Energy and Power Engineering, 2013, 5, 728-732
doi:10.4236/epe.2013.54B141 Published Online July 2013 (http://www.scirp.org/journal/epe)
Multi-level Evaluation of Load Supply Adequacy for
Transmission Network
Zheng-lin Yang, jian-guo Yao, jian Geng
China Electric Power Research Institute, Nanjing, China
Received March, 2013
ABSTRACT
Considering sharp increase of central integration scale of intermittent energy generation, power system couldn’t rely on
the determined model based operation schedule to assure its reliable load supply. Then, this article presents a new load
supply adequacy evaluation method, which based on continuous security constrained generation optimization, and pro-
vides the functional architecture design of online multi-level short term load supply adequacy evaluation system.
Keywords: Load Supply Adequacy; Operation Schedule; Security Analysis; Continuous Security Constrained
Generation Optimization; Unit Commitment; Economic Dispatch
1. Introduction
It is the main objective of short term operation schedule
to secure power system operation, improve load supply
reliability and optimize generation resource configuration.
Outage schedule, one type of operation schedule, should
provide enough margins for reliable power system opera-
tion by optimizing transmission network topology. Gen-
eration schedule, another type of operation schedule,
should optimize multi-periods unit commitment status
and output according to predefined objective, taking con-
sideration of power supply balance, transmission capa-
bility constraints and generation operation constraints. In
traditional deterministic model for short term operation
schedule optimization, outage schedule and generation
schedule are optimized on the assumption that all genera-
tion units meet certain reliability level in operation, and
the negative influence of load forecast bias and forced
outage of transmission and generation device could be
avoided by scheduled regulation reserve and spinning
reserve.
With integration of large scale intermittent energy
generation, such as hydropower, wind power and photo-
voltaic power, its negative influence on operation has far
exceeded load forecast bias. The traditional generation
schedule optimization model, which treat intermittent
energy power as fixed generation and eliminate it from
system load according to forecast of hydropower, wind
power and photovoltaic power, could not cope with the
random fluctuation introduced by large scale intermittent
energy generation. It is important to arrange enough
margins in outage schedule optimization to meet addi-
tional reserve requirement caused by intermittent energy
generation. Therefore, it is of great value for improving
load supply reliability to accurately evaluate maximized
load supply capability for transmission network. Paper [1]
presented load supply adequacy index for bus, which is
defined as the ratio of maximized load supply capability
to current bus load, and applied in load supply capability
evaluation of buses. It is the main objective of outage
schedule optimization and evaluation to arrange enough
available generation capacity and transmission capacity
for coming generation optimization in order to cope with
fluctuation of load. This article applies load supply ade-
quacy index in multi-level evaluation of load supply ca-
pability on every bus according to outage schedule. Load
supply adequacy evaluation could provide quantitative
margin information to assist operator evaluating the
adaptability of power system for load fluctuation.
Through the evaluation of load supply adequacy index,
the transmission network planner and operator could
evaluate the quantitative security margin of transmission
network on the basis of qualitative security [2] evaluation
of operation schedule, and also could evaluate the adap-
tability for integration of intermittent energy generation.
In the reference documents, load supply adequacy in-
dex of every bus is evaluated by increasing load continu-
ally under the assumption that all other bus load remain-
ing unchanged. The unreasonable assumption restricts its
application in engineering. Firstly, there is strong rela-
tionship between bus loads variation. Secondly, it is too
conservative to evaluate load supply adequacy index by
increasing generation at equivalent ratio to meet bus load
growth. Hence, this article presents multi-level load sup-

