ORIGINAL RESEARCH Open Access
Analysis of voltage stability uncertaintyusing stochastic response surface methodrelated to wind farm correlationZhaoxing Ma1* , Hao Chen2 and Yanli Chai3
Abstract
Wind speed follows the Weibull probability distribution and wind power can have a significant influence on power systemvoltage stability. In order to research the influence of wind plant correlation on power system voltage stability, in this paper,the stochastic response surface method (SRSM) is applied to voltage stability analysis to establish the polynomial relationshipbetween the random input and the output response. The Kendall rank correlation coefficient is selected to measure thecorrelation between wind farms, and the joint probability distribution of wind farms is calculated by Copula function. Adynamic system that includes system node voltages is established. The composite matrix spectral radius of the dynamicsystem is used as the output of the SRSM, whereas the wind speed is used as the input based on wind farm correlation. Theproposed method is compared with the traditional Monte Carlo (MC) method, and the effectiveness and accuracy of theproposed approach is verified using the IEEE 24-bus system and the EPRI 36-bus system. The simulation results also indicatethat the consideration of wind farm correlation can more accurately reflect the system stability.
Keywords: Power system, SRSM, Correlation, Wind farm
1 IntroductionAround the world, power systems have witnessed increasedamount of renewable and dispersed generation, especiallywind power and solar power. Renewable energy is a usefulsupplement to traditional energy sources, but is differentfrom the traditional form of energy because of its uncer-tainty and intermittency. The renewable energy is con-nected to power grid by concentrated form or distributedform, bringing many uncertainties to power system voltagestability as well as new problems and challenges to re-searchers. If system voltage stability is evaluated in the mostsevere working model for studying, the results are often tooconservative, and therefore, a new effective way should beinvestigated. This paper propose a method to investigatethe impact of stochastic uncertainty of grid-connected windpower generation on power system voltage stability bystructure dynamic systems that include node voltages.The impact of stochastic power injections on power flows
and voltage profile is a widely studied topic since the 1970s
[1]. The probabilistic analysis was firstly introduced intostudying power system small signal stability by Burchettand Heydt in [2]. A series of work later on [3–6] have fur-ther improved the various aspects of the analytical methodof power system probabilistic small signal stability. In [7, 8],a method of probabilistic analysis was proposed to directlycalculate the probabilistic density function of critical eigen-values of a large scale power system from the probabilisticdensity function of gird connected multiple sources of windpower generation to investigate the impact of stochastic un-certainty of grid-connected wind generation on power sys-tem small-signal stability [9]. Reference [10] presented acomparative analysis of the performance of three efficientestimation methods when applied to the probabilistic as-sessment of small-disturbance stability of uncertain powersystems. In [11], an analytical approach was proposed to in-volve the effects of correlation of wind farms in probabilis-tic analytical multi-state models of wind farms outputgeneration. Reliability models of wind farms consideringwind speed correlation are proposed in [12].In this paper, power system voltage stability is analyzed
using the stochastic response surface method (SRSM).The algebraic equations that contain the voltages are
* Correspondence: [email protected] of Automation Engineering, Qingdao University of Technology,Qingdao 266033, ChinaFull list of author information is available at the end of the article
Protection and Control ofModern Power Systems
© The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, andreproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link tothe Creative Commons license, and indicate if changes were made.
Ma et al. Protection and Control of Modern Power Systems (2017) 2:20 DOI 10.1186/s41601-017-0051-3
http://crossmark.crossref.org/dialog/?doi=10.1186/s41601-017-0051-3&domain=pdfhttp://orcid.org/0000-0003-4833-6630mailto:[email protected]://creativecommons.org/licenses/by/4.0/
converted into a differential system with system nodevoltages. Then, the output of the SRSM is the output ofthe composite matrix spectral radius of the dynamicsystem is constructed, and is used to judge the stabilityof the system voltage. The IEEE 24-node system and theEPRI 36-node system with wind power are considered astwo examples to verify the accuracy of the proposedanalysis method.
