Page 131 - Mathematical Models and Algorithms for Power System Optimization

122 Chapter 5

5.1 Introduction

The bus loads P and Q are derived variables in the traditional power flow algorithm, so the
bus load cannot be optimized. The total load of the urban power grid is the sum of the load
of each bus, because most of the buses are PQ bus, that is, the loads of these buses have
generally specified values in the power flow calculation. Under normal or faulty operating
conditions, to meet certain requirements, the bus loads need to be adjusted manually. However,
if P, Q, U, and θ are all set as independent variables, then P G , Q G , P L , and Q L can be optimized,
respectively. The optimization model based on the AC power flow (OPF) discussed in
Chapter 3 sets P G and Q G as independent variables under the condition of taking P L and Q L as
specified values.
To optimize the bus loads P L and Q L , this chapter considers setting P L and Q L as independent
variables under the given value of P G and Q G conditions. Because an accurately mathematical
description of P L is not required at the planning stage, some simplified power flow methods can
be applied, for example, DC power flow, on which a load optimization model is proposed to
optimize load P L . The optimization calculation of load P L in this chapter includes two aspects:
(1) minimization of load curtailment (LC) under N 1 conditions, and (2) maximization of
load supply capacity (LSC) under a given network structure and power generation conditions.

5.1.1 Description of Minimizing Load Curtailment

The ability to operate with any one major equipment unit out of service is called an N 1
capability, and this requirement is a criterion for the planning and design in the power system.
However, during high-risk periods (summer or winter peak, some components are maintained),
some events of multiple forced outages, N k, may happen. Then the load will have to be
curtailed to ensure system safety. To reduce the loss of power as much as possible when the
system is in a state of N k, it is necessary to choose the best one from all feasible options,
namely, the minimum load reduction option. Rescheduling generation outputs and ensuring
load transfer may avoid the LC as much as possible or minimize the total LC when unavoidable.

LC minimization is a problem of operation planning. It is to minimize total load reduction when
unavoidable, which can give an optimal option for LC, improve the rationality of LC at each
substation, and provide an auxiliary decision option for the dispatcher in the actual restriction
of power.

5.1.2 Description of Maximizing Load Supply Capability

The rational planning of a power system always requires determination of its LSC. The
maximum LSC refers to the maximum load that can be supplied on any branch (line or
transformer) in a power grid without overload. LSC can be used as a key index for assessing the