CHAPTER 5 RESTORATION OF POWER USING MULTI-AGENT...

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CHAPTER 5

RESTORATION OF POWER USING

MULTI-AGENT SYSTEM

5.1 INTRODUCTION

It is a known fact that the restoration of a power system after a

system blackout is a difficult task. It is a complex, delicate, and time-

consuming process. If power is not restored, there will be heavy loss to

consumers. At present, control system software has been developed primarily

to facilitate monitoring and control of the power system. Still the network

undergoes outages. Even at present restoration task is performed manually,

according to guidelines prepared in advance. The power system operators

should execute the restoration plan under a stressful situation. But the

restoration plans are prepared based on the predefined fault conditions and the

plans are complex and convoluted as to be very difficult to interpret for a new

fault condition.

In the era of deregulation on energy policies, the coordination of

independent power producers, generation companies and consumers is

difficult and cannot be solved easily by the conventional approaches such as

Heuristics, Expert systems, Mathematical programming, and Soft computing.

This drawback initiates this study to choose the MAS. This system has many

agents and is defined as one that interacts with external environments,

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cooperates with other ones and processes to be solved autonomously. After

total black-out, power system restoration requires coordination among

generating units, load pick-up characteristics and transmission systems. Since

co-ordination is necessary for the power system restoration, the MAS

technique provides an interesting environment for evaluating agent behaviour.

5.2 ARCHITECTURE OF MULTI-AGENT SYSTEM

The technique, MAS divides the big complex processes into small

process which is assigned to agents, and then the agents interact with each

other to achieve the system goal. The distributed agents perform the task

simultaneously and are believed to increase performance of the system. MAS

technique has been successfully applied in many fields such as power system

and computer network.

The MAS with the two- level architecture consists of a number of

local managing agents (LMAs) and a global managing agent (GMA) which is

shown in Figure 5.1. GMA is an agent that facilitates the negotiation process

of the MAS approach. It classifies the de-energized buses into many groups

based on the power requirement. It sends the start message to one of the

LMAs which is at the higher power level. It repeats the same task

simultaneously with other de-energized groups. After the occurrence of fault,

LMA is developed to decide a suboptimal target configuration by interacting

with other LMAs. Based on the number of buses, the LMAs are allocated. It

has certain rules associated with it to restore the load and are given as follows:

• If there is more number of branches that can energize certain

bus then LMA is allocated to the particular bus which has the

largest amount of available restoration power.

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• If the amount of available power for restoration is insufficient,

LMA tries to restore the bus by negotiating with its

neighbouring LMAs.

• If a load must be shed because of insufficient power, LMA

cuts off the load connected to its own bus to the minimum

possible.

Whenever there is a fault, the LMA corresponding to the

de-energized bus sends a request to the GMA. Many such messages are sent

to GMA if there are many de-energized buses. After receiving the first

message, the multi agent system initiates its function. The GMA then sends a

start message to the LMA, which starts its communication with the

neighbouring LMAs. In this way, MAS recovers the entire network.

Figure 5.1 Architecture of Multi-Agent System

LMA

GMA

LMA

LMA

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5.2.1 Problem Formulation

The objective of the mathematical model (Nagata et al 2002) of a

power system restoration is to maximize the capacity of the served loads in

power system network.

Max Li . yi (5.1)i D

where Li is the load at bus i, yi is the decision variable or expressing its status

(yi=1:restored; yi=0:not restored) and D denotes the set of de-energized loads.

Typical constraints associated with the restoration model taken into

account are:

Limit on the capacity of available power source for restoration

Pb . xb Gq (q S) (5.2) b Fq

where Pb is the power flow in the directed branch b xb is the decision variable

of branch b (xb = 1:b is included in the restoration path; xb = 0:otherwise). Fq

is the set of branches with starting node q, Gq is the restoration power from

the energized bus q, and S is the set of energized buses that can be connected

to de-energized area.

Power balance between supply and demand

Pk - Pk – Li . yi = 0 (i N) (5.3) k Ti k Fi

where Ti is the set of branches incident to bus i, Fi is the set of branches with

originating from bus i, Li is the load at bus i, and N is the set of buses.

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Limits on branch power flow

|Pk| - Uk 0 (k B) (5.4)

where Pk denotes the power flow of branch k, Uk is the capacity of branch k,

and B is the set of directed branches.

