Transcript II. Problem Definition

On Load Tap Changing
Transformer Paralleling
Simulation and Control
OLTC Overview
• Transformer Paralleling
• The need for control
• Current Solutions
• Our Plan and System
2
Parallel Transformers
•
•
•
•
Increase Reliability
Improve Power quality
Prevent voltage sag
Meet increased load
requirements
3
Examples
• Illustrate the need for control
• Present Two Calculation Methods
– Superposition Method
– Admittance Method
4
Grainger Examples
One-Line Diagram Grainger, Example 2.13, pg 78
5
Grainger Examples
Per-Phase Reactance Diagram, Grainger pg 78
6
Superposition Method
j   1
t
pu  1
n
n'
Zload   ( 0 .8  j  0 .6)p u
V2   1 .0  e
j0deg
pu
ZTa   j  0 .1p u
ILoad  
V2
Zload
ZTb   j  0 .1p u
 ( 0 .8  0 .6j)  p u
7
Superposition Method
V   t  1  0 .05
arg( V )  0  d eg
Tap St ep Volt age
By Superpos ition:
Icirc  
ITa  
ITb  
V
ZTa  ZTb
ILoad
2
ILoad
2
 0 .25 j p u
C irc ulating C urrent
 Icirc  ( 0 .4  0 .05)j  p u
 Icirc  ( 0 .4  0 .55)j  p u
8
Superposition Method
Equivalent Circuit
9
Superposition Method

STa   V2  ITa  0 .4  0 .05 j
STb

  V2  ITb  ( 0 .4  0 .55)j  p u
Vars are unbalanc ed
KW s are balanc ed

SLoad   V2  ILoad  ( 0 .8  0 .6j) p u
SLoad  1 p u
STa  STb  SLoad  0 .08 3p u
k VA in t he c irc uit that
s erv es no purpose
at the load
10
Admittance
Method
j0deg
t   1 .05e
 Y Y   1 0j 1 0j 
YTa   

  pu
 Y Y   1 0j 1 0j
 t 2  Y t  Y  1 1.0 25 j1 0.5 j
YTb   

  pu
Y   1 0.5 j 1 0j 
 t  Y
2 1.0 25 j 2 0.5 j

Y   YTa  YTb  

 2 0.5 j 2 0j
Grainger, Example 9.7
11
Admittance Method
 I1 
 V1 
  Y  
 I2 
 V2 
 V1 
    Fin d V1  I 1
 I1 
 I1 a 
 V1 
    YTa   
 I2 a 
 V2 
I2 a  ( 0 .39  0 .04 9j
)  pu
 I1 b 
 V1 
    YTb   
 I2 b 
 V2 
I2 b  ( 0 .41  0 .55 1j
)  pu

STa   V2  I2 a  ( 0 .39  0 .04 9j
)  pu

STb   V2  I2 b  ( 0 .41  0 .55 1j
)  pu
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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
13
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
14
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.
15
Master-Follower
• Desired operation maintains same tap
level on all transformers
• Consists of one control commanding
transformer tap changes to follow
16
Master-Follower
• Positives:
– Appropriate voltage level via load is maintained
• Negatives:
– Does nothing to prevent circulating current
17
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.
18
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.
19
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.
20
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.
21
Var Balancing (∆Var) Method
• Loads transformers by balanced VAr
sharing.
• Ignores KW loading
22
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.
23
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
24
Our Plan
• Goals
– Appropriate voltage level maintained
– Icirc driven to a minimum
– Max variation of tap levels met
– Avoids tap level oscillation
25
System
• Transformers
• 487E Relay
• 3378 Synchrophasor Vector Processor
26
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
27
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
28
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
29
T1 X and R Vs Secondary Nominal Voltage
T1 Leakage Reactance Vs Secondary Voltage
5
4.5
4
3.5
Ohms
3
2.5
X
R
2
1.5
1
0.5
0
0
20
40
60
80
100
Secondary Nominal Voltage
120
140
160
30
Transformers
• The autotransformers do not exhibit
characteristics similar to a typical power
transformer
• Options
– Use these transformers
– Different Transformers, 5 kVA Motor driven
autotransformers
31
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
32
487E Relay
•
•
•
•
Uses Lateral Logic
18 Current Channels
6 Voltage Channels
Synchrophasor data
collected once per
cycle, up to 12
Channels
33
487E Relay
• Control transformer tap level
• Receives commands from SVP
• Displays: voltages, currents, Icirc,
apparent power, real power, reactive
power.
34
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
35
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 realtime circulating current
values.
36
37
Conclusion
Proper transformer control results in
• reduced losses
• increased profits
• maximized quality and reliability
38