Chapter 5 AVAILABLE TRANSFER CAPABILITY …shodhganga.inflibnet.ac.in/bitstream/10603/17295/15/15_chapter5.pdf100 Chapter 5 AVAILABLE TRANSFER CAPABILITY CALCULATIONS AND CONTINGENCY

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Chapter 5

AVAILABLE TRANSFER CAPABILITY

CALCULATIONS AND CONTINGENCY ANALYSIS

IN DEREGULATED POWER SYSTEMS

The available transfer capability work is well established [3, 71-89]

and introduction about ATC and contingency is given in chapter 1. In

this chapter, only methods for computation of ATC, problem

formation and algorithm are given. Also computation of ATC for 7-bus

system, 26-bus system, IEEE 118-bus system and case studies of

124-bus real-time Indian utility power system of Andhra Pradesh

State grid are presented and discussed. A brief description of

contingency analysis along with complete results of Andhra Pradesh

State grid is presented.

5.1. METHODS FOR COMPUTATION OF TRANSFER

CAPABILITY

In recent years, there has been a rapidly growing interest for power

engineers to formulate and solve this complex transfer capability

problem. As a result, many methods and techniques have been

developed; very few methods are practical for large realistic

applications [81]. Only three of them are practical for large realistic

applications. These are follows:

1) Continuation Power Flow (CPF) method [86]

101

2) Optimal Power Flow (OPF) method.

3) Repeated Power Flow (RPF) method.

CPF is first introduced for determining the maximum loadability,

and is also useful for ATC computation. The advantage of CPF is a

successful method even for ill-conditioned power flow equations and

at voltage collapse points. However a major disadvantage is that it

involves complicated implementation of its parameterization, predictor

and corrector and step-size control elements.

OPF and Security-Constrained OPF (SCOPF) are powerful tools [26]

that have been under very active development for the past 30+ years.

OPF can be used to maximize the power transfer between two areas

assuming that all OPF optimized parameters can be centrally

dispatched needs large number of optimal power flows under different

conditions and needs more time.

The RPF method, power flow equations are repeatedly solved at a

succession of points along the specified load/generation increment,

for TTC calculation. Compared with SCOPF and CPF, the

implementation of RPF is much easier [71].

There are a number of methods and algorithms [76, 78] for

computing TTC, which are discussed in the chapter 1.

5.2. PROBLEM FORMULATION

Referring to Figure 5.1, a simple interconnected power system can

be divided into three kinds of areas, which are: receiving area, sending

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area and external area. “Area” can be defined in an arbitrary fashion.

It may be an individual electric system, power pool, control area, sub-

regions, etc. which consist of a set of buses. The transfer between two

areas is the sum of the real powers flowing on all the lines which

directly connect one area to the other area. A base case transfer

(existing transmission commitments) is determined, the transfer is

then gradually increased starting at the base case transfer until the

first security violation is encountered. The real power transfer at the

first security violation is the total transfer capability.

R – Receiving area; S – Sending area;

E – External area; .……transfer path

Figure 5.1. A simple interconnected power system

The objective is to determine the maximum real power transfers

from sending areas to receiving area through the transfer path.

During a transfer capability calculation, many assumptions [82] may

arise that would affect the outcome. The main assumptions used in

this study are as follows:

S S

S R

E E

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• The base case power flow of the system is feasible and

corresponds to a stable operating point.

• The load and generation are changing very slowly so that the

system transient stability is not jeopardized.

• The system steady state stability is maintained with sufficient

damping.

• Bus voltage limits are maintained before the system loses voltage

stability.

Therefore, at this stage only the thermal limits and voltage limits

will be taken into consideration together with generator active and

reactive power limits.

The power flow solution is the most common and important tool in

power system analysis, which is also known as the “load flow”

solution. It is used for planning and controlling to determine the

voltage magnitudes and phase angle of voltages at each bus and

active and reactive power flow in each line. The four quantities

associated with each bus are voltage magnitude, voltage phase angle,

real power injection and reactive power injection.

The Newton-Raphson equations are used in natural power system

form solving for voltage magnitude and angle, given real and reactive

power injections and these can be used in the calculation of transfer

capability [26, 91]. The same thing can express in the mathematical

form as follows [81]:

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Power Flow Equations:

The complex power injected by the source into the ‘i’ th bus of a

power system is

*

iiiii IVjQPS =+= ; i = 1, 2… n (5.1)

The load flow problem is handled more conveniently by use of Ii

rather than Ii*. By taking the complex conjugate of equation (5.1),

iiiii IVjQPS**

=−= ; i = 1, 2… n (5.2)

Then real and reactive powers can be expressed as

)δδ(θ||Y|V|VP jiijijj

n

j

ii +−= ∑=

cos1

(5.3)

)δδ(θ||Y|V|VQ jiijijj

n

j

ii +−−= ∑=

sin1

(5.4)

and Operational constraints

maxmin GiGiGi PPP ≤≤ (5.5)

maxmin GiGGi QQQ ≤≤ (5.6)

maxijij SS ≤ (5.7)

maxmin iii VVV ≤≤ (5.8)

105

The objective function to be optimized is

∑∉∈

=RkRm

kmrPP

,

(5.9)

The control variables in the above formulation are generator real

and reactive power outputs, generator voltage settings, phase shift

angles, transformer taps and switching capacitors or reactors. The

dependent variables in the formulation are slack bus (swing bus)

active and reactive power injections, regulated bus (generator bus)

reactive power injection and voltage angle. All the equality and

inequality constraints considered in this work are given in the above

problem formulation.

5.3. ALGORITHM FOR REPEATED POWER FLOW

METHOD

Repeated power flow (RPF) method [81] involves the solution of a

base case, which is the initial system conditions, and then increasing

the transfer. After each increase, another load flow is done and the

security constraints tested. The computational procedure of this

approach is as follows:

Step 1. Establish and solve for a base case

Step 2. Select a transfer case

Step 3. Solve for the transfer case

Step 4. Increase step size if transfer is successful

Step 5. Decrease step size if transfer is unsuccessful

106

Step 6. Repeat the procedure until minimum step size

reached

The flow chart of this method is given in Appendix-C.2.