2 Discussion2.1 System model analysisIn power system stability analysis, a power system ischaracterized by the set of nonlinear dynamic equationsas:
_x ¼ f x; y; τð Þ ð1Þ
0 ¼ g x; y; τð Þ ð2Þ
where f and g express the system dynamic equations andthe algebraic equations, respectively. x represents thestate variables, y represents the algebraic variables of thenode voltage magnitudes and angles, and τ is the controlparameters.Neglecting the resistances, the algebraic equations that
consist of the algebraic variables of the node voltagemagnitudes and angles can be shown as:
PLi þXmþnj¼1
BijV iV j sin αi−αj� � ¼ ypi ¼ 0 ð3Þ
−QLi þXmþnj¼1
BijV iV j cos αi−αj� � ¼ yQi ¼ 0 ð4Þ
i ¼ nþ 1;⋯; nþm:
where PLi and QLi are the ith node active and reactivepower, respectively. Vi and Vj are the voltage of the ithnode and the jth generator bus, respectively. Bij repre-sents the reactance of the admittance matrix, and αi isthe ith bus phase angle. If 1 ≤ j ≤ n, then αj = δj, where δjis the generator rotor angle of the jth machine. n is thenumber of generator buses and m is the number of loadbuses. PLi and QLi are functions of Vi and αi.Equations (3 and 4) represents a pure dynamic system
and taking derivative of (3, 4) leads to the dynamicquantity _V i and _αi [12, 13] as:
∂yPi�
∂t¼∂yPi
�∂V
_V þ∂yPi�
∂α_αþ
∂yPi�
∂δ_δ¼0
ð5Þ
∂yQi�
∂t¼∂yQi
�∂V
_V þ∂yQi
�∂α
_αþ∂yQi
�∂δ
_δ¼0
ð6Þi ¼ nþ 1;⋯; nþm:
It is more convenient to represent the generatordynamic equations as follows:
_δ i ¼ ωi ð7Þ
Mi _ωi ¼ Pmi− MiMT PCOI−Bi;iþnEgiV iþn sin δi−ψiþn� �
ð8Þ
where δi ¼ δ i−δ0 , ωi ¼ ωi−ω0 , ψi ¼ ψ i−δ0 , MT
¼Xni¼1
Mi , δ0 ¼ 1MTXni¼1
Miδ i , ω0 ¼ 1MTXni¼1
Miωi , PCOI
¼Xni¼1
Pmi−XnþNi¼nþ1
PLi . PLi is the active load at each node
and Pmi is the input mechanical power. δ i and ωi arethe rotor angle and angular speed of the ith machine,respectively. δ0 and ω0 are the centers of angle andangular speed, respectively. Mi and Egi are the ithmachine inertia and internal voltage, respectively. Brepresents the reactance of the admittance matrix. nis the number of generators, Vi+n and ψ iþn are thegenerator bus voltage and phase angle, respectively. Nis the number of non-generator buses in the powersystem.From (5, 6), there is
∂yPi�
∂V_V þ
∂yPi�
∂α_α¼−
∂yPi�
∂δ_δ
ð9Þ
∂yQi�
∂V_V þ
∂yQi�
∂α_α¼−
∂yQi�
∂δ_δ
ð10ÞFrom (9, 10), it yields
A τð Þ B τð ÞC τð Þ D τð Þ
� �_V_α
� �¼ − E τð Þ _δ
F τð Þ _δ:� �
ð11Þ
where, A τð Þ ¼∂yP�
∂V , B τð Þ ¼∂yP�
∂α , C τð Þ ¼∂yQ�
∂V ,
D τð Þ ¼∂yQ�
∂α , E τð Þ ¼∂yP�
∂δ and F τð Þ ¼∂yQ�
∂δ . De-fine τ = (δ′, ω′, V′, α′).
