Energy and Power Engineering, 2013, 5, 785-791
doi:10.4236/epe.2013.54B151 Published Online July 2013 (http://www.scirp.org/journal/epe)
Research on Security Assessment Index System for
Operating Reserve in Large Interc onnected Power Grid
Mingsong Liu1, Huadong Sun1, Jian He1, Hengxu Zhang2, Jun Yi1, Jian Zhang1
1China Electric Power Researc h I n s t itute, Beij i n g, China
2Shandong University, Jinan, China
Received April, 2013
ABSTRACT
Optimization and placement of spinning reserve is an important issue in power system planning and operation. System-
atic way for security assessment of operating reserve needs to study. A security assessment index system for operating
reserve in large interconnected power grids is presented in th is paper. Firstly, classification and determination methods
of operating reserve at home and abroad are investigated, and operating reserve is divided into transient state operating
reserve and quasi-steady state operating reserve from the view of security assessment. Secondly, assessment indexes
and optimization methods for transient state operating reserve are studied. Thirdly, optimization model, deterministic
and probabilistic optimization methods for quasi-steady state operating reserve are explored. Finally, some principles
for determination of operating reserve are suggested, and a security assessment index system is put forward. The pro-
posed index system, considering both transient and quasi-steady state, both deterministic and probabilistic methods,
provides a systematic way to assessment and arrangement of operating reserve.
Keywords: Security Assessment Index System; Operating Reserve; Spinning Reserve; Large Interconnected Power
Grid
1. Introduction
Several major blackouts occurred in the world recent
years, causing great economic losses and severe social
influence [1-5]. Insufficient reserve capacity and im-
proper placement of operating reserve are among several
main reasons. Optimization and placement of spinning
reserve is an important issue in power system planning
and operation [6-10]. In China, power load grows rapidly
with the development of economy, while power supply is
far behind demand. Spinning reserve is often insufficient
in period of peak load in many provinces, which is a
great threat to security and stability of power system op-
eration. How to arrange spinning reserve reasonably and
effectively in a large interconnected power grid becomes
a technical problem urgent to be solved.
The arrangement of spinning reserve in China is
mainly according to a certain proportion of overall gen-
eration load [11], which is relatively rough. As regional
interconnected power grids, intermittent renewable en-
ergy and power electronic devices increase, system
characteristics and operation modes of power grids be-
come more and more complicated. Higher requirements
have been put forward to spinning reserve.
Much work has been done on optimization and place-
ment of operating reserve, which is very helpful to power
system planning and operation. However, systematic way
for security assessment of operating reserve needs to
study. A security assessment index system for operating
reserve in large interconnected power grids is presented
in this paper. Firstly, classification and determination
methods of operating reserve at home and abroad are
investigated, and operating reserve is divided into tran-
sient state operating reserve and quasi-steady state oper-
ating reserve from the view of security assessment. Sec-
ondly, assessment indexes for transient state operating
reserve are studied, and optimization methods are intro-
duced. Thirdly, optimization model, deterministic and
probabilistic optimization methods for quasi-steady state
operating reserve are explored. Finally, some principles
for determination of operating reserve are suggested , and
a security assessment index system is put forward. The
proposed index system, considering transient and quasi-
steady state, deterministic and probabilistic methods,
provide a systematic way to assessment and arrangement
of operating reserve.
2. Classification and Determination Methods
of Operating Reserve
2.1. Classification and Determination of
Operating Reserve in China

M. S. LIU ET AL.
786
 Classification of operating reserve
In China, according to its function, operating reserve is
divided into load reserve, contingency reserve and main-
tenance reserve [11]. Load reserve is used to balance
instantaneous load fluctuation and load forecasting errors,
which should be spinning reserve. Contingency reserve,
activated in a certain period after disturbance, is used to
compensate large active power imbalance caused by con-
tingencies, part of which should be put into operation
automatically when system frequency decreases. Main-
tenance reserve meets the requirements of maintaining all
generato rs per iodicall y.
Spinning reserve usually consists of the following
generators:
1) Generators that are already put into operation,
without reaching their operating limits, including thermal
power generators, hy dro power gen erators, et c .
2) Hydro power generators that can be put into opera-
tion quickly, usually within several minutes.
 Determination of operating reserve
In China, operating reserve is arranged according to a
certain proportion of maximum generation load [11].