5.4. METHODS FOR COMPUTATION OF AVAILABLE

TRANSFER CAPABILITY

A report by NERC in 1996 establishes a framework for determining

ATC of the interconnected transmission networks for a commercially

viable wholesale market. The report defines ATC principles under

which ATC values are to be computed and it permits individual

systems, power pools, regions and sub regions to develop their

procedure for determining ATC in accordance with these principles.

Reference [79] discussed some theoretical aspects of ATC and the

problem of its evaluation under open access environment.

In [86], a method based on continuation power flow incorporating

limits of reactive power flows, voltage limits as well as voltage collapse

and line flow limits is described. However, with this method, the

computational effort and time requirement are large. In [81], the

topological information of a system is stored in matrix form and

constants for different simultaneous cases and critical contingencies

have been calculated before hand and used for determination of ATC

values. For very large systems, the method may be quite cumbersome.

In [83], the localized linearity of the system is assumed and additional

load required to hit the different limits are separately calculated and

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the minimum of all these is taken as ATC. Ashwani Kumar et al. [90]

proposed a simple and efficient non-iterative method to calculate ATC

under bilateral and multilateral contracts based on ACPTDF. The

ACPTDFs have been calculated at base case NRLF results utilizing a

sensitivity-based approach.

METHODS BASED ON DISTRIBUTION FACTORS:

In order to consider line flow (MW) limits for static AC

determination under the system intact, AC power transfer distribution

factors is used. To consider voltage limits in ATC determination under

the normal cases Voltage Distribution Factors [VDF] are used.

1. AC POWER TRANSFER DISTRIBUTION FACTORS

Consider a bilateral transaction ‘tp’ between a seller bus, ‘m’ and

buyer bus, ‘n’. Further consider a line, ‘l’ carrying a part of the

transaction power. Let the line be connected between a bus-i and a

bus-j. For a change in real power transaction between the above seller

and buyer, say, by ‘∆ tp’ MW, if the change in transmission line

quantity ‘ql’ is ‘∆ql’, the AC power transfer distribution factors can be

defined as:

(ACPTDF)ql-tp p

l

t

q

∆

∆ (5.10)

The transmission quantity ‘ql’ is taken as real power flow from bus-

i to bus-j.

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2. VOLTAGE DISTRIBUTION FACTORS

Voltage Distribution Factors (VDFs) are defined as the change in

the bus voltage magnitude ‘∆Vi’ at any bus-i to the change in the pth

transaction, say, by ‘∆ tp’ between seller bus and the buyer bus. Let

the base case voltage magnitude at any bus-i be ‘Vi0’ and, after the

change due to a transaction, the new bus voltage be Vi-tp. The voltage

distribution factors are defined as:

VDFi-tp=p

i

t

V

∆

∆ (5.11)

Where, ∆Vi = Vi-tp – Vi0

5.4.1. PROBLEM FORMULATION FOR DISTRIBUTATION FACTORS

ATC determination using ACPTDF and Voltage distribution factors:

The distribution factors have been computed with the base case

load flow results using the sensitivity properties of the NRLF Jacobin.

The procedure for calculation of these distribution factors is described

below:

Consider the sensitivity relationship provided by the Newton-

Raphson load flow equations in the polar coordinates for a base case

load flow as:

∆V

∆δ = [ ]TS

∆Q

∆P (5.12)

Where, ST = [ ] 1−

TJ is a sensitivity matrix and JT is the full Jacobian

defined for all the buses except for the slack bus.

109

At a base case load flow, if only one of the bilateral transactions,

say the pth transaction, between a seller bus, ‘m’ and a buyer bus, ‘n’

is changed by ‘∆ tp’, only the following two entries in the mismatch

vector [ ∆P, ∆Q ] T of equation (5.12) will be non-zero.

∆Pm = ∆ tp, ∆Pn = -∆ tp (3.13)

With the above mismatch vector, changes in the voltage angle and

voltage magnitude at all the buses can be computed from equation

(5.12), and hence, a new voltage profile can be calculated. These can

be utilized to compute new values of transmission quantity ‘ql’ and

thus the change in the quantity ‘∆ql’ from the base case. Once ‘∆ql’ is

known for all the lines and change in the voltage magnitude is

computed at all the buses corresponding to a transaction ‘∆ tp’, the

ACPTDFs and VDFs for each line and buses, respectively, can be

obtained from (5.10) and (5.11).

ATC from a bus/zone ‘m’ to another bus/zone ‘n’ can be found

using the full AC load flow by varying the amount of transaction until

one or more line flows in the transmission system considered or a bus

voltage at some bus reaches the limiting value. However this method

is computationally involved. Instead, the distribution factors

described above can be used to quickly calculate ATC considering

both the line flow limits and voltage limits, as follows.

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i) ATC for base case, between bus/zone ‘m’ and bus/zone ‘n’

using the line flow limit criterion has been calculated using

ACPTDFs as

ATCmn = min

∈−

l

ij,mn

ijijNij

PTDF

)P(P 0max

(5.14)

Where,

Pijmax is the MW power flow limit of a line between bus-i and bus-j.

Pij0 is the base case power flow in the line between bus-i and bus-j.

PTDFij-mn are the Power Transfer Distribution Factors for the line

between bus-i and bus-j, when ‘Nl’ is the total no. of lines.

ii) Similarly ATC at base case considering voltage limit violation

can be calculated as:

ATCmn = min

∈−

−

B

i,mn

iti NiVDF

)V(Vmin

(5.15)

Where,

Vi-t is the voltage at bus-i under change in the transaction tp.

Vimin is the minimum voltage limit at each bus-i.

VDFi-mn is the voltage distribution factor at bus-i, when the

transaction is taking place between bus/zone ‘m’ and bus/zone ‘n’

and ‘NB’ is the total number of buses.

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CONTINGENCY ANALYSIS

5.5. INTRODUCTION

With the global development towards the deregulation in the power

system industry, the volume and complexity of the CA results in the

operation and the system studies have been increasing. Not only has

deregulation resulted in much larger system model sizes, but also CA

is computed more frequently in the restructured power markets to

monitor the states of the system under “what if” situations in order to

accommodate the maximum number of power transfers. The net

impact of these changes is a need for more effective CA results are

required to help with the comprehension of the essential security

information, information which could be buried in the enormous and

complex CA data sets [29], [30].

