New reliability index for power system protection based on multi-agent technique

2013 3rd International Conference on Electric Power and Energy Conversion Systems, Yildiz Teclmical University, Istanbul, Turkey, October 2-4,2013

New Reliability Indexfor Power System Protection

based on Multi-Agent Technique

Nadheer A. Shalash Faculty of Electrical and Electronics Engineering

University Malaysia Pahang, 26600 Pekan, Malaysia

eng_ n _ a _ [email protected]

Abstract- A new index reliability for power system protection by

optimizing the relay time operating using multi agent system

(MAS) is presented. In the proposed method, the MAS consists of

two agents as follows; first agent is a fault current agent, i.e., to

determine the fault current at all points before and after

grounding. While, the second agent is the time operating, i.e., to

determine the time operating relay before and after modifying

the fault current. The simulation implementation is carried out in

Iraqi national grid system (132 kV) - Najaf city. The results show

that the failure rate is decreased to 50% approximation for over

current and earth fault relays.

Keywords- Multi-agent system; reliability indices; power system protection

I. INTRODUCTION

Reliability assessment has been studied for many years in the field of transmission and distribution of power systems. Most of the related companies had invested a lot of equipment that required for periodically maintenance. In most of maintenance cases, the procedures are based on the historical experiences of maintenance's workers, where they are rarely cases supported by the analytical evidences [1]. However, recently, new concepts to analyze the maintenance procedures have been introduced, such in [2], new models and concepts of Markov model based on the concepts of self-down state and induced-down state are developed that incorporating the effect of protection system failures into power system reliability evaluation. Meanwhile, in [3], a probability based models has been developed to accurately reflect the stochastic nature of a power system behavior and assess its reliability performance. This had improved the common deterministic criteria approach that cannot handle the stochastic nature of power system. Then, in [4], the reliability considerations using synchrophasor monitoring of system conditions that incorporated into protection schemes is discussed, in which the problem stemming of hidden failure modes can be reduced and also can enhance the effectiveness and reliability of protection systems.

Nowadays, agent systems have been used in many areas such as computer science, artificial intelligence and variety of applications [5]. A multi-agent system is a system composed of multiple interacting intelligent agents. An agent can be viewed

Abu Zaharin Ahmad Faculty of Electrical and Electronics Engineering

University Malaysia Pahang, 26600 Pekan, Malaysia [email protected]

as perceiving its environment through sensors and acting upon its environment through actuators. Therefore, a multi-agent system can be used for all type systems composed of multiple autonomous components that demonstrating the following features; 1) each agent has incomplete in terms of global control and capabilities for solving problems, 2) the data are decentralized and its computation is asynchronous, 3) they should be designed according to the problems under consideration [6].

In grid power system, the approach of spread local control must be more robust in order to face the extreme contingencies. Practically, power systems are using many agents such as protective relays and automatic voltage regulators [7]. There are two factors of the relay protection system reliability, i.e., dependability and security. Dependability can be defmed as the ability of the relay to trip for a fault in its protective area while security is the ability to refrain from tripping relay when there is no fault in the protected area [8]. Therefore, in this paper, new approach of application the multi agents system (MAS) for reliability assessment by modify operating time relay is presented. The reliability improvements show the quick promising results.

II. RELIABILITY INDICES

The most common methods used for reliability evaluation, are based on the loss of load or energy (LOLE) approach. In this method, the suitability index that describes the generation reliability level is LOLE. It indicates the time in which the load is more than available generation. The reliability expresses the proportion time of the component if it is "in service" or "available for". Therefore, the availability (A) of a component can be expressed as [9],

A =

I)uptime) (1) (�)uptime)+ �)downtime))

Where, the "up time" is the total time when the component is in service and "down time" is the total time when component not in service. Availability (A) may be expressed in terms of mean time to failure (MTTF) and mean time to repair (MTTR) as given by,

978-1-4799-0688-8/13/$31.00 ©2013 IEEE


MTTF A = ------

MTTF+MTTR (2)

Thus, the mean time between failures (MTBF) is the sum of MTTF and MTTR and can be represented by cycle of time "T" as seen in Fig. 1.

Power system components can be characterized by discrete system states with constant transition rates between these states. State "0" represents the healthy component in an operating condition.

Operating

1 � MITR I- M1TF --I Failed

I I I • Time

I- T • 1 Figure l. Mean time diagram for a two state component

Meanwhile, for state "I" (failed state) when the component cannot perform its intended function. Transition occurs between state "0" and "1" and the transition rates between these states are the failure rate "2 " and the repair rate "II" as shown in Fig. 2.

State "0" :Ie (failure rate! State "1" Component � (repair rate) Component Operating Failed

Figure 2. Diagram of state transition rates

Therefore, Eqn. (1) may be re-expressed in terms of failure rate and repair rate such that,

A=� J.L+A (3)

The fault frequency f is the reciprocal of the average cycle time [10].

