S09 TR Paralleling Tutorial

IEEE/PES Transformers CommitteeSpring 2009 Meeting

Miami, Florida

“Transformer Paralleling”Technical Presentation

Tuesday April 21: 4:45-6:00 p.m.

Tom JauchApplication ConsultantBeckwith Electric Co.

Largo, FL

Jim GrahamElectrical Engineer

Alliant Energy, Cedar Rapids, Iowa

Jin SimVP, Chief Technology Officer Waukesha Electric Systems.

Jim Graham* Reasons for paralleling transformers

* Examples of users concerns & needs

Why Parallel Transformers?

Additional Capacity

Standardization of Xfmr Ratings

Security/Local Back Up

Improved Access for Maintenance

Voltage / Ratio Mismatch

Impedance Mismatch

Unbalanced Loads

Volt/Current Sensors Connected in phase

Incompatible Controllers

Tap Changer Locations

Increased Fault Currents

Do I need Paralleling Controls?

User Issues

Unequal Load Sharing Excessive Circulating Currents

Transformer OverloadingTransformer Loss of Life Due To Overheating

Excessive LTC OperationsHigh Maintenance Costs Lower Transformer ReliabilityVoltage Complaints

Consequences of Improper Paralleling

Jin Sim* Manufacturer Concerns

* Specification needs

Manufacturer’s Request• Existing units with Load Tap Changers

– Serial Numbers of existing units– Test report– LTC Wiring and Schematic Drawings– Method

• Circulating Current• Master Follower• Other• Manual

Impedance Design for Load Sharing When Paralleled

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

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

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

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

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? +

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)

Different Winding Arrangements

Traditional Paralleling Control Methods

Master / Follower – MF

Power Factor - PF

Negative Reactance - NR

Circulating Current - CC

Circulating Reactive Current or vars - CRC

Keeps transformers on “same” tap positions

Master/Follower Method(Direct operation technique)

Feedback

Requires:

Feedback of follower unit(s)

action to master

Usually by external relays or communication

channel(s)

Master/Follower Method(Direct operation technique)

Limitations:Not applicable for:

• Unmatched Z%

• Different Tap Sizes

• Different # of Taps

• Separated HS bus

• Dynamic ∆Z%

•∆ # windings (tertiary)

Application:Z% Matched Transformers

•Solid HS bus

• Different CT ratios OK

Power Factor MethodControl Blocking Technique

I1A, I2A

V1, V2

V1, V2 Limitations:Not applicable for:

• Unmatched Z%

•Separated HS bus

•Dynamic ∆Z%

• ∆ # windings (tertiary)

•Solid HS bus

• Different tap sizes OK

• Different # of taps OK

Biasing of control setpoints technique

(Negative reactance, Circulating & Circulating Reactive Current)

METHODSMaster/Follower

Power FactorNeg Reactance

Circ currentCirc reac current

SPSP V

Importance of SENSITIVITY

SPSP V

SP SPV

SPSP V

OVERSENSITIVE – HUNTING !!

SP SPV

Review of LDC Actions

Negative Reactance MethodSetpoint Biasing Technique

(Using LDC Settings)

Var flow outIncreases setpoint

Review of LDC Actions

NO COMMUNICATIONS REQUIRED

EMERGENCY OPERATION

Sensitivity set by –X LDC setting

LOAD ERROR EXAMPLE

Limited application by compensating

for load error with +R setting

• Unmatched Z%

• Separated HS bus

•Dynamic ∆Z%

• Significant load changes

Application:•Z% Matched Transformers

•Solid HS bus

• Different tap sizes OK

• Different # of taps OK

Circulating Current Method(s)Setpoint Biasing Technique

Removes Load Current errors

Typical Systems

NOTE: I circ = ½ Difference currentLDC current = Total current - I circ

Sensitivity setting by paralleling module

Setting the Sensitivity

1) Set both PBMs sensitivities on neutral

2) Find tap combination that minimizes circulating

current & produces voltage closest to setpoint

3) Raise one transformer one tap and lower on

transformer one tap

4) Adjust sensitivities (together) to the level

where both transformers return to original tap

positions

5) Reduce both sensitivity levels by one

Common Error(STOPPING HERE)

Circulating Current Commissioning

Circulating Current Method(s)METHODS

Master/FollowerPower Factor

Neg ReactanceCirc current

Circ reac current

Typical Backup

External CC Overcurrent relay

Circulating Current MethodSetpoint Biasing Technique

• Unmatched Z% (Comped)

• Different MVA rating

•Separated HS bus

•Dynamic ∆Z%

•Solid HS bus

• Different tap sizes

• Different # of taps

CT ratios need to be in the same relationship as

impedances to balance “difference” current

(Circ I = ½ Diff I)

Different transformer impedances - Example

100 MVA, 10% IZ 100 MVA, 8% IZ

80A 100A

CT ratios need to be in the same relationship as

impedances to balance “difference” current

Different transformer impedances - Example

100 MVA, 10% IZ 100 MVA, 8% IZ

ct = 100/1Act = 80/1A

80A 100A

CT ratios need to be in the same relationship as KVA

ratings to balance “difference” current

Different transformer KVA Ratings - Example

100 MVA, 10% IZ 50 MVA, 10% IZ

50/A100A

ct = 50/1Act = 100/1A

1) Definition of “circulating” current: Ic = ½ difference current2) Definition of “circulating reactive” current: Icr = ½ difference reactive current

EXAMPLE: With either “circulating” current method

100MVA

Ic actual = 2AIcalc ½ (1+2) = 1.5A

∆LDC (½ error) = 0.25A

Circulating Current Method(s)Errors of unequal ct ratios

Voltage ANGLE differenceVoltage ANGLE differenceCauses Circulating KW flow

from VT 2 to VT 1

Circulating Current MethodAPPLICATION PROBLEM

SOLUTION:Circulating Reactive Current

Setpoint Biasing Technique

* Same connections as circulating current

•Control reacts ONLY to circulating reactive current

• Equalizes transformer var flows

* CT ratios equivalent to rating sizes (not %Z)

Circulating Reactive CurrentSetpoint Biasing Technique

Limitations:Requires:

MVA matched ct ratios

(Equalizes var flows by rating)

Application:• OPEN/CLOSED HS bus

• Different HS voltages

• Different Z%

• Dynamic ∆Z%

• Different tap sizes

• Different # of taps

10%10%

+1% Tap

Example

Result: stops Runaway condition

Automatically !!(w/o paralleling control!)

ONE MORE CONCEPTParalleling Self Correction

10%10%

+1% Tap

IN FACT:Could cause OVERSENSITIVE

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)

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

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:

VI. Special System Application Considerations:

VII. Backup protection:

VIII. Typical problems:

IX. Field commissioning / troubleshooting:

X. Conclusions

IEEE/PES Transformers CommitteeSpring 2009 Meeting

Miami, Florida

“Transformer Paralleling”

Questions , Answers & Comments

Tom JauchApplication ConsultantBeckwith Electric Co.

Largo, FL

Jim GrahamElectrical Engineer

Alliant Energy, Cedar Rapids, Iowa

Jin SimVP, Chief Technology Officer Waukesha Electric Systems.

S09 TR Paralleling Tutorial

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