chapter 4 substation configuration reliability estimatio n...

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CHAPTER 4

SUBSTATION CONFIGURATION RELIABILITY

ESTIMATION BY SUCCESSFUL PATH METHOD

Substations are integral parts of a power system. They are

important links between the generating station, transmission systems, distribution

systems and the loads at consumer level. The electrical voltage is stepped up

and down to higher and lower voltage levels several times on its way from the

generation station to the consumer at the substation. An electrical substation is

an assembly of electrical components including bus bars, circuit breakers,

power transformers, instrument transformers, surge arrestors or lightning

arresters, isolators or disconnecting switches, neutral grounding equipments,

power line carrier communication equipments, line traps, tuning units,

coupling capacitor ,protection systems, earthling switches, earth electrodes,

shunt reactors , shunt capacitances, series reactors, series capacitors, isolated

phase bus systems, metering, control and relay panels and d.c. battery system.

The substation’s important functions are establishment of economic

load distribution, protection of transmission systems, controlling the power

exchange, ensuring steady state and transient stability, prevention of loss in

synchronism by load shedding, maintaining the system frequency within ac-

ceptable limits, data transmission via power line carrier communication for

the purpose of network monitoring, control, protection and ensuring reliable

supply to the consumers.

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The substations are classified in various ways as follows:

Outdoor substation or in door substation based on location

E.H.V. substation, H.V. substation, M.V. substation and L.V.

substation based on voltage levels

Grid substation or Distribution substation based on application

Conventional air insulated outdoor substation, or SF6 Gas

Insulated Substation (GIS), or hybrid substations having

combination of the above two based on design.

4.1 AIR INSULATED SUBSTATION (AIS)

AIS has galvanized steel structures for supporting the equipments,

insulators, incoming and outgoing transmission lines. Circuit breakers, isola-

tors, transformers, current transformers, potential transformers are installed in

the outdoor. Bus bars are supported on the post insulators or strain Insulators.

This substation occupies large area.

One of the most important innovations in electrical engineering in

the 20th

century is the launch of gas insulated switchgear in 1965 since a

conventional air insulated substation occupied large area. The dimensions

were reduced from air insulated substation due to the introduction of Gas

Insulated Substation (GIS) technology. The maintenance intervals are also

reduced to once in every ten years. This had improved availability and

reliability with lowered operating costs. The SF6 gas enclosure has made the

switchgear insensitive to pollution like the corrosive effects of salt, sand and

snow.

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4.2 GAS INSULATED SUBSTATION (GIS)

The size of substation reduces to 8% to 10% of the Air Insulated

Substation since circuit breakers, current Transformers, voltage transformers,

bus bars, earth switches, surge arresters and isolators are in the form of metal

enclosed SF6 gas modules. These modules are assembled in accordance with

the required design. The various live parts are enclosed in the metal

enclosures containing SF6 gas at high pressure. GIS sulfur hexafluoride (SF6)

gas. Aluminum is used for the enclosure. This assures freedom from corrosion

and low weight of the equipment. The low weight ensures minimal floor

loading. Gas tight bushings allow subdivision of the bay into a number of

separate gas compartments. Each gas compartment is provided with its own

gas monitoring equipment, a rupture diaphragm and filter material. The static

filter in the gas compartments absorb moisture and decompose it. The rupture

diaphragm prevents build up of high pressure in the enclosure. A gas di-

verter nozzle on the rupture diaphragm ensures that the gas is expelled in a

defined direction in the event of bursting, thus ensuring that the operating per-

sonnel are not endangered. All the modules are connected to one another by

means of flanges. The gas tightness of the flange connections is assured by

proven O ring seals. Temperature related changes in the length of the enclo-

sure and installation tolerances are compensated by bellows type expansion

joints.

Circuit breaker module has a central element of the gas insulated

switchgear .The three pole circuit breaker module enclosures comprises of the

two main components, interrupter unit and operating mechanism. The spring

stored operating mechanism provides the force for opening and closing the

circuit breaker. It is installed in compact corrosion free aluminum housing.

The entire operating mechanism unit is completely isolated from the SF6 gas

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compartments. Antifriction bearings and maintenance free charging mecha-

nism ensures decades of reliable operation.