The CA application is based on detailed electrical model of power

system, called the “network model”. This is a simulated model of the

real power system that is prepared by each utility’s system planners

and network engineer specialists. They translate the real world

equipment and connections of a power system (one –line diagram) into

a mathematical model of the power network that is suitable for

solution by computer algorithms. This network model contains the

connection information and electrical characteristics of the

equipment. The algorithm in contingency analysis uses this network

information and the network model to simulate, and calculate the

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effects of, removing equipment from the power system. With an

initialized power network model, CA now can be executed with a

series of contingency events that is prepared by the CA user. A

“contingency list” contains the each of the elements that will be

removed from the network model, one by one, to test the effects for

possible overloads of the remaining elements. The criteria for selection

of elements for the contingency events are further described below.

In its basic form, CA executes a power flow analysis for each

potential problem that is identified on a contingency list. The voltages,

currents, real and reactive power flows (MW and Mvar) in each part of

the power system can be obtained from power flow solution in

contingency analysis. Results of each contingency test and the

network solution are compared with the limits for every element in the

power system. The lists of violations are saved in the CA data base.

CA actually provides and prioritizes the impacts on an electric

power system when problems occur. Contingency is also called an

unplanned "outage". CA is a computer application that uses a

simulated model of the power system, to evaluate the effects, and

calculate any overloads resulting from each outage event. In other

word, CA is essentially a "preview" analysis tool that simulates and

quantifies the results of problems that could occur in the power

system in the immediate future. This analysis is used as a study tool

for the off-line analysis of contingency events, and as an on-line tool

to show operators what would be the effects of future outages. It

113

allows operators to be better prepared to react to outages by using

pre-planned recovery scenarios.

After a contingency event, power system problems can range from:

• None: When the power system can be re-balanced after a

contingency, without overloads to any element.

• Severe: When several elements such as lines and transformers

become overloaded and have risk of damage.

• Critical: When the power system becomes unstable and will quickly

collapse.

By analyzing the effects of contingency events in advance,

problems and unstable situations can be identified, critical

configurations can be recognized, operating constraints and limits can

be applied, and corrective actions can be planned.

5.6. METHODS OF CONTINGENCY ANALYSIS

The following are the various methods used for contingency

analysis purpose.

• DC load flow method of contingency analysis

• Z-matrix method of contingency analysis

• Voltage stability index (L-index) computation

• Decoupled load flow

• Fast decoupled load flow

114

The tool for detecting overloads is analyzing contingency of the

power system planner which should have execution speed and ease of

detection are the vital considerations. Among the methods mentioned

above, AC power flow calculations methods are accurate when

compared to DC power flow methods but they are a bit complex and

need more execution time.

5.7. RESULTS AND DISCUSSION OF ATC

Results of the ATC for 7-bus, 26-bus, IEEE 118-bus systems and

124-bus Indian utility power system of Andhra Pradesh State grid are

presented and analyzed are given below:

5.7.1. 7-bus System

The computations were done on 7-bus test system with 3-areas as

shown in Figure 5.2. Data for this system is given in Appendix A.1.

Figure 5.2. 7-bus test system

115

The system has been divided into three areas, namely; area A, area

B and area C. Area A includes buses 1, 2, 3, 4 and 5. Area B consists

of bus 6 whereas area C consists of bus 7.

5.7.1.1. Model calculation for PTDF:

The PTDF values are calculated by use of the power world

simulator software with lossless DC approximation and are given in

Table 5.1.

Table 5.1: PTDF values of 7-bus system

S.no. From bus To bus % PTDF

1 1 2 80.41

2 1 3 19.59

3 2 4 2.47

4 2 5 46.47

5 2 6 32.16

6 3 4 18.91

7 4 5 21.38

8 5 7 67.84

9 6 7 32.16

5.7.1.2. Model calculation for ATC:

ATC is calculated by use of the equation (5.14).

ATC =

∈−

l

ij,mn

ijijNij

PTDF

)P(P 0max

Consider a transmission line connected between bus 2 and 6, then

the maximum power limit is max

ijP = 160MW,

116

The power flow in the line in base case is 0

ijP =74.7MW

and PTDF from Table 5.1 is 0.3216

Then ATC = (160- 74.7)/ (0.3216) = 265 MW

Similarly, ATC is calculated for the remaining part of 7-bus system,

26-bus system, IEEE 118- bus system and 124-bus real-time Indian

utility power system of Andhra Pradesh State grid.

ATC values of 7-bus system are shown in Table 5.2.

Table 5.2: 7-bus case, ATC data

Transfer

areas

Transfer

buses

ATC

(MW)

Limiting

line

A-B

1-6 54 1-2

2-6 265 2-6

4-6 281 2-6

A-C

1-7 56 1-2

2-7 106 2-5

4-7 152 4-5

B-C 6-7 111 6-7

5.7.2. 26-bus system

ATC is calculated from all generator buses to normally heavily

loaded load buses by use of PTDF’s, and variation of ATC is given in

Tables 5.3 to 5.8 for different load conditions vary from 50% to 100%.

Thermal limits used for calculating ATC are given in Appendix A.2.

117

Table 5.3: Variation of ATC from bus 1 to other buses of 26-bus

system

Transfer

buses

NR method 100% load 90% load 75% load 50% load

ATC

(MW)

Limiting

line

ATC

(MW)

Limiting

line

ATC

(MW)

Limiting

line

ATC

(MW)

Limiting

line

ATC

(MW)

Limiting

line

1-3 200 17-18 215 17-18 272 16-17 314 16-17 308 16-17

1-5 238 6-18 195 6-18 180 6-18 72 6-18 162 6-18

1-9 197 2-8 195 7-9 153 2-8 154 16-20 232 9-10

1-12 230 17-18 228 2-8 146 2-8 108 12-14 215 12-14

1-15 141 17-18 152 17-18 138 13-15 147 13-15 217 16-17

1-16 80 17-18 86 17-18 101 15-16 99 15-16 117 16-17

1-17 33 17-18 36 17-18 46 17-18 55 1-17 56 16-17

1-18 235 1-18 208 1-18 223 1-18 197 1-18 329 1-18

1-19 201 6-19 203 6-19 200 16-20 94 6-18 212 6-18

1-24 59 22-24 55 22-24 65 22-24 62 22-24 80 22-24

Table 5.4: Variation of ATC from bus 2 to other buses of 26-bus system

Transfer

buses


ATC

(MW)

