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Challenges of Continuous Parallel

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Challenges of Continuous Parallel Operation of GTG Units in DPS A case study & Simulation

Presented By: Sri Rajib Kumar Sarmah Dy.SEE(Gen)

Date: 05-Dec-2014 Venue: Elect Conf Room

Overview of Presentation

• Existing Scheme of Duliajan Power Station.
• Constraints of continuous parallel operation in

existing scheme of DPS. existing scheme of DPS.

• Solutions for continuous parallel operation.
• Comments.

Duliajan Power Station Present Scenario with 2 x 14.45 MW Gas Turbine Units

B/C 1 B/C 2 11 KV Unit 1 Inter. Unit 2 Inter. Unit 2 Gen. Unit 1 Gen. Bus A Bus B Bus C

OFF OFF ON ON ON ON

Gen. Bkr Gen. Bkr GT Unit 1 GT Unit 2 NGR Unit 1 NGR Unit 2

Duliajan Power Station Operating Condition with only One Unit Running

B/C 1 B/C 2 Unit 1 Inter. Unit 2 Inter. Unit 2 Gen. Unit 1 Gen. Bus A Bus B Bus C

ON ON ON ON ON OFF

11 KV Gen. Bkr Gen. Bkr GT Unit 1 GT Unit 2 NGR Unit 1 NGR Unit 2

Constraints of Parallel running of DPS Units

• Existing 2 x 14.45 MW GTG Units cannot be run in parallel for

long duration due to circulating current (zero sequence current) and subsequent Generator Heating.

• Existing scheme is of very old design (1979) Low Reactance

Grounded system and this scheme is obsolete now.

• In present system already fault currents are very high and will
• In present system already fault currents are very high and will

be more in parallel running condition. This pose safety threat.

• New Units are designed with Generator-Unit Transformer

configuration with High Resistance grounded system and this scheme prevents circulating current between parallel units.

Duliajan Power Station Operating Condition With Unit Running in Parallel

B/C 1 B/C 2 Unit 1 Inter. Unit 2 Inter. Unit 2 Gen. Bkr Unit 1 Gen. Bkr Bus A Bus B Bus C

ON ON ON ON ON ON

11 KV Bkr Bkr GT Unit 1 GT Unit 2 NGR Unit 1 NGR Unit 2

Circulating Zero Sequence Current

Eliminating Circulating Zero Sequence Current in Grid Connected Units

• f Various Capacities using Generator-Transformer Arrangement &

High Impedance Neutral Grounding System

Unit 1 Inter. Unit 2 Interrupter Bkr Bus A

ON ON Unit 1Interrupter Bkr

30 MVA, 11/ 132 KV Tr 2 22.6 MVA, 11/ 132 KV Tr 1 132 KV Unit 2 Gen. Bkr Unit 1 Gen. Bkr GT Unit 1, 15 MW GT Unit 2 , 20 MW NGT Unit 1

ON ON

NGT Unit 2 R R No circulating current between parallel running units

Option 1: IDEAL Solution: Scheme for DPS for Eliminating Circulating Zero

Sequence Current Using Unity Ratio Transformer & High Impedance Neutral Grounding System B/C 1 B/C 2 Unit 1 Inter. Unit 2 Interrupter Bkr Bus A Bus B Bus C

ON ON ON ON Unit 1Interrupter Bkr

20 MVA, 11/ 11 KV Tr 1 11 KV 20 MVA, 11/ 11 KV Tr 2 Unit 2 Gen. Bkr Unit 1 Gen. Bkr GT Unit 1 GT Unit 2 NGT Unit 1

ON ON

NGT Unit 2 R R NGT to be designed for Gen Stator Earth Fault < 20 A) There will not be any circulating current between parallel running units

Challenges of Parallel Operation of DPS Units

• To mitigate circulating current between parallel running units, NGR Auto

switching is required and the scheme has to be fail safe design; so that if

• ne of the parallel running unit trips on fault, the system with the healthy

unit does not remain ungrounded.

• During islanding running of two units, NGRs of all the running units to be

connected to ground automatically to prevent ungrounded operation of any unit.

• Automatic Changeover of Droop/ Isoch mode between parallel running

units is required so that only one unit remains in Isoch mode and all

• ther units stays in droops mode.
• Automatic load shedding scheme (Unit based & Frequency based) is

required so that if one of the parallel running unit trips on fault, some non-priority feeders can be segregated automatically & quickly to prevent under frequency trip of the healthy unit. (If the 20 MW unit trips then more load to be segregate corresponding to tripping of any one of 14.45 MW units).

Option 2: Practicable Solution: Eliminating Circulating Zero Sequence Current Using Neutral Switching Breakers B/C 1 B/C 2 Unit 1 Inter. Unit 2 Inter. Unit 2 Gen. Unit 1 Gen. Bus A Bus B Bus C

ON ON ON ON ON ON

11 KV Gen. Bkr Gen. Bkr GT Unit 1 GT Unit 2 NGR Unit 1 NGR Unit 2

No Circulating Zero Sequence Current ON OFF

New GT- Old GT Inter Connection 1- 3 B/C 1 B/C 2 Unit 1 Inter. Unit 2 Inter. Unit 2 Gen. Unit 1 Gen.