Ma et al. Protection and Control of Modern Power Systems (2017) 2:20 Page 2 of 9
Substituting (7, 8) into (11), and solving the dynamicquantity _V i and _αi yield:
_V_α
� �¼ − A τð Þ B τð Þ
C τð Þ D τð Þ� �−1
E τð ÞωF τð Þω
� �ð12Þ
where V = (Vn+1, …, Vn+m)′, α = (αn+1, …, αn+m)′, ω = (ω1,…, ωn)′, δ = (δ1, …, δn)′. The equation can also beexpressed as:
_x ¼ f x;ωð Þ ð13Þ
where x = (V, α)′.Using (13), a dynamic system can be constructed that
contains power system node voltages. A node voltagestate equation and its Jacobian matrix can then be estab-lished and used to meet the uncertainty of wind powergeneration. The uncertain elements that are included inthe wind power are considered as the input, and a singleelement that can measure the system voltage stability isconsidered as the output. After the application of “blackbox algorithm”, it can analyze the influence of the uncer-tainties on system voltage stability.Setting J the Jacobian matrix of system (13) at the
equilibrium. If the real parts of all the eigenvalues of theJacobian matrix J is negative, the system (13) is stable;otherwise, the system can become unstable. Thus, a newsimple and effective lemma [14] is given as follows:Lemma 1 If the spectral radius ρ(J) of matrix J satisfies
ρ(J)
In the following, a 2-rank expansion with n randomvariables is taken as an example for illustration. The2-rank expansion is given as:
y2 φ1;φ2ð Þ ¼ a0 þ a1φ1 þ a2φ2 þ a3 φ21−1� �
þa4 φ22−1� �þ a5φ1φ2 ð20Þ
For (16), six certainty coefficients need be solved in2-rank output model y2. Thus, the number of the
certainty coefficients ai be solved in the 2-rank modelis 1 + 2n +½(n(n-1)) with n random variables.In applying SRSM, the most important task is to solve
the unknown coefficient ai. Both the probability distri-bution method and the efficient regression method canbe used to solve the unknown coefficient, although, theefficiency of the efficient regression method is better andthe results are more robust. As the input variable quan-tities are φ1i and φ2i, (16) can be represented as:
y2i φ1i;φ2ið Þ ¼ a0 þ a1φ1i þ a2φ2i þ a3 φ21i−1� �
þa4 φ22i−1� �þ a5φ1iφ2i
ð21ÞTo calculate the unknown coefficients ai, some sample
points with forms as (φ1m, φ2m) are required to beselected. In this paper, φ describes the wind speedfollowing probability distribution, and y expresses thedegree of voltage instability by (16) and (17). Thus, therelation that is related to power system is established foranalysis.Equation (17) is the 2-rank expansion model, and the
roots (0,ffiffiffi3
p, −
ffiffiffi3
pof the 3-rank Hermite polynomials
can be selected with a total of nine sample points. If thenumber of random variables is more than 3, the numberof sample points is two times larger than that of the un-known factor, and thus large amount of calculation isrequired. However, the selected sample points are in thestandard normal distribution space, and therefore, it isnecessary to convert them to the original space. Thetransformation of the original space sample points corre-sponds to the real response value, and the unknowncoefficients ai can be obtained using the least squaremethod for solving linear equations.
2.3 Copula theory correlation analysis2.3.1 Copula function definitionAssuming H(·,·) is the joint distribution function of F(·)and G(·) with marginal distribution, there exists a Copulafunction C(·,·) satisfying:
H x; yð Þ ¼ C F xð Þ;G yð Þð Þ ð22ÞThe density function of the distribution function H(·,·)
can be derived by the density function C(·,·) of the
Reduction and transformationto m independent, standard
normal variables
End
start
Collocations of sampling points
Found polynomial chaosexpansion equation
Deduction ofcoefficients of y
Calculation of theexpected value of output
response
Calculation the windfarm correlation
Fig. 1 Flow chart of the analysis method
Fig. 2 The IEEE 24-bus system
Table 1 Parameters of wind farms
Wind farms 1 2
Fans 40 20
Rated capacity (MW) 0.6 1.5
Cut in wind speed (m/s) 4 3
Cut out wind speed (m/s) 22 24
Rated wind speed (m/s) 14 15
Ma et al. Protection and Control of Modern Power Systems (2017) 2:20 Page 4 of 9
Copula function and the edge distribution function F(·)and G(·) as:
h x; yð Þ ¼ c F xð Þ;G yð Þð Þf xð Þg yð Þ ð23Þ
where c u; vð Þ ¼∂C u;vð Þ�
∂u∂v , u = F(x), v =G(y); f(·) and
g(·) are the density functions of F(·) and G(·), respectively.In this paper, wind power output sequences of the twowind power plants are x and y, and their distributionfunctions are F(x) and G(y), respectively. u = F(x), v =G(y). H is the copula function of F (x) and G (y).In this paper, Frank Copula function [19–22] is con-
sidered as the connection function of joint probabilitydistribution of wind farms. The respective distributionfunction and density function of Frank Copula can beexpressed as:
CF u; v; βð Þ ¼ − 1βln 1þ e
−βu−1� �
e−βv−1� �
e−β−1
� ð24Þ
cF u; v; βð Þ ¼−β e−β−1
� �e−β uþvð Þ
e−β−1ð Þ þ e−βu−1ð Þ e−βv−1ð Þ½ �2 ð25Þ
where β is the relative parameter and β ≠ 0. If β > 0, ran-dom variables u and v have positive correlation. If β→ 0,random variables u and v tend to be independent. β
κ ¼ P xi−xj� �
yi−yj� �
> 0n o
−P xi−xj� �
yi−yj� �
< 0n o ð26Þ
as the Kendall rank correlation coefficient, and к∈[−1,1],i ≠ j. P indicates probability of occurrence. If к > 0, ran-dom variables X and Y have positive correlation; if к < 0,the random variables have negative correlation. If к = 0,the correlation between random variables X and Y can-not be determined.Random variable P1 and P2 is defined as the output
rates of the two wind farms, respectively. (p11, p12,…,p1n) and (p21, p22,…, p2n) are the respective sample spaceof random variable P1 and P2, n is the sample size, whichestablishes a one-to-one relationship with p1i and p2i.The relation between the Kendall rank related coeffi-
cient к and the related parameters β of Frank Copulafunction can be expressed as:
κ ¼ 1þ 4βDk βð Þ−1½ � ð27Þ
where Dk βð Þ ¼ kβkZ β0
tk
et−1dt, k = 1.