Constraint on radial configuration

The target configuration must be radial. To ensure a radial

configuration, the total number of branches incident to bus i must be almost

unity.

xk 1( i N ) (5.5)k Ti

This MAS technique is applied to 14 bus network and the obtained

results are compared with the work of Nagata et al (2002). The same 14 bus

sample network is extended to 30 bus network and is analysed with three

different fault conditions.

5.3 NEGOTIATION PROCESS BETWEEN AGENTS FOR A 14

BUS SYSTEM

To restore the de-energized areas, the negotiation process is the

main key process. In order to demonstrate the MAS, it has been applied to a

14 bus network which consists of 8 substations and 14 buses as shown in

Figure 5.2. Loads are indicated by arrows together with their magnitude. The

numeral on the branch shows its line flow capacity and s/s refers the

substation. A square in the diagram represents the status of the circuit breaker.

[black square- closed; white square- open]. Simulations are carried out for the

14 bus system in three different cases of fault.

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5.3.1 Case 1 - Fault Between Substations B and D in 14 Bus

It is assumed that the fault occurs between the buses BUS2 and

BUS5. Due to the fault, the buses BUS5, BUS6, BUS11, BUS12, BUS13, and

BUS14 are de-energized which is shown in Figure 5.2. Hence the loads, L2,

L5, and L6 are not supplied with power. The main objective is to re-energize

all the buses and to supply maximum load. Hence the lists of buses that are

de-energized are placed in the DEBLIST.

Figure 5.2 Post Fault Network of 14 Bus for Case 1

DEBLIST is not only used to store the de-energized buses in list,

but also to group the buses according to the power level. Hence, when the

fault occurs between the buses BUS2 and BUS5, LMA5 sends the first signal

to GMA to restore it. GMA sends the signal to restore BUS5. LMA5

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communicates with LMA3 and checks the available power in BUS3 which is

in healthy network. Since the total load in the de-energized area (4.0) is

greater than the line capacity between the buses, BUS3 and BUS5 (2.0), D-G

line is being opened.

When D-G line is opened, the total load in the de-energized area to

be restored through BUS3 is reduced to 2.0. The total power now flowing

through C-D line will be 2.0 which is equal to the capacity of the line thereby

not violating the constraint of the line capacity. While energizing each bus,

the corresponding LMAs send the signal to GMA in order to remove the bus

from DEBLIST.

Now the buses BUS11 and BUS12 are in DEBLIST. Hence, GMA

sends the signal to BUS11 for restoration. LMA11 communicates with LMA9

and checks the available power in BUS9 which is in healthy network, whether

it can supply the load in G substation. The available power in BUS9 (2.0) is

sufficient to supply the load L5 (2.0) and it is less than the capacity of the F-G

line. Hence F-G line is being closed. The buses BUS11 and BUS12 are

energized and are removed from DEBLIST. Since the DEBLIST becomes

empty, the objective of energizing all the buses in the network is achieved. A

program is written in C# language to obtain the output for the 14 bus network

and it is shown in Figure 5.3. The diagrammatic representation of restoration

plan is given in Figure 5.4.

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Figure 5.3 Switching Operation Sequence for 14 Bus Case 1

Figure 5.4 Target Network Configuration of 14 Bus for Case 1

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5.3.2 Case 2 - Fault between Substations B and D by Changing Line

Capacity in 14 bus


In the second case the same fault network is considered as in the

first case, but here the capacity of the line between the buses, BUS9 and

BUS11 is changed from 4.0 to 1.0 as shown in Figure 5.5. As soon as the fault

occurs, DEBLIST is created. Hence the buses to be restored are identified.

The same procedure is followed to re-energize the load in the network for the

14 bus network. The switching operation sequence and the network

configuration are shown in Figures 5.6 and 5.7 respectively.

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Figure 5.6 Switching Operation Sequence of 14 Bus for Case 2


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5.3.3 Case 3 – One Fault Between Substations B and D, Another

Fault Between C and E in 14 Bus

In the third case, two faults are occurred, one fault is between

BUS2 and BUS5, and the other is between BUS4 and BUS7. Hence buses,

BUS5, BUS6, BUS7, BUS8, BUS11, BUS12, BUS13, and BUS14 are de-

energized due to the faults and are stored in DEBLIST. In this case the loads

L5 and L6 are interchanged. Hence L5 has the load 1.0 p.u and L6 has the

load 2.0 p.u. All the other parameters are as same as in first case. The

restoration plan for this fault and the corresponding target network

configuration is given in Figures 5.9 and 5.10 respectively.