The functions of a disconnect switch and an earthling switch are

combined in a three position switching device. The moving contact either

closes the isolating gap or connects the high voltage conductor to the fixed

contact of the earthling switch. Integral mutual inter locking of the two func-

tions is achieved as a result of this design An insulated connection to the fixed

contact of the earthling switch is provided outside the enclosure for test pur-

poses. In the third neutral position neither the disconnect switch contact nor

the earthling switch contact is closed. The three poles of a bay are mutually

coupled and all the three poles are operated at the same time by a motor. The

gas compartments are constantly observed by means of density monitors with

integrated indicators.

4.3 HYBRID SUBSTATION

Hybrid substations are the combination of both AIS and GIS

Some bays in a substation are gas insulated type and some are air insulated

type. The design is based on convenience, local conditions, area availability

and the economics of cost implications.

An important function performed by a substation is switching.

Switching events may be planned or unplanned. A transmission line or other

component may need to be de-energized for maintenance or for

commissioning of equipment. To maintain reliability of supply, no one ever

brings down its whole system for maintenance. In addition, the function of the

substation is to isolate the faulted portion of the system in the shortest

possible time since fault tends to cause equipment damage and destabilize the

whole system.

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The type of high voltage switching scheme may be selected after a

careful study of the flexibility and protection needed in the station for the

initial installation, and also when the station is developed to its probable

maximum capacity. In an ideal substation all circuits and equipments would

be duplicated such that following a fault or during maintenance, one

connection remains available. Practically this is not feasible since the cost of

implementing such a design is very high. Methods have been adopted to

achieve a compromise between reliability of supply and cost. There are four

categories of substation that give varying reliability of supply:

Category 1

No outage is necessary within the substation for either maintenance

or fault conditions.

Category 2

Short outage is necessary to transfer the load to an alternative

circuit for maintenance or fault conditions.

Category 3

Loss of a circuit or section of the substation due to fault or

maintenance.

Category 4

Loss of the entire substation due to fault or maintenance.

4.4 SUBSTATION CONFIGURATION

Substation configuration implies different methods employed to

connect electrical circuits in the power system to transfer the electrical power

in reliable manner. It helps in delivering the electrical power to power system

if any part of the system is faulty or under maintenance.

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Substations use different types of bus bar arrangements, which

depend upon the application, reliability of the supply and cost of installation.

In every substation, bus bar plays a pivotal role to connect different circuits.

However ,switching is possible in the power system with the help of circuit

breakers and isolators.

4.4.1 Considerations for Selection of Bus Bar Arrangement

Different types of bus bar arrangements are employed based on the

voltage, reliability of the supply, flexibility in transmitting power and cost.

The other aspects considered for designing the bus bars arrangements are:

Simplicity in the design

Maintenance of different equipment without interruption of

the power supply

Feasibility in expansion

Economical installation and operational cost

4.4.2 Different Bus Bar Arrangements

Some of the switching schemes used by bus bar arrangements

employed in the substations are listed below:

Single bus-bar configuration

Sectionalized single bus bar configuration

Breaker and a half configuration

Double bus, double breaker configuration

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The substation configuration is investigated since the recent trend

in urban area is to improve the system reliability by adjustment of substation

bus bar configuration with hybrid switchgear within the same space

constraint. The new technique of Successful Path Method (SPM) is proposed

to analyze the reliability of various substation configurations. The results are

compared with the Cut Set Method of Daniel Nack (2005).

The author has published a paper on “A Novel Approach for

Reliability Analysis of Power System Configurations”, International Journal

of College Sciences in India. vol 3, pp 49-72, July 2008.

4.5 DANIEL NACK METHOD

Daniel Nack had presented the minimal cut set method based on the

criteria of continuity or availability of power supply. It considers each failure

state as an exclusive state, so that the probability of occurrence of system

failure is the sum of all the failure event probability. The components

modeled are transformers, bus bars, breakers and outgoing lines from

substation. The incoming lines were assumed to have 100% reliability for

developing substation indices. Daniel Nack (2005) had proposed the

substation component failure rate value as shown in Table 4.1. These are

converted into the corresponding reliability value. The Reliability of the

component is given by the relationship as per equation (2.10).

R(t) = et

(4.1)

where

R(t) = Reliability

= Failure rate

t = Time period

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Table 4.1 Substation component reliability indices

ComponentTotal component failure

rate per year ( )

Total reliability per year

(R) ,R= e- t

Line 0.046 0.955041962

Transformer 0.015 0.985111939

Breaker 0.006 0.994017964

Bus Bar 0.001 0.999000499

The existing method of Daniel Nack had estimated the indices by

cut set method for the four substation configurations including line failures.

These are shown in Table 4.2.

Table 4.2 Daniel Nack reliability indices

Configuration

Total component

failure rate per

year ( )

Total reliability per

year

(R) , R= e- t

Single bus bar 0.0549 0.94657901

Sectionalized single bus bar 0.0459 0.95513747

Breaker and a half bus bar 0.00356 0.99644632

Double bus double breaker 0.00572 0.994296328

4.6 PROPOSED SUCCESSFUL PATH METHOD (SPM)

Boundaries are required to be established before proceeding with

the reliability evaluation of the substation configurations. In this dissertation

work, the boundary is constructed within the substation for reliability

assessment. The components modeled are transformers, bus bars and

breakers.

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4.6.1 Modeling

There are number of methods described in the standard literature to

evaluate the availability of the power system. This work considers the

availability and unavailability as a two state up or down model to represent all

components in the simplest way for the reliability assessment. The level of

performance criteria evaluated is based on the total failure rate per year of

each component. This is converted into reliability or availability of substation

components .The repair time and its duration are ignored. Sudden opening of

circuit breaker online, without any command is known as passive failure. If

the circuit breaker fails to open after the command from the protective relay,

then it is known as stuck condition of breaker. If many circuit breaker failures

occur simultaneously, then it is known an overlapping failure. In modern

substations, the possibilities of multiple failure events are rare due to the

transition from AIS to superior performing GIS. Hence, this dissertation work

considers total failure rate per year occurring in isolation separately for each

component in the substation. The proposed method of SPM for receiving

continuous power supply is expressed using Boolean logic .All the developed

reliability values for the various substation configurations are estimated from

component values listed in the Table 4.1.

The basic difference between FTA and SPM is the direction of the

analysis. A FTA starts with the undesired event and traces backward to the

causes. The fault tree ends with initiating basic events and failures that are

identified as the primary causes. Success path is associated with the degree of

its usefulness. A SPM starts with an initiating cause and traces forward the

resulting consequences. This forward stepping is repeated for different

selected initiating causes. The end consequences can vary depending on the

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initiating cause. Thus the principle of SPM modeling is to identify in each

step the immediate cause of success, which is to be analyzed.

Most failure probabilities are small (less than 0.1), which uses

approximations when combining failure probabilities. Success probabilities

are usually close to 1.0, these approximations cannot be used and the solution

of success models are more accurate than the solution of failure models.

The single bus bar substation, sectionalized bus bar substation,

breaker and a half bus bar sub station and double bus bar double breaker

substations are analyzed for reliability estimation with the following

assumptions:

Boundaries are defined within the perimeter of the substations.

Reliability is defined as the ability of a component to perform a

required function under given environmental and operational

conditions for a specified period of time. The term component is

used to denote any subsystem that can be considered as an

entity. A required function will be necessary to provide a

specified service. The reliability of the components is assessed

based on the required function under consideration.

Set theory is used for event A and event B indicating the

successful paths in the same substation system domain(s) whose

universal set (U) is shown in Figures 4.1 and 4.2.

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A B

U

S

A B

U

S

Figure 4.1 A OR B (A B)

Figure 4.2 A AND B (A B)

A union of set A and set B consists of all components which belong

to either A or B as shown in Figure 4.1. The algebraic operation

with probabilities A B for two A and B events, which are

independent high probability events are given by

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(A B) = P (A) + P(B) – P(A). P (B) (4.3)

This means that the occurrence of set A has no influence on the

successful occurrence or non successful occurrence of set B and

vice versa. In other words, if two successful paths have components

which are operating in parallel and are isolated from one another,

then the success of one event does not affect the success of the

other event. Thus, the success paths of the components of all events

are independent.

The intersection of set A set and set B consists of only elements,

which belong to both sets A and B. This is denoted by A B as

shown in Figure 4. 2.

The algebraic operation with probabilities A B for two A and B

events, which are independent high probability events are given by

P (A B) = P(A) P(B). This is known as the

multiplication rule for probabilities.

The development of a quantitative success model is based on the

need to get the best possible estimate for the top success event

probability.

The success modeling for successful path method includes the

procedure and nomenclature by which events and gates are named

for the specific success of the top event. The process of SPM gives

the information about nature of the basic event and the number of

such events in the combined set of the occurrence of the top event

showing the quantitative importance of each basic event

contributing to the top event.

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Each set is evaluated by probability of its occurrence and it’s

inter relationships.

The quantitative results are interpreted to provide the potential

impact upon the success of the top event.

The immediate cause concept of the successful immediate steps is

determined from the necessary and sufficient occurrence of the next

sequence of its events. The final successful event is achieved

proceeding up the success tree continuously transferring the success

mechanism to success mode till the success tree is completed. The

success mechanisms are evaluated using two basic types of gates,

the OR gate and the AND gate.

The reliable supply is available at the High Voltage transmission

line feeders L1 or L2 in the substations as shown in Figures

4.3- 4.17.

The bus bars, breakers, transformers and lines are considered as

components in the sub-station as shown in Figures 4.3- 4.17. They

are expressed as HV bus no.1, HV bus no.2, LV bus, breakers B1,

B2, B3, B4, B5, B6, B7, B8, B9, B10, B11, B12, B13, B14, B15, B16, B17,

B18, B19, transformers T1 and T2. The substation component

reliability values are substituted from Table 4.1 Each component is

checked for meeting its two criteria. The first criterion checks its

healthy status. If a component is healthy then it can be used. In

other words, it will allow the current to flow. The second criterion

checks whether the current flow is available to the component. If

these two criteria are full filled then the component forms an AND

gate, whose current output is available.

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Considering the worst case, one of the bus bar is functioning

satisfactorily out of the available two bus bar units as shown in

Figures 4.3- 4.17.

Considering the worst case, one of the transformer is functioning

satisfactorily out of the available two transformer units as shown in

Figures 4.3- 4.17.

4.6.2 Single Bus Bar Substation

L1 L2

B2

HV Bus

B4

T2T1

B3

B1

LV Bus

Figure 4.3 Single bus bar configuration

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A single bus bar substation configuration consists of

transformers T1and T2. These transformers are connected in parallel between

the low voltage bus bar (LV bus) and high voltage bus bar (HV bus) through

breakers B3 and B4. The high voltage transmission lines L1 and L2 are con-

nected to the high voltage bus bar (HV bus) through breakers B1 and B2. The

single bus bar substation is shown in Figure 4.3.

This is the simplest bus bar scheme available, which consists of

single bus bar connected to the transformers and load feeders. All the feeders

are connected by circuit breakers and set of isolators. This arrangement helps

to remove the connecting equipments for maintenance by opening the circuit

breaker and isolator contacts.

Single bus bar configuration is lower in installation cost. It requires

less maintenance. It is simple in construction. However, single bus bar

configuration is not very reliable since incase of fault on bus bar the feeders

L1 and L2 connected to bus bar loose supply.

The single bus bar substation reliability is estimated for various

modes of operations as follows:

Mode 1

Successful operation of the single bus bar substation.

The logic for single bus bar configuration during the operation of

transformer T1, when T2 transformer is not available is shown in Figure 4.4.

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Figure 4.4 Logic for single bus bar configuration during T1 operation

Current flow to Line (L1)

Current flow to Breaker (B1)

Current flow to transformer (T1)

Current flow to Breaker(B2)

OR

LV bus allows current to flowCurrent flow to LV bus

Current flow to HV bus


B1allows current to flow

Breaker (B2) allows Current to flow

HV bus allows current to flow

Breaker (B3) allows current to flow

Transformer (T1) allows current to flow

Line L1 allows Current to flow

Line (L2) allows Current to flowCurrent flow to line L2

Current flow to breaker (B3)

AND

AND

AND

AND

AND AND

A AND

AND

AND

AND

AND

AND AND

ANDAND


Current Output


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A reliability value is estimated in mode 1 by substituting the substation

component reliability indices from Table 4.1

The estimated value of reliability in mode 1

= [L1 B1 L2 B2) HV Bus B3 T1 LV Bus

= [L1* B1 L2*B2] *HV Bus* B3.*T1 *LV Bus

= [L1* B1 + L2*B2 – L1* B1 *L2* B2] *HV Bus* B3*T1 *LV Bus

= 0.974753298

Mode 2

Successful operation of the single bus bar substation


transformer T2, when transformer T1 is not available is shown in Figure 4.5.





