Voltage Regulation
&
Parallel Transformer
Operations
Presented By Jesse Young
Objectives
• Review Basic Voltage Regulation principle
•
•
Why do we need voltage regulation
How do we regulate voltage
• Review parallel transformers schemes that have been used at Ameren Missouri
• Given a Standard Circulating Current Paralleling Schematic:
– Explain how current forcing circuit work and what they are used for
– Understand CT polarity and how load current flows in the CT Circuits
•
•
•
•
Circulating Current path
Load current Halving path
LDC (Line Drop Compensation) Path and how it is affect by the Forcing Circuits
Capacitor Bank CT’s and how this affect the LDC current
Voltage Regulation
• What is voltage regulation ?
– the ability of a power system to provide near constant
Voltage over a wide range of load conditions
• Why is it important ?
– Improper voltage can have adverse affect on
customers electrical equipment
• Operating inefficiencies
• Shorten life expectance of electrical equipment
Voltage Regulation
What are the requirements for a well regulated voltage ?
In Missouri the Missouri Public Service Commission (PSC)
requires that we provide our customers voltage within the
following ranges:
• Favorable Range =
+5% / -5%
(normal system serving typical loads (365 days a year))
• Tolerable Range
=
+6% / -8%
(normal system serving maximum or minimum loads (just a few
days a year or contingency lasting more that 24 hours))
• Extreme Range
=
+9% / -10%
((n-1) contingency system serving maximum or minimum loads
(loss of a transformer on the hottest day of the year))
Typical Power System
Gen
GSU
Transmission
Bulk / Sub-T
Distribution
Customer
Power System Losses
Why do we have so transformers?
Economics
• Higher Transmission Voltages reduce
transmission losses and cost
– I2R Losses (real power) decrease
• (less power we have to produce/purchase)
– Lower rated equipment (smaller conductors size/
lower interrupting duty)
• (lower upfront capital investment)
Generator Losses
– At no load
– Under load
VG = Eg
VG = Eg - IL (Rg + jXg)
IL
Rg
+
VF
-
IF
VG
Xg
Eg
Transformer Losses
– At no load
– Under load
Vs = Es
& Vs = Vp/ N
Vs = Es - IL (R + jX) &
Vs = (Vp/N) - IL (R + jX)
X
R
VP
EP
IL
ES
VS
Power Line/Feeders Losses
R
L
R
L
R
L
R
L
R
L
R
L
LOAD
RT
L
Z
T
T OT AL
LOAD
Power Line/Feeders Losses
• SHORT FEEDER 50 MI OR LESS
XT
RT
IL
VS
VR
VS
VR
IL
IL R
IL X
RL
Power Line/Feeders Losses
• SHORT FEEDER 50 MI OR LESS
VS
VR
IL
IL R
IL X
VS
VR
IL
IL R
IL X
Power Line/Feeders Losses
• T-Lines, Long Feeders & Capacitor banks
RT
XT
IL
IT
VS
XC
XC
IC
IT
IL
VR
RL
Power Line/Feeders Losses
• Transmission Lines & Feeders
IC
VS
IL
IT
VL
ITR
ITX
IT
IC
VS
IL
ITX
VL
ITR
Summary of Power System Losses
• Every component of the Power System is
subject to an internal voltage drop. What our
customers receive is the vectorial sum of all
of the losses that occur from Generator to
their facility.