Limiting

line

ATC

(MW)

Limiting

line

ATC

(MW)

Limiting

line

ATC

(MW)

Limiting

line

ATC

(MW)

Limiting

line

2-1 666 1-2 672 1-2 691 1-2 670 1-2 723 1-2

2-3 251 17-18 270 17-18 333 16-17 350 2-8 378 16-17

2-5 266 6-18 218 6-18 201 6-18 81 6-18 181 6-18

2-9 193 7-9 194 7-9 139 2-8 112 2-8 232 9-10

2-12 224 2-8 208 2-8 134 2-8 105 12-14 209 12-14

2-15 156 12-15 175 12-15 135 13-15 144 13-15 230 13-15

118

2-16 86 17-18 90 15-16 98 15-16 96 15-16 123 15-16

2-17 34 17-18 37 17-18 47 17-18 52 16-17 54 16-17

2-18 259 1-18 229 1-18 246 1-18 217 1-18 363 16-17

2-19 204 6-19 205 6-19 189 2-8 109 6-18 246 6-18

2-24 59 22-24 54 22-24 65 22-24 62 22-24 79 22-24


system

Transfer

buses


ATC

(MW)

Limiting

line

ATC

(MW)

Limiting

line

ATC

(MW)

Limiting

line

ATC

(MW)

Limiting

line

ATC

(MW)

Limiting

line

3-1 225 3-13 182 3-13 104 3-13 94 12-14 188 12-14

3-5 206 3-13 166 3-13 95 3-13 76 12-14 152 12-15

3-9 193 3-13 157 3-13 86 12-14 62 12-14 124 12-14

3-12 183 12-14 150 3-13 69 12-14 50 12-14 100 12-14

3-15 139 13-15 124 13-15 72 3-13 76 3-13 158 3-13

3-16 84 15-16 78 15-16 77 3-13 80 3-13 106 15-16

3-17 40 17-18 42 17-18 52 16-17 44 16-17 45 16-17

3-18 185 16-17 167 16-17 99 3-13 89 12-14 155 16-17

3-19 195 6-7 160 3-13 91 3-13 69 12-14 138 12-14

3-24 57 22-24 53 22-24 63 22-24 60 22-24 78 22-24

119


system

Transfer

buses


ATC

(MW)

Limiting

line

ATC

(MW)

Limiting

line

ATC

(MW)

Limiting

line

ATC

(MW)

Limiting

line

ATC

(MW)

Limiting

line

4-1 216 4-12 218 4-12 386 4-12 279 6-7 342 16-17

4-3 194 4-12 195 4-12 346 4-12 357 4-12 359 12-15

4-5 196 6-7 161 6-7 205 6-7 98 6-7 144 6-7

4-9 178 10-12 162 10-12 219 7-9 218 9-10 218 9-10

4-12 162 4-12 163 4-12 289 4-12 299 4-12 313 4-12

4-15 118 12-15 133 12-15 178 13-15 190 13-15 190 12-15

4-16 92 15-16 85 15-16 92 15-16 91 15-16 116 15-16

4-17 38 17-18 40 17-18 52 17-18 47 16-17 48 16-17

4-18 214 4-12 215 4-12 240 16-17 178 6-7 206 16-17

4-19 177 10-12 162 10-12 232 6-19 149 6-7 218 6-7

4-24 56 22-24 52 22-24 62 22-24 59 22-24 76 22-24


system

Transfer

buses


ATC

(MW)

Limiting

line

ATC

(MW)

Limiting

line

ATC

(MW)

Limiting

line

ATC

(MW)

Limiting

line

ATC

(MW)

Limiting

line

5-1 106 5-6 100 5-6 110 5-6 288 5-6 275 5-6

5-3 106 5-6 100 5-6 110 5-6 288 5-6 275 5-6

5-6 106 5-6 100 5-6 110 5-6 288 5-6 275 5-6

5-9 106 5-6 100 5-6 110 5-6 232 9-10 232 9-10

5-12 106 5-6 100 5-6 110 5-6 148 12-14 275 5-6

120

5-15 106 5-6 100 5-6 110 5-6 165 13-15 219 16-17

5-16 85 17-18 92 17-18 104 16-17 103 15-16 118 16-17

5-17 34 17-18 37 17-18 47 17-18 55 16-17 56 16-17

5-18 106 5-6 100 5-6 110 5-6 288 5-6 275 5-6

5-19 106 5-6 100 5-6 110 5-6 237 6-19 272 6-19

5-24 63 22-24 58 22-24 69 22-24 66 22-24 85 22-24


system

Transfer

buses


ATC

(MW)

Limiting

line

ATC

(MW)

Limiting

line

ATC

(MW)

Limiting

line

ATC

(MW)

Limiting

line

ATC

(MW)

Limiting

line

26-1 82 11-26 185 11-26 181 11-26 169 11-26 175 11-26

26-3 81 11-26 183 11-26 180 11-26 168 11-26 173 11-26

26-5 76 11-26 170 11-26 168 11-26 156 11-26 162 11-26

26-6 76 11-26 170 11-26 168 11-26 156 11-26 162 11-26

26-9 80 11-26 178 11-26 175 11-26 163 11-26 169 11-26

26-12 80 11-26 179 11-26 176 11-26 128 11-26 170 11-26

26-15 81 11-26 166 12-15 147 13-15 156 13-15 171 11-26

26-16 80 11-26 94 17-18 102 15-16 100 15-16 123 16-17

26-17 34 17-18 37 17-18 47 17-18 53 16-17 55 16-17

26-18 80 11-26 179 11-26 176 11-26 164 11-26 170 11-26

26-19 76 11-26 171 11-26 168 11-26 156 11-26 162 11-26

26-24 61 22-24 56 22-24 67 22-24 64 22-24 83 22-24

121

ATC is varying with variation of load. This variation of ATC depends

upon generators in operation, load variation, bus voltages, variation of

line flows and line limits.

5.7.3. IEEE 118-bus system

ATC is calculated between the areas, and a variation of ATC is

given Tables 5.9. Thermal limits used for calculating ATC of all the

lines is taken as 500MVA.

