Considerations for Multiple Generators on a Bus
EasyPower allows multiple generators or utilities to be placed on a bus. To provide this flexibility, several guidelines must be followed. These are very similar to the real world requirements normally imposed on a system.
There are three types of generator/utility models for the power flow solution: Swing (Slack), PV, and PQG. Each type has a specific mathematical function and an order of precedence or rank in the solution. The Swing has the highest order because mathematically, it exerts more control over the system. Next in line comes the PV type, and then the PQG type. This order is used in determining how the system is modeled and how the system model changes when breakers are open or closed.
When there are multiple PV type generators on a bus, they must all control the bus to the same voltage. If different values are entered in the database, the program will use the voltage from the largest generator as the control voltage. If the generators are designated as Swing, the size of each generator is irrelevant. The voltage and angle of the first Swing generator read in is used.
PV and Swing generators cannot be placed on the same bus. This is because they will both try to control the bus voltage. PV / PQG or Swing / PQG combinations are acceptable.
Bus control defaults to the highest order generator on the bus.
When multiple generators are present on a bus, the watt and var load must be shared. For a PV / PQG combination, the PQG generators will output their maximum watt and vars. The PV generators will also output their maximum watt capability, but will share the var output. Sharing is based on the size of the generator. For example, suppose three PV generators on a bus have ratings of 10, 40, and 50 MVA. If the var requirements of the bus are 50 MVAR, the smallest will supply 10 percent of the load, the middle 40 percent, and the largest 50 percent.
Share = Generator Rating / Generator Total
This applies only to PV type generators. For multiple Swing generators on one bus, or throughout the system, the share is divided evenly between the generators.
When the var limit of a PV generator is reached, it must switch to a PQG type in order to prevent violating the reactive capability limit of the generator. When the system allows, the generator will automatically switch back to allow a PV type solution. For multiple generators on a bus, the reactive capability limit is the sum of all the PV generators. When this limit is violated, all the generators will switch to PQG type. When the var constraints ease, they will all be switched back.
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