Methods of Voltage Correction
Reduction of Series Resistance
Reduction of Series Reactance
Power Factor Correction
Reduction of the Load Current
In phase voltage control (regulation)
Methods of Voltage Correction
• Reduction of the series resistance
VS
VR
IL
IL R
IL X
VS
IL
VR
IL X
IL R
Methods of Voltage Correction
• Reduction of Series Reactance
VS
VR
IL
IL X
IL R
VS
VR
ILR
IL
ILX
Methods of Voltage Correction
• Power Factor Correction
VS
VR
IL
IL X
IL R
VS
IC
1
2
IT
IL
ITX
VR
IT R
Methods of Voltage Correction
• Reduction of Load Current
VS
VR
IL
IL R
IL X
Methods of Voltage Correction
• In Phase Voltage Control
IT
R
Load
VC
X
Vs
VR
VL
Methods of Voltage Correction
• In Phase Voltage Control to improve line
voltage
VS
VC
ITR
VR = VS + VC
ITX
Voltage Regulators
VP = 100
V S = 10
BASIC
TRANSFORMER
VP = 100
VP = 100
V S = 110
V S = 90
STEP DOWN
AUTOTRANSFORMER
STEP UP
AUTOTRANSFORMER
Voltage Regulators
8
7
6
5
4
3
2
1
0
8
7
6
5
4
3
2
1
0
Voltage Regulators
1
1
0
0
1
0
Voltage Regulator
• Reversing Switch
8
7
6
5
4
3
2
1
0
Load tap changer, reactance type
Changing taps from 11R to 10R
Load tap changer, reactance type
Changing taps from 11R to 10R, Step 1
Load tap changer, reactance type
Changing taps from 11R to 10R, Step 3
Load tap changer, reactance type
Changing taps from 11R to 10R, Step 4
Load tap changer, reactance type
Changing taps from 11R to 10R, Step 4
Load tap changer, reactance type
Changing taps from 11R to 10R, Step 5
Load tap changer, reactance type
Circulating current
Non- bridging
position
Ic = 0
IL/
2
Ic
IL/
2
Bridging position
Ic ≠ 0
Ic
IL/
2
I
I
L
IL/
2
L
Load tap changers
Basic anatomy
LTC compartment
Tap selector
Voltage Regulating Relays
Crude voltage regulating relay
Voltage Regulating Relays
• Crude voltage regulating relay
L
R
Tap Changer Control Circuit
43
M
120
A
84L
84R
LL
UL
P
CS
L
R
L
R
84R
84R
84M
84R
84L
84L
LL
84R
UL
Line Drop Compensation
VS
VR
IL
IL R
IL X
Line Drop Compensation
7200
5%
Without
LDC
7050
DISTANCE
7275
With
LDC
2.5%
7200
2.5%
7125
DISTANCE
Line Drop Compensator Circuit
CT
R
X
I
v
i
CMV
r
PT
VS
vs
r
V
R
x
INDUCTION RE GULA TOR
VOLTA GE CONTROL CIRCUIT
LINE
LOA D CE NTE R
LOAD
Load Tap Changer
Backup Controls
• 3 FUNCTIONS
– PREVENT A DEFECTIVE TAP CHANGER CONTROL
FROM RUNNING OUTSIDE OF PRESET UPPER AND
LOWER VOLTAGE LIMITS
– PREVENT THE LINE DROP COMPENSATOR FROM
RAISING THE VOLTAGE EXCESSIVELY HIGH UNDER
FULL LOAD OR OVERLOAD CONDITIONS
– LOWER VOLTAGE IF THE REGULATED VOLTAGE
EXCEEDS A PRESET BLOCK RAISE SETPOINT BY A FIXED
AMOUNT
LOAD TAP CHANGER BACKUP
CONTROL
LOWER
VOLTAGE
FIXED DEADBAND
VBR
BLOCK RAISE
VL
LOWER
VC
VRR BANDWIDTH
BANDCENTER
BLOCKING BANDWIDTH
VR
RAISE
V BL
BLOCK LOWER
LOAD TAP CHANGER BACKUP
CONTROL
MOTOR
SUPPLY
LOWER
RAISE
BLOCK
RAISE
DEV. 90
LOWER
BLOCK
LOWER
DEV. 90B
84R
COM.
84L
ALARM
Introduction to Transformer
Paralleling
• Why do we operate transformers in parallel?
– Increase the load carrying capability of the substation
– Improve efficiencies
– Improve reliability
• What is circulating current and what causes it?
– Circulating current flows between XFMRs when their secondary voltage are not matched
– Typically this occurs when transformers are on different voltage taps
– Difference in Load Tap Changer speeds can cause this
• Consequences of circulating current
– LTC will run apart
– Increased heating of the XFMR windings ( Loss of XFMR life)
Paralleling Problems
WAT - 74
WAT - 78
WAT - 76
TRANSF. W
IW
TRANSF. D
ID + I C
TRANSF. V
IV - IC
IC
Phasors for LDC Circuits
• Balanced Load
Equal Current
flowing in both
transformers
• Different reaction
time can causes
Circulating Current
VD
VV
ID
IV
VD
VV
IV'
IC
ID'
ID
IC
IV
Line Drop Compensator Circuit
CT
R
X
I
v
i
CMV
r
PT
VS
vs
r
V
R
x
INDUCTION RE GULA TOR
VOLTA GE CONTROL CIRCUIT
LINE
LOA D CE NTE R
LOAD
4 METHODS TO SOLVE PARALLELING
PROBLEM
• Electrical Interlocking (Master/Slave
Leader/Follower)
• Cross Current Compensation
• Current Balancing
• Delta-Var
Basic Parallel Voltage Regulation premise:
• The Voltage Regulator must continue its basic function of
controlling bus voltage as described by LDC Settings (Band Center,
Band Width, R, and X)
• Voltage Regulator must act to minimize circulating currents
between Paralleled Transformers
Electrical Interlocking Schemes
The Leader/Follower or Master /Slave scheme uses one controller to
control all of the Paralleled Transformers
• Advantages
– Principle of operation is simple
– Comparatively inexpensive
– Ensures that tap changers are never more than one tap apart
• Disadvantages
– Requires a relatively complex interlocking scheme which can be difficult
to troubleshoot
– Once tap changers are one tap apart no Voltage regulation will occur until
they are but back in step
• Required Components
– Cam operated tap position switches
– In-step relays (raise and lower)
– Selector switch
Out-of-Step Circuit
• Heart of leader-Follower Scheme
• Ensures that all paralleled transformers remain
in step with each other.