Load reserve is 2%-5% of maximum generation load.
Lower proportion is suitable for large systems, while
higher proportion is for small systems. Contingency re-
serve is about 10% of maximum generation load, which
should be greater than the largest generator of a system.
Maintenance reserve is about 8%-10% of maximum gen-
eration load.
2.2. Classification and Determination of
Operating Reserve in America
 Classification of operating reserve
In America, according to its function, operating re-
serve is divided into regulating reserve, contingency re-
serve, additional reserve for interruptible imports, and
additional reserve for on-demand obligations [12].
Regulating reserve is similar to load reserve in China,
and the same is contingency reserve. Additional reserve
for interruptible imports and on-demand obligations
should be made effective within ten minutes, which meet
interruptible imports and on-demand obligations to other
entities or balancing authorities.
Operating reserve consists of spinning reserve and
nonspinning reserve. Spinning reserve means unloaded
generation which is synchronized and ready to serve ad-
ditional demand. Nonspinning reserve means that oper-
ating reserve not connected to the system but capable of
serving demand within a specified time, or interruptible
load that can be removed from the system in a specified
time.
 Determination of operating reserve
In America, operating reserve is arranged in a deter-
ministic way [13]. Regulating reserve is to provide suffi-
cient regulating margin to meet NERC's control per-
formance criteria, while contingency reserve is to meet
NERC disturbance control standard, considering the most
severe single contingency and a certain proportion of
hydro, thermal generation load. The combined unit ramp
rate of each balancing authority's on-line, unloaded gen-
erating capacity must be capable of responding within
ten minutes.
2.3. Classification and Determination of
Operating Reserve in Europe
 Classification of operating reserve
In Europe, according to its function, operating reserve
is divided into primary control reserve, secondary control
reserve, and tertiary control reserve, as shown in Figure
1 [13]. Primary control reserve is activated within sec-
onds, while secondary control reserve is activated within
minutes. Tertiary control reserve implies changes in gen-
eration or load on a contractual, market or regulatory
basis, activated for a period of time, e.g. 15 minutes.
 Determination of operating reserve
In Europe, operating reserve is determined as follows
[13]. Primary control reserve, proportionally distributed
among each control area, is agreed to be 3000 MW,
which is the maximum instantaneous power deviation for
the UCTE synchronous area, based on operational char-
acteristics concerning system reliability and size of load s
and generation units. Secondary control reserve must be
available to cover expected demand and generation fluc-
tuations. A total tertiary control reserve must be available
to cover the largest expected loss of power in the control
area.
Figure 1. Principle frequency deviation and subsequent activation of reserves.

M. S. LIU ET AL. 787
2.4. Classification of Operating Reserve
According to Response Rate
There are some other kinds of classification of operating
reserve. In [14], reserve is divided into 10-minute spin-
ning reserve, 10-minute nonspinning reserve, 30-minute
reserve, 60-minute reserve and cold reserve. In [15], re-
serve is divided into instantaneous reserve, fast reserve,
slow reserve and cold reserve. It is obvious that response
rate is the fundamental characteristics of operating re-
serve. Therefore in power market, it is prone to divide
operating reserve into instantaneous reserve, 10-minute
spinning reserve, 10-minute nonspinning reserve, 30-
minute reserve, 60-minute reserve and cold reserve, as
shown in Table 1.
2.5. Classification of Operating Reserve from the
View of Security Assessment
Instantaneous reserve takes effect during transient state
process immediately after a contingency, which affects
system frequency deviation much. While other kinds of
operating reserve take effect during quasi-steady state
process, usually at least several minutes after a distur-
bance, which mai nl y affect power flow regu l a t ion.
From the view of security assessment for power sys-
tem operation, operating reserve can be divided into
transient state operating reserve and quasi-steady state
operating reserve. Transient state operating reserve con-
sists of instantaneous reserve, while quasi-steady state
operating reserve consists of other kinds of operating
reserve.
3. Transient State Operating Reserve
3.1. System Frequency Response after Loss of
Generation
System frequency response after loss of generation is
introduced in [16]. When a large generator trips, the dis-
turbance spreads to each generator immediately. Due to
dead zone and delay block, generator governors do not
Table 1. Classification of reserve according to response
rate.
