SOUTHERN REGIONAL LOAD DESPATCH CENTREBANGALORE

ONE DAY WORKSHOP ON

AVAILABLE TRANSFER CAPABILITY(ATC)

IN INDIAN CONTEXT

14TH AUGUST 2007

Power Grid Corporation of India Limited

ATC FUNDAMENTALS

WHY ATC?

Presentation Road Map• What is Transfer Capability• Difference between Transfer Capability

and Transmission capacity• Assessment of Transfer Capability • What is reliability Margin why are they

required• What are the risks associated with

violation of transfer capability in real time• How to Improve Transfer Capability

AIM OF POWER SYSTEM ENGINEERS

• EARLIER STATEMENT– To provide Reliable, Stable and Secured

Power supply to the end user with Least possible cost

• PRESENT STATEMENT– To provide Reliable, Stable and Secured

Power supply to the end user with Least possible cost WITH Maximizing profit to all stake holders

Electricity is a scientific phenomenon• EMF travels at the speed of light• Available ‘just-in-time’• Delivered to the customers fresh • No one get placed on hold• Impartial in its benevolence and wrath• Good servant but a ruthless master• Interconnected systems with thousands of

kilometers of transmission lines and hundreds of generators operating with split second synchronism

• The largest single machine ever created

Grid operation is a continuous interplay of technical phenomena and natural/ human intervention

Power flow characteristics • Is directional • Does not recognize geographical

boundaries, asset ownership• Does not check the map to determine the

shortest route• Flows are dictated purely by

– Impedances of the transmission lines– Point of injection by generators– Point of consumption loads

“Time & Location matter is fundamental to operation” -Shmuel Oren & Fernando Alvarado

Some Definitions• ‘TTC is the amount of electric power that can be transferred over the

interconnected transmission network in a reliable manner based on all of the following conditions:1. For the existing or planned system configuration, and with normal (pre-contingency) operating procedures in effect, all facility loadings are within normal ratings and all voltages are within normal limits.2. The electric systems are capable of absorbing the dynamic power swings, and remaining stable, following a disturbance that results in the loss of any single electric system element, such as a transmission line, transformer, or generating unit.3. After the dynamic power swings subside following a disturbance that results in the loss of any single electric system element as described in 2 above, and after the operation of any automatic operating systems, but before any post-contingency operator-initiated system adjustments are implemented, all transmission facility loadings are within emergency ratings and all voltages are within emergency limits.

Some Definitions continued

4.With reference to condition 1 above, in the case where pre-contingency facility loadings reach normal thermal ratings at a transfer level below that at which any first contingency transfer limits are reached, the transfer capability is defined as that transfer level at which such normal ratings are reached.5 In some cases, individual system, power pool, subregional, or Regional planning criteria or guides may require consideration of specified multiple contingencies, such as the outage of transmission circuits using common towers or rights-of-way, in the determination of transfer capability limits. If the resulting transfer limits for these multiple contingencies are more restrictive than the single contingency considerations described above, the more restrictive reliability criteria or guides must be observed.’

TRANSFER CAPABILITY• Transfer Capability’ is

the measure of the ability of interconnected electric systems to reliably move power from one area to another over all transmission lines (or paths) between those areas under specified system conditions

Transfer Capability is different from ‘Transmission Capacity’, which usually refers to the thermal limit or rating of a particular transmission element or component

TRANSMISSION CAPACITY vs TRANSFER CAPABILITY

S No. Transmission Capacity Transfer Capability

1 Is a physical property in isolation Is a collective behaviour of a system

2 Depends on design only Depends on design, topology, system conditions, accuracy of assumptions

3 Deterministic Probabilistic

4 Constant under a set of conditions Always varying

5 Time independent Time dependent

6 Non-directional Directional

7 Determined directly by design Estimated indirectly through simulation studies

8 Declared by designer/ manufacturer

Declared by the System Operator

9 Generally Understood by all Frequently misunderstood

10 Considered unambiguous & sacrosanct

Subject to close scrutiny by all stakeholders

Power System

Stability

Thermal

Overloading

Rotor Angle

Stability

Small-Disturbance

Angle Stability

Transient

Stability

Frequency

Stability

Voltage

Stability

Large-

Disturbance

Voltage Stability

Small-

Disturbance

Voltage Stability

Cascading Blackouts

A CHAIN IS ONLY AS STRONG AS ITS WEAKEST LINK

IN A GRID WITH ELEMENTS IN SERIES AND PARALLEL, THE WEAKEST LINK IN

SERIES WOULD DETERMIN THE STRENGTH OF THE NETWORK

Transfer Capability Limits

Thermal limit• Thermal Limits establish the maximum electrical

current that a transmission line or electrical facility can conduct over specified time periods before it sustains permanent damage by overheating or before it violates public safety requirements