Z.-L. YANG ET AL. 729
ply adequacy evaluation method, which evaluates load
supply adequacy from bus, zone and system level, and
integrates sensitivity factor based evaluation method and
continuous security constrained generation optimization
based evaluation method[3-8] to make the index more
accurate reflect load supply adequacy of transmission
network. Based on multi-level load supply adequacy
evaluation, the online short term load supply adequacy
evaluation system is designed and implemented.
2. Theory of Load Supply Adequacy
Evaluation
2.1. Sensitivity Factor Based Evaluation Method
Paper 1 presents bus load supply adequacy index and
sensitivity factor based evaluation method for the first
time. The theory is as follows.
First of all, ij , the sensitivity of load j on branch i,
is calculated. Then, the maximized load supply capability
,max on bus j is calculated considering available
transmission capability of branch i by increasing genera-
tion output by equivalent ratio and assuring balance be-
tween generation and load. Considering all branch, the
minimized
sens
Pij

j, max1j, max2j, maxij, max
PminP,P P

is defined as load supply capability of bus j and j

is
defined as load supply adequacy of bus j.
i,max i
ij, maxj
ij
ff
P-P
sens
 (2.1)
j, max
j
j
P
P
  (2.2)
i,max represents thermal limit of branch i, represents
the power flow of branch i at initial state, ij, max repre-
sents the maximized available load supply capability of
bus load j considering the transmission constraint of
branch i.
fi f
P
ij Indicates the relationship between branch i and
load j, larger ij represents stronger relationship be-
tween them. The difference between ,max and
sens
sens
fi
j
f
represents the available transmission capability of branch
i considering its thermal limit. The smaller difference
between ,max and
fi
j
f
, indicates the smaller load sup-
ply adequacy j

of load j, which has strong relationship
with branch i.
Considering thermal limit of branch and its relation-
ship between bus load, sensitivity factor based evaluation
method evaluates the load supply adequacy by selecting
the minimized load supply capability among all the
maximized load supply capability considering thermal
limit constraints of all branches. In sensitivity factor
based evaluation process, not only thermal limit of
branch, but also the transient limit of interface and limi-
tation of generation capability has great influence on load
supply adequacy of bus load.
Sensitivity factor based evaluation method has two
shortcomings in engineering application. Firstly, it is
only suitable for single bus load supply adequacy evalua-
tion. Secondly, it is more conservative since it rely on
proportional generation growth other than consider more
rational generation output and unit commitment.
2.2. Continuous Security Constrained
Generation Optimization Based Evaluation
Method
The theory of continuous security constrained generation
optimization based method is to calculate the maximized
load supply capability through continuous load growth
and corresponding generation optimization to find feasi-
ble generation schedule, which could satisfy all the pow-
er balance constraints, transmission security constraints
and generation security operation constraints. The evalu-
ation process of continuous security constrained genera-
tion optimization based method is demonstrated as Fig-
ure 1.
Ld0 represents initial load of evaluation object, and the
growth ratio of each round is δ. The first round evalua-
tion starts after the setting of objective and constraint
model for generation optimization. In every round, base
Figure 1. Diagram of continuous security constrained gen-
eration optimization based evaluation method.