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Figure 5.9 Switching Operation Sequence of 14 Bus for Case 3

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5.4 NEGOTIATION PROCESS BETWEEN AGENTS FOR A 30

BUS SYSTEM

The negotiation process is the main method of restoring the

de-energized areas. In order to demonstrate the MAS, it has been applied to a

30 bus network which consists of 16 substations and 30 buses as shown in

Figure 5.11. Simulations are carried out for the 30 bus system in three

different cases of fault.

5.4.1 Case 1- Fault Between Substations B and D in 30 Bus

It is assumed that the fault occurs between the buses BUS2 and

BUS5. Due to the fault, the buses BUS5, BUS6, BUS11, BUS12, BUS13,

BUS14, BUS23, BUS24, BUS25, BUS26, BUS27, BUS28, BUS29, and

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BUS30 are in the de-energized bus list (DEBLIST) which is indicated in

dotted background and the loads, L2, L5, L6, L11, L12, L13 and L14 are not

supplied with power. The main objective is to re-energize all the buses with

maximum load.


DEBLIST is not only used to store the de-energized buses in list,

but also to group the buses according to the power requirement. Hence, when

the fault occurs between the buses BUS2 and BUS5, BUS5 sends the first

signal to GMA to request for restoration. GMA sends the signal to restore

BUS5. LMA5 communicates with LMA3 and checks the available power in

BUS3 which is in healthy network. Since the total load in the de-energized

area (8.0) is greater than the line capacity between the buses 3 and 5 (5.0),

D-G line is being opened.

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be restored through BUS3 is reduced to 5.0. The total power now that need to

flow through C-D line will be 5.0 which is equal to the capacity of the C-D

line thereby not violating the constraint of the line capacity and hence C-D

line is closed. While energizing each bus, the corresponding LMAs send the

signal to GMA in order to remove the bus from DEBLIST. Now the buses

BUS11, BUS12, BUS23, BUS24, BUS25, and BUS26 are included in the

DEBLIST. Hence, GMA sends the signal to BUS11, for restoration. LMA11

communicates with LMA9 and analyses the available power in BUS9 which

is in healthy network, whether it can supply the load in G substation. Since

the total load in the de-energized area (3.0) is greater than the line capacity

between the buses, BUS9 and BUS11 (2.0), G-M line is opened.

When G-M line is opened, the total de-energised load to be restored

through BUS11 is reduced to 2.0. Since the power that needs to flow through

F-G line and capacity of F-G line are equal, the same line is being closed. The

buses BUS11, BUS12, BUS25 and BUS26 are energized and are removed

from DEBLIST.

Still buses BUS23 and BUS24 are in DEBLIST. LMA23 sends the

signal to GMA in order to restore BUS 23. GMA checks the available power

in LMA21. Since the available power is enough to restore the load and the

power needs to flow through L-M line (1.0) is less than the capacity of L-M

line (6.0), L-M line is closed. Hence buses BUS23 and BUS24 are energized

and are removed from the DEBLIST. Since the DEBLIST becomes empty,

the objective of energizing all the buses in the network is achieved. The

restoration plan and the target network reconfiguration for case 1 is given in

Table 5.1 and Figure 5.12 respectively.

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Table 5.1 Switching Operation Sequence of 30 Bus for Case 1

S.No Station Operation Content

1 B S/S B-D Line Open

2 D S/S B-D Line Open

3 G S/S D-G Line Open

4 D S/S C-D Line Close

5 M S/S G-M Line Open

6 G S/S F-G Line Close

7 M S/S L-M Line Close


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5.4.2 Case 2- Fault Between Substations B and D By Changing Line

Capacity in 30 Bus

In the second case, it is assumed that the fault is between the buses

2 and 5 but the capacity of the line between the buses, BUS3 and BUS5 is

changed from 5.0 to 3.0 which is shown in Figure 5.13. As soon as the fault

occurs, DEBLIST is created. Hence the buses to be restored are identified.


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GMA sends the signal to restore BUS5. LMA5 communicates with

LMA3 and checks the available power in BUS3, which is in healthy network.

Since the total load in the de-energized area (8.0) is greater than the line

capacity between the buses, BUS3 and BUS5 (3.0), D-G line is opened.


be restored through BUS3 is reduced to 5.0. However, the load to be met (5.0)

through the bus, BUS5 from BUS3 is greater than the capacity of C-D line

(3.0). Hence, H-O line is opened which has the load, 2.0 and thereby the total

load that has to be restored by closing C-D line becomes 3.0. Now C-D line is

closed. The total power, now flowing through C-D line becomes 3.0, which is

equal to the capacity of the line, thereby not violating the constraint of the line

capacity.