Unit 3

• Gen. Bkr

N B/C 1

Bus A Bus B Bus C

New Bus 3

Tie 3 Bkr Tie 1 Bkr Tie 4 Bkr Tie 2 Bkr 11 KV 11 KV

Duliajan Power Station Switchgear Arrangement with Three Units

New Bus 1 N B/C 2 New Bus 2

Gen. Bkr Gen. Bkr GT Unit 1 GT Unit 2 NGR Unit 1

URT, 31.5 MVA 11/ 11.5 KV

GT Unit 3 NGTr. Unit 3 High Imp Grnd. NGR URT 3 NGR Bkr 1 NGR Bkr 2 NGR Bkr 3 NGR Unit 2

Path of Zero Sequence Currents with Any Combination of two units or Three Units Running Parallel

B/C 1 B/C 2 Unit 1 Inter. Unit 2 Inter. Unit 2 Gen. Unit 1 Gen.

Unit 3

• Gen. Bkr

N B/C 2

Bus A Bus B Bus C

NBus 2 NBus 3

Tie 3 Bkr Tie 1 Bkr Tie 4 Bkr Tie 2 Bkr

ON ON ON ON ON ON ON ON ON ON ON ON

11 KV 11 KV

ON NBus 1 N B/C 1

Path 1 Path 3 Gen. Bkr Gen. Bkr GT Unit 1 GT Unit 2 NGR Unit 1 URT Unit 3 GT Unit 3 NGTr. Unit 3 High Imp Grnd. NGR URT 3 Path 2

ON

NGR Unit 2

Operation with Unit #3 Running Alone

B/C 1 B/C 2 Unit 1 Inter. Unit 2 Inter. Unit 2 Gen. Unit 1 Gen.

Unit 3

• Gen. Bkr

N B/C 2

Bus A Bus B Bus C

NBus 2 NBus 3

Tie 3 Bkr Tie 1 Bkr Tie 4 Bkr Tie 2 Bkr New GT- Old GT Inter Connection 1-3

ON ON ON ON ON ON ON ON ON ON

11 KV 11 KV

ON NBus 1 N B/C 1

OFF OFF

Gen. Bkr Gen. Bkr NGR Unit 1 NGR Unit 2 URT Unit 3 GT Unit 3 NGTr. Unit 3 High Imp Grnd. NGR URT 3

ON

NGR Bkr 1 NGR Bkr 2 Unit 2 Not Running Unit 1 Not Running

ON ON

Operation with Unit #3 & Unit #1 Running In Parallel and Unit #3 is Selected as Master

B/C 1 B/C 2 Unit 1 Inter. Unit 2 Inter. Unit 2 Unit 1

Unit 3

• Gen. Bkr

N B/C 2

Bus A Bus B Bus C

NBus 2 NBus 3

Tie 3 Bkr Tie 1 Bkr Tie 4 Bkr Tie 2 Bkr

ON ON ON ON ON ON ON ON ON ON

11 KV 11 KV

ON NBus 1 N B/C 1

OFF

ON

Unit 2 Gen. Bkr Unit 1 Gen. Bkr

• Gen. Bkr

NGR Unit 1 NGR Unit 2 URT Unit 3 GT Unit 3 NGTr. Unit 3 High Imp Grnd. NGR URT 3

ON

NGR Bkr 1 NGR Bkr 2 Unit 2 Not Running Unit 1 Running

ON

OFF

ON

OFF

Operation with Unit #3 & Unit #1 Running In Parallel and Unit #1 is Selected as Master B/C 1 B/C 2 Unit 1 Inter. Unit 2 Inter. Unit 2 Unit 1

Unit 3

• Gen. Bkr

N B/C 2

Bus A Bus B Bus C

NBus 2 NBus 3

Tie 3 Bkr Tie 1 Bkr Tie 4 Bkr Tie 2 Bkr

ON ON ON ON ON ON ON ON ON ON

11 KV 11 KV

ON NBus 1 N B/C 1

OFF

ON

Unit 2 Gen. Bkr Unit 1 Gen. Bkr

• Gen. Bkr

NGR Unit 1 NGR Unit 2 URT Unit 3 GT Unit 3 NGTr. Unit 3 High Imp Grnd. NGR URT 3 NGR Bkr 1 NGR Bkr 2 Unit 2 Not Running Unit 1 Running

ON

OFF

ON ON

OFF

Comments

• Big challenge is to integrate old equipments with technologies.
• More in depth analysis is required by in-house & external

experts before going for any up-gradation/ modification.

• Techno- commercial feasibility study is required whether to go
• Techno- commercial feasibility study is required whether to go

for up-gradation of existing system or go for replacement of old units.

• At present, space constraint is there to adopt new system.
• Detailed analysis of system fault levels, protection & interlock

requirements is required.