HUNAN POWER GRID BLACKOUT RESTORATION

DECISION SUPPORT SYSTEM He Pengcheng1, Chen Hao1, Zhou Yunhai2, Wu Jianrui2, Tang Lichun1, Wang Zhenggang1, Chen Zhuo1 (1. Dept. of dispatch, Power Company of Hunan Province, Changsha 410007, 2. College of Electrical Engineering and renewable energy, China Three Gorges University, Yichang 443002)

Abstract—This article describes an online blackout restoration decision support system， which includes black start optimization strategy, calculation steps, the main function and structure of the software platform. By common information platform, this system automatically and efficiently collect real time power system information&data from EMS, enabling the yearly rolling revision of the black-start scheme, or online adjustment, and provide a black start analysis and decision making tools for dispatcher. The system integrate network data management and scientific computing, graphical modeling input and output tools, providing a flexible platform for power system dispatch department to develop a high-quality black-start scheme by detailed data analysis.

Keywords: black-start；blackout；decision support system；graphical modeling；dynamic coloring

I. RESEARCH SIGNIFICANCE Ice disaster happened in 2008 has resulted a

large area blackout in many provinces. According to system black start process in the ice disaster, off-line formulation of black start scheme is based on equipments are fully available, it can not adapt to such large natural disaster in 2008. During the black start process, the availability of power generation equipment and power transmission equipment is complex, the original black start scheme cannot be adjusted in real time. Furthermore, construction of power grid is faster and faster, network structure change rapidly. Increasing a fire or water power plant, 500kV or 220kV substation or transmission line have dramatically changed the grid operation characteristics and caused significant adjustment of regional power grid operation mode and loads of main power supply mode change. So that target network is bound to be changed in black start scheme, the original fast start channel will also be unrealistic. Putting any new equipment into operate, dispatchers must adjust the black start plan. Regardless of time or manpower consideration, there is no enough time, even new program has not yet been completed, the network structure has been updated. Black start scheme requires real-time

rolling revision. Therefore, the concerned leaders and dispatchers proposed the needs of real-time rolling revision, security and stability checking, the impact of large disturbance.

II. BLACK START OPTIMIZATION ALGORITHMS AND IMPLEMENTATION A. black start optimization strategy

Black start optimization algorithm divides system recovery into two stages:

The first stage: several small system black start scheme will produce when black start power supplies and other constraints are satisfied, the programs go hand in hand and relatively independent.

The second stage: after each subsystem is restored, nodes of the network which has not been restored must develop transmission paths, while the power supply points are charged nodes, and considering the importance of nodes, giving priority to vital load transmission.

B. The power of Black start selection Black start power supply should have

self-starting ability, easy grid-connected and loading faster. Such as hydropower unit, gas turbine unit, oil fuel unit, and so on.

Hydroelectric generating unit can be controlled and adjusted flexibly, started and stopped quickly, and has self-starting ability, its start-up time is also shorter. Therefore, most black start powers are hydroelectric generating units

Gas fuel generating unit, its boiler ignition is quick and easy, may fast start in a short time.

Oil fuel generating unit has fast starting and power regulation capability, and can be used as black-start power supply.

Considering the current partition slices parallel recovery strategy in Hunan power grid, black start power not only include the hydroelectric power plant, but also support from adjacent power grid or the next higher level power grid (500kV). When black start procedure running, the procedure may obtain the unit type through the EMS real-time data, it will judge the hydropower unit is available or in overhaul state, through judge the fuel units whether is islanding

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remnants unit (with the local load) to determine whether the unit is a black start unit. Through the substation bus bar charging state to decide whether it can be started as a power option.

C. The Selection of black - start path For each of the starting units, we must

calculate the shortest path between it and all powers. The system uses the Bellman－Ford algorithm for finding shortest path: it calculates the starting point to all other vertices and finds out the shortest path, this algorithm is iterative equation and can be expressed as follows:

For an undirected network N=(U,A,W), arcs(i,j)∈A, the corresponding right wij also

known as arc-length or cost, u j is the distance

marks, The recording is from the beginning 1 to this apex most short circuit length upper boundary; pred(j) is the predecessor marking, when starting point 1 to the vertex j and a way to get to this long upper bound, the record is the point in front of apex j . When the iteration is end, the shortest path can be determined from the recording information of array pred.

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is the temporary label of vertex j which is gat from the k-th iteration.

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starting point 1 to vertex j D. The Verification of feasibility of black start scheme

Black start and accident treatment need to consider a variety of security and stability constraints, such as self-excitation, overvoltage problems of transformers and line when they are charging with no-load, etc. In addition, the connection mode is continuously changing in the system recovery process, and the stability limit is not the same. If stable limit of each connection mode and black-start solution need online calculation, there will be many difficulties:

The stability limit involves electromagnetic transient calculation and electromechanical transient calculation, the professional software does not provide the calling interface, Procedures must be started manually, so it can not meet the needs of online applications.

If we develop the electromagnetic transient calculation and electromechanical transient calculation program by ourselves, the work load is huge, the required time and funding is difficult to guarantee, and also cannot guarantee the correctness of the program.

This project uses a proven professional software, such as PSASP，EDSA, according the different capacity of black-start unit, they work out the corresponding limit indexes, several typical black start of the system, the stability limit of recovery mode, such as the self-excitation criterion of the unit in black start process, the inrush current criterion of no-load transformer, the starting impact of large motor, the load restoration impact, and so on. Furthermore, these criterions are expressed in physical quantity, such as voltage level, the capacity of electric motor, the capacity of load, and so on. In this way, there is a corresponding limit value index for hydropower unit or thermal power unit with a different capacity. The online black start plan and system recovery plan will run on these stability limits to ensure system security, professional software can guarantee the validity of the calculation results.