2.4 Wind power uncertainty analysisThe relationships between active power Pe that is sup-plied by the wind generation source and wind speed vare expressed as [14, 20]:
Pe ¼cþ dv; asvin≤v≤vrP0; asvr≤v≤vout0; others
8<: ð28Þ
where vin and vout are respective cut-in wind speed andcut-out wind speeds, vr is the rated wind speed, Pe is theactive power generated by the wind farm, and P0 is therated active power. c and d are constants.In this paper, the wind speed is assumed to follows a
Weibull distribution and is shown by variation η ofstandard normal distribution as:
v tð Þ ¼ − ln 12þ 1
2gerg
ηffiffiffi2
p� �� � � �1
t
ð29Þ
where gerg is the Gaussian error function.The analysis process with SRSM is expressed in the
next section.
Fig. 6 Cumulative density distribution of voltage instability for correlation κ = 0.257
Table 2 Average error indices of IEEE 24-node system in differentcorrelation
Correlation coefficient Average error
0.162 0.039
0.257 0.041
~
~
~
~~
~ ~ ~
24 25
51
11 2612
2728
13
6
29
34
171819
151420
21
22 30 31 33
873
23
1
9
2
4 5
52
Fig. 7 The single-line diagram of the 36-node system
Ma et al. Protection and Control of Modern Power Systems (2017) 2:20 Page 6 of 9
3 Method3.1 Voltage uncertainty analysis with SRSMAs for actual systems that contains wind power, whosewind speed follows Weibull distribution rather than thenormal distribution, it should convert the wind speed asthe standard normal distribution to analyze the impact ofwind power uncertainty on voltage stability based onSRSM. Some researchers also apply SRSM to analyze un-certainty of power system dynamic simulation [23]. Theflow chat for the calculation and analysis of the uncertaintyof wind power on stability using SRSM is shown in Fig. 1.
4 Results4.1 Case studiesIn the test cases, dispersed wind generation is consi-dered in the IEEE 24-bus system shown in Fig. 2 andtwo wind farms are added into the system at node 1 and7, respectively. In all cases presented below, comparisonsare made to the Monte Carlo (MC) numerical approach.
4.2 Case 1In this test case, the outputs of the two wind farms are in-dependent from each other. In this paper, MC simulated6000 times to verify the accuracy and efficiency of SRSM.
To enable a balanced comparison of the accuracy betweenSRSM and MC, the same number of uncertainties areused for each moment model. The parameters of the twowind farms are shown in Table 1. In study, the mode ofload change is that increase the whole network load si-multaneously, with the load of each node increased by thesame proportion. In the load direction, the load compo-nent of the partial load follows normal distribution.Figure 3 shows the cumulative probability curve ob-
tained by SRSM and MC method for the IEEE 24-nodesystem, and the voltage instability probability of the sys-tem are determined under different load levels.In Fig. 4, the probability density of spectral radius with
380 MW wind power generation is given. The calcula-tion results also show that SRSM has high accuracy thanthe MC method, and it can meet the needs for the prac-tical applications.