Table 5.9: Variation of ATC between the areas for different cases

of IEEE 118-bus System

Transfer

areas

Case 1 Case 2 Case 3 Case 4

ATC

(MW)

Limiting

area/line

ATC

(MW)

Limiting

area/line

ATC

(MW)

Limiting

area/line

ATC

(MW)

Limiting

area/line

1-2 750.68 Area 2 67.46 Area 1 231.08 Area 1 49.27 Area 1







3-2 717.31 Area 2 735.76 65-68 133.11 Area 2 234.71 Area 3





122

5.7.4. CASE STUDY OF APSEB

5.7.4.1. Data collected in March and September 2011

The assessment of ATC using PTDF has been conducted on 124

bus real-time Indian utility power system of Andhra Pradesh State

220-kV transmission grid. For this case study all generating stations

and 400-kV buses are taken as sellers and some of the load busses

are taken as buyers. The sellers and buyers combination is taken

according to the Distribution Companies (DISCOMs) already available

in the State.

In this case study, generation and load data is taken at two

different instants and the analysis was done as two different cases.

They are

1. March 2011

2. September 2011.

Combinations of buses used to calculate ATC are given in Table

5.10. The ATC for these combinations are tabulated in Appendix-B,

Table B.3. The variation of ATC is depending on already committed

dispatches and thermal limits.

Table 5.10: Sets of buses used to calculate ATC for the case of March 2011 and September 2011

Transfer buses

From To

1 3,5,6,8,10,13,15,16,18,19,20,21,22,24,30,36,37,39

4 3,5,7,13,15,16,19,22,30,35,37,39

27 3,6,7,10,16,18,19,20,22,24,30,35,39

123

29 5,6,8,10,15,16,18,20,22,24,30,35,36,37,39

30 3,6,7,10,13,15,16,18,19,20,21,22,24,35,36,37,39

35 3,5,6,7,10,13,15,16,18,19,20,21,22,24,30,36,37,39

37 3,5,6,8,10,13,15,16,18,19,20,21,22,24,35,36,39

9 3,5,6,8,10,13,16,18,19,20,21,22,24,36,39,42,43,51,53,57,59

11 3,5,10,13,16,19,22,36,42,43,51,57,59

15 5,6,8,10,16,18,19,20,21,22,24,36,39,42,43,51,53,57,59

20 3,5,6,7,10,13,16,18,19,22,24,36,39,42,43,51,53,57,59

45 3,5,6,7,10,13,16,18,19,20,21,22,24,36,39,42,43,51,53,57,59

46 3,5,6,7,10,13,16,18,19,20,21,22,24,36,39,42,43,51,53,57,59

50 3,5,6,8,10,13,16,18,19,20,21,22,24,36,39,42,43,51,53,57,59

54 3,5,6,7,10,13,16,18,19,20,21,22,24,36,39,42,43,51,53,57,59

56 3,5,6,8,10,13,16,19,22,24,36,42,43,51,53,59

61 3,5,6,7,10,13,16,18,19,20,21,22,24,36,42,43,53,57,59

128 3,5,6,8,10,13,16,18,19,20,21,22,24,36,39,42,43,51,53,57,59

96 95,98,100,101,106,108,109,112,123

97 95,98,100,101,103,106,108,109,112,123

100 95,98,101,103,106,108,109,112,123

102 95,98,100,101,103,106,108,109,112,123

104 95,100,103,106,109,112

105 95,98,101,103,106,108,112,123

106 95,98,100,101,103,108,109,112,123

107 95,98,100,106,109,112,123

111 95,98,100,101,103,106,108,109,123

112 95,98,103,106,109,123

115 95,98,100,101,103,106,108,109,112,123

68 71,77,82,83,86,88,89,125

74 71,77,81,82,83,86,88,89,125

75 71,77,81,82,83,86,88,89,125

78 71,77,81,82,83,86,88,89,125

124

84 71,81,83,86,88,89,125

85 71,81,83,86,88,89,125

87 71,77,81,82,83,86,88,89,125

91 71,81,82,86,88,125

Variation of ATC for above mentioned cases; the difference is more

100MW is shown in from Figures 5.3 to 5.6.

Figure 5.3. Variation of ATC from March to September 2011


125



From the above figures it is observed that variation of ATC is very

high from March to September 2011 for the following bus

combinations: 1-20,21; 27-3; 30-3; 35-3,21; 37-21; 11-42,57; 20-3;

50-3; 54-3; 128-20,21,36,39,57; 105-103; 115-100,101,103,108, 112,

123; 68-77 ,82 , 88, 89; 75-71; 84-71; 85-71; 87-89.

The results are certainly useful in an online environment of

deregulated power system to perform the transactions between buyer

and seller.

126

5.7.4.2. Case 3: With zero interstate power transfer

For this case study, total APGENCO, private sector and two

generating stations (bus 1 (2600MW), bus 115 (1500MW)) from CGS

being considered, similarly as the case 1 and 2 but interstate power

from other states as CGS share is set to zero. The analysis was done

at different load conditions varying from 50% to 125% as follows.

ATC is calculated between the areas for different load conditions of

case 3 are tabulated in Table 5.11.

Table 5.11: Variation of ATC between the areas for different cases of Case 3

Transfer

areas

Case 3 (a)

100%(AG)

Case 3 (b)

125%

Case 3 (c)

100%

Case 3 (d)

75%

Case 3 (e)

50%

ATC

(MW)

ATC

(MW)

ATC

(MW)

ATC

(MW)

ATC

(MW)

1-2 1019 1944 1941 1160 353

1-3 1199 1503 1436 1247 353

1-4 1520 1790 537 1057 353

2-1 528 101 36 913 28

2-3 748 101 36 881 1211

2-4 801 101 36 473 673

3-1 734 620 541 458 28

3-2 1019 1428 941 485 358

3-4 795 1359 545 480 368

4-1 607 21 411 224 11

4-2 636 23 1668 216 11

4-3 610 228 1515 1042 111

127

5.7.4.3. Case 4: With interstate power (CGS share) transfer

In this case study, analysis was done with total APGENCO

(8923.86MW), IPPs (3286.3MW) and CGS share (3229.04) being

considered. The details of AP share from CGS are given in chapter 3.

The analysis was also done at different load conditions varying

from 50% to 125% and ATC is calculated between the areas for

different load conditions of case 4 are tabulated in Table 5.12.