Voltage limitSystem voltages and changes in voltage must be maintained

within the acceptable range as defined in the Grid Codes. For example, minimum voltage limits can establish the maximum amount of electric power that can be transferred without causing damage to the electric system or customer facilities. A widespread collapse of system voltage can result

in a black out of portions or the entire interconnected network

• Stability LimitsThe transmission network must be capable of surviving disturbance

through the transient and dynamic time periods (from milliseconds to several minutes respectively) following a disturbance. All generators connected to ac interconnected transmission system operate in synchronism with each other at the same frequency. Immediately following a system disturbance, generators begin to oscillate relative to each other, causing fluctuations in system frequency, line loadings, and system voltages. For the system to be stable the oscillations must diminish as the electric systems attain a new, stable operating point. If a new, stable point is not quickly established, the generators will likely lose synchronism with one another, and all or a portion of the interconnected system may become unstable. The result of generator instability may damage equipment and cause uncontrolled, widespread interruption of electric supply to customers.

Total Transfer Capability: TTC

Voltage Limit

Thermal Limit

Stability Limit

Total Transfer Capability

Total Transfer Capability is the minimum of the Thermal Limit, Voltage Limit and the Stability Limit

Time

Power Flow

• “Non-simultaneous Transfer Capability is the amount of electric power that can be reliably transferred between two areas of the interconnected electric system when other concurrent normal base power transfers are held constant.”

• “Simultaneous Transfer Capability is the amount of electric power that can be reliably transferred between two or more areas of the interconnected electric system as a function of one or more other power transfers concurrently in effect.”

TTC assessment block diagram

TTC

AnticipatedNetwork topology +Capacity additions

Anticipated Substation Load

Anticipated Ex bus

Thermal Generation

Anticipated Ex busHydro generation

LGBR

Last Year

Reports

WeatherForecast

CEACTUSTU

Last Year

patternOperator

experience

Planning criteria

Operating limits

Credible contingencies

Simulation

Analysis

Brainstorming

Stakeholders

Reliability Margins

Short Term Open Access

Long Term Open Access

Reliability Margin

TTC

ATC

Need for Reliability Margins

– Peculiarity in Indian power grids– Difference in Planning assumptions and

operating conditions– Forecasting errors– Outage of units etc

Peculiarity in Indian power grids• Haulage of power over long distances• Resource inadequacy leading to high uncertainty in

adhering to maintenance schedules • Pressure to meet demand even in the face of acute

shortages and freedom to deviate from the drawal schedules.

• A statutorily permitted floating frequency band of 49.0 to 50.5 Hz

• Non-enforcement of mandated primary response, absence of secondary response by design and inadequate tertiary response.

• No explicit ancillary services market• Inadequate safety net and defense mechanism

Difference in Planning assumptions and operating conditions

• Planning criteria– The ISTS shall be capable of withstanding and be secured against a selected

list of credible contingency outages without necessitating load shedding or rescheduling of generation during Steady State Operation.

– The credible contingencies considered are• Outage of a 132 kV D/C line or,• Outage of a 220 kV D/C line or,• Outage of a 400 kV S/C line or,• Outage of single Interconnecting Transformer, or• Outage of one pole of HVDC Bipole line, or• Outage of 765 kV S/C line• Outage of a single largest in feed

– Planning is carried out on regional self sufficiency basis– In the proposed Planning criteria six dispatch scenario’s are considered

Difference in Planning assumptions and operating conditionsOperating conditions not accounted during planning

– Simultaneous outage of more elements like Bus bar operation in a station

– Simultaneous outage of generators in a station due to auxiliary supply problem or evacuation line outages

