S. Y. HONG ET AL.

Copyright © 2013 SciRes. ENG

(1)

where x is the maintenance time vector,

is the

weight of optimize objective function i, l is the total

number of inequality constraints, m is the total number

of equality constraints.

However, weighting single objective optimization

method has the following defects:

• Enough prior knowledge is required to determine

the weight of each objective function.

• Only one Pareto optimal solution can be obtained in

each optimization time, which is difficult to judge

the reliability and optimality of the optimization

results.

• Each objective function has different dimension.

• Considering about all these, this paper adopts mul-

ti-objective optimization model to optimize several

objective functions and requires that all objective

functions meet the condition of setting constraints,

which is shown as follow:

12

Min ( )(( ),( )...,( ))

k

XR

FXf XfXfX

∈

=

12

()((), (),...,())0

m

gXgXgXgX= ≤

(2)

where F(X) is the optimization target vector, g(X) is the

constraint vector, X is the decision variable.

2.2. Optimization Objective Functions

The purpose of arranging maintenance scheduling is not

only to transfer load as much as possible, but to consider

the economy and reliability of distributio n network oper-

ation. Therefore, maintenance scheduling of distribution

is a combinatorial optimization problem of multi-objec-

tive and multi-constraint , which is related to the objective

functions including the following aspects:

1) Load Loss

(3)

where

λ

denotes average electricity price, N means the

assemblage of transfer nodes, Pi is the load loss, Ti is the

maintenance continuous time.

2) Grid Active Power Loss

In order to avoid the outage of distribution network

caused by equipment maintenance, we should conduct

the network load transfer and besides, choose the optimal

transfer path to reduce the grid active power loss, which

is the target of load transfer in equipment maintenance.

(4)

where

denotes the grid active power loss of transfer

path k, M denotes the assemblage of all transfer paths.

3) System Risk

Generally, maintenance scheduling optimization mod-

el requires only that transfer strategy meet the network

power flow constraint, seldom considering the problem

of load equalization. The risk value of power distribution

system is calculated as follow:

(5)

where Pj denotes the load of node j, Re is the failure rate

of main equipments on trans fe r pa t h .

The risk assessment value can be divided into three

levels as “Low Risk”, “Medium Risk” and “High Risk”,

corresponding to evaluation score 0 - 0.3, 0.3 - 0.7, 0.7 -

1.0 respectively.

The selection of power load transfer paths is closely

related to the reliability of transfer lin e. If the power load

of maintenance line is transferred to another line of low

reliability in distribution network, the failure risk of

transfer line will greatly increase, which will impact the

reliable operation of distribution network. Therefore, we

should conduct the calculation of line risk and transfer

the power load to a high reliability line as far as possible.

In this paper, combining with Condition Based Main-

tenance (CBM) conducted by Power Supply Company,

the health status of distribution equipments on transfer

path are evaluated and then, make a prediction of equip-

ment failure rate according to the health evaluation re-

sults. After that, Per-unit value of the line load is calcu-

lated based on the max line load. In the following, sys-

tem risk is calculated and the level of risk assessment is

set up according to results of risk value.

2.3. Health State and Failure Rate

Health evaluation is a comprehensive evaluation process,

which means that the electrical equipment’s health state

is evaluated by various state parameters, according to the

health state, the hidden defects of equipment are found

out in time and Power Supply Company can conduct the

maintenance to make sure that the equipment is in

healthy condition [6].

This paper adopts Fuzzy Variable Weight Analysis

method to evaluate the health degree of distribution

equipments. The method can adjust the weights of

equipments’ state parameters automatically according to

the relationship and quality of different parameters. The

procedure of distribution equipments’ health evaluation

can be described as follows: