IEEE/PES Transformers Committee Spring 2009 Meeting Miami, Florida “Transformer Paralleling” Technical Presentation Tuesday April 21: 4:45-6:00 p.m. Tom Jauch Application Consultant Beckwith Electric Co. Largo, FL Jim Graham Electrical Engineer Alliant Energy, Cedar Rapids, Iowa Jin Sim VP, Chief Technology Officer Waukesha Electric Systems.
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ZA, ZB = per Unit Impedance of Transformers A & BIA, IB = per Unit Load Current of Transformers A & B
IL = per Unit load current of Transformers A & B in parallel
Assuming the voltage drop thru both transformer is equal
Then: IA x ZA = IB x ZB and IL = IA + IB
And
IA = ZB/(ZA+ZB) & IB = ZA/(ZA+ZB)
PARALLEL OPERATION CASE 1Different Cooling Classes
Bank A 10/12.5 MVA; Z @ 10 MVA base = 0.08/unitBank B 12/16/20 MVA; Z @ 12 MVA base = 0.08/unit
On the same base (12.5 MVA)
Bank A 10/12.5 MVA; Z @ 12.5 MVA base = 0.10/unitBank B 12/16/20 MVA; Z @ 12.5 MVA base = 0.083/unit
Transformers share load inversely to the ratio of the bank to the sum of the impedances of the banks in parallel.
Bank A Loading = ZB/(ZA+ZB) = 0.83/(0.10+0.083) = 0.454/unitBank B Loading = ZA/(ZA+ZB) = 0.10/(0.10+0.083) = 0.546/unit
Note: Since the max rating of Bank A is 12.5 MVA and it carries 0.454/unit of substation capacity:
The max total load of bank A and B paralleled w/o overloading bank A is 12.5/0.454 = 27.5 MVA.
Therefore, the max loading of Bank B w/o overloading Bank A is 27.5 – 12.5 = 15.0 MVA (less than 20 MVA rating).
PARALLEL OPERATION CASE 2 Different Cooling Classes
(Modified for optimum load sharing)
Assuming that Bank A exists and the need is to purchase and install a new transformer rated 12/16/20 MVA to operate in parallel with Bank A while
providing a substation capacity of 32.5 MVA.
The specified impedance of the new transformer for Bank B is determined as follows to utilize the full nameplate capacity of both transformers when they
are paralleled.
* Bank A: 10/12.5 MVA; Z @ 20 MVA base = 0.16/unit
* Anticipated total load = 32.5 MVA
* Bank A’s rated per unit load capacity of 12.5 MVA is: 12.5/32.5 = 0.385/unit of the total bank loading of 32.5 MVA.
•Bank B’s rated load capacity of 20 MVA is 20/32.5 = 0.615 per unit of the paralleled bank rating of 32.5 MVA; therefore Bank B’s impedance needs to
be calculated to carry 0.615 per unit of the bank capacity.
Bank B loading 0.615 per unit = 16/(16+X) and solving for X.
X=0.10 per unit on 20 MVA base. Converting to a 12 MVA base, theimpedance needs to be 0.06 per unit on the self cooled nameplate 12 MVA
rating.
PARALLEL OPERATION CASE 3Same Cooling Classes, Different RatingsIf the transformers are both rated with two identical stages of cooling and both have identical impedances on there self cooled bases, each
will share load according to its rating:
Bank A 12/16/20 MVA; Z @ 12 MVA base = 0.8/unitBank B 24/32/40 MVA; Z @ 24 MVA base = 0.8/unit
On the same base (40 MVA)
Bank A = 0.267/unitBank B = 0.133/unit
Transformers share load inversely to the ratio of the bank to the sum of the impedances of the banks in parallel.
Bank A Loading = ZB/(ZA+ZB) = 0.133/(0.133+0.267) = 0.33/unitBank B Loading = ZA/(ZA+ZB) = 0.267/(0.133+0.267) = 0.67/unit
This validates that transformers of equal per unit impedances (expressed on their own base will load proportionally to their ratings)
CFVV - Constant Flux Voltage Variation Load Tap Changer Operation
CFVV - LTC operation regulates the transformer secondary
by increasing or decreasing the turns in the secondary
winding while the primary winding turns are constant.
Impedance is “Constant”Step Voltage is “Constant”
Load Tap Changer is installed in the LV winding to vary
the output by varying the turns in the LV winding.
VFVV - Variable Flux Voltage Variation Load Tap Changer Operation
VFVV - LTC operation regulates the transformer secondary by
increasing or decreasing the turns in the primary winding
while the secondary winding turns are constant.
> Impedance is Variable> Step Voltage is Variable
Load Tap Changer is installed in the HV winding resulting in a variable flux regulation.
> Increase output voltage by reducing HV turns Step> Decrease output voltage by increasing HV Turns
Paralleling Issues
Constant Flux Voltage Variationvs
Variable Flux Voltage Variation
Tom JauchTransformer Paralleling Application
BASICSLTC Control basicsParalleling basics
Control paralleling techniques
VARIABLESSystem variables – configuration
Transformer Differences
CHOOSING BEST METHODParalleling Control methodsApplications & Limitations
SETTING & COMMISSIONINGCommon Errors
NEW TERMParalleling self-correction
CONCLUSIONS
Control Basics
Control Basics
Two or more transformers connected in such a manner that they share in the supply of a common load bus."
Note: Any system operation that removes the supply source from a paralleled transformer(s) or separates a transformer load winding from a common load bus ends the parallel operation of the transformer(s).
The paralleling guide describes and compares controlmethods of paralleling power transformers equipped with load tap changers (LTC) or series regulators.
"Paralleled Transformers: LINES
• These functions must operate correctly and automatically regardless of system configuration changes or breaker operations.
ThreeMajorPremises:
The transformers must continue their basic function of controlling the regulated bus voltage as prescribed by the basic settings on the control (band center, bandwidth and line drop compensation).
The tap changers must operate to maintain tap position so as to minimize the current that circulates between them. Depending upon the designs of the transformers, the appropriate tap positions on the paralleled transformers are not necessarily on the same tap to achieve this.
Setpoints SP
Need for paralleling equipment
Example 1:
TIMING ERROR: Onetapchanger faster than other (tolerances)– causes one transformer to do all voltage regulation
V1,2
SetpointsSP
Need for paralleling equipment
Example 2:
VOLTAGE ERROR: Onetapchanger voltage magnitude higher than other – causes one transformer to do all raising and other do all lowering –(tolerances)
RESULT: tap position runaway
V1V2
Effects of “off-tap”positions
Inserts voltage source --------------
Develops Circulating current =
Reactive Power or Vars(∆V in reactive circuit)
----------------
Results in unbalanced
transformer loading-----------------
Circulating current calculation(1/2 difference current)
There are three basic control Techniques for controlling paralleled transformers.
a)Direct operation technique (from one control)(Master / Follower)
b) Blocking technique which blocks controls from operating in an inappropriate direction
(Power Factor)
c) Biasing technique for adjusting control set points (Negative Reactance, Circulating Current,
Circulating reactive current or vars)
Two important Factors in Paralleling
1)System & Bus Configuration Variables
* Normal Conditions* Emergency Operation
* Contingency Conditions
2) Transformer Variables
CONFIGURATION (System Variables)
Who is in parallel with who? +
LOADS
LINES
CONFIGURATION (System Variables)
Who is in parallel with who? +
LOADS
LINES
CONFIGURATION (System Variables)
Who is in parallel with who? +
LOADS
LINES
CONFIGURATION (System Variables)
Who is in parallel with who? +
LOADS
LINES
CONFIGURATION (System Variables)
Who is in parallel with who? +
TRANSFORMER VARIABLES
Transformer Impedance - ∆Z%(Matched: Equal Z% @ max rating)
Transformer Rating – ∆MVA
Tap Size – ∆ tap size
Number of taps – ∆ #taps
Number of windings - # windings
Winding configuration – Dynamic ∆Z%
CT Ratios – ∆ ct ratios
Voltage Ratings – ∆V RatingVoltage Ratio – ∆V ratio
= ∆ Primary Voltage
As Taps change – impedances change(dynamic changes in difference current)
Operation (hunting) if used withParalleling control !
ONE MORE CONCEPTParalleling Self Correction
Example
The paralleling guide describes and compares controlmethods of paralleling power transformers equipped with load tap changers (LTC) or series regulators.
Suggestion:
Two questions must be asked to determine if this Guide (paralleling control) is applicable.
#1 Is there any system condition that will cause this transformer to be paralleled (in parallel) with another? (IF YES)
#2 Will all system conditions cause paralleling self correction? (IF NO)
Then
Transformer will require parallel control equipment
"Paralleled Transformers:
Two or more transformers connected in such a manner that they share in the supply to a
common load bus."
BASICSLTC Control basicsParalleling basics
Control paralleling techniques
VARIABLESSystem variables – configuration
Transformer Differences
CHOOSING BEST METHODParalleling Control methodsApplications & Limitations
SETTING & COMMISSIONINGCommon Errors
NEW TERMParalleling self-correction
CONCLUSIONS
Conclusions
• Several methods of paralleling to choose
from depending on needs of the application
• In choosing consider:
– all possible configurations*
– all future possibilities
IEEE
Guide for
Transformer ParallelingPC57.153
The paralleling guide describes and compares control methods of paralleling power transformers equipped with load tap changers (LTC) or series regulators.
(PC57.153)
I. Definition & Purpose of a Paralleling Guide:
II. General Overview of Paralleling Requirements:
III. Basic Tapchanger Control:
IV. Basic Paralleling Method Descriptions/Applications:
V. Special Transformer Application Considerations: