By
Dr. Hidaia Mahmood Alassouli
Email: hidaia_alassouli@hotmail.com
The objective of this project is to investigate the usefulness of the power system simulator PowerWorld program developed by “Glover, Overbye &Sarma”. The results obtained from the power simulator program were presented for different case studies. The following power system network was used in this study. The system consists from 6 buses. Area 1 includes bus 1-5 while Bus 6 will be part of Area 1 in some case studies, or will form separate area 2 in other case studies for comparison purpose. Note that the Available Transfer Capability (ATC) analysis add-on which determines the maximum MW transfer possible between two parts of a power system without violating any limits, and the voltage adequacy and stability tool (VAST) add-on that can solve multiple power flow solutions in order to generate a PV curve for a particular transfer or a QV curve at a given bus, was not studied here because we don’t have yet VAST add-on and the ATC add-on packages.
2.1 Load frequency control:
The turbine governor control eliminated the rotor acceleration and deceleration following a load changes during normal operation. However there is a steady state frequency error . One of the objective of load frequency control is to return the to zero.
In power system consisting of interconnected areas, each area agrees to export or import a scheduled amount of power through a transmission line connections or tie line to neighboring area. Thus , a second objective is to have each area absorb its own load during normal operation. This is achieved by maintaining the net tie line line power flow out of each area at it is scheduled value
The following summarizes the the two basic LFC objectives for an interconnection power system:
2.2 Economic Dispatch:
The previous section described how LFC adjusts the reference power settings of the turbine governors in an area to control frequency and the net tie line power flow out of the area. This section describes how the real power output of the controlled generating unit in the area is selected to meet a given load and minimize the total operating costs in the area. This is the economic dispatch.
2.3 Coordination of Economic Dispatch with LFC:
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2.4 Optimal Power Flow:
The solution to the problem of optimizing the generation while enforcing the transmission lines is to combine economic dispatch with power flow. The results is known as optimal power flow. There are several methods for the solving the OPF, with the linear programming (LP) approach. The LP OPF solution iterates between solving power flow to determine the flow of power in the system devices using LP to economically dispatch the generation subject to transmission lines limits. In absence of the system limits the OPF generation dispatch is identical to economic dispatch.
The PowerWorld Simulator (Simulator) is an interactive power system simulation package designed to simulate high voltage power system operation. In the standard mode Simulator solves the power flow equations using a Newton-Raphson power flow algorithm. However with the optimal power flow (OPF) enhancement, Simulator OPF can also solve these equations using an OPF. In particular, Simulator OPF uses a linear programming (LP) OPF implementation.
The objective of the OPF algorithm is to minimize the OPF objective function, subject to various equality and inequality constraints. Currently two objective functions are available in Simulator OPF: Minimum Cost and Minimum Control Change. Minimum Cost attempts to minimize the sum of the total generation costs in specified areas or super areas. Minimum Control Change attempts to minimize the change in the generation in the specified areas or super areas.
In solving a constrained optimization problem, such as the OPF, there are two general classes of constraints, equality and inequality. Equality constraints are constraints that always have to be enforced. That is, they are always "binding". For example in the OPF the real and reactive power balance equations at system buses must always be satisfied (at least to within a user specified tolerance); likewise the area MW interchange constraints. In contrast, inequality constraints may or may not be binding. For example, a line MVA flow may or may not be at its limit, or a generator real power output may or may not be at its maximum limit.
An important point to note is because the OPF is solved by iterating between a power flow solution and an LP solution, some of the constraints are enforced during the power flow solution and some constraints are enforced during the LP solution. The constraints enforced during the power flow are, for the most part, the constraints that are enforced during any power flow solution. These include the bus power balance equations, the generator voltage set point constraints, and the reactive power limits on the generators. What differentiate the LP OPF from a standard power flow are the constraints that are explicitly enforced by the LP. These include the following constraints:
(Area MW Interchange, Bus MW and Mvar power balance , Generator Voltage Setpoint , Super Area MW Interchange , Transmission Line and Transformer (Branch) MVA limits )
In Simulator OPF the LP OPF determines the optimal solution by iterating between solving a standard power and then solving a linear program to change the system controls to remove any limit violations. The basic steps in the LP OPF algorithm are :
· Solve the power flow
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