– Odd and Even Cam Op. SW's complete permissive
circuit for raise and lower operations
– 43T SW bypasses Odd/Even SW's (Allows
continued paralleled operation of Tap Changers)
– 43S Selector Switch (Independent, off, XFMR #)
– 86 (In-Step relay)
Out-of-Step Circuit
To other
Xfmr(s)
43A
86X
86
X
Warson Paralleling Scheme
Warson Paralleling Scheme
A
184-1
32
184-1
16
184-1
OS2
184-1
OS4
184-1
OS1
184-1
OS3
133-1
I
XFMR-1
184-1
32
133-1
I
184-1
32
184-1
16
184-2
OS1
133-2
I
124K-2-3
Out of Step Circuit
for XFMRs 1 & 2
N
EVEN T APS
ODD T A P S
COMMON
184-2
OS2
124K-2-3
133-2
I
XFMR-2
Warson Paralleling Scheme
A
133-1
I
184-1
OS4
184-1
OS2
184-2
OS1
124K-2-3
N
184-1
OS3
133-2
I
184-1
OS1
184-2
OS2
124K-2-3
133-1
I
133-2
I
Warson Paralleling Scheme
N
110-1-2
1
110-1-2
1
143-1
A
133-1
P
TRANSF. #1
TRANSF. #2
BOTH
127T#3-1
190-1
L
184Y-1
L
190-1
R
184-1
LL
184-1
RL
184Y-1
L
133-1
P
133-2
P
184Y-1
R
133-2
P
184Y-1
R
133-1
P
184Y-2
L
184-2
LL
184Y-2
R
184-2
RL
133-2
P
133-1
I
184-1
OS4
184-1
OS2
184-2
OS2
124K-2-3
N
184-1
OS3
133-2
I
184-1
OS1
184-2
OS1
124K-2-3
133-1
I
133-2
I
Warson Paralleling Scheme
A
1 3 3 -1
I
1 8 4 -1
OS 4
1 8 4 -1
OS 2
1 8 4 -2
OS 1
1 2 4 K -2 -3
1 8 4 -1
OS 3
1 3 3 -2
I
1 2 4 K -2 -3
1 8 4 -1
OS 1
1 8 4 -2
OS 2
1 3 3 -1
I
1 3 3 -2
I
1 2 4 K -2 -3
1 2 4 K -2 -3
N
1 3 3 -3
I
1 8 4 -3
OS 4
1 8 4 -2
OS 1
1 8 4 -3
OS 2
1 3 3 -4
I
N
E V E N TA P S
ODD TA P S
COMMON
1 8 4 -3
OS 3
1 3 3 -3
I
1 8 4 -3
OS 1
1 8 4 -2
OS 2
1 3 3 -4
I
Modern Day Leader-Follower
With the advent of modern Computers and Communication
scheme we are now seeing new Leader Follower schemes being
used that use LTC Tap position and Breaker Status information to
determine if units are in parallel and on same tap
This scheme relies on Breaker position & tap positions being
communicated to the controllers for proper operation. The scheme
should alarm and block tap changes if the units get out of step.
Current Balancing Method
• Used in both Transmission and Distribution
Substation
• Advantages
– Allows Transformers to be operated in parallel with
minimal circulating currents
– Allows one transformer to be taken out of service
without over-compensating for load
• Disadvantages
– Requires additional wiring and Forcing CT's
– Difficult to troubleshoot
– High side of transformers must be tied together
Current Transformer Fundamentals
5: 5
IP
2
IS
2
Np I p =Ns I s
Ip = Is
2
Current Balancing Method
TRANSF. 2
TRANSF. 1
5 2 -1
a
P1
P2
5 2 -1 -2
a
K1
VRR
5 2 -2
a
5 2 -1
b
5 2 -1 -2
b
L1
VRR
5 2 -2
b
5 2 -1 -2
a
P1
K2
L2
P2
5 2 -1 -2
b
COMP.
COMP.
COMP.
COMP.
52-1
52-2
52-1-2
Current Balancing
• Split Bus
Operation
T RANSF . # 1 L OAD CURRENT
T RANSF . # 2 L OAD CURRENT
BAL ANCED CURRENT (L DC CURRENT )
UNBAL ANCED CURRENT
TR AN SF. 2
TR AN SF. 1
5 2 -1
a
P1
P2
5 2 -2
a
4
4
5 2 -1 -2
a
K1
4
K2
4
4
VRR
5 2 -1
b
5 2 -1 -2
b
L1
5 2 -1 -2
a
P1
4
5 2 -2
b
VRR
L2
P2
5 2 -1 -2
b
4
4
COM P.