= [L1*B1 L2*B2] *HV Bus* B3* T2 *LV Bus

= [L1*B1 + L2*B2 – L1*B1*L2.*B2] *HV Bus*B3*T1*LV Bus

= 0.974753298

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Current flow to Breaker(B2)

OR

LV bus allows current to flowCurrent flow to LV bus





HV bus allows current to flow




Line (L2) allows Current to flowCurrent flow to line L2


AND

AND

AND

AND

AND AND

A AND

AND

AND

AND

AND

AND AND

ANDAND


Current Output

Figure 4.5 Logic for single bus bar configuration during T2 operation

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Mode 3

Operation of single bus bar configuration in

Mode 1 Mode 2.

Model 3 is a logic of operation in mode 1 OR mode 2.

The reliability value in mode 3

= 0.974753298 0.974753298

= 0.999362604

A reliability value of 0.999362604 is obtained for single bus bar

configuration.

4.6.3 Sectionalized Single Bus Bar Substation

Sectionalized single bus bar substation configuration is shown in

Figure 4.6.

Figure.4.6 Sectionalized single bus bar configuration

HV Bus

LV BusLV Bus

HV Bus

L1 L2

B2

B4

T2T1

B3

B1

B5

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In a sectionalized single bus bar configuration, the bus bar is split

into sections by means of a bus coupler (B5). A sectionalized single bus bar

configuration is flexible in operation. It is higher in reliability than single bus

bar configuration. Isolation of bus sections for maintenance is possible in this

scheme. However, it has a higher cost than a single bus bar configuration as

additional circuit breaker and isolator is required.

A logic for sectionalized single bus bar configuration during the

operation of transformer T1 is shown in Figure 4.4.

Mode 1

Successful operation of the sectionalized single bus bar substation.

The logic for single bus bar configuration during the operation of transformer

T1, when bus coupler B5 is on and transformer T2 is not available is shown in

Figure 4.4.

A reliability value is estimated in model 1 by substituting the

substation component reliability indices from Table 4.1


= [(L1 B1 L2 B2) HV Bus B3 T1 LV Bus

= [L1*B1 L2 *B2] *HV Bus* B3* T1 *LV Bus.

= [L1*B1 + L2*B2 – L1*B1*L2*B2] * HV Bus * B3 *T1 *LV Bus.

= 0.974753298

Mode 2

Successful operation of the sectionalized single bus bar substation

in Mode 2.

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transformer T2, when bus coupler is on and transformer T1 is not available is

shown in Figure 4.5.

A reliability value is estimated in mode 2 by substituting the

substation component reliability indices from Table 4.1



= [L1*B1 L2 * B2] HV Bus* B3*T2.* LV Bus.

= [L1*B1 + L2*B2 – L1*B1*L2.*B2] *HV Bus *B3 *T2 *LV Bus.

= 0.974753298

Mode 3

Operation of sectionalized single Bus Bar configuration in

Mode 1 Mode 2

Mode 3 is a logic of operation in Mode 1 OR Mode 2.


= Mode 1 Mode 2

= 0.974753298 0.974753298

= 0.999362604

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AND AND

LV Busallows current to flowCurrent flow to LV Bus

Current flow in line (L2)

(L1) Line allows current

to flow

(B1)Breaker allows

current toi flow

Current to flow

(T1) TransformerallowsCurrent to flow

(T2) Transformer allows

current to flow

HV Busallows current

to flow

(B2) Breaker allows

currrent to flow


Current flow toHV Bus

Current flow to (T2)

Transformer)


Current flow to

HV Bus


AND

AND

ANAND

AND

OR

AND

AND AND

AND AND

Breaker (B4) allows current flow

Current flow to

breaker (B4)(B3 )Breaker allows

current flow

Current flow

to Breaker (B3)

AND AND

AND

HV Bus allows

L2 Line allows Current to flow

Current output

Current flow in line (L1)

Mode 4

The successful operation of sectionalized single bus bar configuration

logic during B5 off in mode 4 is shown in Figure 4.7.

Figure 4.7 Logic for Sectionalized Single bus bar configuration during

B5 off

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A reliability value is estimated in Mode 4 by substituting substation



= [(L1 B1 HV BUS B3 T1) (L2 B2 HV BUS B4

T2)] LV BUS.

= [(L1*B1*B3 *HV BUS *T1) (L2*B2*B4*HV BUS*T2) ]*LV

BUS

= 0.993917857

Mode 5

Model 5 is a logic of operation in Mode 3 OR Mode 4.


