Algorithm Study and Software Design of District Grid On-Line Risk Assessment Based on Fuzzy Theory

In the background of the design and construction of Smart Grid Operation Supporting System for District Power Networks, this paper established the weighted fault probability model of the overhead line which is based on equipment operating status, utility theory and fuzzy theory. In this model, the proper membership function is adopted to describe the influence of lightning, wind speed, line ice and temperature, and the outage rate of overhead line, derived from historical statistics, is amended. Based on this model, the power supply risk analysis software is developed to calculate the online risk indicators of district grid, and provide real-time decision support information based on risk theory for scheduling operations personnel.


Introduction
The failure of power system is a collection of possibility and severity.When analyzing power system faults, the traditional EMS only took the most likely contingency list, and also did not quantify the effect.In contrast, online operational risk analysis is more scientific, which can not only reflect possibility of contingency, but also severity of power system faults by establishing risk indices [2].Regional grids are mostly radial network structure.When faults occur, power grid splitting or loss of load would happen, so it is very essential to study online risk assessment model and algorithm applicable to regional grid, which can assist dispatchers making security decision, guarantee sufficient security margin and make full use of power system transmission capacity.It can be a security barrier for power system safety and stability operation.
This paper aims to design online risk analysis software of power supply for the district grid based on the realtime possibility model of elements' faults, and using the fuzzy theory to deal with the uncertainties factors.Otherwise, it can provide real-time risk-based decision-making information for dispatchers.

Definition of Operation Risk Assessment
The basic definition of operational risk assessment of power system is: giving comprehensive measurement of possibility and severity of uncertainty power system faces to [1].Risk-based security assessment describes possibility of contingency by probability, and denotes the severity of failure by severity function.Then we can get the quantitative risk indices by integrating the two aspects.
The basic formula for calculating risk indices is: Here: f X is power system's operational status; is the ith failure and S E denotes the severity level of Ei under the operational status of f X , is a risk index of the operational status ( )

Contents of Power System Risk Assessment
Figure 1 shows main content of domestic and foreign study currently about power system operational risk assessment and relationship between them.Risk-based security assessment can be divided three categories: element-level risk assessment, system-level risk assessment

Risk Assessment Methods of Power System
Re ission e-scale grids, regional grid is closedlo rom 10 kV/6 kV to 66 kV source, including large grid and a va nes; ble lines; op or w increasing capacity of single load and dy ct grid equipments lack some data acquisi assessment is mainly used for dispatching de

Status Selection
ion for planning department se-

Network Modeling
traditional reliability-based etwork.The fu risk-based decision optimization.The method of power system risk e cludes 4 steps: determining ① component outage models; ② selecting system states and calculating their probabilities; ③ evaluating the consequences of selected system states; ④ calculating the risk indices.
According to different objects' characteristic, different risk assessment methods should be adopted [5].For simple systems, there are 4 fundamental approaches: the probability convolution, series and parallel networks, Markov equations, and frequency-duration approaches.For a large-scale and complex system, risk assessment methods include status enumeration and Monte Carlo simulation.The latter can be divided into sequential and non-sequential sampling method.

Algorithm Design of Online Power Supply
Risk Assessment for Regional Grid gional grid is a composite generation and transm system, whose risk assessment includes 4 main aspects: determination of component failure and load curve model, selection of system status, identification and analysis system problems, and calculation of reliability indices.Both the status enumeration and Monte Carlo simulation can be applied to regional grid risk evaluation.The two methods use different approaches to select system status and have different forms of formulas to calculate risk indices.The techniques of identifying and analyzing problems in system status are the same.These include power flow calculation and expected contingency analysis for problem recognition and the optimal power flow for remedial actions.
Compared with larg op design but open-loop operation, and has more complex wiring modes and operation modes.In this paper, we study the grid between 500kV substation and the load supplied by the substation directly or indirectly.The characteristics are as follows [3]: 1) Complicate voltage level, f / 220 kV/500 kV; 2) Various power riety of distributed power source; Online risk partment, only considering steady analysis.Based on present research, the calculation process of online power supply risk analysis for regional grid is illustrated in Figure 2, which doesn't consider the impact of human decision.
Operational risk evaluat lects system status with state enumeration or Monte Carlo simulation method.The software in this paper is based regional smart grid dispatching technical supporting system, so it can get operation state by fully using telemetry and teleindication data.The probability of present system status is 1.
It is not suitable to copy "branch-bus model" when modeling power network structure, which is feasible for planning and designing department when approximate model (node model), based on improved state estimation, we obtain network computing model (bus model) by network connectivity analysis, which can be changed with switch state so it can meet the demand of real-time condition.
Regional grid is high voltage distribution n ndamental task of regional grid dispatching is to assurance grid's security, economic, high-quality operation, to guar needs ponent Failure Models controlled by regional gri ly include overhead lin ime Reliability Model D head lines htning and Line Icing is fu antee the interests of users, to adapt to the of economic construction and people's living, in particular to ensure safety and sustainable power supply for high-risk customers and important users.However, lots of high risk customers and important users connect to grid by low-voltage side, so under current technology, it is difficult to get detailed physical model of the whole grid by SCADA, especially low-voltage side grid.Corresponding to the network which can not model by SCADA, we developed associated model interface of high risk customers and key users by introducing database and visualization technology and making fully use of staff report and statistical data in order to assurance the model of customers in low-voltage side more closely to reality.Associated modeling refers to obtain information of devices being associated by the information of associated devices.Here, associated devices are the devices with telemetry and teleindication data, and the devices being associated are high-risk and important customers.