Z.-L. YANG ET AL.
730
case security analysis and contingency analysis should be
carried out on achieved feasible generation schedule after
generation optimization. And if any new violated trans-
mission devices are detected, the new violations should
be sent to generation optimization engine as additional
transmission security constraints until no other violations
appear. Then increase the load by & and move to next
round evaluation. On the other hand, if generation opti-
mization engine could not find feasible generation sche-
dule that satisfy all constraints, the constraints are re-
laxed and the ld of last round should be taken as the
maximized load supply capability, and should
be load supply adequacy of evaluation object. Then the
evaluation process ends. Since evaluation load grows and
security constrained generation optimization run every
round, it is called continuous security constrained gen-
eration optimization based evaluation method.
/0Ld ld
In generation optimization model, the constraints can
be divided into system power balance, generation opera-
tion and transmission security. For unit commitment, the
constraints include system power balance, minimized
regulation reserve, maximized unit output, minimized
unit output, ramp up and ramp down speed, initial unit
status, minimized run time, minimized offline time, unit
regulation reserve, must run, fixed unit commitment or
output schedule for generation, transmission capability in
base case and contingency analysis, and also include the
maximized output, minimized out, maximized energy,
minimized energy, minimized regulation reserve and
spinning reserve for a group of generations.
Any generation schedule, which satisfy all the con-
straints listed above for certain load level, is feasible for
load supply adequacy evaluation, which has no specific
requirement on optimization objective. According to
available cost model of generation, the optimization ob-
jective could be one of minimized energy cost, mini-
mized fuel cost or minimized bias of yearly energy con-
tract accomplishment between generation providers.
Maximized transmission margin could also be taken as
optimization objective by converting from load ratio of
transmission device into virtual dispatch cost. Maximized
transmission margin optimization could achieve the best
performance with moving of transmission capability
constraints to optimization objective model.
Compared with sensitivity factor based evaluation
method, continuous security constrained generation op-
timization based evaluation method not only could be
used for single period load supply adequacy evaluation,
but also could be used for accurate multi-period coupled
evaluation, and the evaluation result could be more rea-
sonable. For single period evaluation, the ramp speed of
generation need not be considered in generation optimi-
zation. But for multi-period evaluation, all the constraints
listed above should be taken into consideration for con-
tinuous load growth. Considering high strength optimiza-
tion, the calculation of continuous security constrained
generation optimization based evaluation method need
more time than sensitivity factor based evaluation me-
thod.
3. Multi-level Load Supply Adequacy Eval-
uation of Transmission Network
3.1. Level of Short Term Load Supply Adequacy
Evaluation
The level of short term load supply adequacy evaluation
could be divided into bus, zone and system. Bus level
evaluation takes single bus load as evaluation object,
calculates maximized load supply capability of bus con-
sidering continuous load growth and assumption of any
other bus load remaining unchanged. For the strength
relationship between bus loads, the situation that one bus
load grows continuously and all other remain unchanged
seldom appears. The result of bus load level evaluation
could not reflect load supply adequacy of transmission
network intuitively and is inconvenient to apply in engi-
neering. Power system operator care more about the load
growth margin of a zone or even all the system than bus
level. Power system operator not only care load supply
adequacy of one period, but also care the load supply
adequacy of all periods of a day to evaluate the maxi-
mized margin of load supply for random fluctuation of
all kinds such as cased by intermittent energy resource.
Zone level evaluation takes single zone as evaluation
object, calculates maximized load supply capability of
zone considering continuous load growth and assumption
of any other zone load remaining unchanged. According
to evaluation time scope, zone level evaluation could be
divided into single period and multi-periods evaluation.
System level evaluation takes all the system as evalua-
tion object, calculates maximized load supply capability
of system considering continuous system load growth
and assumption of tie line exchange remaining un-
changed. System level evaluation also could be divided
into single period and multi-periods evaluation.
3.2. Load Growth Model
In order to reduce the evaluation times, this article sets 2
as the maximized evaluable load supply adequacy limit
and should stop evaluating if current index has already
exceeded the limit. According to the limit setting of load
supply adequacy index, this article adopts dichotomy
method to determine load growth of next round.
In order to reflect the relativity of load growth among
bus load in zone level and system level load supply ade-
quacy evaluation, the growth of load should be allocated
to every bus load by rational proportion, which assures
evaluation result could reflect the load supply capability