Transfer

areas

Case 4 (a)

100%(AG)

Case 4 (b)

125%

Case 4 (c)

100%

Case 4 (d)

75%

Case 4 (e)

50%

ATC

(MW)

ATC

(MW)

ATC

(MW)

ATC

(MW)

ATC

(MW)

1-2 2061 483 1674 999 486

1-3 1040 483 1531 991 204

1-4 838 483 1572 638 450

2-1 681 491 258 56 427

2-3 1355 692 457 607 612

2-4 1408 610 1018 607 606

3-1 1146 413 553 54 381

3-2 1299 1573 1435 979 697

3-4 869 610 1018 647 510

4-1 676 413 314 56 426

4-2 2576 2174 901 363 513

4-3 675 1168 1155 363 1403

ATC is also calculated for the Case 4(a) (with 100% load, all

generators are operating) and Case 4(c) (with 100% load, some costly

128

generators are off) for the combination of buses are given in Table

5.13. ATC for the given combination is tabulated in Appendix-B, Table

B.22.

Table 5.13: Sets of buses used to calculate ATC for Case 4(a) and

4(c)

Transfer buses

From To

1 2,3,5,12,24,25,26,27,28,35,129

4 2,3,12

27 1,28,35,37

29 30,31,32,33,34,35,36,38,39,40,42,113

30 6,19,29,31,32,33,34,35,36,38,39,40,42,87,113

31 29,30,32,33,34,35,36,42,87,111,113

32 29,30,31,35

34 29,30,33,35,36

35 1,6,19,28,37

37 1,6,19,28,35

9 6,7,8,16,19,20,21,22,23,35,36,39,42

11 1,2,3,4,12

15 6,9,10,13,16,17,18,19,20,21,23

20 9,15,19,21,35,46,47

45 15,20,21,44,46,47,54,68

46 21,45,47,49,52,54,68,82,84,85

50 45,46,48,49,52,57,61,64,66,72

52 46,48,49,50,61,68,128

129

54 15,21,46,50,52,66,68,82,84,85,77

56 46,48,49,50,52,128

128 46,49,50,52,56

96 31,87,95,98,100,101,102,106

97 85,98,100,101,102,106

100 85,96,97,98,101,102,106

102 98,100,101,108,110,112,115,118,123

104 98,100,101,102,106,112

105 98,100,101,102,106

106 85,87,96,97,98,101,102,108

107 85,87,96,97,98,100,101,102,106,108

111 100,101,102,106,108,110,112,115

112 100,101,106,108,110,111,115

113 29,30,31,102,108,111,112

115 100,101,102,106,108,110,111,112

118 102,106,112,115

66 21,46,49,54,68,71,72,74

68 21,46,49,54,66,71,72,74

72 57,59,66,68,71,74,75,78

74 66,68,69,71,77,78,82

75 66,68,69,71,77,78,82

78 66,71,75,77,80,81,82,85

84 36,39,40,54,85,88,100,101

130

85 39,77,78,81,82,84,87,88,89,100,101,102

87 77,81,82,84,85,100,102

91 39,84,85,88,89

The variation of the ATC for these two cases, the difference is

more than 100MW is shown in Figures 5.7 to 5.11. The change in ATC

is due to some costly generators off and that causes to change the

loading of the lines.

Figure 5.7. Variation of ATC for Case 4 (a) and Case 4 (c)


131




132

From the above figures, it is observed that variation of ATC is very

high, between Case 4(a) and case 4(c) for the following bus

combinations: 29-36; 30-36; 31-36,42,87; 9-35,36; 15-6; 20-46; 46-

54,84; 50-49,52,57,72; 52-46,49,50; 54-46,50,52,84,85,77; 100-85;

102-100; 112-100; 66-46,49; 72-57; 78-82,85; 84-54,85,100,101; 85-

77,82,84,100,101,102; 87-77,100,102.

5.7.4.4. Case 5: During summer

In this case study, analysis was done with part of APGENCO

(8923.86MW), IPPs (3286.3MW) and CGS share (3229.04) being

considered. In the summer, most of the hydropower plants are not in

operation due to low head of water. According to the data taken from

GENCO, generators at bus 54 (Nagarjuna Sagar) are operating under

peak load are considered for this case.

The analysis was also done at different load conditions varying

from 50% to 125% as follows. ATC is calculated between the areas for

different load conditions are tabulated in Table 5.14. This variation of

ATC is depends on the generators operating in that particular area

and loading of the lines.


Transfer areas

Case 5 (a) 100% (AG)

Case 5 (b) 125%

Case 5 (c) 100%

Case 5 (d) 75%

Case 5 (e) 50%

ATC

(MW)

ATC

(MW)

ATC

(MW)

ATC

(MW)

ATC

(MW)

1-2 226

NP

226 739 1066

1-3 1660 1660 1087 206

1-4 1660 1660 235 1281

133

2-1 00 00 297 90

2-3 359 355 981 19

2-4 788 893 1435 1502

3-1 626 219 297 90

3-2 226 219 226 226

3-4 631 219 717 533

4-1 677 307 08 96

4-2 226 226 08 144

4-3 441 448 08 206

5.7.4.5. ATC Calculations for tie-lines

AP State grid operating at 220kV consists of total 27 tie lines

between the four Areas or DISCOs. There are 10 tie lines between area

1 – area 2 (area 2 – area 1), 2 tie lines between area 1 – area 3 (area 3

– area 1), 1 tie line between area 1 – area 4 (area 4 – area 1), 10 tie

lines between area 2 – area 4 (area 4 – area 2), and 4 tie lines between

area 3 – area 4 (area 4 – area 3) and no tie lines are existing between

area 2 – area 3 (area 3 – area 2).

ATC is calculated for all these tie lines in both the directions is

calculated for the Case 4(a) (with 100% load, all generators are

operating) and Case 4(c) (with 100% load, some costly generators are

off) and are tabulated in Table 5.15.

134

Table 5.15: ATC of tie-lines between areas for Case 4(a) and 4(c) (with CGS share)

From area 1 to area 2

Transfer buses Case 4(a) Case 4(c)

S.