Reserve Response Time Synchronous
instantaneous reserve several seconds Yes
10-minu te spinning reserve <10min Yes
10-minut e nonspinning reserve <10min No
30-minute reserve (10min, 30min) No
60-minute reserve (30min, 60min) No
cold reserve >60min No
take effect at once. System frequency decreases sharply.
Several seconds later, governors start regulating, and
active power of each generator is adjusted according to
its moment of inertia and system frequency deviation.
Ten or more seconds later, primary frequency control is
over, but system frequency does not recover to normal
value. Several minutes later, AGC takes effect, restoring
system frequency to normal value.
3.2. Assessment Index for Transient State
Operating Reserve
Dynamic characteristics of system frequency and assess-
ment index for system frequency and spinning reserve
have been studied [17-23]. There are mainly three kinds
of index shown as follows.
 Transient frequency deviation index
Overall spinning reserve capacity affects system
frequency response after loss of active power. On the
other hand, transient frequency deviation reflects overall
spinning reserve capacity. If overall spinning reserve
capacity is small, transient frequency deviation becomes
large. If overall spinning reserve capacity is large,
transient fr equency deviati on becom e s small.
There are several indexes for transient frequency
deviation shown as follows.
1) Maximum transient frequency deviation. This index,
intuitive and easy, is widely adopted in power system
analysis.
2) Transient frequency deviation index using two-
element table [18]. A set of two-element tables, made up
of transient frequency deviation and its duration, is used
to describe transien t frequ ency dev iatio n accep tab ility for
individual bus or gener ator.
3) Transient frequency deviation index considering
cumulative effect [19]. The index, defined by integration
of frequency deviation and its duration, is used to
quantitatively assess transient frequency deviation
security by taking into account the frequency deviation
cumulative effect．
 Transient frequency time-space distribution index
After a disturbance causing large active power
imbalance, transient frequency presents time-space
distribution features. Since network structure, load level,
generation capacity, spinning reserve capacity, dynamic
model parameters are different among areas, transient
frequency deviation may be different at different
locations.
Some transient frequency time-space distribution
indexes are used in [21], shown as follows.
1) Mean changing rate of frequency. Shortly after a
disturbance, the frequency response curve is usually
linear. The index reflects type and severity of the
disturbance and information of network and generators.

M. S. LIU ET AL.
788
2) Maximum transient frequency deviation at different
locations. The index is used to assess influence degree on
different locations by the disturbance.
3) Transient frequency response delay at different
locations. This index reflects frequency spreading
process.
 Response rate of generators in primary frequency
control
After a large disturbance of active power, different
generators may response in different rates. Generators of
different types, years and capacity have different re-
sponse rate. Generally speaking, thermal generators of
new type, with large capacity response fast. The faster
the generators with reserve capacity response, the less the
maximum transient frequency deviation is, and the
shorter the duration to maximum deviation is. Response
rates of generators in primary frequency control are re-
lated to difference coefficients and dead blocks of their
governors [17, 24].
3.3. Optimization of Transient State Operating
Reserve
In [22], an optimization method for spinning reserve,
considering transient frequency security, is proposed. The
algorithm is separated into two steps. Critical spinning
reserve capacity is calculated based on sensitivity
analysis. Then critical spinning reserve capacity is put
into optimization model as a constraint, and th e model is
solved using priority list method.
In [23], an optimization meth od in which the transient
frequency deviation of power system is taken into
account is proposed. The maximum active power of each
generation unit that is allowed to undertake during
primary frequency regulation is calculated. Then both
spinning reserve cost and moment of inertia of each
generation are comprehensively considered to give the
optimization configuration scheme of spinning reserve.
4. Quasi-Steady State Operating Reserve
4.1. Optimization Model of Quasi-steady State
Operating Reserve
When quasi-steady state operating reserve takes effect, a
system is usually in a quasi-steady state. Transient state
process can be ignored, and it becomes a problem of
power flow. Optimization of quasi-steady state operating
reserve is a problem of optimal power flow with several
constraints, as shown in (1).


min
=0
..
FX
GX
st
A
XB
(1)
The objective function


F
X represents network
loss, generation cost, reserve procurement cost, social
cost, or some other kinds of cost. The objective is to
minimize give n ki n d of cost.
The equality constraint represents power
flow equations.

=0GX
The inequality constraint
A
XB represents a set of
constraints, shown as follows.
1) Constraints of generator operation limits.