= Mode 3 Mode 4

= 0.999362604 0.993917857

= 0.99999612

The reliability value of 0.99999612 is obtained for sectionalized

single bus bar configuration.

4.6.4 Breaker and a Half Bus Bar Substation

The Breaker and a half bus bar configuration are shown in

Figure 4.8.

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Figure 4.8 Breaker and half bus bar configuration

Figure 4.8 shows two main buses, which are normally energized.

There are three circuit breakers and two feeder circuits between the buses.

This arrangement allows for breaker maintenance without interruption of

service. A fault on either bus may cause no feeder interruption. This

configuration has high reliability, operational flexibility, capability of isolating

any circuit breaker either of the main bus for maintenance without service

interruption. However it has higher cost and protection and control schemes

are more complex

Mode 1

Successful operation logic for breaker and a half bus bar configuration

during transformer T1 and HV bus bar no 1 in operation, when HV bus No 2

and transformer T2 are not available is shown in Figure 4.9

B7

B9B8

B6

B10 B11

T1 T2

L1 L2

HV Bus no 1

HV Bus no 2

LV Bus

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Figure.4.9 Logic for breaker and half bus bar configuration during T1

and HV bus bar no1 in operation




= [L1 L2 B7 HV BUS no 1 B6] B8 T1 LV bus

= [L1 L2 *B7 *HV Bus no 1 *B6] *B8 *T1 *LV BUS

= 0.999000499



Current flow to (T1) Transformer

Current flow to (B8) Breaker


Current flow to (B8)

Current flow to HV Bus no 1

(T1)Transformer allows to current flow

Current flow to LV Bus

(B8) Breaker allows current to flow

(B6) Breaker allows current

to flow


Current flow to

Line (L2) (L1) Line allows current to flow (L2) Line allows current to flow


HV Bus no 1 allows current to flow

AND

AND

AND

OR

AND

AND

AND

ANDAND

LV Bus allows current to flow

Current Output

85

HV Bus no 1allows current

to flow

AND

AND

AND

AND

AND

AND

AND

OR

AND

LV Bus allows current to flow


Current to Breaker (B9)

(L2) allows current to flow


Current flow to Transformer (T2)


(T2) Transformer allows current to flow


(B6) Breaker allows current

to flow

(L1) Line allows current to flow

(Current to flow to Breaker (B7)

(Current to flow to HV Bus no 1

(Current to flow to Breaker (B6)

(Current to flow to Line (L1)



Mode 2

Successful operation of logic for Breaker and a half bus bar

configuration during transformer T2 and HV Bus bar no 1 in operation, when

HV bus bar no 2 and transformer T1 are not available is shown in Figure 4.10.

Figure 4.10 Logic for breaker and half bus bar configuration during T2


Current Output

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A reliability value is estimated in mode 2 by substituting the

substation component reliability indices from Table 4.1.


= [L2 L1 B6 HV BUS no 1 B7] B9 T2 LV bus

= [L2 L1 *B6 *HV Bus no 1* B7] *B9 *T2 *LV BUS

= 0.999000499

Mode 3

Operation of Breaker and half configuration in

Mode 1 Mode 2

Mode 3 is a logic of operation in mode 1 OR mode 2.


= Mode 1 Mode 2

= 0.999000499 0.999000499

= 0.999999001

Mode 4



HV bus bar no.1 and transformer T2 are not available is shown in Figure 4.11.

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and HV bus bar no2 operation




(B11)Breaker allows current to flow

Current flow to Transformer

to transformer (T1)





(B8) Breaker allows current flow


HV bus no 2 allows

current flow

(T1) transformer allows current to flow

LV bus allows current to flow



Current flow to HV Bus no2




AND

AND

AND

AND

AND

AND

AND

AND

AND

OR

Current Output

(B11) Breaker allows

current to flow

Current flow to Line L2

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= (L1 B8 L2 B9 B11 HV bus no 2 B10) T1 LV bus

= (L1*B8 L2 *B9 *B11 *HV bus no 2 *B10) T1 *LV bus

= 0.980989071

Mode 5



HV bus bar no 1 and transformer T1 are not available is shown in Figure 4.12.



Current Output



AND

AND

AND

AND

AND

AND

AND

AND

AND

OR


(B8) allows current to flow

HV bus no 2 allows current flow

(T2) Transformer allows current to flow


2)


9 Current flow to Breaker (B8)



Current flow to HV Bus no 2



Current flow to LV bus



current to flow


current to flow

(B9) Breaker allows

current to flow


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component reliability indices from Table 4.1.


= (L2 B9 L1 B8 B10 HV Bus no 2 B11) T2 LV BUS

= (L2 B9 L1 *B8 *B10 *HV Bus No2 *B11)* T2 * LV BUS

= 0.980989071

Mode 6


Mode 4 Mode 5



= Mode 4 Mode 5

= 0.980989071 0.980989071

= 0.999638585

Mode 7


Mode 3 Mode 6

Mode 7 is a logic of operation in Mode 3 OR Mode 5. The reliability

value in mode 7

90

= Mode 3 Mode 6

= 0.999999001 0.999638585

= 1.0

A reliability value of 1 is obtained for breaker and a half bus bar

configuration.

4.6.5 Double Bus Bar Double Breaker Substation

A Double Bus bar double breaker configuration is shown in

Figure 4.13

Figure 4.13 Double bus bar double breaker configuration

B12

T2

HV Bus no 1

LV Bus

B13 B14 B15

B16 B17 B18 B19

T1

L1 L2

HV Bus no 2

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AND

AND

AND

AND

AND AND

AND AND

OR




Current flow to HV bus no1


Current flow to Breaker ( B12)


B12 allows current

to flow

Breaker ( B15) allows Current to flow

HV bus no.1 allows current to

flow




Line L1 allows Current

L to flow

Line L2 allows Current to flow Current flow to Line L2


Figure 4.13 consists of two main buses, both are normally

energized. Between the main buses are two breakers and one circuit. This

arrangement allows for any breaker to be removed from service without

interruption to its circuit. A fault on either of the main bus may not cause

circuit outage. A breaker failure will result in the loss of only one circuit. A

double bus bar double breaker configuration has higher reliability and operational

flexibility. However it is highest in cost due to the requirement of two breakers per

circuit.

Figure 4.14 Logic for Double bus bar double breaker configuration

during T1 and HV bus no 1 in operation

Current Output

92

Mode 1

Successful operation logic for double bus bar and double breaker

configuration during transformer T1 and HV Bus bar no 1 in operation when

HV bus bar 2 and transformer T2 are not available is shown in figure 4.14. A

reliability value is estimated in mode 1 by substituting the substation component

reliability indices from Table 4.1

The estimated value of reliability in Mode 1

= (L1 B12 L2 B15) HV BUS no 1 B13 T1 LV Bus

= (L1*B12 L2*B15)*HV BUS No 1* B13 *T1* LV Bus.

= 0.974753298.

Mode 2

Successful operational logic for double bus bar and double breaker

configuration during transformer T2 in operation and HV Bus bar no 1 when,

HV bus bar no 2 and transformer T1 are not available is shown in figure 4.15.



The estimated value of reliability in Mode 2

= (L1 B12 L2 B15) HV bus no1 B14 T2 LV bus.

= (L1 *B12 L2 *B15) *HV bus no 1* B14 *T2 *LV bus.

= 0.974753298.

93

Figure 4.15 Logic for Double bus bar double breaker configuration

during T2 and HV bus no 1 in operation

AND

AND

AND

AND

AND AND

AND AND

OR






Current flow to Breaker ( B12)


B12 allows current

to flow

Breaker ( B15) allows Current to flow

HV bus no.1allows current to

flow




Line L1 allows Current

L to flow

Line L2 allows Current to flow Current flow to Line L2


Current to flow to B15

Current Output

94

Mode 3


Mode 1 Mode 2



= Mode 1 Mode 2

= 0.974753298 0.974753298

= 0.999362605

Mode 4



bus bar no 1and transformer T2 are not available is shown in Figure 4.16.




= (L1 B16 L2 B19) HV bus no 2 B17 T1 LV bus.

= (L1 *B16 L2* B19) *HV bus no 2* B17.*T1 *LV bus.

= 0.974753298

95

Figure 4.16 Logic for double bus bar double breaker configuration

during T1 and HV bus no2 in operation

Mode 5


configuration during transformer T2 and HV Bus bar no 2 in operation when

HV bus bar no. 1 and transformer T1 are not available is shown in

Figure 4.17.

Current Output

AND

AND

AND

AND

AND AND

AND AND

OR






Current flow to HV bus no 2







HV bud no.2 allows current to flow


B16 allows current to flow


Line L1 allows current to flow

Line L2 allows Current to low

96

Figure 4.17 Logic for double bus bar double breaker configuration

during T2 and HV bus no2 in operation

Current Output

AND

AND

AND

AND

AND AND

AND AND

OR







Current flow to HV bus no.2








B16 allows current to flow

Line L1 allows current to flow

HV bus no.2 allows current to flow


97




= (L1 B16 L2 B19) HV BUS no 2 B18 T2 LV BUS

= (L1*B16 L2 *B19) *HV BUS no 2 *B18 *T2 *LV BUS.

= 0.974753298

Mode 6

Operation of double bus bar double breaker configuration in Mode

4 Mode 5



= Mode4 Mode 5

= 0.974753298 974753298

= 0.999362605

Mode 7

Operation of double bus bar double breaker configuration in

Mode 3 Mode 6



= Mode 3 Mode 6

98

= 0.999362605 0.999362605

= 0.999999594.

A reliability value of = 0.999999594 is obtained for double bus bar

double breaker configuration.

4.7 RESULT AND DISCUSSIONS

A comparison is done between the proposed and the convention

methods to establish the usefulness of the proposed SPM method.

4.7.1 Comparison of Proposed and Daniel Nack Method for Reliability

Assessment of Various Substation Configurations

Table 4.3 shows the comparison between the proposed SPM and

Daniel Nack method for reliability assessment of various substation

configurations.

Table 4.3 Proposed and Daniel Nack method for reliability values of

various substation configurations

Configuration

Estimated

reliability value as per

existing method

Estimating reliability

value as per pro-

posed method

Breaker and a half bus bar 0.99644632 1.00000000

Double bus bar double breaker 0.994296328 0.999999594

Sectionalized single bus bar 0.95513747 0.99999612

Single bus bar 0.94657901 0.999362604

The proposed method estimates reliability value of 1 for one and a

half bus bar configuration, where as Daniel Nack method estimates a value of

0.99644632. The proposed method estimates reliability value of 0.999999594

99

for double bus bar double breaker configuration, whereas Daniel Nack

method estimates a value of 0.994296328. The proposed method estimates

reliability value of 0.99999612 for sectionalized single bus bar configuration,

whereas Daniel Nack method estimates a value of 0.95513747. The proposed

method estimates reliability value of 0.999362604 for single bus bar

configuration, where as Daniel Nack method estimates a value of 0.94657901.

Daniel Nack failure values for various substation configurations

were less than 0.01.Lower numerical values obtained for failure will tend to

encourage approximations. The failure values are converted to reliability

values for comparing with the proposed method. Therefore these minor

differences may be due to the effect of approximations. The proposed SPM

method has estimated reliability close to one and thus avoided approximations.

Hence, the proposed reliability estimates are more accurate. In addition,

proposed SPM has less computational time. It is easy to understand as it is

based on Boolean logic.

4.8 CONCLUSION

The proposed method of SCADA short term forecasting improves

load forecasting as given below.

It uses 30 samples for estimating the short term load forecasting

in advance for the next 30 minutes on one minute interval. This

is an improvement over the other standard methods given in

literatures, where one sample is taken for one hour.

The result shows consistent average variation of actual and

estimated load , which are very encouraging.

100

MAPE value shows improvement in forecasting the load

compared to other method. Thus, the developed projection

statistics are useful and powerful diagnostic tool.

The estimated frequency is computed at every point in real

time with reasonable accuracy. This has resulted in the

frequency stability of the system.

A special feature of continuously self correcting mechanism

built in the proposed algorithm to assess the deviant points

gives better accuracy.

It used a salient feature of maximum and minimum forecast

user points in the algorithm, which can be changed by a

forecaster due to the changes in weather. Thus it has improved

load forecasting accuracy compared to other conventional

methods.

The proposed method of SPM usefulness is given below:

It uses a simple method of Boolean Logic for reliability

assessment of substation configuration.

It requires less computational time than the existing method of

Daniel Nack.

It is easier to understand and simple to implement.

It has estimated reliability values accurately.

The breaker and a half scheme are generally recommended in

the field, for continuity of power supply. The

proposed SPM method proves this field utility requirement by

estimating its reliability as 1.0.

chapter 4 substation configuration reliability estimatio n...

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