No. From To

ATC

(MW)

Limiting

line

ATC

(MW)

Limiting

line

1 1 28 1080 28-1 1310 28-1

2 12 11 349 11-12 546 11-12

3 30 19 586 19-9 610 19-9

4 24 25 704 24-25 704 24-25

5 27 28 500 27-28 500 27-28

6 37 28 815 37-28 575 37-28

7 30 36 713 40-84 277 40-84

8 35 42 850 35-38 1094 35-38

9 35 36 681 35-36 299 40-84

10 38 42 479 38-42 581 38-42


11 28 1 516 28-1 276 28-1

12 11 12 451 11-12 251 11-12

13 19 30 1637 29-30 675 40-84

14 25 24 896 24-25 896 24-25

15 28 27 1099 27-28 1099 27-28

16 28 37 785 28-37 1024 28-37

17 36 30 1044 35-36 859 35-36

18 42 35 1170 29-35 694 40-84

135

19 36 35 908 35-36 747 35-36

20 42 38 620 35-38 696 38-42


21 31 113 471 31-113 338 31-113

22 31 101 647 101-31 279 101-31


23 113 31 375 31-113 508 31-113

24 101 31 996 101-31 1089 40-84


25 30 87 996 30-87 823 101-31


26 87 30 1232 40-84 478 40-84

From area 2 to area 3 and from area 3 to area 2

NIL


27 39 89 1555 89-39 1426 89-39

28 40 84 723 40-84 871 40-84

29 46 120 412 46-120 415 46-120

30 46 84 926 54-46 1250 54-46

31 46 82 758 82-46 767 82-46

32 46 55 615 46-55 731 68-55

33 54 84 1344 84-54 825 84-54

34 57 72 1120 72-57 1474 72-57

136

35 124 69 598 69-124 604 69-124

36 124 71 690 124-71 683 124-71


37 89 39 782 89-39 913 89-39

38 84 40 250 40-84 97 40-84

39 120 46 692 82-120 697 82-120

40 84 46 1857 54-46 1204 40-84

41 82 46 1870 82-46 1862 82-46

42 55 46 598 68-55 644 66-68

43 84 54 1336 84-54 1855 84-54

44 72 57 764 72-57 394 72-57

45 69 124 534 69-124 528 69-124

46 71 124 642 66-69 632 66-69


47 99 85 722 85-99 795 97-99

48 95 85 1096 95-85 1474 95-85

49 95 87 1197 95-87 1530 95-87

50 93 92 959 92-93 1112 92-93


51 85 99 716 85-99 579 85-99

52 85 95 1127 95-85 743 95-85

53 87 95 1242 95-85 819 95-85

54 92 93 1013 92-93 859 92-93

137

The concept of ATC requires the determination of what is available

from a particular condition. If the exact conditions were known in

advance and a specific transaction was in question, the burden would

be significantly less than that encountered in the attempt to predict

what will be available at a future time.

The ATC computations presented should give the market

participants enough and quick information to make bidding decisions.

In a competitive dynamic power market, ATC depends on the

generation pattern and loading pattern as determined by the bidding

results. Yet market participants also need ATC information before

bidding strategically. The fast ATC calculation method will make more

efficient rounds of bidding for both market participants and ISO.

5.8. RESULTS AND DISCUSSION ON CONTINGENCY

ANALYSIS

There are different methods are available to give contingency

ranking. For this analysis, tie-line MVA Loading is considered for

contingency ranking. Contingency ranking of all the tie-lines

according to MVA loading is given in Table 5.16.

Table 5.16: Contingency ranking of tie-lines

From

bus

To

bus Circuit

Power flow MVA

rating

%MVA

loading

Contingency

rank (MW) (Mvar)

40 84 1 -217.1 3.3 400 55.1 1

1 28 2 -189.8 35 400 48.2

2 1 28 1 -189.8 35 400 48.2

138

46 82 1 183.1 -27.5 400 47.4 3

30 19 1 174.4 8.7 400 43.6 4

57 72 2 -169.3 -19.4 400 42.6

5 57 72 1 -169.3 -19.4 400 42.6

35 42 1 163.3 2.4 400 42.3 6

46 120 1 165.6 -12.3 400 41.5 7

27 28 2 150 -7.8 400 37.6 8

27 28 1 150 -7.8 400 37.6

12 11 1 -149 -4.6 400 37.3 9

30 87 1 138.6 38.4 400 35.9 10

54 84 3 92.2 -11 525 35.5

11 54 84 2 184.3 -22 525 35.5

54 84 1 184.3 -22 525 35.5

38 42 1 119.2 -9.4 400 29.9 12

95 85 1 108.3 -32.8 400 28.2 13

95 87 1 90.1 -31.2 400 23.7 14

39 89 2 -93.1 9.2 400 23.4 15

39 89 1 -93.1 9.2 400 23.4

46 55 1 79.8 -23.3 400 20.8 16

30 36 1 73.9 19.1 400 20.7 17

31 113 1 -49.4 8.2 400 19.4 18

35 36 1 45.6 51.8 400 19.4 19

31 101 1 73.2 -7.2 400 19.1 20

139

99 85 1 53.8 -12.1 400 15 21

93 92 2 56.3 -9.5 400 14.8

22 93 92 1 56.3 -9.5 400 14.8

37 28 2 40.2 -26.9 400 12.5

23

37 28 1 40.3 -29.6 400 12.5

24 25 2 48.2 -2.8 400 12.1

24

24 25 1 48.2 -2.8 400 12.1

124 69 1 -18 26.7 400 8.1 25

124 71 1 7.9 -12.7 400 4.3 26

46 84 2 9.7 6.2 400 3.2

27 46 84 1 9.7 6.2 400 3.2

Variation of the ATC compared to base case between the areas

for single line outage at a time is given in the Tables 5.17(a) to 5.17(d).

Table 5.17: (a) ATC between areas with contingencies

Transfer

areas

Base

case

Contingency line

40-84 1-28 46-82 30-19 57-72 35-42 46-120

ATC

(MW)

ATC

(MW)

ATC

(MW)

ATC

(MW)

ATC

(MW)

ATC

(MW)

ATC

(MW)

ATC

(MW)

1-2 2061 1873 NE 1346 1736

System Blackout

1563 1368

1-3 1040 1086 NE 1019 947 976 1023

1-4 838 834 NE 754 841 844 770

2-1 681 675 NE 630 737 728 640

2-3 1355 971 1286 1145 1350 1376 915

140

2-4 1408 1408 1408 1408 1406 1408 1162

3-1 1146 1146 NE 1146 1146 1146 1146

3-2 1299 1288 1323 1341 1296 1284 1334

3-4 869 856 845 795 877 874 808

4-1 676 676 NE 676 676 676 676

4-2 2576 2580 2587 2577 2578 2593 2581

4-3 675 675 675 675 675 676 675

Table 5.17: (b) ATC between areas with contingencies

Transfer

areas

Base

case

Contingency line

27-28 12-11 30-87 54-84 38-42 95-85 95-87

ATC

(MW)

ATC

(MW)

ATC

(MW)

ATC

(MW)

ATC

(MW)

ATC

(MW)

ATC

(MW)

ATC

(MW)

1-2 2061 1518 2217 1730 2054 1720 2010 1997

1-3 1040 1101 1128 785 1048 992 1017 1021

1-4 838 852 848 814 807 842 835 637

2-1 681 524 540 682 -66 705 691 687

2-3 1355 1017 1075 1367 -29 1408 1286 1322

2-4 1408 1404 1408 1408 -118 1408 1408 1408

3-1 1146 1079 802 1136 1146 1146 1141 1022

3-2 1299 1269 1255 1325 1296 1294 1203 1087

3-4 869 882 880 857 837 872 830 713

4-1 676 676 676 676 676 676 676 676

4-2 2576 2573 2723 2574 2597 2583 2577 2577

4-3 675 675 675 676 675 675 675 675

141

Table 5.17: (c) ATC between areas with contingencies

Transfer

areas

Base

case

Contingency line

39-89 95-87 39-89 46-55 30-36 31-113 35-36

ATC

(MW)

ATC

(MW)

ATC

(MW)

ATC

(MW)

ATC

(MW)

ATC

(MW)

ATC

(MW)

ATC

(MW)

1-2 2061 2235 1997 2235 1156 1895 2033 1854

1-3 1040 1068 1021 1068 1037 1014 917 1007

1-4 838 851 637 851 473 843 838 844

2-1 681 590 687 590 675 663 652 900

2-3 1355 1287 1322 1287 1343 1362 1383 1408

2-4 1408 1408 1408 1408 1362 1408 1408 1408

3-1 1146 1090 1022 1090 1146 1146 1146 1146

3-2 1299 1246 1087 1246 1169 1296 1249 1294

3-4 869 897 713 897 493 873 872 872

4-1 676 676 676 676 676 676 676 676

4-2 2576 2495 2577 2495 2577 2576 2576 2581

4-3 675 676 675 676 675 675 676 675

Table 5.17: (d) ATC between areas with contingencies

Transfer

areas

Base

case

Contingency line

31-101 99-85 93-92 37-28 124-69 124-71 46-84

ATC

(MW)

ATC

(MW)

ATC

(MW)

ATC

(MW)

ATC

(MW)

ATC

(MW)

ATC

(MW)

ATC

(MW)

1-2 2061 1756 2028 1959 NE 2056 2036 2043

1-3 1040 768 1023 1020 NE 1040 1040 1040

1-4 838 805 837 836 NE 836 831 845

142

2-1 681 665 686 693 NE 680 681 720

2-3 1355 1374 1312 1303 1408 1353 1353 1379

2-4 1408 1408 1408 1408 1408 1408 1408 1408

3-1 1146 1080 1146 742 NE 1146 1146 1146

3-2 1299 1346 1257 954 1281 1296 1303 1287

3-4 869 871 867 723 864 865 860 878

4-1 676 676 676 676 NE 676 676 676

4-2 2576 2576 2577 2577 2578 2576 3577 2582

4-3 675 676 675 675 675 675 675 675

From the above results, out of all these line outages, line 57-72

outage is more severe and causes system blackout. Outage of lines 1-

28 and 37-28, isolate the Area 1 from the remaining system,

calculation of ATC is Not Existing (NE). Outage of line 54-84 causes

the overloading of the line 54-46 by 16% due to that ATC from Area 2

to remaining areas becomes negative.

ATC calculations are extended for the some of the limiting lines

also and all these lines are loaded in between 60 – 70%. Outage of

lines 49-46 is more severe, cause’s system blackout. The details of

variation of ATC for these lines are given in Table 5.18.

143

Table 5.18: ATC Between the areas for other limiting lines

Transfer

areas

Bas

case

Contingency line

49-46 102-115 21-47 29-35 100-101 42-35 54-46 97-98

ATC

(MW)

ATC

(MW)

ATC

(MW)

ATC

(MW)

ATC

(MW)

ATC

(MW)

ATC

(MW)

ATC

(MW)

ATC

(MW)

1-2 2061

Blackout

2052 1343 1269 2086 1563 2048 2048

1-3 1040 992 1147 562 1063 978 1040 1042

1-4 838 838 771 441 838 844 843 837

2-1 681 681 132 297 672 728 690 685

2-3 1355 1356 237 719 1366 1376 1388 1352

2-4 1408 1408 504 1312 1408 1408 1408 1408

3-1 1146 1146 300 506 487 1146 1146 1044

3-2 1299 1238 1344 1347 795 1284 1293 1143

3-4 869 869 790 909 521 874 874 804

4-1 676 676 179 370 676 676 676 676

4-2 2576 2576 2581 2364 2577 2593 2579 2577

4-3 675 675 446 675 675 675 675 675

To find secured operating of the system the above given ATC’s are

more useful of the operators of the Andhra Pradesh State electricity

market.

5.9. SUMMARY

In this chapter, basic concepts of ATC and ATC calculations are

discussed. ATC is calculated from all generator buses to normally

heavily loaded load buses for IEEE 26-bus system and between the

144

areas of IEEE 118-bus system for different load conditions, and results

are compared and analyzed. Variation of ATC between different buses

for the data of March, September 2011 is calculated for 124-bus real-

time Indian utility system of Andhra Pradesh State grid and ATC is

calculated between the different distribution companies for different

cases considering each company as an area.

And it also presents the concepts of Contingency Analysis (CA) and

methods to analyze the CA problem. Contingency ranking is taken

according to the percentage loading of the lines. For this analysis, tie-

line MVA Loading is considered for ranking. A 124-bus real time

Indian utility power system of Andhra Pradesh State grid is considered

to find the variation of ATC between the areas for transmission line

outage of each tie-line.

Documents

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