ltc theory application and maintenance 4l

8/13/2019 Ltc Theory Application and Maintenance 4l

http://slidepdf.com/reader/full/ltc-theory-application-and-maintenance-4l 1/58

Load tap changersTheory & application

Jon Brasher, Technical Specialist, Alamo Tennessee - USA



Load tap changer Why ?

§© ABB Group

§June 11, 2013| Slide 2

§6 AM §12 N§12 M §4 PM §8 PM

To keep the system voltage constant for all conditions of total system load.

System load changes throughout the 24 hour day.



Load tap changersWhy ?

§ Voltage regulation

§ Control of circulating current

§ Minimize losses between parallel transformers

§ Control power flow§ Phase angle shift using PST

§ Amount and direction of power exchanged over linesconnecting two networks

§ Combination of controlling power flow &circulating current

§© ABB Group

§June 11, 2013| Slide 3



Load tap changersThe theory

Change the turns ratio between windings in a transformer.

Changes output voltage of transformer

Change under load without disturbing the customer

§© ABB Group

§June 11, 2013| Slide 4

V1V2

2

1

12 N

N

V V ´=

Vi = Voltage

Ni = Number of winding turns



Load tap changersThe rules

§ Taps changed (output voltage raised or lowered) while under load

§ No interruption in load current

§ Never dead short any part of winding

§ Fully automatic

§ Sense need for voltage change

§ Make the tap (voltage) change

§© ABB Group

§June 11, 2013| Slide 5



Load tap changersWhere ? Typical US application

DeTC on high voltage side5 position

LTC on low voltage side33 Position



Load tap changersThe theory

Transformer has tapped or regulating winding.

Taps are connected to the LTC

Change under load without disturbing the customer

The LTC must:1) Select the tap connection and

2) Divert the load current to the selected tap

3) Maintain the flow of power at all times

4) Prevent shorting the tap winding

§© ABB Group

§June 11, 2013| Slide 7



Load tap changer details

Application to thetransformer



For two-winding power transformers,the most popular arrangement is theregulation at the neutral end of thewinding.

This solution provides the mosteconomical tap winding arrangementbecause of the relatively low dielectricstress at the neutral end combined witha compact three-phase neutral pointOLTC.

§© ABB Group

§June 11, 2013| Slide 10

OLTC in Y or Star Connected Windings

Load tap changersThe application



§© ABB Group

§June 11, 2013| Slide 11



§© ABB Group

§June 11, 2013| Slide 12



§© ABB Group

§June 11, 2013| Slide 13

3-pole line-end arrangement2-pole + 1-pole

line-end arrangement3-pole mid-winding arrangement

OLTC in delta-connected windings

Load tap changersThe application



A = Linear switching

§ Not very common§ Simple design trafo & LTC§ Smallest regulating range§ Smallest losses

B = Plus/minus switching§ Most common (~90%)§ Additive (+) & subtractive (-)§ Reversing switch

C = Coarse/fine switching

§

Less common (~10%)§ Coarse winding insertion§ Change over selector § Small losses

§© ABB Group

§June 11, 2013| Slide 14

A B C

10

9 9M M

CoarseWinding

Reversingswitch

Selector switch

R e g u l a t i n g W i n d i n g

Load tap changersConfigurations



Load tap changersBasic types

Tap-changers can be located in the main transformer tank, or on the outside

§ In-tank types can be used up to the largest transformerssizes and voltages

§ On-tank types can be used up to approximately 80 MVA /145 kV

§© ABB Group

§June 11, 2013| Slide 15



Load tap changersBasic types

§© ABB Group

§June 11, 2013| Slide 16

In-tankvacuum/resistanceExample: type VUCG

Application: HV winding

On-tankvacuum/reactanceExample: type VRLTC

Application: LV winding

On-tankresistanceExample: type UZ

Application: LV & HVwinding



Load tap changers, reactance typeHistorical perspective

W type: UTS & URSGE type: LR72 & LR68Reactor only

Analog drive system

W type: UTT & URTGE type: LR65 & LR83Reactor plustransfer switch

Analog drive system

ABB type VRLTCReactor plus by-passswitch plus vacuuminterrupter Digital drive system

W type UVTGE type LR400……………Reactor plus by-passswitch plus vacuuminterrupter

Analog drive system



LTC history video

§ Video 1

§© ABB Group

§June 11, 2013| Slide 18



Load tap changers, reactance typeReactor or preventive autotransformer (PA)

Why?

§ The PA providesimpedance for the tapcircuit

§ Prevents shorting thewinding when inbridging position

What is it?

§ Gapped core reactor (insures very exact split

of load current)§ Wound on its own core

§ Mounted inside of thetransformer

§© ABB Group

§June 11, 2013| Slide 19



I cI c

Load tap changer, reactance typeCirculating current

§© ABB Group

§June 11, 2013| Slide 20

I L/2

Non- bridging positionI c= 0

IL/2

I L

I L/2

I L/2

IL

Bridging positionI c ≠ 0



Load tap changer, reactance typeChanging taps from 11R to 10R, Step 1

§© ABB Group

§June 11, 2013| Slide 21

Steady state on position 11 R

11 R is a bridging position

Step 1: open by-pass switch



Load tap changer, reactance typeChanging taps from 11R to 10R, Steps 2 & 3

§© ABB Group

§June 11, 2013| Slide 22

Step 2: open vacuum interrupter Step 3: move tap selector to non-bridging position



