Chapter 6 TRANSMISSION LOADING RELIEF - …shodhganga.inflibnet.ac.in/bitstream/10603/17295/16/16_chapter6.pdf · Chapter 6 TRANSMISSION LOADING RELIEF 6.1. INTRODUCTION Transmission

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Chapter 6

TRANSMISSION LOADING RELIEF

6.1. INTRODUCTION

Transmission Loading Relief (TLR) is a sequence of actions taken

during operations planning or during real-time operation to avoid or

remedy security violations associated with the transmission system.

Informal or “local” TLR has been in existence so long as

transmission lines have been subject to limitations. System planners

and operators have developed unique systematic procedures to

prevent transmission loading problems and they vary across the

interconnected grid of each control area. Issues of security raised due

to financial transactions are resolved by formal TLR procedures with

consistency and fairness. These procedures are briefly summarized

below.

The process starts when a security coordinator identifies a

transmission facility within its security area that is about to, or has

exceeded the operating security limit. At this point the security

coordinator may invoke TLR.

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6.2. TLR PROCEDURES

This NERC TLR procedure involves following “levels” [37]:

TLR Level 1: Notify security coordinators of situation

In this level transmission system is secure. In this level, some of

the transmission facilities are exceed or approach their security limits

causes contingency of a transmission or generation, or other operating

problems The reliability coordinator will inform condition to other

reliability coordinator.

TLR Level 2: Hold interchange transactions at current level

In this level also transmission system is secure and one or more

transmission facilities are expected to approach, or are approaching,

or are at their operating security limit. The reliability coordinator

holds interchange transactions and allowed to change additional

interchange transactions with constraints. In this stage quick decision

required to move higher TLR Levels (3 and above) to allow interchange

transactions because it is a transient state. Time for this level is not

more than 30 minutes.

TLR Levels 3a: Reallocate firm and non-firm transmission service

In this level also transmission system is secure and one or more

transmission facilities are expected to approach, or are approaching,

or are at their operating security limit. The reliability coordinator shall

give preference to those interchange transactions using firm

transmission service, followed by those using higher priority non-firm

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transmission service. According to the transmission service priorities

interchange transactions can be held or curtailed, shall be reallocated

(reloaded) within operating conditions permit.

TLR Level 3b: Curtail non-firm transactions

In this level also one or more transmission facilities are expected to

approach, or approaching, or at their operating security limit.

Curtailments are required for two reasons:

a) Interchange transactions for a higher priority transmission

service are allowed.

b) To mitigate an existing operating security limit violation.

During the period of operating security limit violation, all new

interchange transactions are held by the reliability coordinator using

non-firm transmission. Interchange transactions using firm

transmission service will be allowed.

TLR Level 4: Reconfigure transmission

In this level one or more transmission facilities are operating above

their security limit. Before the reliability coordinator orders

curtailment of interchange transactions using firm transmission

service, he will request the transmission providers in his reliability

area to attempt to reconfigure their transmission systems to allow the

interchange transactions to continue. Transmission reconfiguration

may be implemented as long as it does not jeopardize the operating

security of the interconnection. Transactions using non-firm

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transmission service will be curtailed or held from starting. Some

transactions using firm transmission service will be allowed to start.

TLR Level 5a: Reallocation of transmission services

In this level one or more transmission facilities are operating at

their security limit and the system is secure. At this point, the

reliability coordinator shall begin the process of curtailing interchange

transactions using firm transmission service until the operating

security limit violation has been mitigated. Further reconfiguration of

transmission services is not possible or effective. Available re-dispatch

options will continue to be implemented.

TLR Level 5b: Curtail firm transmission service


their security limit and all non-firm interchange transactions are at or

above curtailment threshold. Further reconfiguration of transmission

services is not possible or effective. A three-step process is followed for

curtailment of firm interchange transactions:

• Identifying the known re-dispatch options.

• Calculating the percent of overload due to constrained

transactions.

• Curtailments of firm interchange transactions.

Available re-dispatch options will continue to be implemented.

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TLR Level 6: Implement emergency procedures


their security limit due to the removal from service of a generating

unit or another transmission facility. If the transmission loading

condition is deemed critical to bulk system reliability by a reliability

coordinator, the reliability coordinator has the authority to

immediately direct the control areas in his reliability area to re-

dispatch generation, or reconfigure transmission, or reduce load to

mitigate the critical condition until interchange transactions can be

reduced utilizing the TLR procedures or other procedures to return the

system to a secure state. All control areas should comply with all

requests from their reliability coordinator. However, the control area

operator should immediately notify his reliability coordinator if the

reliability coordinator’s request is unclear or would seem to cause an

operating problem.

TLR Level 0 – TLR concluded

The TLR procedures initiated by the reliability coordinator should

be notified to all other reliability coordinators within the

interconnection to operate the system in a “normal” state, allowing

reestablishment of interchange transactions according to the

transmission priorities.