2) Constraints of generator reserve capacity.
3) Constraints of generator ramp rates.
4) Constraints of line curren t limits.
5) Constrains of section transfer capab ility.
6) Constrains of overall reserve capacity.
7) Constrains of area reserve capacity limits.
8) Constrains of system stability.
These inequality constraints can be used as security
assessment indexes for quasi-state operating reserve.
4.2. Deterministic Optimization Methods
In traditional power industry, generation, transmission
and distribution belong to one power company. Reserve
capacity is provided and used by the same company.
Therefore economic affairs are easy to be coordinated
within the compan y. Power supply reliability and system
operation security are the most important issues. Spinning
reserve capacity is usually determined according to a
certain proportion of maximum generation load, or the
largest generator of a system. These methods are easy
and widely used. However, they are not economically
optimal.
Some optimization methods are studied to minimize
reserve capacity, reserve procurement cost, etc. In [25],
optimal reserve model of interconnected regional power
systems is built up. The adaptive immune genetic algorithm
is used to calculate the optimal reserve capacity, taking
into account offset frequency, load fluctuation and security
reliability constraints of the regional inter- connected
power system. In [26], two approaches are adopted, the
independent modeling of multi-step optimization and
unified modeling of joint optimization for daily spinning
reserve and generation scheduling. Different energy-
saving generation dispatching models are established.
4.3. Probabilistic Optimization Methods
Power supply reliability is a most important issue for
both traditional power industry and power markets.
Security and adequacy are two aspects of reliability.
Probabilistic indexes, such as loss of load probability and
value of lost load, are often used to evaluate system reli-
ability.
Much has been done on probabilistic optimization
methods of spinning reserve. In [27], a stochastic variable
called spinning reserve's gain or loss is defined, to

M. S. LIU ET AL. 789
analyze the risks of spinning reserve scheme from the
prospective of power generating system. A utility
expectation decision-making model is proposed, reflect-
ing both the objective risk of a spinning reserve scheme
and the decision-maker's attitude to risks. In [28], a
method and standards of shareable operational reserve
reliability evaluation for multi-area power networks are
proposed. Such random factors as the constraints of tie-
lines, the status of on-line generations, loss of network
elements, load fluctuations are cons idered. The reliability
of each area and the whole power system are evaluated
separately. In [29], an extended state-space partitioning
based method to assess operating reserve of power grid
connected with large-scale wind farm is proposed.
Multi-state model of wind turbine generator is adopted,
and the total state space, divided into two subspaces, can
be chosen by state enumeration method and Monte Carlo
simulation respectively. In [30], a mathematical model
for deploying distributed generation as reserve is built u p.
Loss of load prob ability is used to evaluate the reliab ility
of power supply, and different optimal deployment
scheme for distributed generators are given under
different reliability targets.
5. Security Assessment Index System for
Operating Reserve
5.1. Some Principles for Operating Reserve
Requirements of operating reserve capacity are closely
related to system scale, network structure, characteristics
of load and generation, etc. It is better to make system-
atic assessment for a power grid before arranging its re-
serve capacity, instead of using a certain proportion roughly.
When a DC line with large active power is connected
to a power grid, severe impacts on the grid may be
caused if an outage occurs in one pole of the DC line.
Spinning reserve capacity needs improving, according to
frequency response characteristics after the outage.
When large amount of wind power is connected to a
power grid, wind power, with characteristics of fluctua-
tion and intermittence, requires even higher spinning
reserve capacity. Spinning reserve capacity needs im-
proving, and the spinning reserve for wind power should
be close to wind power.
Transient frequency deviation is close related to active
power disturbance. For a power grid, the active power
disturbance should be maximum active power imbalance
considering N-1 contingencies. However, if control
measures are designed for a contingency, it should be
analyzed separately.
5.2. Recommended Security Assessment Index
System
A recommended security assessment index system for
operating reserve in large interconnected power grid is
shown in Table 2.
For transient state operating reserve, deterministic as-
sessment methods are mainly adopted. Transient fre-
quency deviation indexes, such as maximum transient
frequency deviation, index using two-element table and
index considering cumulative effect, are used to assess
overall spinning reserve capacity. Transient frequency
time-space distribution indexes, such as mean changing
rate of frequency, maximum transient frequency devia-
tion and transient frequency response delay at different
locations, are used to assess spinning reserve distribution
characteristics. Response rate of generators in primary
frequency control is used to assess spinning reserve re-
sponse.
For quasi-steady state operating reserve, both determi-
nistic and probabilistic methods are adopted. Indexes
such as available transfer capability and generator ramp
rates are used to assess security of operating reserve.
Indexes such as loss of load probab ility and value of lost
load are used to assess operating risk considering operat-
ing reserve.
6. Conclusions
Optimization and placement of spinning reserve is an
important issue in power system planning and operation.
Classification and determination methods of operating
reserve at home and abroad are investigated at first, and a
new classification of operating reserve from the view of
security assessment is presented. Then assessment in-
dexes and optimization methods for both transient state
and quasi-steady state operating reserve are studied. At
last a security assessment index system for operating
reserve in large interconnected power grids is put for-
ward. The proposed index system takes into account
Table 2. Recommended security assessment index system.
Reserve TypeAssessment
Methods Assessment Object Assessment Indexes
Capacity Transient frequency
deviation index
Distribution Transient frequency time-space di s tribution
index
Transient
state
operating
reserve
Deterministic
Response Response rate of
generators in primary
frequency control
DeterministicSecurity Available transfer capability, generator
ramp rates, etc.
Quasi-steady
state
operating
reserve Probabilistic Operating
risk loss of load probability, value of lost load, etc.

M. S. LIU ET AL.
790
transient and quasi-steady state, deterministic and prob-
abilistic methods, and provides a systematic way to as-
sessment and arrangement of operating reserve.
REFERENCES
[1] Y. Xue and S. Xiao, “Comprehensively Defending High
Risk Events with Low Probability,” Automation of
Electric Power Systems, Vol. 35, No. 8, 2011, pp. 1-11.
[2] G. Andersson, P. Donalek, R. Farmer, et al., “Causes of
the 2003 Major Grid Blackouts in North America and
Europe, and Recommended Means to Improve System
Dynamic Performance,” IEEE Transactions on Power
Systems, Vol. 20, No.4, 2005, pp. 1922-1928.
doi:10.1109/TPWRS.2005.857942
[3] Y. Tang, G. Bu and J. Yi, “Analysis and Lessons of the
Blackout in Idian Power Grid on July 30 and 31, 2012,”
Proceedings of the Chinese Society for Electrical Engi-
neering, Vol. 32, No. 25, 2012, pp. 167-174.
[4] W. Lin, Y. Tang, H. Sun, et al., “Blackout in Brazil
Power Grid on February 4, 2011 and Inspirations for
Stable Operation of Power Grid,” Automation of Elec-
tric Power Systems, Vol. 35, No. 9, 2011, pp. 1-5.
[5] Y. Fang, “Lessons from September 8, 2011 Southwest
America Blackout for Prevention and Control of Cas-
cading Outages,” Automation of Electric Power Systems,
Vol. 36, No. 15, 2012, pp. 1-7.
[6] X. Shu, Z. Ming, L. Gengyin, et al., “On Spinning Re-
serve Determination and Power Generation Dispatch
Optimization for Wind Power Integration Systems,” in
Power and Energy Society General Meeting, 2012, pp.
1-7.
[7] M. Q. Wang, H. B. Gooi and S. X. Chen, “Optimising
Probabilistic Spinning Reserve using an Analytical Ex-
pected-energy-not-supplied Formulation,” Generation,
Transmission & Distribution, IET, Vol. 5, No.7, 2011,
pp. 772-780. doi:10.1049/iet-gtd.2010.0805
[8] A. Hoballah and I. Erlich, “Dynamic Stability and Net-
Work Constrained Optimal Spinning Reserve Alloca-
tion,” in Power and Energy Society General Meeting,
2011, pp. 1-5.
[9] C. Wang, Y. Qiao and Z. Lu, “A Method for Determina-
tion of Spinning Reserve in Wind-thermal Power Sys-
tems Considering Wind Power Benefits,” Automation of
Electric Power Systems, Vol. 36, No. 4, 2012, pp. 16-21.
[10] G. Zhao, T. Liu and X. Li, “Optimal Deployment of
Distributed Generation as Backup Generators,” Automa-
tion of Electric Power Systems, Vol. 33, No. 1, 2009,pp.
85-89.
[11] “Guide on Security and Stability for Power System,” DL
755-2001, Beijing: China Electric Power Press, 2001.