Since the buses BUS11, BUS12, BUS23, BUS24, BUS25, BUS26,

BUS27 and BUS28 are in DEBLIST, GMA analyses the available power in

BUS9, which is in healthy network. The available power in BUS9 (2.0) is not

sufficient to supply the load (3.0) that has to be restored through BUS11.

Hence, G-M line is opened thereby reducing the load to be met through

BUS11 to 2.0. Hence, the power to flow through F-G line (2.0) in order to

supply the load is equal to the capacity of F-G line. Now F-G line is closed.

The buses BUS11, BUS12, BUS25 and BUS26 are energized and are

removed from DEBLIST.

Still buses BUS23, BUS24, BUS27 and BUS28 are in DEBLIST.

LMA23 sends the signal to GMA in order to restore. GMA checks the

available power in LMA21. Since the available power is enough to restore the

load and the power needs to flow through L-M line (1.0) is less than the

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capacity of L-M line (6.0), L-M line is closed. Hence buses BUS23 and

BUS24 are energized and are removed from the DEBLIST.

Buses BUS27 and BUS28 yet to be energized are in the DEBLIST.

BUS27 can be energized only through BUS25. Hence LMA27 sends the

signal to GMA. GMA checks the availability of power in BUS25. Since the

load in O substation (2.0) cannot be met with the available power in BUS25

(0.0), the load L13 is cut and then the BUS27 and BUS28 are energized by

closing the N-O line and are removed from DEBLIST. Now, the DEBLIST

becomes empty. Though L13 load is cut, all the de-energized buses are re-

energized. The restoration plan for case 2 and the diagrammatic representation

of the network is shown in Table 5.2 and Figure 5.14 respectively.


S.No Station Operation Content




4 O S/S H-O Line Open





9 O S/S Load Cut

10 O S/S N-O Line Close

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5.4.3 Case 3 – One Fault Between Substations B and D, Another

Fault Between C and E in 30 Bus

In the third case, it is assumed that two faults occur between the

buses BUS2 and BUS5, and BUS4 and BUS7. In this case the loads L13 and

L14 are interchanged compared to the previous case. Hence L13 has the load

1.0 p.u and L14 has the load 2.0 p.u. All the other parameters are same as in

the second case as shown in Figure 5.15. Hence buses, BUS5, BUS6, BUS7,

BUS8, BUS11, BUS12, BUS13, BUS14, BUS15, BUS16, BUS17, BUS18,

BUS23, BUS24, BUS25, BUS26, BUS27, BUS28, BUS29, and BUS30 are

de-energized due to the faults and are stored in DEBLIST.

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GMA sends the signal to restore LMA5. LMA5 communicates with

LMA3 and checks the available power in BUS3 which is in healthy network.

Since the total load in the de-energized area (8.0) is greater than the line

capacity between the buses, BUS3 and BUS5 (3.0), D-G line is opened. When

D-G line is opened, the total load in the de-energized area to be restored

through BUS3 becomes less (5.0). However, the load to be met (5.0) through

the bus, BUS5 from BUS3 is greater than the capacity of C-D line (3.0).

Hence, H-O line is opened which has the load 1.0 and thereby the total load

that has to be restored by closing C-D line becomes 4.0.

Since E substation is under de-energized area due to the fault

between buses BUS4 and BUS7, GMA sends the signal to restore BUS7.

LMA7 communicates with LMA9 and checks the available power in BUS9 to

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restore BUS7. Since all the constraints are met, E-F line is closed and hence E

substation, I substation and J substation buses are energized.

Now the buses BUS5, BUS6, BUS11, BUS12, BUS13, BUS14,

BUS23, BUS24, BUS25, BUS26, BUS27, BUS28, BUS29 and BUS30 are in

DEBLIST. The total load under BUS5 that is to be restored is 4.0, which is

greater than the power that can be obtained from BUS3 (3.0). Hence H-P line

is opened. When the C-D line is closed, the buses, BUS5, BUS6, BUS13 and

BUS14 are energized and the loads L2 and L6 are supplied with power. Now

these buses are removed from DEBLIST.