– Weather disturbance causing multiple outage of lines in the same corridor

– Depletion in Hydro storage and less generation due to fuel shortages– Variations in interregional exchanges– Forecast errors– Transmission lines and generators not coming up as per plan– Re configuration of switching arrangements due to constraints like

overloading of lines and transformers– Socio-economic uncertainties in a progressive economy

The above causes the difference in transfer capability in real time compared to Planning assumptions

Likely consequences of contingency during various operating conditions

S No. Scenario Likely consequences

1 Real time transfers > TTC System might not survive even a single element outage what to talk of a multiple contingency

2 ATC < Real time transfer< TTC

System might survive a single element tripping. But the chances of a cascading failure are high in case of a multiple contingency.

3 Real time transfer < ATC Chances of survival are high for single contingency and moderate for multiple contingency.

Providential escape from ‘the valley of death’ on certain occasions cannot be a justification to operate the system at that edges. Luck is a not a part of operating procedure

Methods to improve TTCWe should of strong defence mechanisams like

– System Protection schemes– Effective under frequency and under voltage

protections– Auto re-closing schemes– Tools for damping the oscillations like TCSC’s– Wide area monitoring and measurement equipment

for quick action taking– Improved visualisation to the system operator to take

immidiate corrective action– Empowerment of SLDC/Generator operators to take

immidiate corrective actions

KOLAR SPECIAL PROTECTION SCHEME

Performance of the SchemeFREQUENCY DIP DURING KOLAR HVDC TRIPING AND DURING SIMHADRI GENERATION LOSS

48.5

48.7

48.9

49.1

49.3

49.5

49.7

49.9

50.1

T-30 Minutes T-25 Minutes T-20 Minutes T-15 Minutes T-10 Minutes T-5 Minutes T=0 Minutes T+5 Minutes

Time

FR

EQ

IN

HZ

TAL-KOL TRIP ON15-09-06 AT 16:52 HRS LOSS IS 1887 MW

SIMHADRI GEN LOSS OF APPROX 950 MW ON 16-01-07

AT 1812 HRS

Frequency Trend during the Tal-Kolar pole 2 trip

0200400600800

100012001400160018002000

0:01

0:03

0:05

0:07

0:09

0:11

0:13

0:15

0:17

0:19

0:21

0:23

0:25

0:27

0:29

Time

Pow

er fl

ow

49.25

49.3

49.35

49.4

49.45

49.5

49.55Talcher-Kolar power flow

Frequency

Line loading as function of length

0.50

0.75

1.00

1.25

1.50

1.75

2.00

2.25

2.50

2.75

3.00

3.25

0 1 2 3 4 5 6 7 8 9 10

Length in x100 kM

Tim

es

SIL

Voltage Level (kV)Number and

size of Conductor

S.I.L. (MW)

765 4 x 686 2250

765 4 x 686 614

Op at 400

400 2 x 520 515

400 4 x420 614

400 3 x420 560

400 2 x 520 155

Op at 220

220 420 132

132 200 50

SIL of different voltage level and conductor configuration

St. Clair’s curve

REGIONAL GRIDS

QUICK FACTS

SOUTHERN REGION

WESTERNREGION

EASTERN REGION

NORTHERN REGION

NORTH-EASTERN REGION

INSTALLED CAPACITY

NORTHERN :- 36,547 MW

EASTERN :- 17,159 MW

SOUTHERN :- 37,592 MW

WESTERN :- 40,280 MW

NORTH-EASTERN :- 2,506 MW

TOTAL 134,084 MW

REGIONAL GRIDS

Area : 889,000 SQ KMSPopulation : 307 Million

Peak Demand : 28,000 MW :560 MU / Day

Area : 951,488 SQ KMSPopulation : 230 Million

Peak Demand : 29,000 MW :640 MU / Day

Area : 636,249 SQ KMSPopulation : 223 Million

Peak Demand : 25,000 MW :470 MU / Day

Area : 425,432 SQ KMSPopulation : 227 Million

Peak Demand : 10,000 MW :200 MU / Day

Source:

Powerline

(Siemens Ad),

Oct-2006

HYDRO RESOURCES

COAL BELT

MUMBAI

DELHI

CHENNAIBANGALORE

KOLKATTA

RESOURCES ARE FAR AWAY FROM LOAD CENTERS.