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6.3. LIMITATIONS

The transmission relief procedure depends on the results of off-line

ATC, which has a number of limitations:

a) Computation of off-line ATC doesn’t consider the effect of the

actual transactions in/out of the region. It is a non-

simultaneous computation.

b) The non-peak hour operating conditions differ from that at peak

hour. Generally, the magnitude of flows will be larger at the

peak hour and the most restrictive condition is computed for

ATC. Sub-utilization of system capabilities is resulted when ATC

is underestimated during non-peak hours.

c) Numerical errors are present in off-line ATC because it often

neglects the influence of reactive power, uses linear models and

assumes constant distribution factors.

6.4. PROBLEMS IN TLR DUE TO THE ABOVE

LIMITATIONS

These limitations create the following problems in TLR:

a) By using daily offline ATC computations can result in

inaccurate TLR decisions.

b) New constrained facilities can be originated when TLR

procedures are implemented.

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c) There is a need of precise and fast coordination among all the

coordinators and participants in the operating grid since TLR is

an iterative process that changes from one level to another.

d) Market’s efficiency is affected due to the opens opportunities for

economic gaming of TLR procedures.

6.5. METHODS TO ALLEVIATE TRANSMISSION LOADING RELIEF

The following are some of typical means of controls to mitigate the

transmission emergencies.

1. Generator active power adjustment

2. Phase angle regulator adjustment

3. Interchange schedule adjustment

4. Generator reactive power adjustment

5. Transformer tap adjustment

6. Shunt capacitor/reactor switching or synchronous condenser

adjustment

7. Transmission line switching

8. Pumped storage generator operation

9. Economic load Management/Customer load shedding

10. Distributed generation

11. Transmission System Expansion

The following are the methods listed and explained below, to

alleviate transmission loading relief.

1. Sensitivity based load curtailment

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2. Economic load management

3. VAR support

4. Re-dispatching and coordinated re-dispatching

5. Counter trading

1. Sensitivities based load curtailment:

In this method, load curtailment is done by use of the system

sensitivities those are PTDFs, LODFs and TLRs. PTDFs determine the

sensitivity of the flow on an element such as transmission line to a

single power transfer. TLR Sensitivities determine the sensitivity of the

flow on the single monitored element such as a transmission line to

many different transactions in the system. In other words, TLR

sensitivities gauge the sensitivity of a single monitored element to

many different power transfers. Transmission Line Relief (TLR)

sensitivities can be considered as the inverse of the Power Transfer

Distribution Factors (PTDFs). Both TLR sensitivities and PTDFs

measure the sensitivity of the flow on a line to load curtailment.

2. Economic load management for transmission loading relief:

Another possible solution for TLR management is to find customers

who will volunteer to lower their consumption when transmission

congestion occurs. By lowering the consumption, the congestion will

“disappear” resulting in a significant reduction in bus marginal costs.

A strategy to decide how much load should be curtailed for what

customer is discussed here. The anticipated effect of this congestion

relief solution is to encourage consumers to be elastic against high

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prices of electricity. Hence, this congestion relief procedure could

eventually protect all customers from high electricity prices in a

deregulated environment. A set of indices are used to represent the

level of effective and agreeable load curtailment in congested

conditions.

This method has three indices that are computed to calculate the

overall index for the load management. The three factors considered

for the index calculations are:

1. Power Flow Effect through Sensitivity Index,

2. Economic Factor for Locational Marginal Price (LMP) Index, and

3. Load Reduction Preference for customer load curtailment index.

3. VAR support:

In the present day scenario, unplanned power transactions are

rapidly increasing due to the competition among utilities to meet

increasing demand and if transactions are not properly controlled,

transmission lines are often operated and stressed to the limit. The

increased use of existing transmission is made possible, in part, by

reactive power compensation. The role of VAR support in the open

power market is to help manage congestion. Better utilization of the

existing power system to increase power transfer capability by

installing VAR support such as capacitor banks and FACTS (Flexible

AC Transmission Systems) devices becomes imperative. Capacitors,

Static VAR Compensator (SVC), Thyristor Controlled Series Capacitor

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(TCSC), Unified Power Flow Controller (UPFC), are some of the

examples of FACTS devices used for VAR support.

The main advantage of FACTS devices is the possibility of their

installation for a short period compared to the planning and

construction of new transmission lines. FACTS not only improve the

transmission capacity but also reduce the losses. However, FACTS

devices are expensive.

4. Re-dispatching and coordinated re-dispatching:

Re-dispatching and coordinated re-dispatching involve re-

dispatching of generating units to relieve part of congestion. It comes

down to the introduction of corrections to the initial generation

dispatch, usually based on the prices that generators communicate to

the System Operator (SO) for up and down regulation. It might also be

a separate market. Two alternatives can be considered. If the SO only

intervenes within its own control area, the system can be optimized

locally (internal re-dispatch). A more comprehensive approach involves

several SOs and tries to find a global optimum by re-dispatching units

on both sides of the congested interconnector (coordinated re-

dispatch). The latter has the advantage of being more efficient, since

there are more nodes where power injections can be modulated.

However, it requires a strong co-ordination between SOs involved and

harmonization of market rules in the areas involved.

Re-dispatching, however, means incurring costs that cover

payments to generators for up and down regulation. This cost is borne

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first by the SO, and can be charged in a second phase to the market

players. Re-dispatching costs are usually socialized (for example

included in transmission tariffs), but they can also be charged to

specific users causing congestion.

