General Guidelines

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INTERFACE TRANSFORMER WINDING CONFIGURATION
Pankaj Sharma, P. Eng. Sustainment Manager (PC
Planning) May 13th, 2009
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Topics to be Covered

• Importance
• Types
• DG Contribution to Utility System fault
• Temporary Over-voltage
• Summary

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Importance of DGIT Winding Connection

• Impacts Power Quality and Reliability
• Impacts Distribution System grounding
• Impacts total ground fault levels and
  distribution of ground fault in-feeds
• Impacts Protection System design

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Types of Winding Connections

• Star ground / star ground
• Star ground / star ungrounded
• Delta / star ground
• Star ground / delta
• Delta / delta
• Note
• All connections are in System / generator order
• System side Neutral must be effectively or low
  impedance grounded

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STAR GROUND / star ground

• Advantages
• Standard utility transformer replacements
  available
• No voltage phase shift simple relaying and
  fault detection
• Ferro-resonance less probable
• Low TOV on distribution system (Dx) if Gen is
  solidly gnd

• Disadvantages
• DG can feed any type of Dx faults
• May pass zero-sequence currents
• High TOV if Gen neutral is grounded thru high
  impedance

Most common transformer (typically up to 500kVA)
connection in North America for load
distribution If Gen is solidly grounded ? IG
issues Generally Gen are grounded thru impedance
? TOV issue
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STARGROUND / starunground

• Advantages
• -- Inverters may require an ungrounded
  connection on LV side
• -- No passage for IG and also no IG source
• -- No voltage phase shift less vulnerable to
  Ferro resonance

• Disadvantages
• -- High TOV on Dx

Not a common transformer connection
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DELTA / star ground

• Advantages
• Low DG contribution to Dx ground faults (via
  ve and ve seq networks)
• Does not pass zero sequence I, harmonics from
  the DG are blocked
• Primary SLG faults do not have severe impact
  on the secondary voltages

• Disadvantages
• More difficult for generator protection to
  detect SLG faults on Dx
• Triplen harmonics from DG circulate in Delta
  and low-impedance secondary neutral
• Possible ferro-resonance and the need for 3Ø
  switches on the primary side of transformer
• High TOV on Dx

Second most common Xmer connection in North
America Most common transformer connection in
Europe High TOV on Dx can be mitigated by
connecting Grounding Xmer on HV side of the
DGIT For 3-Ø, 3-W Dx this connection is suitable
and preferred
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STAR GROUND / delta

• Advantages
• Primary Dx faults are easily detected by the
  generators interconnection protection system
• Blocks triplen harmonics from Gen
• Protection scheme is standardized based on
  utility-owned generator protection systems
• -- No TOV on Dx system if Xmer neutral is solid
  or grounded through appropriately sized
  reactor/resistor

• Disadvantages
• Source for ground fault current
• May cause sympathetic tripping of other
  feeders during ground faults
• Interferes with existing protection systems
  and makes fuse saving impossible or extremely
  difficult (if solidly grounded)
• Transformer may overheat due to zero-sequence
  currents

• Usual connection for generator transformers
• IG contribution has to be optimized by inserting
  impedance in the neutral connection.
• Low enough to reduce TOV to acceptable levels
• High enough to maintain feasible protection
  schemes.
• For 3-Ø, 4-W Dx system this connection is
  preferred

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DELTA / delta

• Advantages
• Primary SLG faults not affected
• Low DG contribution to Dx ground faults (via
  ve and ve seq networks)
• Some inverters may require an ungrounded
  connection
• Could center-tap one-leg of delta secondary to
  provide single- and three-phase service

• Disadvantages
• Ferro-resonance issues may require 3Ø switches
  and protection on primary side
• Detection of primary SLG faults is difficult
• Non-standard equipment for most utilities
• High TOV on Dx system

Uncommon connections but used in some industrial
facilities to reduce impact from SLG faults of
the system
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TRANSFORMER CONNECTIONS AND ZERO SEQUENCE
CURRENTS
Xmer can pass I0 only if I0 circulates in
primary loop on the source side Secondary loop
(between Xmer and fault) is driven by this
loop Open loop results in Xmer unable to pass
I0
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How to Restrict Ground Fault Contribution at HV
side

• By inserting Neutral Grounding Reactor (NGR)
  between neutral of star connected HV winding and
  ground
• NGR needs to be carefully sized higher values
  will reduce the ground current contribution but
  increase the TOV
• Optimize NGR such that TOV remains under 130

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Temporary Over-voltage (TOV)

• TOV is a fundamental frequency O/V associated
  with
• switching (for example load rejection)
  nonlinearities (ferro-resonance effects,
  harmonics)
• ground faults (SLG faults)

• If the ratio of X0 / X1 is positive 1 System
  is solidly grounded
• 3 System is effectively grounded (TOV lt 130
  of L-N voltage)
• gt 3 System is no more suitable for 4-W
  multi-grounded Dx

TOV vs. X0/X1 ratio
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How to Restrict TOV at HV side

• TOV at 3-Ø, 3-W Dx are not an issue (L-L rated
  insulation)
• TOV must be restricted to lt 130 in 3-Ø, 4-W Dx
  to keep the system effectively grounded as surge
  arrestors, 1-Ø Xmers rated for L-N volts and L-N
  connected customers are all vulnerable to TOV
• By connecting Grounding Xmers (GT) at HV side of
  DGIT the TOV can be restricted to safe levels
• GT must be sized for a) continuous current
  rating and b) fault current rating (10 sec)

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SUMMARY

• There is no best winding connection which is
  commonly suitable for all types sizes of DG
• There is always a trade off in selection of
  winding connection
• IG contribution and TOV at PCC are two critical
  aspects to be sincerely considered in selection
• - ? on HV ?TOV issues ? mitigation ? Gnd
  Xmer
• - Star-gnd on HV ? IG constraint ? mitigation
  NGR
• Balance between IG TOV can be achieved by
  appropriately sizing of Grounding Xmer
  or Neutral Grounding Reactor (NGR)

T H A N K S ??? QUESTIONS ???