E. The selection of black-start scheme

All of the black start plans must through feasibility checking, the black-start scheme which is not out of bounds can be seen as possible candidate. The best plan is chosen according to the black start plan technical specification. For the program, capacity of the start unit, production of the started unit in the previous 3 hours, the number of transformers in the charging path, the maximum capacity of the transformer which can be charged with no-load, the charging reactive power of the transmission path, the number of the line in the charging path, the breakers and switches operation numbers and other indexes, and the weight that can be chosen by dispatchers must be considered.

Current acquiescent program optimization principle is as follow: taking the largest generating capacity in the previous three hours as target to select the started unit, taking the largest capacity unit as target to select start power source, taking the shortest transmission path as target to select the power transmission path between the power of the black start and start the unit. Program allows to dispatchers set its own technical indexes weight and consider a variety of factors through a reserved interface.

F. The calculation steps The calculation steps are summarized as

follows: 1) Pick out the black start unit and the started unit;

2）Rank the started unit according to production of the started unit in the previous 3 hours

3) Match power for the started unit from the big capacity to the small, If there is more than one available black start power, the technical indexes of the program must be calculated one by one to select the best one as black start power, if there is no viable black-start scheme, the started unit is skipped, and we must seek the best black start power for the next started unit.

4） When a black-start scheme for one started unit is gat, the start power and path which have been used were marked, and do not participate in the optimization process of the next start unit to ensure the black-start schemes are relatively independent, and recovery can go hand in hand.

G. Other optimization features for improving system availability

1) Optimized index calculation, comparison and adjustment

The above indexes are calculated, and normalized separately, the dispatcher may assign these indexes with weight, and set them as the start power supply optimization objective function.

2) Achieve the whole constraint of the black start scheme through the target power grid

Property is set to determine whether the bus belongs to the target network node, if it is, first to recover its power supply even if the bus is not the power plant bus.

3) The preferential recovery of the important load

Property is defined to judge the importance of the bus loads, the most important one must be recovered firstly.

4) The power source and the target point must designated by people and the procedure seeks the power transmission way automatically

5) Online adjustment of the program

Each component of the system has condition attribute, such as if equipment is not available， its property is exit or maintenance, it will not participate in the optimization of the black start program.

III. THE DEVELOPMENT OF SOFTWORE SUPPORT PLATFORM

The calculation and analysis process of the accident treatment auxiliary decision support system of the black start and large-area blackout in Hunan power grid needs to know the entire network topology and related device parameters, the results of the calculation also need to be presented on graphics in a visual way, so the development of system software support platform is needed to provide a graphical modeling tool used to draw a variety of structured graphics of the whole system, input

the parameters and dynamically displays the analysis results on the graphics.

The structure of the supporting platform is as follows:

Figure 1: the system function structure

The function of the supporting platform is as

follows:

1) Mapping electrical network structure / the object electrical network structure of the black start

Electrical network structure is the reflection of the entire power grid, including the important substation and plant. The object power grid is a simplified power grid, which reflects the object electrical network structure after the black start.

This subsystem should be able to provide simple, intuitive, flexible and standard mouse operation mode to complete the drawing and modification of the electrical network and the object electrical network structure of the black start. Through mouse operation you can add plant/substation nodes, names and connecting lines to the graph and set the graphic appearance property neatly.

2) Mapping one-line diagram for plant and substation

The subsystem should be able to complete the plant / substation one-line diagram through mouse operations, and add the following graphic symbol to it:

• Commonly used preliminary main equipment: bus, generator, transformer, circuit breakers,, and so on.

• The text annotation objects playing the role of annotation.

• A variety of simple two-dimensional graphics: line, rectangle, ellipse, and so on.

• You can select, move and zoom in/out the entity through convenient mouse operations and set graphical appearance attributes

3）Hierarchical management of graphic files

Stratified association management is used to manage electrical network structure and the one-line diagram for plant/substation.

We can set associated settings for the nodes of the electrical network structure/ object electrical network structure of the black start. After you complete the lamination connection setting between the corresponding graphic file, clicking a node on the electrical network structure/ the object electrical network structure of the black start in a graphical interface, the corresponding one-line diagram will be displayed in the multi-window.

4) Getting the existing electric parameters from a particular EMS

The existing EMS may have part of electrical data which black start decision required, therefore, the subsystem should be able to obtain the corresponding electrical parameters from the corresponding EMS, and store the data into system database.

5) Visual management of electrical parameters

You can set the electrical parameters for the selected device entity in the one-line diagram editing interface.

6) The graphical output of the auxiliary decision analysis results

You can output the auxiliary decision analysis results in an intuitive graphical way and print the results.

7) Output the black start step obtained from analysis

You can output the black start step obtained from analysis with forms and print the result.

8) Dynamic coloring function

The system can realize the dynamic coloring function on a primary grid wiring diagram or a geographical system connection diagram, and use different colors or flashing to highlight the difference between the transmission path, the coming power transmission line, the not available line (component) and the charged or uncharged component, etc. The related personnel can know the system recovery state intuitively, and know what is about to do and what is the results of operations.

IV. SUMMARY

The accident treatment auxiliary decision support system of the black start and large-area blackout in Hunan power grid automatically and efficiently collect real time power system information and data from EMS by common information platform; the black start optimization algorithm was used to propose the black start optimization plan, the security and

stability checking algorithm, as well as decision support tools were used to provide a black start analysis and decision making tools for dispatcher. The System integrate network data management and scientific computing, and provide a graphical modeling input and output tools, providing a flexible platform for power system to develop a high-quality black-start scheme .The system has been running on line, the effect is good.

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