Re-dispatching is an effective method to solve congestion if enough

resources are available to change the load flow pattern and balance

the line loading better. If more than one SO is involved the

effectiveness increases even more, as more units are available for the

purpose. However, in some cases re-dispatching can fail to alleviate

congestion, which calls for measures as pro rata curtailment.

5. Counter trading:

The basis of Counter Trading is to make use of the electricity laws,

namely of the fact that electricity flows in opposite directions can be

net off, allowing alleviation of congestion. In order to provoke an

electricity flow in the direction opposite to congestion, the SO steps

into the market and intervenes. It buys electricity in the control zone

downstream of congestion and sells it back in the control zone

upstream. However, this intervention comes at a price. The control

zone downstream of congestion is usually a high price area. Therefore,

counter-trading involves buying expensive electricity in order to sell it

back in the low price area, making a loss. This loss is covered by

transmission tariffs.

The method, though simple, has some important drawbacks. First

of all, it assumes that a counter-trade between zones can always solve

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inter-zonal congestion. This assumption does not have to be correct.

Zones are the aggregation of nodes. A zonal approximation thus

assumes that any transaction between zones has an identical effect on

the interconnection between them. This is obviously not true for a

network in which zones are interconnected by more than one line and

the error increases with the increased level of meshing. In order to be

able to effectively solve congestion, the SO would have to know the

topology of the network and the exact location of the involved power

injection and sink points.

In this thesis, sensitivities based load curtailment method is used.

In this method, first PTDFs are calculated and ATC is calculated by

use of these PTDFs. The ATC is calculated and updated for each

transaction on hour-ahead based.

6.6. HOUR-AHEAD ATC

Computation of hour-ahead ATC is same as off-line ATC and for

the next hours it operates on the planned model. It is used in the

Energy Management System (EMS) study mode to meet data

integration challenges.

The difficult part of hour-ahead ATC is data management and

modeling that is because the system consists thousands of buses, and

hundreds of directions and contingencies in few minutes.

To accurately compute ATC, the following information must be

provided to the solver:

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System and transaction data: network topology, equipment

parameters, control settings, load profile, generation profile, external

model description, interchange schedule, contingency description,

subsystem description, direction description.

From existing technologies, ATC for the next hour can be computed

as follows.

Step 1. Obtain a base system case. This would give a base

topology description, generation schedule and base power

flows.

Step 2. Using system and bus load forecast functions, system

model modifications are also included to the loads.

Update generation settings and base flows, run for OPF.

Step 3. For next hour case, compute the PTDFs by solving the

power flows.

Step 4. Compute ATC based on linear methods for confirmed

reservations using a transaction analyzer which could

also be used for computations such as contingency

analysis, sensitivity analysis, interchange distribution

calculations, transaction arrangement and transmission

loading relief.

For over future hour models, hour-ahead ATC would run cyclically

and also on operator demand or new reservations are confirmed. The

economic and reliability goals of the hourly-market are achieved by

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this core function of information system. A flow chart of the functions

is given in Appendix C.3.

6.7. TYPES OF TRANSACTIONS

In open-access electricity markets transactions can be done in

three different ways. They are:

A. Simultaneous transactions

B. Sequential transactions: off-line ATC

C. Sequential transactions-updated ATC

6.7.1. Simultaneous transactions

In a system, suppose that the transmission reservations occur

simultaneously [47], [48]. If these transactions are implemented, there

is a chance that one or more lines may be overloaded. When this

schedule is analyzed prior to implementation, TLR will have to be

invoked to relieve the overloading of these lines. Each participating

control area for the transaction will implement a local TLR procedure,

which would imply the rearrangement of its own resources, or ask the

security coordinator for a TLR procedure, which would require the

rearrangement of the resources of all the interconnection entities

involved in the transaction. Once TLR is initiated, the process

determines for each type of transaction the impact of those that affect

the loading of the facility in 5% or more based on PTDFs [51]. Then

transactions are curtailed in a progressive, prioritized manner. If no

more transactions can be curtailed, the process goes to the next TLR

level.

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6.7.2. Sequential transactions: off-line ATC

As an alternative approach, consider a sequence of transactions

starting from base case. In this case, ATC values remain as those

computed by the off-line ATC process as the transactions take place.

1). At time t1 marketer M1 proposes a transaction in one

direction. The transfer should create no problem if there is

ATC available after the transaction is implemented.

2). At time t2 marketer M2 reads ATC for another direction and

proposes a transaction T2. There may be a chance of

overload if the transaction T2 is made considering the ATC

values prior to transaction T1 that needs to be relieved.

So, TLR procedure should be invoked to relieve this overload.

Note that in this case that for proposed transactions the values

of unreserved ATC do not change and therefore the capability for each

direction is assumed to be the one that was computed for the base

case. As this is a non-simultaneous process, new transactions can

overload the system. TLR will be needed even though the transactions

were based on posted ATC values.

6.7.3. Sequential transactions-updated ATC

Because of the disadvantage of invoking TLR procedures in the

sequential off-line ATC method, we go for this case. Start again with

the system at the initial operating point.

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1). At time t1 marketer M1 proposes a transaction in one

direction. The transfer should create no problem if there is

ATC available after the transaction is implemented.

2). ATC values are recomputed and updated.

3). At time t2 marketer M2 reads ATC for another direction and

proposes a transaction T2. As the transaction is made based

on the updated ATC values, there should be no overload.

4). ATC values are recomputed and updated after each

transaction.

This case is an ideal case in which ATC is updated each time a

transaction takes place, is proposed, or a reservation is made. Note

that ATC values remain positive. For this ideal case, no TLR procedure

is required [93].

If a continuous ATC computation is possible, and transactions are

based on updated ATC data we can think of transactions as moving

within a secure transactional space.

6.8. RESULTS AND DISCUSSION

6.8.1. 7-bus system

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

shown in Figure 6.1. Data for this system is given in Appendix A.1.

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Figure 6.1: 7-bus test system

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. ATC values at initial

operating are shown in Table 6.1.

Table 6.1: 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

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6.8.1.1. Example-I of 7-bus system

Case A: Simultaneous transactions

Given this system data, suppose that the following transmission

reservations occur, which are all possible after the initial posting:

Direction 1-6:50MW

Direction 2-6:200MW

Direction 4-6:200MW

If these transactions are implemented, line 1-2 will have an over

load of 22% and line 2-6 will have an over load of 95%. When this


invoked to relieve the overloading of these lines.

Case B: Sequential transaction: off-line ATC

As an alternative approach, consider a sequence of transactions

starting at the initial operating point used above. In this case, ATC

values remain as those computed by the off-line ATC process as the

transactions take place.

1. At time t1 marketer M1 proposes a transaction T1 of 50 MW from

A-B in the direction 1-6. As ATC is 54MW, the transfer should

create no problem and there should be 4MW available for

direction 1-6 after the transaction is implemented.

2. At time t2 marketer M2 reads ATC for direction 2-6 to be 265MW

and proposes a transaction T2 of 200MW. As a result, line 1-2

and 2-6 are operating near full load.

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3. At time t3 marketer M3 reads ATC for direction 4- 6 as 281MW

and proposes a transaction T3 of 200 MW. The final results will

be overloads of 22% in line 1-2, 95% in line 2-6. TLR must be

applied in order to relieve the loading of these lines. Sequential

ATC data for this example is given in Table 6.2.

Table 6.2: Sequential ATC data of Example-I of 7-bus system

Transfer t0 t1 t2 t3

Area Bus ATC

(MW)

Limit

line

ATC

(MW)

Limit

line

ATC

(MW)

Limit

line

ATC

(MW)

Limit

line

A-B 1-6 54 1-2 4 1-2 4 1-2 4 1-2

2-6 265 2-6 265 2-6 65 2-6 65 2-6

4-6 281 2-6 281 2-6 281 2-6 81 2-6

A-C 1-7 56 1-2 56 1-2 56 1-2 56 1-2

2-7 106 2-5 106 2-5 106 2-5 106 2-5

4-7 152 4-5 152 4-5 152 4-5 152 4-5

B-C 6-7 111 6-7 111 6-7 111 6-7 111 6-7

The numbers highlighted in Tables are ATC remaining after a

transaction is performed. If all the ATC numbers were computed after

t3 the results would be as shown in Table 6.3.

Table 6.3: ATC for final conditions of Example-I of 7-bus system

Transfer t3

Areas Buses ATC (MW) Limiting line

A-B

1-6 -168 1-2

2-6 -166 2-6

4-6 -186 2-6

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A-C

1-7 -448 1-2

2-7 -438 2-5

4-7 -513 4-5

B-C 6-7 87 6-7

Case C: Sequential transaction-update ATC

Start again with the system at the initial operating point given in

Table 6.1.

1. As in Case B, at time t1 marketer M1 proposes a transaction T1

of 50 MW from A-B in the direction 1-6. As ATC is 54MW, 4MW

remain shown in column t1.Other ATC values are recomputed

and updated in column t1, the constraining elements can also

change.

2. At time t2 marketer M2 proposes a transaction in direction 2-6

of 200 MW. As ATC is 220MW there should be no problem. All

other ATC values are recomputed.

3. At time t3 marketer M3 proposes a transaction in direction 4-6

of 10 MW. As ATC is 26MW there should be no problem. All

other ATC values are recomputed.

4. At time t4 transaction T1 is cancelled relieving 50MW

indirection 1-6. All other ATC values are recomputed.

Sequential transactions-update ATC data for this example is given

in Table 6.4.