1 =_1_ J.L+A (4)

The other two reliability indices can be computed from the previously defined ones, as follows:

T=� (5) f

D=� W f

Where, T is a time cycle, 0 is a mean duration and P is an individual probability

III. EARTH FAULT AND OVERCURRENT RELAY

In protection power system, the time overcurrent devices are used as the primary scheme, the fault signal in this relay is tripped in a time specified by a delay-time versus overcurrent

(OC) fault curves [11]. The characteristics of each relay have been used by manufacturers as a set of curves conformable to different values of time dial settings. For the time inverse relay characteristics, the relay starts to operate after an intended time delay [12]. A typical inverse time OC relay has two values to be set, i.e., the pick-up current (Ip) and time multiplier setting (TMS). The Ip current value is the minimum current value for which the relay operates. And, for TMS, it should be in the range of 0.1 to 0.9. The tripping currents of the relays are varied according to the tripping time required and the characteristics of other protection devices used in the network. According to the type of relay as given in Table I, the operating time of protection (OTP) can be adjusted using the curve characteristics [13] as shown in Figures 3 and 4 for transformer (90 MY A). These characteristics are calculated by the following Eqn. (7).

TABLE !. TYPE OF RELAY BASED ON TRANSFORMER CAP AClTY

Transformers relay CT Ip TMS

specifications

Transformer 90 MVA ,( OC ) 400/5 5 0.3

Transformer 90 MVA ,( OC ) 300/5 5 0.3

Transformer 90 MVA ,(EF ) 400/5 1.5 0.3

Transformer 90 MV A , (EF) 300/5 1.5 0.3

3.5r--�-�-�-�-�-�-�--' OTP

3

2.5

2

1.5

0.50 1 000 2000 3000 4 000 5000 6 000 7 000 8000 I f. kA

Figure 3. Characteristics of OC relay for power transformer ( 90 MVA)

3 . 5,...-----.-------.---...,....-----.-----,

OT P 3

2.5

2

1 .5

O.5 0L---�5 0'-O---1 ..... 00-0---1 ..... 5 0-0 ---2 0 ...... 0-0----'25 00

I f, kA

Figure 4. Characteristics of earth fault (EF) relay for power transformer ( 90 MVA)


OTP = [ If JTMS 1pxCT (7)

Where, [ f = fault current and CT = current transformer ratio

IV. PROPOSED INDEX OTP

Usually, accords to MTBF. The protective relay manufacturers will describe the reliability of their apparatus with the relays may vary depending on the relay installation and operation over a period of time. The reliability indices determined by using real-time data analysis that provides more accurate or realistic information as it uses the past failure data of the relays [14].

The basis for all points in the system effective grounding or in a specific part of the system is the ratio of zero-sequence reactance (X a) to positive-sequence reactance (Xl) is not more than three; meanwhile the ratio of zero-sequence resistance (Ra) to Xl is not more than one for any operational condition. The system may be considered as effectively grounded [15] when the following conditions are met;

X 1<-°<3 Xl

& R

0< _0 <1 Xl The expression of the Zo/Z\ ratio can be expressed as:

(8)

(9)

By expressing the residual current during the term of the three phase current and Zr/ZI ratio, the single phase to ground fault current can be expressed as;

If 3V 3V

(10)

Where K=Zo/Z\

Thus

V 13ph = ZI

3 (2+K)

(11)

(12)

In all simulated cases, the value of the ratio (Rr/XI) is not changed. Therefore, the resistance was neglected during the calculations [16].

Zo Xo K=-=- (13) ZI XI

That means the value of K should be in the range of 1 to 3. According to Eqn. (8), hence, Eqn. (12) can be rewritten in terms of application value of K such that,

I 0.6 < _"_f < 1 (14) 13 ph

From Eqn. (7) & Eqn. (14), it can be noted that the proposed new index reliability of OTP can be calculated by using the following equation.

OTP = ( 0.613Ph <If< 13ph ITMS 1p xCT ) (15)

Eqn. (16) is then used to calculate the expected failure rate according to Eqn. (15);

N A,( t) = -:-tv:--, --

I OTPm 111=1

Where, N is the total number of failures.

V. MULTI-AGENT SYSTEM

(16)

Technically, the agents or providers act autonomously on the platform of collecting information. A number of agents are identified as multi agents, with individual subtasks are synchronized so as to attain a particular common goal. There are generally two configurations of agents, i.e., first the agents with different subtasks and second is agents of the same kind and on the same hierarchical level or order. A multi agent system is a computational program in which several agents unite to accomplish some tasks or projects. Tn the power system, there could be a different number of multi agent systems structured [17].

