APTPower Flow
Part 2- A more complicated system
Purpose: This page is not intended to teach the topic of power flow, but rather to introduce the topic. After reading the information on this page, the reader should have a intuitive understanding of why calculating power flow in a networked transmission system can be challenging. EE454 is an excellent course for students who would like to learn more about power flow analysis. It is assumed that the reader has an understanding of basic circuit analysis.
2 - A look at a more complicated system
While simple DC circuits offer useful insight into the power flow problem, calculating a real life power flow solution is a good deal more involved. Recall that the transmission lines are not simple resistive elements. Also note that loads are rarely resistive. Almost all powers system loads contain a mix of load types that include many electric motors and various other items. The result - loads are both resistive and inductive, meaning that they demand both real and reactive power.
A real power system will have several sources of power, many power lines and many loads scattered about. The loads are not constant; they vary as the needs of consumers change. This adds an additional variable to the power flow problem that must be contended with. An electric utility must estimate the demand for power (an educated guess) and arrange the power system to accommodate the expected load. Nevertheless, there will be many possible contingencies that can change the systems operating state.
Operating States
In the execution of power system control it is useful to define some power system operating conditions or operating states. According to T. Dy Liacco's security assessment framework, operating states might be defined as follows:
|
Normal
|
Abnormal
|
|
secure
|
emergency
|
|
insecure
|
restorative
|
Figure 2-1
Power System operating states and state transitions due to contingencies and control functions
Example 2-1- A small power system
Figure 2-2 is the one line diagram for a small fictitious utility transmission system. The loads shown the are loads at the buses of distribution substations throughout the utility system:
Figure 2-2 - Small fictitious power system
XX% indicates line current loading as a percent of line current capacity.
Assume the system has been modeled and all the line impedances are known. Also, assume it is possible to run a power flow solution for a given situation using some computer software. Suppose the system is operating normally. At the dispatch center the operator(s) may be looking at a mimic board that shows the status of the system in a graphic format similar to what is shown above. Additionally, the operator may have information on line loading (the percentages shown next to the lines), load buses and generator outputs. There may be other information presented. For example, the operator may have data on the voltage at each bus. How many contingencies are possible? What contingencies will result in an emergency situation? Suppose all the loads in the system must maintain voltage levels of .95pu. What possible problem(s) will cause low voltage at a bus?
Some possibilities are simple to recognize. Clearly the load at Hope will be lost if something happens to the line from Millers Point to Hope. If the load at Hope grows to 7MVA the line serving the bus may be overloaded. The possible results of some other contingencies are not clear at all. Suppose the line serving Millers Point from Grand View is out of service. Given the current loading, will the other lines be able to accommodate the new flow situation? Will the bus voltage at Hope remain within limits? What other possibilities could cause problems? Even though the system shown in figure 2-2 is fairly simple, it is not simple or trivial to quickly predict what will happen if something changes. An experienced operator will undoubtedly have a good feel for what will cause problems but as the system grows in complexity and as situations become more confused, the problem becomes increasingly difficult.
If transmission system is operating in a secure state and a hypothetical but likely casualty occurs, perhaps a tree falls on a power line; there should be no immediate loss of loads. However once the casualty has occurred, it is necessary to determine the new operating state of the system. Is the system still secure? What contingency will cause the system to fail or to degrade into the emergency state? How can this be quickly assessed?
To learn more about power flow, please check out EE454 notes -pfnote.doc
Links