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Table 6.4: Sequential transactions- update ATC of Example-I of 7-

bus system

Transfer t0 t1 t2 t3 t4

Area Bus ATC

(MW)

Limit

line

ATC

(MW)

Limit

line

ATC

(MW)

Limit

line

ATC

(MW)

Limit

line

ATC

(MW)

Limit

line

A-B

1-6 54 1-2 4 1-2 9 1-2 4 1-2 54 1-2

2-6 265 2-6 220 2-6 25 2-6 13 2-6 59 2-6

4-6 281 2-6 28 1-2 26 2-6 14 2-6 62 2-6

A-C

1-7 56 1-2 6 1-2 9 1-2 5 1-2 56 1-2

2-7 106 2-5 98 2-5 54 2-5 34 2-6 63 2-5

4-7 152 4-5 34 1-2 57 1-2 30 1-2 90 2-5

B-C 6-7 111 6-7 123 6-7 68 2-5 69 2-5 79 2-5

Figure 6.2: Secure transactional space for transactions in Example I

T2 (2-6)

T1 (1-6)

to

t1

T3 (4-6)

281

54

265

26

225

t 2

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Secure transactional space: A secure transactional space (as

mentioned earlier) is now created based on the above sequential

updated ATC data. This space gives the limits for future transactions

and all transactions are to be made within its limits to reduce the

chance of overloading. It can be seen that each time a transaction is

proposed, the secure transactional space changes (it can be reduced

or augmented).

For the transactional space shown in the above Figure 6.2, the

coordinates are the transactions in directions 1-6, 2-6 and 4-6. The

center of coordinates (0, 0, 0) represents the initial operating point

where no transactions take place. At this point the secure

transactional space is given by cube t0 with dimensions {54, 265,

281}. Then T1 results in a secure transactional space given by cube t1

with dimensions {4, 220, 28}. T2 moves the operating point to (50,

200, 0) and ATC results in a secure transactional space given by cube

t2 with dimensions {9, 25, 26}. T3 moves the operating point to (50,

200, 10) and so on. As it can be seen, each time a transaction is

proposed, the secure transactional space changes (it can be reduced

or augmented) after ATC values and the distances to each boundary

are given by the ATC value for that direction.

6.8.1.2. Example-II of 7-bus system



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Direction 1-7: 50MW

Direction 2-7: 90MW

Direction 4-7: 140MW

If these transactions are implemented, line 1-2 will have over load

of 7%, line 2-5 will have over load of 64%, line 5-7 will have over load

of 29% and line 6-7 will have an over load of 45%. Sequential ATC

data for this example is given in Table 6.5.

Table 6.5: Sequential ATC data of Example-II of 7-bus system


Area Bus ATC

(MW)

Limit

line

ATC

(MW)

Limit

line

ATC

(MW)

Limit

line

ATC

(MW)

Limit

line

A-B 1-6 54 1-2 54 1-2 54 1-2 54 1-2

2-6 265 2-6 265 2-6 265 2-6 265 2-6

4-6 281 2-6 281 2-6 281 2-6 281 2-6

A-C 1-7 56 1-2 6 1-2 6 1-2 6 1-2

2-7 106 2-5 106 2-5 16 2-5 16 2-5

4-7 152 4-5 152 4-5 152 4-5 12 4-5

B-C 6-7 111 6-7 111 6-7 111 6-7 111 6-7

If all the ATC numbers were computed after t3 the results would be

as shown in Table 6.6. Sequential transactions-update ATC data for

this example is given in Table 4.7. Secure transactional space is

shown in the Figure 6.3.

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Table 6.6: ATC for final conditions sequential ATC data of

Example-II of 7-bus system

Table 6.7: Sequential transactions - update ATC sequential ATC

data of Example-II of 7-bus system


Area Bus ATC

(MW)

Limit

line

ATC

(MW)

Limit

line

ATC

(MW)

Limit

line

ATC

(MW)

Limit

line

ATC

(MW)

Limit

line

A-B

1-6 54 1-2 7 1-2 5 1-2 3 1-2 51 1-2

2-6 265 2-6 248 2-6 37 2-5 15 2-5 239 2-5

4-6 281 2-6 36 1-2 25 1-2 17 1-2 258 2-6

A-C

1-7 56 1-2 07 1-2 5 1-2 3 2-5 50 2-5

2-7 106 2-5 61 2-5 08 2-5 3 2-5 48 2-5

4-7 152 4-5 44 1-2 10 2-5 05 2-5 68 2-5

B-C 6-7 111 6-7 78 2-5 9 2-5 4 2-5 60 2-5

Transfer t3


A-B 1-6 -852 2-5

2-6 -651 2-5

4-6 -67 1-2

A-C 1-7 -136 1-2

2-7 -130 2-5

4-7 -185 2-5

B-C 6-7 -162 2-5

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Figure 6.3: Secure transactional space for transactions in Example II

6.8.1.3. Example-III of 7-bus system





Direction 6-7: 80MW

If these transactions are implemented, line 1-2 will loaded about

95% and line 2-6 will have an over load of 52%. Sequential ATC data

for this example is given in Table 6.8. If all the ATC numbers were

computed after t3 the results would be as shown in Table 6.9.