NECESSITATES LONG TRANSMISSION LINKS FOR EVACUATION

AREAS SHOWN ARE APPROXIMATE AND INDICATIVE

EASTERN REGION

SOUTHERN REGION

WESTERNREGION

NORTHERN REGION

NORTH-EASTERN REGION

THE NATIONAL GRID : PHASE 1

ER

500 MW VINDHYACHAL

WR-NR HVDC B2B LINK

Commissioned in Nov. 1989

BIRPARA(ER) – SALAKATI(NER) 220 KV AC LINK in April 87

500 MW SASARAM

WR-NR HVDC B2B LINK

Commissioned in June 2001

500 MW GAZUWAKA

ER-SR HVDC B2B LINK

Commissioned in Sep. 1999

500 MW BHADRAWATI

WR-SR HVDC B2B LINK

Commissioned in Sept. 1997

NATIONAL GRID PHASE-1 COMPLETE

400 KV Siliguri-Boangigaon in April 2000

Bhadrawathi 2nd pole in March, 1998

SOUTHERN REGION

WESTERNREGION

EASTERN REGION

NORTHERN REGION

NORTH-EASTERN REGION

‘ELECTRICAL’

REGIONS

1

2SOUTHERN REGION

WESTERNREGION

EASTERN REGION

NORTHERN REGION

NORTH-EASTERN REGION

‘ELECTRICAL’

REGIONS

1

2

1

SR INTERCONNECTION BY 2012

KOLAR

TALCHER

RGM

NARENDRA-KOLHAPUR D/C AND BACK TO

BACK 2X 500 MW HVDC SYSTEM

PROPOSED

KOLHAPUR

SR WOULD BE SYNCHRONOUSLY CONNECTED WITH REST OF INDIA THROUGH 765 KV D/C RAICHUR-

SHOLAPUR-PUNE LINK

11850 MW

1200 MW6050 MW

5500 MW

1400 MW6150 MW

36,700 MW OF INTER-REGIONAL POWER BY 2012

EASTERN REGION

SOUTHERN REGION

WESTERNREGION

NORTHERN REGION

NORTH-EASTERN REGION

INTER-REGIONAL TRANSFER BY END OF 11th PLAN (2012)

4000 MW

41203%

95427%

3348625%

8693665%

HydroThermalNuclearWind & Others

Source wise composition of installed capacity in India (1,34,084 in 2007) AS on 30-06-07

ALL INDIA GENERATION COMPOSITION

16.8, (3%)

486.1, (83%)

84.5, (14%)

ThermalHydroNuclear

Total Market Size = 587.4 BU

Total Installed Capacity 1,34,084 MW

Sector wise consumption of electricity in India

22%

29%

5%

35%

6%

3%

IndustryDomesticRailwaysAgricultureCommercialOthers

Total Installed Capacity 1,34,084 MW

ALL INDIA MARKET COMPOSITION

5%9%

3%

46%

37%

State Sector long termPPACentral Sector longterm PPAIPP generation

Short Term Trading

Balancing market

(1,34,084 in 2007) AS on 30-06-07

THE SOUTHERN REGION GRID

ATC ISSUES AND HOTSPOTS

SOUTHERN REGION

WESTERNREGION

EASTERN REGION

NORTHERN REGION

NORTH-EASTERN REGION

1

2

TWO ELECTRICAL REGIONS w.e.f Aug. 2006

‘NEW’ GRID

HVDC INTERCONNECTS

AC INTERCONNECTS

MAJOR INTERCONNECTIONS

2X500 MW BACK TO BACK STATION AT

GAZUWAKA(SR)

1000 MW BACK TO BACK STATION AT

BHADRAWATI(WR)