Load tap changer, reactance typeChanging taps from 11R to 10R, Steps 4 & 5

§© ABB Group

§June 11, 2013| Slide 23

Step 4: close vacuum interrupter Step 5: close by-pass switch

Steady state on position 10R

Complete the tap change

Elapsed time = 2 seconds



VM Vout = VM + VT

Plus configuration

VMVout = VM - VT

Minus configuration

VT

Transformer windingPolarity / plus or minus tapping

VT



Load tap changersPlus/minus & the reversing switch

High voltage winding

Regulating windingLow voltage winding

Reversing Switch

Load tap changer

Reversing switch in the RAISE position.Tap winding adds to LV winding

B

AR



Regulating WindingLow Voltage Winding

High Voltage Winding

Reversing Switch

Load Tap Changer

Load tap changersPlus/minus & the reversing switch

Reversing switch in the LOWER position.Tap winding subtracts from the LV winding

B

AR



Load tap changersThe reversing switch

§© ABB Group

§June 11, 2013| Slide 27




Tap changer compartment



Quality assuranceType tests, IEEE C57.131 and IEC 60214

1. Temperature rise of contacts

2. Switching tests

§ Service duty test

§ Breaking capacity test

3. Short-circuit current test

4. Transition impedance test

5. Mechanical tests

6. Dielectric tests

§© ABB Group

§June 11, 2013| Slide 29



Load tap changersTap changing vs motor driving

§ On tank tap changers

§ Oil filled tap changingcompartment

§ Motor drive

compartment§ Connecting drive shaft

§ Connecting electronics

§© ABB Group

§June 11, 2013| Slide 30



Vacuum / reactance LTCLTC compartment

§© ABB Group

§June 11, 2013| Slide 31



LTC compartmentDiverter assemblies



LTC compartmentDiverter detail

§© ABB Group

§June 11, 2013| Slide 33



Vacuum interrupter video

§ Video 2

§© ABB Group

§June 11, 2013| Slide 34



Vacuum Interrupter arc video

§ Video 3

§© ABB Group

§June 11, 2013| Slide 35



LTC compartmentBy-pass switch

§© ABB Group

§June 11, 2013| Slide 36

Closed

OpenClosed

Closed



§© ABB Group

§June 11, 2013| Slide 37



LTC compartmentDiverter assembly and tap selector

§© ABB Group

§June 11, 2013| Slide 38



§© ABB Group

§June 11, 2013| Slide 39

LTC compartmentTap selector




Motor and drive controls



Motor drive and tap changer control

20th Century

§ Induction motor

§ Electromechanical

controls§ Cam switches

§ Selsyn’s

21st Century

§ Digital drive system

§ Digital control

system§ No

electromechanicalcontrols

§ No cam switches

§ No Selsyn’s

§© ABB Group

§June 11, 2013| Slide 41



Motor driveMajor components

§© ABB Group

§June 11, 2013| Slide 43

TLMS

Digitaldrive

Servomotor

Gear head

Multi-turnabsolute encoder



Brushless AC servomotor

Digital accuracy

Continuous positional feed back

Precise speed control

Active on-position holding

Resolver feed back loop

§© ABB Group

§June 11, 2013| Slide 44



Digital servo drive

§ Digital servo drive

§ Ruggedized

§ Vibration/shock

§ Temperature

§ EMC hardened

§ Instructs the servo motor

§ Monitors the servo motor

§ Communicates with TLMS

§© ABB Group

§June 11, 2013| Slide 45



Multi-turn absolute encoder

Knows position of the drive shaft

Supplies shaft data to TLMScontroller

Accurate to 0.00001 degrees

Non-volatile memory

§© ABB Group

§June 11, 2013| Slide 46



Tap Logic™ module (TLMS)

Monitors

Decides

Commands

Alerts

Alarms

Transmits

Records

Reports

§© ABB Group

§June 11, 2013| Slide 47



Tap Logic™ module (TLMS)Percent of time on tap position

§© ABB Group

§June 11, 2013| Slide 51

1551869%

694831%

Bypass switch operations

P2 Bypass (Upper)

P3 Bypass (Lower)



Vacuum interrupter Monitoring current through VI

§© ABB Group

§June 11, 2013| Slide 52

Current detector module

Fiber optic signalto optical receiver



Typical analog control set-upLTC to tap changer control unit

§© ABB Group

§June 11, 2013| Slide 55

Transformer Control Cabinet LTC Motor Drive Compartment

VoltageController

(“90” device)

Current Loop Module(Beckwith M-2025C)

Gear Box

LTC Drive Shaft

And/or

Potentiometer PositionTransducer

Power Supply(Incon 1945)

Tap PositionIndicator

(Incon 1250-B)

Selsyn PositionTransducer (Incon 1292-KS)

T i l di it l t l t



Typical digital control set-upLTC to tap changer control unit

§© ABB Group

§June 11, 2013| Slide 56

VoltageController

(“90” device)

Current Loop Module

(Beckwith M-2025C)

Digital Motor Controller TLMSTM

Transformer Control Cabinet LTC Motor Drive Compartment

L d t h



Load tap changersMaintenance and duty cycle reality

§© ABB Group

§June 11, 2013| Slide 57

Operations

per day

Operations

per year

Number of operations

15 years 20 years 30 years 40 years 50 years

25 9,125 136,875 182,500 273,750 365,000 456,250

35 12,775 191,625 255,500 383,250 511,000 638,750

45 16,425 246,375 328,500 492,750 657,000 821,250

70 25,550 383,250 511,000 766,500 1,022,000 1,277,500

90 32,850 492,750 657,000 985,500 1,314,000 1,642,500

100 36,500 547,500 730,000 1,095,000 1,460,000 1,825,000

140 51,100 766,500 1,022,000 1,533,000 2,044,000 2,555,000

Maintenance at 1,000,000 operationsInspection at 500,000 operations

ltc theory application and maintenance 4l

Documents