T3 (4-7)

152

t0

t 1 44

56

61

50

6

T1 (1-7)

T2 (2-7)

106

170

Table 6.8: Sequential ATC data sequential ATC data of Example-III

of 7-bus system


Area Bus ATC

(MW)

Limit

line

ATC

(MW)

Limit

line

ATC

(MW)

Limit

line

ATC

(MW)

Limit

line

A-B 1-6 54 1-2 54 1-2 54 1-2 54 1-2

2-6 265 2-6 265 2-6 65 2-6 65 2-6

4-6 281 2-6 81 2-6 81 2-6 81 2-6

A-C 1-7 56 1-2 56 1-2 56 1-2 56 1-2

2-7 106 2-5 106 2-5 106 2-5 106 2-5

4-7 152 4-5 152 4-5 152 4-5 152 4-5

B-C 6-7 111 6-7 111 6-7 111 6-7 31 6-7

Table 6.9: ATC for final conditions of Example-III of 7-bus

system


in Table 6.10. Secure transactional space is shown in the Figure 6.4.

Transfer t3


A-B 1-6 -75 2-6

2-6 -75 2-6

4-6 -79 2-6

A-C 1-7 -201 2-6

2-7 -196 2-6

4-7 -231 2-6

B-C 6-7 -8 2-5

171

Table 6.10: Sequential transactions- update ATC of Example-III

of 7-bus system


Area Bus ATC

(MW)

Limit

line

ATC

(MW)

Limit

line

ATC

(MW)

Limit

line

ATC

(MW)

Limit

line

ATC

(MW)

Limit

line

A-B

1-6 54 1-2 14 1-2 9 1-2 14 1-2 54 1-2

2-6 265 2-6 79 2-6 19 2-6 71 2-6 143 2-5

4-6 281 2-6 74 1-2 21 2-6 73 1-2 273 2-6

A-C

1-7 56 1-2 15 1-2 10 1-2 14 1-2 30 2-5

2-7 106 2-5 121 2-5 50 2-6 43 2-5 28 2-5

4-7 152 4-5 91 1-2 59 2-6 61 2-5 38 2-5

B-C 6-7 111 6-7 151 2-5 135 2-5 54 2-5 36 2-5

Figure 6.4: Secure transactional space for transactions in

Example III

T3 (6-7)

111

t0

t1

281

135

21

19

T1 (4-6)

T2 (2-6)

265

54

81 200

79

172

6.8.1.4. Example – IV of 7-bus system





Direction 6-7: 80MW


of 18%, line 2-5 will have over load of 31%, line 4-5 will have over load

of 9% and line 5-7 will have an over load of 33%.Sequential ATC data

for this example is given in Table 4.11. If all the ATC numbers were

computed after t3 the results would be as shown in Table 6.12.

Sequential transactions-update ATC data for this example is given in

Table 6.13. Secure transactional space is shown in the Figure 6.5.

Table 6.11: Sequential ATC data of Example-IV of 7-bus system


Area Bus ATC

(MW)

Limit

line

ATC

(MW)

Limit

line

ATC

(MW)

Limit

line

ATC

(MW)

Limit

line

A-B

1-6 54 1-2 54 1-2 54 1-2 54 1-2

2-6 265 2-6 265 2-6 265 2-6 265 2-6

4-6 281 2-6 81 2-6 81 2-6 81 2-6

A-C

1-7 56 1-2 56 1-2 56 1-2 56 1-2

2-7 106 2-5 106 2-5 106 2-5 106 2-5

4-7 152 4-5 152 4-5 12 4-5 12 4-5

B-C 6-7 111 6-7 111 6-7 111 6-7 31 6-7

173

Table 6.12: ATC for final conditions sequential ATC data of

Example-IV of 7-bus system

Table 6.13: Sequential transactions- update ATC sequential ATC data of Example-IV of 7-bus system


Area Bus ATC

(MW)

Limit

line

ATC

(MW)

Limit

line

ATC

(MW)

Limit

line

ATC

(MW)

Limit

line

ATC

(MW)

Limit

line

A-B

1-6 54 1-2 14 1-2 2 1-2 2 1-2 16 1-2

2-6 265 2-6 78 2-6 60 2-6 80 2-5 128 2-5

4-6 281 2-6 73 1-2 7 1-2 8 1-2 87 2-6

A-C

1-7 56 1-2 15 1-2 2 1-2 2 1-2 17 2-5

2-7 106 2-5 121 2-5 84 2-5 16 2-5 56 2-5

4-7 152 4-5 90 1-2 10 1-2 10 1-2 80 2-5

B-C 6-7 111 6-7 151 2-5 105 2-5 20 2-5 70 2-5

Transfer t3


A-B

1-6 -75 2-6

2-6 -75 2-6

4-6 -79 2-6

A-C

1-7 -201 2-6

2-7 -196 2-6

4-7 -231 2-6

B-C 6-7 -8 2-5

174

Figure 6.5: Secure transactional space for transactions in Example IV

6.8.2. 26-bus system

A 26-bus power system, taken from an example in a book by Sadat

is considered as a single area. Suppose that the following

transmission reservations occur, which are all possible after the initial

posting:





of 7% and line 7-9 and 4-12 will have an over load of 8%. When this


invoked to relieve the overloading of these lines. Sequential

transactions-update ATC data for this case is given in Table 6.14.

T3 (6-7)

151

t0

t1

281

90

T1 (4-6)

T2 (4-7)

152

111

200 81

t2

175

‘Secure transactional space’ for this case is shown in Figure 6.6.