[12] NERC, “Operating Reserves,” WECC Standard
BAL-STD-002-0, 2007.
http://www.nerc.com/page.php?cid=2|20
[13] ENTSO-E, “Load-Frequency Control and Performance
[C],” UCTE OH – Policy 1, 2009.
https://www.entsoe.eu/publications/system-operations-r
eports/operation-handbook/
[14] J. Wang, “Study on Reserve Capacity in Power Market,”
Ph.D. Thesis, Xian Jiaotong University, Xian, 2003.
[15] Y. Lai, Y. Xue, D. Wang, et a1., “Risk Deci-
sion-marking for Reserve Capacity Market,” Automation
of Electric Power Systems, Vol. 30, No. 16, 2006, pp.
1-5.
[16] B. Cai, “Power System Frequency,” Beijing: China
Electric Power Press, 1998.
[17] H. Zhou, L. Ni and T. Xu, “Study on Power-Frequency
Dynamic Characteristic of Power Grid,” Power System
Technology, Vol. 33, No. 16, 2009, pp. 58-62.
[18] T. Xu and Y. Xue, “Quantitative Assessments of Tran-
sient Frequency Deviation Acceptability,” Automation
of Electric Power Systems, Vol. 26, No. 19, 2002, pp.
7-10.
[19] H. Zhang, Y. Liu and Y. Xue, “Quantitative Assessment
of Transient Frequency Deviation Security Considering
Cumulative Effect,” Automation of Electric Power Sys-
tems, Vol. 34, No.24, 2010, pp. 24.
[20] H. Zhang, and Y. Liu, “Disturbance Identification Con-
sidering Differences in Frequency Deviation Propaga-
tion Speed,” Automation of Electric Power Systems, Vol.
34, No. 22, 2010, pp. 62-66.
[21] Z. Hou, “Dynamic Characteristic Analysis of Power
System Frequency,” Master Thesis, Shandong Univer-
sity, Jinan, 2012.
[22] M. Liu, J. He, H. Zhang, et al., “Study on Optimization
and Implementation of Spinning Reserve in Large In-
terconnecting power grid,” China Electric Power Re-
search Ins titute, 2011.
[23] M. Liu, H. Sun and J. He, “An Optimization Method for
Spinning Reserve in Primary Frequency Control Con-
sidering Transient Frequency Deviation,” Power System
Technology, Vol. 35, No. 8, 2011, pp. 129-133.
[24] Q. Zhao, L. Wang, Z. Liu, et al., “Study on Dynamic
Frequency Characteristics and Coordinative Un-
der-Frequency Load Shedding Scheme for Nationwide
Interconnected Power Grid of China,” Power System
Technology, Vol. 33, No. 8, 2009, pp. 35-40.
[25] J. Wu, M. Shi, G. Chen, et al., "Immune Genetic Algo-
rithms for Modeling Optimal Reserve Capacity of In-
terconnected Regional Power Systems,” Proceedings of
the CSEE, Vol. 29, No. 1, pp. 14-20, 2009.
[26] G. Xie, L. Zhang and J. Ge, “Optimization of Spinning
Reserve Scheduling of Energy-saving Generation Dis-
patching,” Automation of Electric Power Systems, Vol.
33, No. 13, 2009, pp. 43-46.
[27] X. Qi and M. Ding, “Risk Analysis and Utility Deci-
sion-making of Spinning Reserve Scheme in Power
Generating System,” Automation of Electric Power Sys-
tems, Vol. 32, No. 3, 2008, pp. 9-13.
[28] Y. Jia, Z. Yan, Y. Sun, et al., “Shareable Operational
Reserve Reliability Evaluation for Multi-area Power
Networks,” Automation of Electric Power Systems, Vol.
33, No. 16, 2009, pp. 13-18.
[29] J. He, H. Sun and M. Liu, "Extended State-Space Parti-

M. S. LIU ET AL.
791
tioning Based Operating Reserve Risk Assessment for
Power Grid Connected With Wind Farms,” Vol. 36, No.
3, 2012, pp. 257-263.
[30] G. Zhao, T. Liu and X. Li, “Optimal Deployment of
Distributed Generation as Backup Generators,” Automa-
tion of Electric Power Systems, Vol. 33, No. 1, 2009, pp.
85-89.