Since the buses BUS11, BUS12, BUS23, BUS24, BUS25, BUS26,

BUS27, BUS28, BUS29 and BUS30 are in DEBLIST, GMA analyses the

available power in BUS9, which is in healthy network. The available power in

BUS9 (2.0) is not sufficient to supply the load (3.0) which is to be restored

through BUS11. Hence, G-M line is opened thereby reducing the load to be

met through BUS11 to 2.0. Hence, the power to flow through F-G line (2.0) in

order to supply the load is equal to the capacity of F-G line. Now F-G line is

closed. The buses BUS11, BUS12, BUS25 and BUS26 are energized and are

removed from DEBLIST.

Still buses BUS23, BUS24, BUS27, BUS28, BUS29 and BUS30

are in DEBLIST. LMA23 sends the signal to GMA in order to restore. GMA

checks the available power in LMA21. Since the available power is enough to

restore the load and the power needs to flow through L-M line (1.0) is less

than the capacity of L-M line (6.0), L-M line is closed. Hence buses BUS23

and BUS24 are energized and are removed from the DEBLIST.

Buses BUS27, BUS28, BUS29 and BUS30 that are yet to be

energized are in the DEBLIST. BUS27 can be energized only through

BUS25. Hence LMA27 sends the signal to GMA. GMA checks the

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availability of power in BUS25. The load in O substation (1.0) cannot be met

with the available power in BUS25 (0.0). Hence the load L13 is cut and then

the BUS27 and BUS28 are energized and are removed from DEBLIST.

Finally, BUS29 and BUS30 are to be energized only through

BUS27. Since the available power in BUS27 is not sufficient to meet the load

in P substation (2.0), the load is being cut and then BUS29 and BUS30 are

energized by closing O-P line and are removed from DEBLIST. Now, the

DEBLIST becomes empty. Thus the objective function is satisfied. Table 5.3

includes the switching sequence for case 3 and Figure 5.16 explains the

implementation of restoration plan.


S.No Station Operation content



3 C S/S C-E Line Open

4 E S/S C-E Line Open


6 O S/S H-O Line Open

7 F S/S E-F Line Close

8 P S/S H-P Line Open





13 O S/S Load Cut

14 O S/S N-O Line Close

15 P S/S Load Cut

16 P S/S O-P Line Close

Generally, the restoration plan is obtained for the predetermined

condition of the electrical network, and the optimal path for the transmission

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of power is not discussed in the available literature. But there are several

reports about MAS-based approaches for power system restoration. Takeshi

Nagata et al (2002) proposed MAS which consists of number of bus agents

and a single facilitator agent. In this process, the restoration model considers

the transmission capacity, power balance between the supply and demand,

available generation and finally the radial constraint. But the process does not

consider the safe operating voltage limit and the shortest path for power

transmission that may be helpful for loss reduction.


Takeshi Nagata et al (2004) proposed a new decentralized multi-agent

simulator for a bulk power system restoration. The proposed MAS is

constructed with four-level hierarchical architecture with agents in each level.

After negotiating with agents, the restoration plan is obtained.

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This initiates the need for modified MAS with the separate

restorative path search guidance tool which suggests the optimal path through

which the power can be transmitted. The optimal path considers the shortest

path by distance (minimum losses), capacity of the transmission line, power

balance between source and demand, priority of loads and allowable voltage

limits of the network. So, there will not be cascaded outages which help the

MAS to be more effective.

5.5 ARCHITECTURE OF MODIFIED MULTI-AGENT SYSTEM

The multi-agent approach consists of several Demand Agents

(DAs), Source Agents (SAs), Load Dispatching Agents (LDAs) and a single

Management Agent (MA) as shown in Figure 5.17. The system is constructed

with three-level architecture. MA is located in the upper level. LDAs are

located in the second level and several DAs and SAs are implemented at the

third level. DAs correspond to the individual load while the SAs include the

generator which has the different generator characteristics such as ramp rate

and capacity. LDAs are equipped with the restorative path search guidance

tool to identify the optimal path. The LDAs are given suggestion for

connecting the electrical subsystem to the actual network by the MA.

Figure 5.17 Architecture of Modified Multi-Agent System

MA

LDA LDALDA

SA DAASA SADAA DAA DAA DAA DAA

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Source Agents (SAs) have the characteristics of each generator, the

capacity of the generator and the type of plant. They offer this information to

the LDAs and manage the supply of power to the load. Table 5.4 shows the

generator characteristics. It is assumed that the uniform ramp rate as

10MW/min.

Table 5.4 Generator Characteristics

Name of the

Power Station

Unit

ID

Type of

Plant

Capacity

(MW)

NCTPS

(station 1)

G1 Gas 210

G2 Steam 210

G3 Steam 210

ETPS

(station 2)

G4 Gas 110

G5 Steam 110

G6 Steam 110

G7 Steam 60

G8 Steam 60

MTPS

(station 3)

G9 Gas 220

G10 Steam 220

MAPS

(station 4)

G11 Steam 220

G12 Gas 220

NEYVELI

(station 5)G13 Steam 220

Demand Agents (DAs) are implemented in the third level. They

know the characteristics of each load, such as load demand. It sends the

request to LDAs to restore the load. The important loads are given priority

among the de-energized loads. An important step to pick up the load is to

connect the load nearest to the power sources in a power system network. A

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load is an integration model of more number of customers. The load pickup

characteristic is shown in Figure 5.18.

Steps of Restoration

0 1 2 3 4 5 6

loa

d(%

)

0

20

40

60

80

100

120

Figure 5.18 Characteristics for Load Pick Up

Load Dispatching Agents (LDAs) are allocated to each electrical

zone. Power system network is divided into a number of electrical zones in

order to enhance the restoration effectively. The SAs and DAs within that

zone send the message to the corresponding LDA. LDA is equipped with

restorative path search guidance tool which is solved by the SPF algorithm

which was discussed in Chapter 3. The generator characteristics and load

characteristics are also stored here as data base. Based on this algorithm after

checking the constraints the optimal path for the power flow is decided.

Management Agent (MA) is located in the first level. LDAs send

all information to MA about the electrical zone such as the amount of load

restored, the available generation, optimal path through which power is

transmitted. On the whole, the MA will check the constraints by conducting

the load flow in order to validate the restoration plan.

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5.6 EXECUTION PROCESS BY MULTI-AGENT SYSTEM

In this section, the restoration process using multi agent approach is

explained.

1. The black start generator starts in each electrical zone and the

current status of the generators are updated to LDAs by SAs.

2. LDAs calculate the serving electrical power and reply

message forwarded to SAs to supply power to the non-black

start generator.

3. At the same time, the DAs send information to LDAs such as

required power to restore the load. During black-out

condition, an important rule is to pick up the load nearest to

the power source.

4. LDAs acquire the available power of started SAs and figures

out a scheme of power assignment of each DA. The load

restoration process should proceed with the constraint and the

restorative path search guidance tool suggests the optimal path

for transmission of power. Then LDAs update this information

to MA.

5. MA validates the restoration plan by conducting the load flow

and sends the reply message to LDAs. If the limits are

violated, then the LDA will instruct the DA to shut down the

load. Only after getting the reply message, LDAs send the

information to SAs and DAs.

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6. In each electrical zone more and more SAs are cranked, the

skeleton transmission lines are energized. In order to stabilize

generation and maintain normal voltage, sufficient load must

also be restored. The same process is continued until 60% of

the load in each electrical zone is supplied. Then the MA

decides about the interconnection of subsystem in order to

share the available source of power and the reconfiguration of

the entire power system network.

Different kinds of agents have different tasks during restoration.

The above mentioned process is only a frame work for power restoration.

The key process for the MAS is negotiation only. With the proper negotiation

between the agents, an efficient restoration plan is developed. In order to

enhance the restoration, the restorative path search guidance tool is used.

This will check the constraints like capacity of the transmission line, shortest

path, priority of loads and power balance between the source and the demand.

Based on the above mentioned procedures, the restoration plan is

obtained. Each LDA provides the values of generation and load to SAs and

DAs respectively. SAs increase its output power according to its

characteristics. It is to be noted that the transfer of power does not violate the

constraints since the optimal path is obtained from the restorative path search

guidance tool and the complete plan is validated by the MA.

5.6.1 Restoration Process for the Practical Network

The MAS is applied and a restoration plan is obtained for the 19

bus practical network. Initially, none of the generators are functioning and all

circuit breakers are open. Considering crew constraints, in every power

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station it is assumed that the generators are turned on one after another with

the ramp rate of 10 MW per minute. The SAs and DAs are numbered

corresponding to their respective bus numbers. The SAs send information

about the available power and DAs send message about the requirements of

power to the zone LDA. The available power in the generating station is

allocated to the load based upon the load characteristics. The optimal path

through which power can be transmitted is given by the restorative path

search guidance tool.

Initially, the practical network is divided into two electrical zones

and it is separated based on the available generation in each electrical zone so

that the generation matches the demand. After the zone recovers 60% of the

load, it can be connected together. The power station NCTPS (bus no 1) and

ETPS (bus no 2) along with the load buses 3, 4, 5, 6, 8 and 9 are grouped as

one zone. The remaining load buses 7,11,12,13,15,16 and 17 along with the

generating stations 14,18 and 19 are grouped as second zone. The load bus 10

will act as a boundary bus between the zones. The GAs at Bus1, Bus2, Bus14,

Bus18 and Bus19 start communicating the amount of power generated by the

respective generating stations to the corresponding zone LDA. At the same

time, all the DAs send a request to the LDA to start the restoration process.

On receiving this request, the LDA decides to energize the buses that are

directly connected in one step to the generating stations.

In the first step (1-4 minutes), the power generated by the stations is

given in Figure 5.19. The load bus which is nearest to the generating stations

is picked up first. So the generating sources in NCTPS (bus no 1) supply the

load to DA10, DA4, DA5, DA6 and DA9. The generating sources in ETPS

(bus no 2) supply the load DA3 and DA4. Thus in electrical zone 1, the

largest power consumer bus 4 (Mylapore) is supplied by two generating

132

stations. Similarly, in the other electrical zone, the three generating stations

MTPS (bus no 14), MAPS (bus no 19) and NEYVELI (bus no 18) energize

the load DA10, DA11, DA12, DA13, DA15, DA16 and DA17. At the end of

the first step, all the loads are supplied 20% of their respective load and is

shown in Figure 5.19.

Figure 5.19 Reconfiguration of the Network for I Step

In the second step (5- 8 minutes) all the load buses recover 40% of

their load. The power generated by each station is given in the Figure 5.20. In

the fifth minute, the load buses DA3, DA5, DA7, DA13, DA15 and DA17

recovers their 40% of load. After energization the DAs inform the LDA about

the updation of load. The load buses DA4, DA6, DA12 and DA16 obtain 40%

of their load in the sixth minute.

133

Figure 5.20 Reconfiguration of the Network for II Step

In the third step (9-11 minute), all the loads are supplied 60% of

load so the corresponding LDA decides about the interconnection of electrical

zones at the end of the 11th minute and it is shown in Figure 5.21. The entire

network is reconfigured but the partial load is supplied.

134

Figure 5.21 Reconfiguration of the Network for III Step

In the fourth step (12-18 minute) the load buses are energized 80%

of load and it is shown in Figure 5.22. Before updating the network, the MA

conducts the load flow to validate the plan and then only the SAs and DAs are

informed about the energisation of load.

135

Figure 5.22 Reconfiguration of the Network for IV Step

In the final step (19-22 minute) all the loads are supplied the full

load and the entire network is reconfigured through the optimal path which

considers the shortest path by distance, priority of loads, power balance

between generator and demand, voltage limit and capacity of the transmission

line. The complete reconfiguration of the network is shown in Figure 5.23.

With the integration of optimal path guidance with the MAS enhances the

network restoration and reduces the cascaded outages.

136

Figure 5.23 Reconfiguration of the Network for V Step

The optimal path is given in Table 5.5, and it is clear that some of

the load buses are not supplied from a single generating source. For example,

the load bus 3 is energised by the sources 1 and 2. For all the load buses the

optimal path for the power transmission is given in the same table. The

restorative path search guidance tool could suggest the other available paths

also for the transmission of power. If the power system operator is not

available or the equipment is not working, then the other paths could be

considered for power transmission which is also given in the same table. For

load buses 13, 15, 17 there is only one path for the supply of power. Hence

that line is in fault condition, then the area around that station could not be

restored.

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Table 5.5 Optimal Path for the 19 Bus Practical Network

Generat

ing

station

Loads to

be

connected

Bus Optimal path Other available path

From To

1, 2,

4,18,19

3, 4, 5, 6,

7, 8, 9, 10,

11, 12, 13,

15, 16, 17

1 3 1 3 3

2 3 3

1 4 1 4 4

2 4 4

1 5 1 5 5

1 6 1 6 6

14 7 14 15 17 7 14 15 13 17 7

19 19 13 17 7 19 13 10 7

1 8 1 8 10 8

1 9 1 9 10 9

1 10 10 10

14 11 14 15 13 10

11

14 15 17 10 11

19 19 13 10 11 19 13 17 10 11

18 18 17 10

11

18 17 16 10 11

19 12 19 13 10 12 19 13 17 10 12

19 13 19 13 Only one path

18 18 17 13 18 17 10 13


19 19 13 15 19 13 17 15

14 16 14 15 17 16 14 15 13 10 16

19 19 13 17 16 19 13 10 16

18 18 17 16 18 17 10 16


138

Based on the optimal path guidance tool, the restoration plan is

obtained for the 19 bus practical network. In Table 5.6 the load bus is given in

numerals without the bracket and the numerals within the bracket represent

the generating station which is supplying the load. For example, 10(1) denotes

that load bus 10 is supplied by the generating station 1. For all the generation-

demand pair, the optimal path is given in the Table 5.5.

Table 5.6 Restoration Plan for 19 Bus Practical Network

Time (minute) Major Events

1 Energise load bus 10(1), 5(1), 3(2), 16(14),

15(19), 13(18)

2 Energise load bus 6 (1), 4(2), 7(14), 13(19),

17(18)

3 Energise load bus 4(1), 9(1), 11(14), 7(19),

12(19), 11(19), 16(18)

4 Energise load bus 8(1), 7(14)

All the load buses attains 20% of their load

8 All the load buses attains 40% of their load

11 All the load buses attains 60% of their load.

MA decides for interconnection of electrical

zone



The load flow is conducted for each minute in MATLAB and the

voltage profile of all the buses for the five steps are included in the Table 5.7.

The voltage of all the buses lies in the safe operating limits.

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Table 5.7 Voltage Profile of All Buses in the Practical Network

Bus NoVoltage Profile (p.u)

Step 1 Step 2 Step 3 Step 4 Step 5

1 1.000 1.000 1.000 1.000 1.000

2 1.000 1.000 1.000 1.000 1.000

3 0.998 0.997 0.996 0.995 0.994

4 0.997 0.993 0.992 0.989 0.988

5 0.999 0.997 0.995 0.991 0.988

6 0.998 0.996 0.993 0.986 0.981

7 0.997 0.991 0.985 0.966 0.950

8 0.996 0.991 0.985 0.976 0.968

9 0.996 0.991 0.986 0.977 0.968

10 0.996 0.991 0.984 0.968 0.955

11 0.992 0.982 0.970 0.948 0.927

12 0.996 0.990 0.983 0.965 0.952

13 0.995 0.983 0.971 0.929 0.901

14 1.000 1.000 1.000 1.000 1.000

15 0.994 0.991 0.967 0.916 0.951

16 0.994 0.996 0.976 0.950 0.953

17 0.995 0.995 0.974 0.937 0.966

18 1.000 1.000 1.000 1.000 1.000

19 1.000 1.000 1.000 1.000 1.000

140

Table 5.8 Computation of Losses for the 19 Bus Practical Network

Time

(minute)

Total Losses (MW)

MAS with Optimal

Path

Power World Simulator

1 1.29 29.40

2 6.56 33.83

3 17.07 42.91

4 30.72 54.71

5 43.89 66.24

6 65.81 91.31

7 93.91 109.96

8 117.53 130.86

9 149.72 158.99

10 186.88 192.20

11 229.75 248.55

12 259.47 275.56

13 304.15 317.84

14 361.09 371.95

15 418.17 426.57

16 469.15 475.19

17 528.27 530.41

18 597.89 602.71

19 673.75 675.64

20 764.38 766.84

21 859.75 869.44

22 935.83 937.82

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The important criteria while transmitting power is to select the path

with minimum losses. Since MAS is integrated with the optimal path search

guidance tool, the shortest path is selected for the power transmission which

automatically minimizes the losses. The total loss in the network is computed

using the MAS with optimal path search tool and the Power world simulator.

From the Table 5.8, it is signified that the losses in the MAS is very much

reduced.

5.7 CONCLUSION

The three-level architecture MAS along with the restorative path

search guidance tool is explained with the 19 bus practical network. In

today’s power system operation, the restoration plan is prepared only for the

predetermined fault conditions of the network. Even for a slight change in the

status of the power system network, the restoration plan has to be updated

under the stressful condition. Under such situation this guidance tool can be

used effectively for changing the network topology and it has the provision to

update the present status of the network.Outages and faults in interconnected

power systems may cause cascading sequences of events, and catastrophic

failures of power systems. So that the cascaded outages can be greatly

reduced with the help of this technique since it considers the entire essential

constraints.

The SPF algorithm based MAS technique provides the guidance to

the power system operators to solve the power system restoration problem for

any type of network topology.

CHAPTER 5 RESTORATION OF POWER USING MULTI-AGENT...

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