4.3 Case 2In this test case, two wind farms are correlated each other.Figures 5 and 6 compare the cumulative probability curveobtained by SRSM and MC for the IEEE 24-node system,and the system voltage instability probability are deter-mined under different load levels.In Figs. 5 and 6, MC was used to verify the accuracy of
the proposed SRSM method in the paper, and is can beseen that SRSM method has good accuracy. Under thecondition of different correlation coefficients, the ave-rage error of voltage instability analysis with SRSM isgiven in Table 2. Calculation results shown that Kendallrank correlation coefficient method in analyzing voltageinstability has good accuracy, and can satisfy the engi-neering requirement.The results of numerical calculation show that wind
farm correlation had a significant influence on systemvoltage stability. According to Figs. 5 and 6, the greaterthe correlation between the wind farms has, the morenoticeable influence on system voltage stability. The
100 150 200 250 300 350 400 450 5000
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Critical power(MW)
Vol
tage
uns
tabi
lity
prob
abili
ty
SRSMMC
Fig. 8 Cumulative density distribution of voltage instability for correlation κ = 0.280
0 50 100 150 200 250 300 3500
0.2
0.4
0.6
0.8
1
Critical power(MW)
Vol
tage
uns
tabi
lity
prob
abili
ty
MCSRSM
Fig. 9 Cumulative density distribution of voltage instability forcorrelation κ = 0.794
Ma et al. Protection and Control of Modern Power Systems (2017) 2:20 Page 7 of 9
simulation results of case 1 and case 2 shown that thesystem voltage may reached an instability state is under-estimated without considering the correlation, that isunderestimate the potential risks. From the simulationresults, it also got that the proposed method can reflectaccurately the system voltage stability as analyze thevoltage stability uncertainty problems.
4.4 Case 3In the test case, dispersed wind generation is consideredin the EPRI 36-node system shown in Fig. 7. Two corre-lated wind farms whit the same parameters as shown inTable 1 are added to the system and are connected atnode 4 and 5, respectively.In this case, MC is simulated 4000 times to verify the
accuracy and efficiency of SRSM. The system referencepower is 100 MW and the 2-rank SRSM polynomial isused for calculation. In the simulation, the load of eachnode is again increased by the same proportion.For correlation coefficient κ = 0.280 and κ = 0.794, the
cumulative density distributions of voltage instability areexpressed in Figs. 8 and 9, respectively.As can be seen from the Figs. 8 and 9, the larger the cor-
relation between the wind farms has, the lower the voltageinstability critical power is, and the probability of instabilityis greater under the same power condition. Under the con-dition of strong correlation, it is also indicated that more at-tention should be paid to the voltage instability problem.
5 ConclusionsThis paper presents a method that establishes a dynamicsystem including node voltage to study power system volt-ages stability incorporating wind farm uncertainty. Ratherthan the eigenvalues of the Jacobi matrix, the criterion ofpower system voltage stability is given by the spectral ra-dius of the composite matrix. In the study process, thecorrelation of wind farms is considered, such that the un-certainty of the wind farms and the analysis method arecloser to actual systems. The proposed method which usesSRSM to study the uncertainty can provide power systemoperators with useful real-time estimation of the powersystem voltage stability with wind power integration.Compared to the traditional methods, e.g. the MonteCarlo method, the proposed one is more efficient.The analysis and the simulation results also shown
that the proposed method has a higher accuracy and hasa good application prospect to actual system operationand stability analysis. The effect of the correlation be-tween multiple distributed energy source on system vol-tage stability will be considered in future research.
AcknowledgementsThis work is supported by project of the Jiangsu Province University NaturalScience Research Foundation (14KJB470003).
Authors’ contributionsZM contributed to the study design and analysis and drafted the manuscript;HC worked on aspects of the study relating to wind farm correlation; YC wasinvolved in data acquisition and revision of the manuscript. All authors haveread and approved the final manuscript.
Competing interestsThe authors declare that they have no competing interests.
Author details1College of Automation Engineering, Qingdao University of Technology,Qingdao 266033, China. 2Jiangsu Electric Power Company, Nanjing 210024,China. 3Jiangsu Normal University, Xuzhou, China.
Received: 6 January 2017 Accepted: 9 May 2017
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AbstractIntroductionDiscussionSystem model analysisStochastic response surface method analysisCopula theory correlation analysisCopula function definitionCorrelation analysis
Wind power uncertainty analysis
MethodVoltage uncertainty analysis with SRSM
ResultsCase studiesCase 1Case 2Case 3
ConclusionsAcknowledgementsAuthors’ contributionsCompeting interestsAuthor detailsReferences