4
COM P.
COM P.
4
4
4
52-1
52-2
52-1-2
COM P.
Current Balancing
• Split Bus
Operation
T RANSF . # 1 L OAD CURRENT
T RANSF . # 2 L OAD CURRENT
BAL ANCED CURRENT (L DC CURRENT )
UNBAL ANCED CURRENT
TR AN SF. 2
TR AN SF. 1
5 2 -1
a
P1
P2
5 2 -2
a
4
4
5 2 -1 -2
a
K1
4
K2
4
4
VRR
5 2 -1
b
5 2 -1 -2
b
L1
5 2 -1 -2
a
P1
4
5 2 -2
b
VRR
L2
P2
5 2 -1 -2
b
4
4
COM P.
4
COM P.
COM P.
4
4
4
52-1
52-2
52-1-2
COM P.
Current Balancing
• Parallel Operation (Balanced Load)
T RANSF . # 1 L OAD CURRENT
T RANSF . # 2 L OAD CURRENT
BAL ANCED CURRENT (L DC CURRENT )
UNBAL ANCED CURRENT
TR AN SF. 2
TR AN SF. 1
5 2 -1
a
P1
5 2 -2
a
P2
4
4
K1
5 2 -1 -2
a
4
4
4
K2
4
4
VRR
5 2 -1
b
L1
5 2 -1 -2
b
4
5 2 -1 -2
a
P1
4
5 2 -2
b
4
VRR
L2
P2
5 2 -1 -2
b
4
4
COM P.
4
COM P.
COM P.
4
4
4
5 2- 1
5 2- 2
52-1-2
COM P.
Current Balancing
• Parallel Op (Unbalanced Load)
T RANSF . # 1 L OAD CURRENT
T RANSF . # 2 L OAD CURRENT
BAL ANCED CURRENT (L DC CURRENT )
UNBAL ANCED CURRENT
TR AN SF. 2
TR AN SF. 1
6
2
5 2 -1
a
P1
5 2 -2
a
P2
2
2
6
44
6
VRR
5 2 -1 -2
a
4
K1
4
5 2 -1
b
5 2 -1 -2
b
4
L1
4
44
VRR
L2
P2
5 2 -1 -2
b
6
2
COM P.
4
6
2
2
K2
5 2 -2
b
5 2 -1 -2
a
P1
4
4
2
COM P.
COM P.
4
2
2
6
2
5 2- 1
2
5 2- 2
52-1-2
COM P.
Parallel Operation
• Circulating Current
T RANSF . # 1 L OAD CURRENT
T RANSF . # 2 L OAD CURRENT
BAL ANCED CURRENT (L DC CURRENT)
UNBAL ANCED CURRENT
TR AN SF. 2
TR AN SF. 1
5
5 2 -1
a
P1
5 2 -2
a
P2
5
8
5 2 -1 -2
a
3
K1
3
2
2
K2
8
3
VRR
5 2 -1
b
L1
5 2 -1 -2
b
3
5 2 -1 -2
a
P1
3
VRR
3
5 2 -2
b
L2
P2
5 2 -1 -2
b
8
2
COM P.
COM P.
3
8
COM P.
COM P.
5
52-1
3
2
52-2
52-1-2
One Line Operation
• Loss of Transformer
T RANSF . # 1 L OAD CURRENT
T RANSF . # 2 L OAD CURRENT
BAL ANCED CURRENT (L DC CURRENT)
UNBAL ANCED CURRENT
TR AN SF. 2
TR AN SF. 1
5 2 -1
a
P1
5 2 -2
a
P2
6
5 2 -1 -2
a
K1
4
K2
6
6
VRR
5 2 -1
b
5 2 -1 -2
b
VRR
5 2 -2
b
3
L1
3
5 2 -1 -2
a
P1
3
3
L2
P2
3
5 2 -1 -2
b
6
COM P.
COM P.
3
6
COM P.
COM P.
3
3
52-1
52-2
52-1-2
Bulk SS Paralleling
Power Factor Paralleling
• Power factor paralleling works by monitoring
the load power factor difference between
paralleled units and incrementing or
decrementing the LTCs as necessary to
minimize the difference.
Cross Current Compensation
TRANSF. D
VRR
ID
TRANSF. V
VRR
IV
Cross Current Compensation
• LDC CT of one XFMR are connected to the LDC
of the other XFMR
• Advantage
– No additional equipment needed
• Disadvantage
– The transformer must be close enough to make
the necessary low impedance connections