1) Com
The generation unit capacity d EMS is generally small, so we utilize the two-state (up and down) model as the failure model of generating units, not considering dated states.
Transmission components main es, cables, transformers, capacitors, and reactors.In general, these components are presented using two-state (up and down) model.
2) Component Real-T uring the operation of the power system, over operation conditions are more complex and most severely affected by uncertain factors such as climate environmental and so on, which have different influence characteristics to the overhead lines.In this paper, a method of dealing with uncertainty information based on the fuzzy theory was adopted of appointment, and combined with the actual operation conditions of the power system; the overhead line fault probability model is established.That the failure rate of the overhead lines is the overhead line outage probability multiplied by a correction factor of the weather on the outdoor component outage probability impact.Weather factors affect the rate of overhead line fault considering temperature, wind speed, lightning and Line Icing.
Temperatures, wind speed, lig zzy uncertain factors, which are different from random factors, there is no exact probability distribution and classical probability statistical methods can not be used to describe it.The fuzzy set theory introduced by Zadeh Professor is a powerful tool to deal with and descript the fuzzy uncertain factors.The fuzzy set allows for the description of concepts in which the boundary is not sharp.Besides, a fuzzy set concerns whether an element be-longs to the set and to what degree it belongs.It does not consider the situations where elements do not belong to.As a result, the range of fuzzy set is in [0, 1].A fuzzy set is mathematically defined by Zadeh as: where is the membership function of For the fuzzy set A, the value of the membership functio lishing the membership function in unction of lightning impact tor to de of lightning disasters impact on in A, and X is the universe of objects with elements x.In the case of the classical "crisp" set A, membership of x in A can be viewed as a characteristic function that can obtain two discrete values: n can be anywhere between 0 and 1, making it different from a crisp set.Membership function of a fuzzy set expresses to what degree the value of x is compatible with the concept of A.
The method of estab clude weighted method, fuzzy statistics, expert scoring method, interpolation ,standard function method and so on.There is strong uncertainty to the impact of climate change for overhead lines running.In this paper, based on the long-term experience of dispatcher to judge for these types of environmental factors and determine the membership function.
a) The membership f The density of lightning is an important indica termine the lightning degree of a region.Lighting Location System (LLS) can automatically measure and record the density of lightning.Lightning protection design standards also adopt lightning density as a reference.The membership function of the lightning effects identified here as shown in Figure 3: The membership function overhead lines running as: where in (4) a and b is the lightning density threshold determined according to the experiences of the dispatching personnel, In other words, it does not affect while the lightning density is less than the lower limit threshold value a, and the influence coefficient is 0, otherwise, higher than the high limit threshold value b is considered a greater impact, influence coefficient is 1.
b) The membership functions of wind speed and line Ic d speed can be obtained by the meteorological de he wind speed, in μ2 (x), A is the impact threshol membership function of temperature impact on ov ure forecast information can be obtained by perature impact on ov ing Win partment forecast; while ice thickness for the line, air humidity, temperature and wind size the extent of ice damage has a larger impact, not yet theoretical or empirical model to predict the extent of ice cover based on meteorological conditions, we use the actual ice thickness measurement indicators to assess the severity of the ice storm.Wind speed and line of ice thickness with the fall line health density similar to lighting, the same form of the membership function μ2(x), μ3(x), shown in Fig- ure 4: For t d value determined according to the experiences of the dispatching personnel, b is the critical value determined catastrophe occur; Line Icing μ3 (x), a and b are respectively the upper and lower critical value of ice thickness impact.
c) The erhead lines The temperat contact with the meteorological department.Within the normal temperature range, the temperature did not affect the line running, so the value is set at 0; when the temperature is too low or too high, its influence is large, and the function value is set to 1.The membership function shown in Figure 5 below: The membership functions of tem erhead lines as: where in formula 5, a, b, c and d determined according to e segmentation, the fuzzy number ve the experiences of the dispatching personnel are the impact threshold value that air temperature impact on overhead lines running.
Any overhead lin ctor composed by the membership function of various impact factors is determined as follows R [ , , , ] r r r r where r i1 , r i2 , r i3 and r i4 successively corresponds the fuzzy numbers that lightning, wind speed, line icing and temperature affect the segment i line outage probability , Definition B = [B1, B2, B3, B4] as the weight coefficients of line fault outage rate considering four influence factors, then: where, i  in is the impact factor of the i-th overhead lines consider g the four influential factors.Set the outage rate of overhead lines acquired from historical statistics is  , and after correction by the influencing factors of ove ead line outage rate is λ i , then: rh