Table 6.14: Sequential transactions-update ATC for 26-bus system

Transferbus

t0 t1 t2 t3 t4

ATC (MW)

Limitingline

ATC (MW)

Limitingline

ATC (MW)

Limiting line

ATC (MW)

Limitingline

ATC (MW)

Limitingline

1-18 231 1-18 31 1-18 5 1-18 1 1-18 205 1-18

1-12 240 2-8 74 1-18 15 1-18 3 1-18 165 2-8

2-12 220 2-8 99 1-18 19 1-18 4 1-18 151 2-8

3-9 113 3-13 93 3-13 40 1-18 7 1-18 86 3-13

4-9 175 12-10 162 10-12 146 1-18 46 1-18 95 7-9

4-18 190 16-17 42 1-18 9 1-18 2 1-18 112 4-12

26-18 130 5-6 47 1-18 10 1-18 2 1-18 83 11-26

Figure 6.6: Secure transactional space for transactions of 26-bus system

6.8.3. IEEE 118- bus system

Suppose that the following transmission reservations occur, which

are all possible after the initial posting:


T2 (2-12)

T1 (1-18)

220

t0

T3 (4-9)

231

175

160

99

200

145

80

t1

t2

176




of 24% and line 68-81 and 80-81 will have an over load of 31%. When

this schedule is analyzed prior to implementation, TLR will have to be

invoked to relieve the overloading of these lines. Sequential

transactions-update ATC data for this case is given in Table 6.15.

‘Secure transactional space’ for this case is shown in Figure 6.7.

Table 6.15: Sequential transactions-update ATC for IEEE 118-bus system


Area Bus ATC

(MW)

Limiting line

ATC

(MW)

Limiting line

ATC

(MW)

Limiting line

ATC

(MW)

Limiting line

A-B

25-59 765 64-65 068 64-65 065 64-65 060 64-65

26-62 841 64-65 352 64-65 335 64-65 246 64-65

A-C

12-77 794 65-68 862 65-68 965 65-68 1048 65-68

26-90 836 65-68 909 65-68 1000 89-90 1007 89-90

B-A

54-11 861 38-65 835 38-65 770 38-65 665 38-65

65-27 842 38-65 816 38-65 754 38-65 650 38-65

B-C

54-77 697 65-68 758 65-68 848 54-56 864 68-69

65-82 580 65-68 630 65-68 705 65-68 673 68-69

C-A

80-11 801 65-68 739 65-68 90 81-80 89 81-80

92-32 910 65-68 835 65-68 358 81-80 100 81-80

C-B

87-54 300 86-87 300 86-87 300 86-87 93 81-80

100-45 808 86-87 700 65-68 327 81-80 77 81-80

177

Figure 6.7: Secure transactional space for transactions of IEEE 118-bus system

6.8.4. 124-bus real-time Indian utility power system of Andhra

Pradesh State grid

For this analysis in each area three independent transactions are

considered. ATC numbers used in this case are from the case 4 (a) i.e.

all generators are operating with CGS share and 100% load.

6.8.4.1. Transactions of area 1



Direction 30-31: 1100 MW



t1

T1 (25-59)

T3 (100-45)

T2 (80-11)

808

801

768

700

739

700

650

t0

t2

178

If these transactions are performed simultaneous transactions and

sequential transactions with off-line ATC, line 29-31 will have over



invoked to relieve the overloading of these lines.

Start with the system at the initial operating point as shown in

column t0.

1. At time t1 marketer M1 proposes a transaction T1 of 1100 MW in

the direction 30-31. As ATC is 1157MW, 57MW remain shown in

column t1.Other ATC values are recomputed and updated in

column t1, the constraining elements can also change.

2. At time t2 marketer M2 need to propose a transaction less than

140 MW in direction of 35-42 and the proposed transaction is

125MW. As ATC is 140MW there should be no problem. All other

ATC values are recomputed.

3. At time t3 marketer M3 need to propose a transaction less than

29MW in direction of 4-6 and the proposed transaction is 25

MW. As ATC is 29MW there should be no problem. All other ATC

values are recomputed.

Sequential transactions-update ATC data for this example is

given in Table 6.16. Secured transactional space for these

transactions is shown in Figure 6.8.

179

Table 6.16: Sequential transactions- update ATC in area 1 of 124-

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

Transfer

Bus

t0 t1 t2 t3

ATC

(MW)

Limiting

Line

ATC

(MW)

Limiting

Line

ATC

(MW)

Limiting

Line

ATC

(MW)

Limiting

Line

1-3 585 1-2 585 1-2 585 1-2 585 1-2

1-12 466 3-12 466 3-12 466 3-12 466 3-12

29-38 345 35-38 174 29-31 29 29-31 04 29-31

30-31 1157 29-31 57 29-31 03 29-31 01 29-31

30-36 626 40-84 147 29-31 25 29-31 06 29-31

29-33 389 33-30 389 33-30 389 33-30 389 33-30

35-42 776 35-38 140 29-31 15 29-31 06 29-31

Figure 6.8: Secure transactional space for transactions in Area 1

For the transactional space shown in the above Fig. 6.7, the

coordinates are the transactions in directions 1-6, 2-6 and 4-6. The

center of coordinates (0, 0, 0) represents the initial operating point

t1

T1 (30-31)