Z.-L. YANG ET AL. 731
in future operation. General growth modes of bus load
are listed as follows.
1) Proportional growth
All bus loads grow proportionally. This mode is suit-
able when the bus load is same in type and has same va-
riety rule, or has no scrutiny load model.
2) Proportional growth by load group
Separating bus load into several group by load type,
bus loads in same group apply the same growth ratio, and
every group could have different growth ratio. This mode
is suitable when every type load has different variety rule
or integration of intermittent energy generation centrally.
3) Proportional growth by load type
As for zone or system which has scrutiny load model
for every bus load, the bus load could be divided into
group logically by load type. Then, appropriate growth
ratio is selected for every load type to simulate load vari-
ation of zone or system.
For multi-period evaluation, the same load growth
mode applies for every period generally.
3.3. Zone Level Load Supply Adequacy
Evaluation
For zone level short term load supply adequacy evalua-
tion in every round, the total load growth is allocated to
every bus according to selected load growth mode and
load of other zone remaining unchanged. Then optimize
unit commitment and output with continuous security
constrained generation optimization based evaluation
method to satisfy all the constraints. If feasible genera-
tion schedule is achieved, then increase load and move to
next round evaluation. Otherwise terminate evaluation
process and select load supply adequacy index of last
round as result for evaluated zone. As for same load
growth ratio in every period, the load supply adequacy of
every period is same.
Repeating above evaluation process for every zone,
the load supply adequacy index of every zone could be
achieved.
3.4. System Level Load Supply Adequacy Evalu-
ation
For system level short term load supply adequacy evalu-
ation in every round, the system load growth is allocated
to every bus according to selected load growth mode and
keeping tie line exchange fixed. Then optimize unit
commitment and output with continuous security con-
strained generation optimization based evaluation me-
thod to satisfy all the constraints. If feasible generation
schedule is achieved, then increase load and move to
next round evaluation. Otherwise terminate evaluation
process and select load supply adequacy index of last
round as result for evaluated system. As for same load
growth ratio in every period, the load supply adequacy of
every period is same.
4. Design of Online Multi-level Short Term
Load Supply Adequacy Evaluation System
The functional infrastructure of online short term load
adequacy evaluation system is demonstrated as Figure 2,
the main functions are data management, bus level
evaluation, zone level evaluation, system level evaluation,
sensitivity factor based evaluation, continuous security
constrained generation optimization based evaluation and
parameter management. The continuous generation op-
timization based evaluation module is composed of gen-
eration optimization and security analysis.
Data management module serves for model and opera-
tion data input, which include network model, outage
schedule of generation and transmission devices, trans-
mission capability, system load forecast, bus load fore-
cast, from operation schedule application to form base
case for evaluation. Bus level evaluation module imple-
ments evaluation data preparation, calling of sensitivity
factor based evaluation service , continuous security con-
strained generation optimization based evaluation service
and demonstration of evaluation result. Zone level
Figure 2. Functional infrastructure of online multi-level
short term load supply adequacy evaluation system.

Z.-L. YANG ET AL.
732
evaluation module implements load growth management
of zones, calling of continuous generation optimization
based evaluation service and demonstration of evaluation
result of every zone. System level evaluation module
implements load growth management of system, calling
of continuous generation optimization based evaluation
service and demonstration of evaluation result of system.
Sensitivity factor based evaluation module provides sen-
sitivity calculation service and sensitivity factor based
load supply adequacy evaluation service. Continuous
security constrained generation optimization module
provides multi-objective, security constrained unit com-
mitment and economic dispatch service, provides single
period and multi-periods coupled load supply adequacy
evaluation service and calling of base case power flow
analysis, contingency analysis and sensitivity calculation
service provided by security analysis module automati-
cally. Parameter management module implements load
growth mode management, objective model and con-
straint model management for generation optimization,
and parameter management for security analysis. Visu-
alization of load supply adequacy module demonstrates
load supply adequacy distribution of transmission net-
work by contour and three dimensional bar methods.
Continuous security constrained generation optimiza-
tion based evaluation method not only could apply for
single period and multi-periods zone level evaluation and
system level evaluation for large scale transmission net-
work, but also could apply for bus level evaluation for
small scale transmission network. As for enormous cal-
culation of bus level evaluation, only sensitivity factor
based evaluation method could be use for online evalua-
tion in engineering for large scale transmission network.
For example of provincial power grid with 700 calcu-
lating node, the evaluation time of 96 periods system
level evaluation is about 8 minutes, and the evaluation
time of 96 periods zone level is about 5 minutes by one
ThinkPad computer equipped with 256G hard disk and
6G memory. The performance could meet the require-
ment of online application in engineer.
5. Conclusions
This article presents multi-level, including bus level,
zone level and system level, load supply adequacy evalu-
ation method, which is able to evaluate load supply ade-
quacy of transmission network at any operation status
and provide reliable technical support for coping with
random fluctuation caused by intermittent energy genera-
tion. After analysis of sensitivity factor based evaluation
method, it presents a high performance continuous secu-
rity constrained generation optimization based evaluation
method, which is more feasible in engineering applica-
tion for zone level and system level load supply ade-
quacy evaluation. And provide the functional design of
online multi-level short term load supply adequacy
evaluation system, which could be integrated into opera-
tion schedule applications. Though the evaluation fo-
cuses on load growth mode analysis, same theory could
be used in load decreasing analysis so as to cope with the
influence of bidirectional fluctuation of large scale in-
termittent energy generation.
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