A thorough investigation on the various modeling, both protection and reliability indices are necessary for developing an effective detection technique. In order to achieve the reliability assessment, it is essentially necessary to develop each agent the effective communication within the platform and globally build a working relationship as they negotiate towards the laid criterion [18]. Therefore the design starts with the setting of protection system. When a fault occurs in power system such as short circuits, this fault will be cleared by over OC, earth fault (EF) or differential protection (DP). OC relays are used as short circuit protection and some aspects must be considered in the setting of such protection. For example, the current settings shall be high enough to not risk during maloperation. On the other hand, it is also must be setting to low enough to give a secure operation at minimum fault current (tripping requirement).

The proposed techniques for protection and grounding based on MAS can be validated on a real practical power system to measure the response of multi-agent, the accuracy of the


developed reliability index and the sensitivity of the protection system. The procedures of the proposed framework are surmnarized by the flow chart as given in Fig. 5.

Data Colle ction of practic al Iraqi power system

Data Selection

Si m ulati on of the gi yen power system in different conditions

Development of the Modeling

Risibility Indices Proposed Index

I ntell i gent Multi ·Agent System

Calculation

Verification

Figure 5. Flow chart of agent-based new index with reliability indices.

VI. DESCRIPTION OF TEST SYSTEM

In order to evaluate the OTP index with indices reliability, the 132 kV transmission system ofNajaf City is used which is one sector of Traqi national grid (132 kV) as shown in Fig. 6. This system consists of 3 generators, 8 lines, 10 transformers and 3 feeders. The data on demand, reliability and generation was registered in the Iraqi national dispatch center during January 2007. Table II depicted the reliability data of Najaf

city.

TABLE II. RELIABILITY DATA OF NAJAF CITY

Component No. A(t) Il Comp

Generator 3 3 321

Transformer 10 2 595

Circuit breaker 16 4 662

Relay OC Bus 1 2 - 532 REAL Y OC Bus 2 2 - 532 REAL Y OC Bus 3 2 542 REAL Y OC Bus 4 2 - 533 REAL Y EF Bus 1 2 - 631 REAL Y EF Bus 2 2 - 647 REAL Y EF Bus 3 2 - 664 REAL Y EF Bus 4 2 - 647

LINE 8 0.053 0.0830 Feeder 3 0.093 0.0830 Busbar 4 0.3 22

r-nJaf U.P.S.

2

shnilliyali( 1 + 2)

J'.<ajaf['l;ol11i ';ajafTowII 1 3

Jwohery Til't Factor'\'

t-------41 �----41 4

I I I

Figure 6. Single line diagram of Najaf city.

VIT. PROPOSED METHOD

The proposed method is employed to the system by first, the positive, negative and zero sequence of admittance matrices are composed from the network including the grounding of transformers and generators. After that, the fault currents are calculated, then, the operating times of OC and EF relays during fault are computed according to the type of ground in and operation characteristic of relays as presented in section m. Then, determine all the possibilities of grounding in and the other substations. Afterwards, the ratios X/Xl is calculated. The simulation is repeated to calculate the fault current and OTP according to Eqn. (16).

The reliability indices can be evaluated by affecting all bus bar faults. The OTP of EF and OC relays are depending on the fault current magnitude. So, the multi agents can be used for recalculated the reliability indices before and after modified

fault current between (O.6I3Ph < If < I3Ph ) in the buses.

VIII. RESULTS AND DISCUSSION

TABLE III. CASES OF OPERATING OF EF RELAY BEFORE MODIFIED

Location If Amp. OTP(EF) A(t) Fault sec.

Bus 1 1503.25 0.8099 2.469 Bus 2 3887.09 0.5830 3.42 Bus 3 5667.87 0.5240 3.813 Bus 4 3305.51 0.6125 3.26

TABLE IY. CASES OF OPERATING OF oC RELAY BEFORE MODIFIED.

Locatiou If Amp. OTP(Oq sec. A(t) Fault

Bus 1 1503.25 1.2821 1.57 Bus 2 3887.09 0.7989 1.704 Bus 3 5667.87 0.7713 2.57 Bus 4 3305.51 0.9735 2.056


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L,-�-'-� I ��==�===="3' =C" =T :8="=O �. =)== =±====� __ _ .. � L"':CI

Figure 7. Visualization of multi agent for OTP.

TABLE V. CASES OF OPERATING OF EF RELAY AFTER MODIFIED.

Location 0.6/3 ph <If </3 ph A(t) Fault OTP(EF) sec.

Bus I 1503.25 -1498.1 0.8099 -0.811 1.243 Bus 2 3887.09 -3880.3 0.5830 -0.59 1.7 Bus 3 5667.87 -5658.4 0.5240 -0.525 1.9 Bus 4 3305.51 -3312.2 0.6125 -0.612 1.64

TABLE VI. CASES OF OPREATTNG OF oC RELAY AFTER MODIFIED.