T3 (29-38)

T2 (35-42)

345

776

140 57

174

1157

t0 t2

180

where no transactions take place. At this point the secure

transactional space is given by cube t0 with dimensions {1157, 776,

345}. Then T1 results in a secure transactional space given by cube t1

with dimensions {57, 140, 174}. T2 moves the operating point to

(1100, 125, 29) and ATC results in a secure transactional space given

by cube t2 with dimensions {3, 15, 29}. T3 moves the operating point

to (1100, 125, 4) and so on. As it can be seen, each time a transaction

is proposed, the secure transactional space changes (it can be reduced

or augmented) after ATC values and the distances to each boundary

are given by the ATC value for that direction.








sequential transactions with off-line ATC, line 15-16 will have over

load of 13%, line 50-59 will over load of 34%, line 57-58 will have over

load of 30%, and line 58-59 will have an over load of 6%. When this


invoked to relieve the overloading of these lines. To avoid over loading

of lines need to perform following transactions.

181





in Table 6.17.

Table 6.17: Sequential transactions- update ATC in area 2 of 124-

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

Transfer

bus

t0 t1 t2 t3

ATC

(MW)

Limiting

line

ATC

(MW)

Limiting

line

ATC

(MW)

Limiting

line

ATC

(MW)

Limiting

line

15-6 739 19-9 617 15-16 143 15-16 152 15-16

15-16 437 15-16 392 15-16 91 15-16 97 15-16

9-40 291 40-84 197 40-84 196 40-84 198 40-84

9-21 1120 19-9 1016 20-21 1009 19-9 1010 19-9

46-49 205 49-46 194 49-46 194 49-46 -150 57-58

46-21 572 21-47 72 21-47 141 21-47 171 21-47

50-59 682 57-58 682 57-58 685 57-58 80 57-58

50-63 324 57-63 324 57-63 324 57-63 210 50-59






Direction 115-110:300 MW

182


sequential transactions with off-line ATC, line 102-115 over load by 3

% (double circuit), line 109-110 will over load of 6%, line 100-101will

have over load of 13% (double circuit), line 101-97 will have an over









in Table 6.18.

Table 6.18: Sequential transactions- update ATC in Area 3 of 124-bus real-time Indian utility system of Andhra Pradesh

Transfer

bus

t0 t1 t2 t3

ATC

(MW)

Limiting

line

ATC

(MW)

Limiting

line

ATC

(MW)

Limiting

line

ATC

(MW)

Limiting

line

115-110 337 109-110 337 109-110 337 109-110 59 109-110

115-117 196 117-102 190 117-102 190 117-102 133 102-115

100-116 337 109-110 315 100-101 227 100-101 222 100-101

100-98 397 97-98 77 97-98 07 97-98 36 97-98

100-97 525 101-97 239 101-97 178 100-101 174 100-101

104-123 446 101-103 432 101-103 432 101-103 463 101-103

106-101 910 100-101 330 106-101 295 100-101 288 100-101

106-95 737 97-98 137 97-98 13 97-98 67 97-98

183








sequential transactions with off-line ATC, line 85-88 over load by 54%,

and line 83-85 will operates at 98% of it MVA rating. When this








in Table 6.19.

Table 6.19: Sequential transactions- update ATC in area 4 of 124-bus real-time Indian utility power system of Andhra Pradesh State

Transfer

bus

t0 t1 t2 t3

ATC

(MW)

Limiting

line

ATC

(MW)

Limiting

line

ATC

(MW)

Limiting

line

ATC

(MW)

Limiting

line

85-89 1288 85-89 183 85-89 220 85-89 60 85-88

85-88 667 85-88 127 85-88 154 85-88 29 85-88

85-92 1093 87-92 1086 87-92 1093 87-92 1093 87-92

184

85-81 1042 81-82 1053 81-82 69 83-85 68 83-85

85-77 1292 81-77 1284 81-77 89 83-85 89 83-85

78-79 477 77-79 475 77-79 680 78-79 680 78-79

78-81 1119 81-82 1132 81-82 250 83-85 249 83-85

78-83 778 83-85 764 83-85 62 83-85 32 83-85

With these case studies and by developing more case studies for

different possible transactions, it is possible to incorporate hour-

ahead ATC functions that minimize the need for TLR as well as

enhance the utilization of transmission resources. The reduction of

the need for TLR may also be achieved by minimizing misuse of ATC,

or through alternative concepts that are more accurate and flexible for

use in reserving contract paths.

6.9. SUMMARY

In this chapter, different levels (stages) of North American Electric

Reliability Council (NERC) TLR procedures and their effects are

discussed. Different types of transactions that can be performed in

deregulated power system and their effects on transmission loading

are discussed and finally an algorithm using hour-Ahead ATC to

invoke TLR procedures is proposed. The results are discussed and

analyzed for basic 7-bus, 26-bus, IEEE 118-bus, and 124-bus-real

time Indian utility power system of Andhra Pradesh State grid.

185

Secured transactional space is defined for different transactions using

sequential transactions with update ATC to invoke TLR procedures.

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