Selection and Analysis of Expected Fault
We can fault collection by integrat-

Collection
obtain the expected ing the following three ways: 1) Scanning the whole grid by N-1; 2).Scanning the special operation mode with potential power supply risk, such as single line to single substation, single power source to substation, then it can be got the fault group which would influence the security of power supply; 3) Defining the fault group by experienced dispatchers and operation analysts via visual manmachine interface; The expected fault set formatted by 1) and 2) has incl requirements for co ally ha

Conclusions
to determine the possibility of com-uded most accidents of high frequency and high risk.3) is only as a necessary complement, which can reduce the manual workload and maintenance.
Online calculation software has high mputing speed.In paper [6], the author combined fault enumeration and probability sampling method, and improved computing speed by parallel computing in the foreground and background.This approach has two shortcomings: first, it increased hardware cost; second, it raised inaccuracy by adopting fault sampling mode.We absorb results of existing "static security analysis" research to analyze fault, which can satisfy computing speed requirement.This fault analyzing method utilized AC-DC hybrid algorithm, and has introduced parallel computing technology based on multi-processor workstation.The approach has been improved by combining with node optimization, matrix inversion and node type conversion, etc, which has greatly improved computing speed.The correctness has been verified by the application in regional-level scheduling, provincial scheduling and city-level scheduling.For the test of 2000 nodes system, it only needs 3s scanning whole grid lines, transformers and units.The computing time would slightly increase with the increase of limit violation number.
As important users and high risk customers gener ve double power source, if not considering hot backup source in fault analysis, the risk indices of loss of load computed would deviate greatly with actual situation.So when the main source of important users or high risk customers break down, it should be analyzed after putting into backup source.
It is presented three grid in this paper according to formula(1): ①line overload risk; ②bus low-voltage risk; ③loss of load risk.
We compute risk indices ①,② by the formula, here notes the limit violation of line power flow and bus voltage; X denotes security upper limit or lower limit; the superscript 2m is used to overcome the "shelter" defect [4].
The power s k customers is related to a range of social, political and economic issues.The outage severity of these users depends not only on the district grid's own characteristic, but also on the users' property.We introduced an importance factor to classify these users, and because of lacking outage time, we only compute loss of load risk indi-ces, not outage cost evaluation indicators.The computing formula of risk ③ is as, here: is the importance of the jth load; denotes the reduction amount of the jth load after failure I; is the load number of reduction.We adopted real-time failure probability model denotg the probability.For outdoor components, the weather condition value is set by dispatchers; for indoor components, the weather condition value is constantly equal to 0.
The risk indices c dispatchers, it is more expected that the risk indices can directly show the system security condition.So we classified three risk grades according the risk value: security level, alerting level and over standard level.
Distract grid is closed-loop design but open-lo tion.When recovering, the distract high-voltage grid (220 kV and above) and radial distribution grid lower than 110 kV, different correction and control measures should be adopted.a) Correction an rid Bas ethod and objective function selecting, then giving the control target and control variable, the correction measure can be obtained for load and generation unit.b) Distract Low-voltage Radial Grid Correctio l Fo eration under close-loop.It is generally adopted adjusting operation mode as effective measure to ensure continuous power supply and eliminate limit violation.The correction strategy includes load balance, single power source switch, multi-source load transfer, etc.In extreme cases, it can be adjusted by removing load according to load importance.After eliminating fault, system recovery takes into account recovery path constraints and risk indices constraints.
It is often difficult ponent failure due to lack of statistical data.And the possibility of outdoor component failure has closely relationship with climatic conditions.For power dispatching, it is originally difficult to get weather forecast parameters.Even if it can be obtained the weather forecast parameters, it has possibly an error.In this paper, based on utility theory and probability theory, we have established the three-dimensional model of failure probability by making fully use of the dispatchers' operation experience and Copyright © 2013 SciRes.EPE

Figure 1 .
Figure 1.Contents of power system risk assessment.

3 )
Coexistence of long and short li 4) Coexistence of overhead lines and ca 5) Complex network structure, running in open-lo eak-link style; 6) Constantly namic load; 7) Part distri tion or have poor data acquisition such as voltage, reactive power, active power transformer tap because of in-terest, skill level or some special connection, T tie line, for example.

Figure 2 .
Figure 2. Calculation process of risk indic es.

Figure 3 .
Figure 3.The membership function of the lightning e cts.ffe

Figure 4 .
Figure 4.The membership function of the wind speed and line Icing.

Figure 5 .
Figure 5.The membership functions of air temperature.
an quantify the risk of system, but for 3.6.Risk-based Correction Strategy Set op operad Control for High-voltage Distract G ed on sensibility calculation, integrating planning m n Contro r distract grid, the operation mode is usually radial op