On Load Tap Changing

Transformer Paralleling

Simulation and Control

2

OLTC Overview

• Transformer Paralleling

• The need for control

• Current Solutions

• Our Plan and System

3

Parallel Transformers

• Increase Reliability• Improve Power quality• Prevent voltage sag• Meet increased load

requirements

4

Examples

• Illustrate the need for control

• Present Two Calculation Methods– Superposition Method– Admittance Method

5

Grainger Examples

One-Line Diagram Grainger, Example 2.13, pg 78

6

Grainger Examples

Per-Phase Reactance Diagram, Grainger pg 78

7

Superposition Methodj 1 pu 1

tn

n'

Zload 0.8 j 0.6( )pu

V2 1.0 ej 0 deg pu

ZTa j 0.1 pu ZTb j 0.1 pu

ILoad

V2

Zload

0.8 0.6j( ) pu

8

Superposition Method

V t 1 0.05 arg V( ) 0 deg Tap Step Voltage

By Superposition:

IcircV

ZTa ZTb0.25j pu Circulating Current

ITa

ILoad

2Icirc 0.4 0.05j( ) pu

ITb

ILoad

2Icirc 0.4 0.55j( ) pu

9

Superposition MethodEquivalent Circuit

10

Superposition Method

STa V2 ITa

0.4 0.05j

Vars are unbalancedKWs are balancedSTb V2 ITb

0.4 0.55j( ) pu

SLoad V2 ILoad

0.8 0.6j( ) pu

SLoad 1pu

kVA in the circuit thatserves no purpose at the load

STa STb SLoad 0.083pu

11

Admittance Methodt 1.05e

j 0 deg

YTa

Y

Y

Y

Y

10j

10j

10j

10j

pu

YTbt 2

Y

t Y

t Y

Y

11.025j

10.5j

10.5j

10j

pu

Y YTa YTb21.025j

20.5j

20.5j

20j

Grainger, Example 9.7

12

Admittance MethodI1

I2

YV1

V2

V1

I1

Find V1 I1

I1a

I2a

YTa

V1

V2

I2a 0.39 0.049j( ) pu

I1b

I2b

YTb

V1

V2

I2b 0.41 0.551j( ) pu

STa V2 I2a

0.39 0.049j( ) pu

STb V2 I2b

0.41 0.551j( ) pu

13

Problem Definition

• We want to minimize the circulating current.

• Why?– Increased total losses of the two transformers– Unable to fully load one transformer without

over-loading or under-loading the other– This current is parasitic, serving no benefit– The transformer is not operating at optimum

14

Project Objectives

• Build and test an experimental system– Measure the circulating current

• Build a mathematical model of the system• Design a control scheme that utilizes SEL

technology• Refine the System to minimize circulating

current over a variety of conditions

15

Popular Solution Methods.

• Master- Follower Method

• Power Factor Method

• Circulating Current Method

• Var Balancing (∆Var) Method TM

Source: Advanced Transformer Paralleling  Jauch, E. Tom: Manager of Application Engineering, Beckwith Electric Co., Inc.

16

Master-Follower

• Desired operation maintains same tap level on all transformers

• Consists of one control commanding transformer tap changes to follow

17

Master-Follower

• Positives:– Appropriate voltage level via load is maintained

• Negatives:– Does nothing to prevent circulating current

18

Power Factor (PF) Method

• Desired tap positions provide equal PF

• Done by comparing angle of currents

• Does not operate controls, Just prevents them from operating in the wrong direction.

19

Power Factor (PF) Method

• Positives:– Keeps PF in desired range.

• Negatives:– Difficult to apply to more than 2 parallel

transformers.– If VAr flow, tap level changed is blocked to

minimize PF difference.– If transformers have different impedances, Highest

KW loaded transformer is forced to have highest VAr load.

20

Circulating Current Method

• Assumes continuous circulating current path

• Controls are biased to minimize Icirc.• Higher tap lowered, as lower tap increased

the same amount to make equivalent tap level.

• Relay used to block operation if tap level variation becomes to great.

21

Circulating Current Method

• Positives:– Icirc is put to a minimum– Initial voltage level maintained– Max difference in tap levels maintained

• Negatives:– Auxiliary CT’s are required– Flow of KW can not be fixed by changing taps

» This causes oscillation of tap levels.

22

Var Balancing (∆Var) Method

• Loads transformers by balanced VAr sharing.

• Ignores KW loading

23

Var Balancing (∆Var) Method

• Positives:– Balanced VArs make Icirc a min or 0– No auxiliary CT’s are needed

• Negatives:– Method is patented by Beckwith Electric Co.

INC.

24

Our Plan

• SEL 3378 SVP assumes control of system• Provided with phasors from the relay• SVP calculates optimal tap levels• SVP directs tap changers through SEL

487E relay

25

Our Plan

• Goals– Appropriate voltage level maintained– Icirc driven to a minimum– Max variation of tap levels met– Avoids tap level oscillation

26

System

• Transformers

• 487E Relay

• 3378 Synchrophasor Vector Processor

27

Transformers

• Two Autotransformers will be used to simulate two parallel power transformers

• Voltage controlled motors on the tap changers

• Transformer secondary will feed an external load from unity to 0.5 lead/lag

28

Transformers

• Superior Electric Type 60M21• Single Phase• Input Voltage: 120V• Output Voltage: 0V-140V• KVA: 0.7• Toroidal Core• Synchronous Motor

– 120VAC, 60Hz, 0.3A, 3.32 RPM

29

Transformers

• Short Circuit Tests– The resistance of the tap contact is larger

than the reactance of the winding– The MVA imbalance of the parallel

combination is expected to be dominantly Watts, rather than Vars

• Verified through no-load Paralleling test

30

T1 X and R Vs Secondary Nominal Voltage

0 20 40 60 80 100 120 140 1600

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

T1 Leakage Reactance Vs Secondary Voltage

X

R

Secondary Nominal Voltage

Ohm

s

31

Transformers

• The autotransformers do not exhibit characteristics similar to a typical power transformer

• Options– Use these transformers– Different Transformers, 5 kVA Motor driven

autotransformers

32

Calculations

• The Superposition method will support the real component while the Admittance method will not– The real component will create a negative

resistance in the PI equivalent

33

487E Relay

• Uses Lateral Logic• 18 Current Channels• 6 Voltage Channels• Synchrophasor data

collected once per cycle, up to 12 Channels

34

487E Relay

• Control transformer tap level

• Receives commands from SVP

• Displays: voltages, currents, Icirc, apparent power, real power, reactive power.

35

3378 SVP

The SVP time aligns synchrophasor messages, processes them with a programmable logic engine, and sends controls to external devices to perform user defined actions.

-SEL 3378 data sheet

36

3378 SVP

• Interface with the 487E Relay via serial connection.

• Phasor input to calculate circulating current.

• Control output to relay to minimize circulating current.

• Display output with real-time circulating current values.

37

38

Conclusion

Proper transformer control results in • reduced losses • increased profits• maximized quality and reliability