). 
(Left): Example entry for a PQ generator in the PTW Component Editor.   For clarity, only the fields unique to the PV model are shown. 
Both the PQ option and the PV option could result in more power being produced than is needed on the network, so this excess power would be absorbed by the swing bus or by the Utility (assuming a fixed system frequency). The exception is for islanded power systems with no Utility or Swing Bus (Version V8.0 onwards).  In this case, the model will only work if the "Consider System Freq Change" option is enabled in the Load Flow study (DAPPER).  More information is available on page 8 of the PTW V8.0 Enhancements brochure, available for download here
Both DAPPER and TMS assume a constant frequency (with the exception of DAPPER in V8.0 onwards, as mentioned above), which is fine for networks with a strong grid infeed. However, this assumption isn't acceptable for studies looking at, for example, starting large motors with no grid infeed. For these types of studies, I*SIM should be used instead of TMS, in which case I*SIM's governor and exciter models are used. 
Click here for more details about running a TMS study.