Location E 0.6/3ph<lf</3ph A(t) Fault OTP(OC) sec.

Bus I 1503.25 -1498.1 1.2821 -1.2849 0.781

Bus 2 3887.09 -3880.3 0.7989 -0.7995 0.852

Bus 3 5667.87 -5658.4 0.7713 -0.7718 l.293 Bus 4 3305.51 -3312.2 0.9735 -0.9725 1.027

Tn this section, the results of the proposed method are delivered. From Tables (TIT to VI), shown the decreasing of employ the new index reliability failure rates approximately to 50% when compared to before modified fault current. For example in bus 1, the fault current is 1503.25A at before modifYing the fault current. While after modified, the fault current is limited between the range of 1498.lA and 1503.25A. From that, it can be seen that the results of failure rates of EF and OC have been decreased to approximately 50%.

Figure 7 shows an evolution of OTP after modifYing the fault current for all bus bars using JADE (Java Agent Development Framework) package. The result of failure rates are sent to calculate indices reliability by agent communication language (ACL), that is a message class, which is responsible for sending and receiving the information between the agents. Developing a JADE agents are, by developing a java class that runs (import jade.core.agent) and applying the setupO method. This approach is where the actual agents are inserted .The agent OTP can communicate with other agents from buses for making a decision (refuse / accept) to take a minimum fault current. According to Eqn. (16), the value of reliability indices will decrease and therefore the reliability of the power system will also improve.

The indices reliability results are shown in the tables VII and VTIT for before and after modifYing fault current. The fault frequency is constant in all cases and the cycle time, mean duration is decreed for all relays. For example, OC relay in bus I, that has been shown in table VII, the f = 0 .0019, D=196. 93 and T=535.14 for failure rate = 1.57. Then, when the limited

fault between (O.613ph < It < 13ph ) the failure rate has been

decreased to 0.781 in table VIII, which is the f= 0.0019, D=195.3 and T= 533.56 are computed. Therefore, the reduction in failure rates of the OC and EF relays are leading to the improvement of reliability and protection quality.

TABLE VII. SIMULATION RESULT OF RELIABILITY INDICES BEFORE MODIFIED FAULT CURRENT

Component A (t) I-t f D T Generator 3 321 .0031 120.33 327

Transformer 2 595 0.0016 151.9 615

Circuit Breaker 4 662 .0014 218.52 726

Relay OC Bus t 1.57 532 0.0019 196.93 535.14

Relay OC Bus 2 1.704 532 .0019 197.03 535.4

Relay OC Bus 3 2.57 542 .0018 201.34 547.14

Relay OC Bus 4 2.056 533 0.0019 197.65 537.11

Relay EF Bus 1 2.469 631 0.0016 117.01 635.93

Relay EF Bus 2 3.42 647 0.0015 120.3 653.84

Relay EF Bus 3 3.813 664 0.0015 123.57 671.62

Relay EF Bus 4 3.26 647 0.0015 120.24 653.52

Line 0.053 0.0830 1.9724 0.1227 0.507

Feeder 0.093 0.0830 2.7624 0.13 0.362

Busbar 0.3 22 0.431 8.004 23.2


TABLE VIII. SIMULATION RESULT OF RELIABILITY INDICES AFTER MODIFIED FAULT CURRENT

Component 1 (t) 11 f D T Generator 3 321 .0031 120.33 327

Transformer 2 595 0.0016 151.9 615

Circuit Breaker 4 662 .0014 218.52 726

Relay OC Bus 1 0.781 532 0.0019 195.3 533.56

Relay OC Bus 2 0.852 532 .0019 196.4 533.7

Relay OC Bus 3 1.293 542 .0018 200.4 544.58

Relay OC Bus 4 1.027 533 0.0019 196.89 535.05

Relay EF Bus 1 1.243 631 0.0016 116.56 633.48

Relay EF Bus 2 1.7 647 0.0015 119.67 650.4

Relay EF Bus 3 1.9 664 0.0015 122.875 667.8

Relay EF Bus 4 1.64 647 0.0015 119.65 650.28

Line 0.053 0.0830 1.9724 0.1227 0.507

Feeder 0.093 0.0830 2.7624 0.13 0.362

Bus bar 0.3 22 0.431 8.004 23.2

IX. CONCLUSION

In this paper, the use of agents for solving power system problems has been reviewed. New index reliability for power system protection is proposed in order to improve the quality and efficiency of the power system according to failure rates that has been greatly decreased. With the using of MAS, the high speed rate of protection will facilitate the incorporation of smart grid system by increasing the security of power system based on the developed techniques and decreasing the economic loss due to damaged equipment.

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New reliability index for power system protection based on multi-agent technique

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