TALCHER

KOLAR

TALCHER-II TO KOLAR

2000 MW BIPOLE LINK

INTER REGIONAL TRANSFER CAPACITY SR WITH OTHER REGIONS

• WITH ER• JEYPORE-GAZUWAKA 1000 MW• TALCHER-KOLAR 2000 MW

• WITH WR• RAMAGUNDAM-CHANDRAPUR 1000 MW

TOTAL CONCURRENT CAPACITY IS 4000 MW

220 KV LINKS ARE IGNORED BECAUSE THEY ARE NOT IN ACTIVE USE

SR GRID MAP

GAZUWAKA

RAICHUR

GOOTY

SALEM

UDUMALPETTRICHU

R

MADURAI

TRICHY

MADRAS

NEYVELI

GUTTUR

KAIGA

BHADRWATHI

MUNIRABAD

P

P

P

P

P

P

P

P P

P

P

N

KOLAR

HOSUR

THIRUVANANTHAPURAM

NELLOREN

LM

SIMHADRI

HIRIYURTALGUPP

A

KADAPA

NARENDRA

CHITTOOR MA

PS

KALPAKA

GAZUWAKA

VEMAGIRI

NUNNA

KHAMMAM

RAMAGUNDAM

MBN

KNL

GHANAPUR

SSLM

MMDPL

I

NSR

S'HALLI

HOOD

Y

35

160607

105155

49.42

43

197

1

195

225

227258

242 240242 236

303

21471471

96108

318110

37

218

34

0

278

252252

229341

107

267123

20

11952

32068

388

420 419

143 14171

265

119 122

280

0

1 401

151158

221

321

300

257

17

389 381209

243

272

197 187

343329 343344 205348343

78

133131

123123

251

253

0198 202

v 299314

200

1

0 0

273

185

RAYALASEEMA AXIS

404

402

395

405

408

409

405391

403

232

406409

410

409

403

0

406

396

402

404

406403

397

406

386

407

401

384396

399

391

398

397

404

1542

284

GENERAL DIRECTION OF POWER FLOW IS

FROM NORTH TO SOUTH

0

200

400

600

800

1000

1200

1400

1600

1800

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

HOURS --->

MW

---

->

49.0

49.5

50.0

50.5

51.0

51.5

52.0

52.5

53.0

53.5

FR

EQ

(H

Z)

---

->

UI IMPORT BY SR FROM CG ON 05-MAR-07

CG FREQ

SR FREQ

UI IMPORT

FROM CG

AMOUNT SAVED FOR SR CONSTITUENTS = 2.20 CRs

20.31 MUs IMPORTED FROM CG

PEAK IMPORT OF 3670 MW FROM ‘NEW’ GRID

AREAS OF CONSTRAINT• HYDERABAD URBAN AREA

– HIGH 400/220 KV ICT LOADINGS– 220 KV LINE OVER LOADING– DEC TO FEB– SENSITIVE TO IMPORT FROM WR AT RAMAGUNDAM– NEW STATIONS PLANNED………..WOULD BE IN PLACE BY 2008-9

• SRISAILAM EVACUATION PROBLEMS– DEPENDS ON RAINFALL IN CATCHMENT AREA (N KARNATAKA, SW

MAHARASHTRA)– OVERLOADING OF SRISAILAM-KURNOOL AND KURNOOL-GOOTY 400 KV S/C

LINKS– SENSITIVE TO IMPORT FROM GAZUWAKA AREA– NO TIME FRAME AS YET FOR AUGMENTATION

• NUNNA-NELLORE D/C LINK – WOULD BE SOLVED WITH NEW GENERATION COMING UP SOUTH BY 2009

• GOOTY-BANGALORE CORRIDOR– FULL GENERATION AT RAICHUR, ALMATTI, BTPS AND IMPORT FROM WR/ER

• WIND ENERGY EVACUATION ISSUES IN SOUTH TAMILNADU– 2000 MW WIND IN TN, PARTICULARLY ALONG KERALA BORDER AND IN

KANYAKUMARI AREA– EVACUATION PROBLEMS AS NETWORK WAS NOT DESIGNED FOR THIS– DEDICATED SS AND TL IN PROGRESS– WOULD SATURATE AT 4000-5000 MW– SEASONAL AND UNPREDICTABLE– CONSTRAINT – KERALA HAS TO MAINTAIN HYDRO TO PREVENT LINE

OVERLOADING

HOT SPOTS• COIMBATORE AREA

– LINE OVERLOADING PROBLEMS

• MADRAS CITY– 110 KV GMR VASAVI EVACUATION– ROW PROBLEMS

• BANGALORE CITY SUBTRANSMISSION

• RELIABILITY ISSUES