oltc final final

8/8/2019 Oltc Final Final

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On Load Tap Changing

Transformer Paralleling

Simulation and Control


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2

OLTC Overview

Transformer Paralleling

The need for control

Current Solutions

Our Plan and System


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Parallel Transformers

Increase Reliability

Improve Power quality

Prevent voltage sag

Meet increased loadrequirements


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Examples

Illustrate the need for control

Present Two Calculation Methods

Superposition Method

Admittance Method


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Grainger Examples

One-Line Diagram Grainger, Example 2.13, pg 78


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Grainger Examples

Per-Phase Reactance Diagram, Grainger pg 78


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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

Zload0.8 0.6j( ) pu=:=


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

By Superposition:

Icirc

VZTa ZTb+ 0.25j pu=:= Circulating Current

ITa

ILoad

2Icirc 0.4 0.05j( ) pu=:=

ITbILoad

2Icirc+ 0.4 0.55j( ) pu=:=


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Equivalent Circuit


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STa V2 ITa 0.4 0.05j+=:=Vars are unbalanced

KWs are balancedSTb V2 ITb 0.4 0.55j+( ) pu=:=

SLoad V2 ILoad 0.8 0.6j+( ) pu=:=

SLoad 1 pu=

kVA in the circuit that

serves no purposeat the load

STa STb+ SLoad 0.083pu=


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Admittance Methodt 1.05e

j 0 deg:=

YTaY

Y

Y

Y

10j

10j

10j

10j

pu=:=

YTbt( )

2Y

t Y

t

Y

Y

11.025j

10.5j

10.5j

10j pu=:=

Y YTa YTb+21.025j

20.5j

20.5j

20j

=:=

Grainger, Example 9.7


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Admittance Method

I1

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=:=


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Problem Definition

We want to minimize the circulatingcurrent.

Why?

Increased total losses of the twotransformers

Unable to fully load one transformer

without over-loading or under-loading theother

This current is parasitic, serving no benefit

The transformer is not operating at

optimum


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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 SELtechnology

Refine the System to minimize circulatingcurrent over a variety of conditions


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Popular Solution Methods.

Master- Follower Method

Power Factor Method

Circulating Current Method

Var Balancing (Var) Method TMSource:Advanced Transformer Paralleling Jauch, E. Tom: Manager of

Application Engineering, Beckwith Electric Co., Inc.


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Master-Follower

Desired operation maintains same taplevel on all transformers

Consists of one control commandingtransformer tap changes to follow


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Master-Follower

Positives: Appropriate voltage level via load is

maintained

Negatives: Does nothing to prevent circulating

current


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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 wrongdirection.


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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 blockedto minimize PF difference.

If transformers have differentimpedances, Highest KW loadedtransformer is forced to have highestVAr load.


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Circulating Current Method

Assumes continuous circulating currentpath

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 levelvariation becomes to great.


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Var Balancing (Var) Method

Loads transformers by balanced VArsharing.

Ignores KW loading


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Var Balancing (Var) Method

Positives: Balanced VArs make Icirc a min or 0

No auxiliary CTs are needed

Negatives: Method is patented by Beckwith

Electric Co. INC.


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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 SEL487E relay


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Our Plan

Goals Appropriate voltage level maintained

Icirc driven to a minimum

Max variation of tap levels met

Avoids tap level oscillation


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System

Transformers

487E Relay

3378 Synchrophasor Vector Processor


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Transformers

Two Autotransformers will be used tosimulate two parallel power transformers

Voltage controlled motors on the tapchangers

Transformer secondary will feed an

external load from unity to 0.5 lead/lag


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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 dominantlyWatts, rather than Vars

Verified through no-load Paralleling test


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T1 X and R Vs Secondary Nominal Voltage


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Transformers

The autotransformers do not exhibitcharacteristics similar to a typical powertransformer

Options

Use these transformers

Different Transformers, 5 kVA Motor drivenautotransformers


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Calculations

The Superposition method will support thereal component while the Admittance

method will not The real component will create a negative

resistance in the PI equivalent


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487E Relay

Control transformer tap level

Receives commands from SVP

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

power.


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3378 SVP

The SVP time aligns synchrophasormessages, processes them with a

programmable logic engine, and sendscontrols to external devices to performuser defined actions.

-SEL 3378 data sheet


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3378 SVP

Interface with the 487ERelay via serialconnection.

Phasor input to calculate

circulating current. Control output to relay to

minimize circulatingcurrent.

Display output with real-time circulating currentvalues.


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Conclusion

Proper transformer control results in

reduced losses

increased profits

maximized quality and reliability

oltc final final

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