After reading this chapter, you should be able to:
The problem of optimizing the performance of a power system network is formulated as a general optimization problem. It is required to state from which aspect the performance of the power system network is optimized.
In optimization problem, the objective function becomes ‘to minimize the overall cost of generation in economic scheduling and unit commitment problem’:
The optimal power flow problem:
The solution technique for an optimal power flow problem was first proposed by Dommel and Tinney and has following three major steps:
The optimization problem of minimizing the instantaneous operating costs in terms of real and reactive-power flows is studied in this unit.
The optimal power flow problem without considering constraints, i.e., unconstrained optimal power flow problem, is first studied and then the optimal power flow problem with inequality constraints is studied. The inequality constraints are introduced first on control variables and then on dependent variables.
The primary objective of the optimal power flow solution is to minimize the overall cost of generation. This is represented by an objective function (or) cost function as:
subject to power (load) flow constraints:
and
where Vi = |Vi|∠δi is the voltage at bus ‘i’, Vj = |Vj|∠δj the voltage at bus ‘j’, Yij = |Yij|∠θij the mutual admittance between the i th and j th buses, Pi the specified real-power at bus i, Qi the specified reactive power at bus i, the net real power injected into the system at the i th bus, Pi = PGi – PDi, and the net reactive power injected into the system at the i th bus, Qi = QGi – QDi.
For load buses (or) P–Q buses, P and Q are specified and hence Equations (5.2) and (5.3) form the equality constraints.
For P–V buses, P and |V| are specified as the function of some vectors and are represented as:
f(x, y) = 0
where x is a vector of dependent variables and is represented as
and y is a vector of independent variables and is represented as
Out of these independent variables (Equation (5.6)), certain variables are chosen as control variables, which are to be varied to yield an optimal value of the objective function. The remaining independent variables are called fixed or disturbance or uncontrollable parameters.
Let ‘u’ be the vector of control variables and ‘p’ the vector of fixed or disturbance variables.
Hence, the vector of independent variables can be represented as the combination of vector of control variables ‘u’ and vector of fixed or disturbance or uncontrollable variables ‘p’ and is expressed as
The choice of ‘u’ and ‘p’ depends on what aspect of power system is to be optimized. The control parameters may be:
Now, the optimal power flow problem can be stated as
subject to equality constraints:
Define the corresponding Lagrangian function by augmenting the equality constraint to the objective function through a Lagrangian multiplier λ as
where λ is a vector of the Lagrangian multiplier of suitable dimension and is the same as that of equality constraint, f (x, u, p).
The necessary conditions for an optimal solution are as follows:
Equation (5.13) is the same as equality constraints, and the expressions for and are not very involved.
Consider the general load flow problem of the N–R method by considering a set of ‘n’ non-linear algebraic equations,
fi(x1, x2 … xn) = 0 for i = 1, 2 … n (5.14)
Let x10, x20, … xn0 be the initial values.
and let ∆x10, ∆x20, … ∆xn0 be the corrections, which on being added to the initial guess, give an actual solution:
∴ fi(x10 + Δx10, + x20 + Δx20, …, xn0 + Δxn0) = 0 for i = 1, 2 … n (5.15)
By expanding these equations according to Taylor’s series around the initial guess, we get
where are the derivatives of fi with respect to x1, x2 … xn evaluated at (x10, x20, … xn0).
Neglecting higher order terms, we can write Equation (5.16) in a matrix form as
or in a vector matrix form as
where J 0 is known as the Jacobian matrix and is obtained by differentiating the function vector ‘f ’ with respect to ‘x’ and evaluating it at x 0.
By comparing Equations (5.11), (5.12), and (5.13) with Equations (5.17) and (5.18), it is observed that is the Jacobian matrix and the partial derivatives of equality constraints with respect to dependent variables are obtained as Jacobian elements in a load flow solution.
The equality constraints are basically the power flow equations, i.e., real and reactive-power flow equations:
equality constraints for P–Q bus
Pi is the equality constraint for P–V bus
‘x’ is the dependent variable like |Vi|, δi
Then, may be expressed as partial derivatives of
The Equations (5.11), (5.12), and (5.13) are non-linear algebraic equations and can be solved iteratively by employing a simple technique that is a ‘gradient method’ and is also called the steepest descent method.
The basic technique employed in the steepest descent method is to adjust the control parameters ‘u’ so as to move from one feasible solution point to a new feasible solution point in the direction of the steepest descent (or negative gradient). Here, the starting point of feasible solution is one where a set of values ‘x’ (i.e., dependent variables) satisfies Equation (5.13) for given ‘u’ and ‘p’. The new feasible solution point refers to a location where the lower objective function is achieved.
These moves are to be repeated in the direction of negative gradient till minimum value is reached. Hence, this method of obtaining a solution to non-linear algebraic is also called the negative gradient method.
The algorithm for obtaining an optimal solution by the steepest descent method is given below:
Step 1: Make an initial guess for control variables (u0).
Step 2: Find the feasible load flow solution by the N–R method. The N–R method is an iterative method and the solution does not satisfy the constraint equation (5.13). Hence, to satisfy Equation (5.12), ‘x’ is improved as follows:
xr + 1 = xr + Δx
∆x is obtained by solving the set of linear equations of the Jacobian matrix of Equation (5.18) as given below:
f (xr + Δx, y) = f (xr, y) + (xr, y) Δx = 0
The final results of Step-2 provide a feasible solution of ‘x’ and the Jacobian matrix.
Step 3: Solve Equation (5.11) for λ and it is obtained as
Step 4: Substitute λ from Equation (5.19) into Equation (5.12) and calculate the gradient:
For computing the gradient, the Jacobian matrix, , is already known from Step 2.
Step 5: If the gradient ∇ is nearly zero within the specified tolerance, the optimal solution is obtained. Otherwise,
Step 6: Find a new set of control variables as
where Δu = − α∇. (5.22)
Here, ∆u is a step in the negative direction of the gradient.
The parameter α is a positive scalar, which controls the step i’s (size of steps), and the choice of α is very important.
Too small a value of α guarantees the convergence but slows down its rate. Too high a value of it causes oscillations around the optimal solution. Several methods are suggested for determining the best value of α for a given problem and for an optimum choice of step size.
α is a problem-dependent constant. Experience and proper judgment are necessary in choosing a value of it. Steps 1, 2, and 5 are repeated for a new value of ‘u’ till an optimal solution is reached.
In Section 5.2, the unconstrained optimal power flow problem and the computational procedure for obtaining the optimal solution are discussed. Now, in this section, the inequality constraints are introduced on control variables, and then the method of obtaining a solution to the optimal power flow problem is discussed.
The permissible values of control variables, in fact, are always constrained, such that
For example, if the real power or reactive-power generation are taken as control variables, then inequality constraints become
|
PGi (min) ≤ PGi ≤ PGi (max) |
|
|
QGi (min) ≤ QGi ≤ QGi (max) |
(5.24) |
In finding the optimal power flow solution, Step 6 of the algorithm of Section 5.2.1 gives the change in control variable as
Δu = − α∇
where and the new control variable, unew = uold + ∆u.
This new value of control variable must be checked whether it violates the inequality constraints on the control variable or not:
i.e., ui(min) ≤ ui(new) ≤ ui(max)
If the correction ∆u causes to exceed one of the limits, ‘ui’ is set equal to the corresponding limit, i.e., the new value of ui is determined as
otherwise set ui(new) = ui(old) + ∆ui
After a control variable reaches any of the limits, its component in the gradient should continue to be computed in later iteration, as the variable may come within limits at some later stages.
The optimality condition under inequality constraints can be rewritten as Kuhn–Tucker conditions given below:
Therefore, now, in Step 5 of the algorithm of Section 5.2.1, the gradient vector has to satisfy the optimality condition given by Equation (5.26).
In this section, the optimal solution to an optimal power flow problem will be obtained with the introduction of inequality constraints on dependent variables and penalties for their violation.
The inequality constraints on dependent variables specified in terms of upper and lower limits are
where x is a vector of dependent variables.
For example, if the bus voltage magnitude |Vi| is taken as a dependent variable, the inequality constraint becomes
The above-mentioned inequality constraints can be handled conveniently by a method known as the penalty function method. In this method, the objective function is augmented by penalties for the violations of inequality constraints. Due to this augmented objective function, the solution lies sufficiently close to the constraint limits when the violations of these limits have taken place. The penalty function method, in this case, is valid since these constraints are seldom rigid limits in the strict sense but are, in fact, soft limits (e.g., |V| ≤ 1.0 on a P–Q bus really means |V| should not exceed 1.0 too much and |V| = 1.01 may still be permissible).
When inequality constraints are violated, the objective function can be modified by augmenting penalties as
where ωj is the penalty introduced for each of the violated inequality constraints.
A suitable penalty function is defined as
where γj is called a penalty factor since it controls the degree of penalty and is a real positive number.
A plot of the penalty function, which is proposed, is shown in Fig. 5.1. The plot clearly indicates how the rigid limits are replaced by soft limits. The necessary optimality conditions:
FIG. 5.1 Penalty function
would now be modified as given below, while the condition of Equation (5.13), i.e., load flow equations, remains unchanged:
In the above equations, the vector can be calculated from the penalty function ωj.
The vector can be obtained from Equation (5.30) and would contain one non-zero term corresponding to dependent variable xj .
The vector , since the penalty functions on dependent variables are independent of control variables.
If we choose a higher value for γj, the penalty function ωj can be made steeper so that the solution lies closer to the rigid limits, but the convergence becomes poorer. In normal practice, it is required to start with a lower value of γj and then increase it during the optimization process if the solution violates constraints above a certain tolerance limit.
It is concluded that the solution to optimal power flow problem can be achieved by superimposing the N–R method of load flow on the optimal power flow problem with respect to relevant inequality constraints. These solutions are often required for system planning and operation.
min C = C(x, u)
subject to equality constraints:
f(x, u, p) = 0
Partial derivatives of equality constraints with respect to dependent variables. i.e.,
To adjust the control variables so as to move from one feasible solution point to a new feasible solution point where the lower objective function is achieved.
The moves from one feasible solution point to a new feasible point are to be repeated in the direction of negative gradient till a minimum value is reached.
The too small value of α guarantees the convergence but slows down the rate of convergence, whereas too high a value of it causes an oscillatory solution around the optimal solution.
While introducing inequality constraints on control variables.
While introducing inequality constraints on dependent variables.
The penalty function ωj can be made steeper so that the solution lies closer to the rigid limits, but convergence becomes poorer.
y is a vector of an independent variable.
y is a vector of a dependent variable.
y is a vector of a constant.
y is a vector of an uncontrolled variable.