Power Quality Impacts of Distributed Generation

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Power Quality Impacts of Distributed Generation

• Roger C. DuganSr. Consultant
• Electrotek Concepts, Inc.
• Tennessee Tech.
• February 9, 2004

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Introduction

• Renewed interest in Grid-connected DG
• Utility Perspective
• T D Capacity Relief
• Hedge against high market prices
• End User Perspective
• CHP and Improved Reliability
• Incentives from Utility
• Commercial Power Producer
• Sell kWh or ancillary services

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Emphasis of This Presentation

• Units connected to distribution (MV)
• Typically lt 10 MW
• Commonly 1-2 MW
• Frequently owned by end user
• Impact of these units on Power Quality
• Operating conflicts
• We will look at a few of the many

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OverCurrent Protection Conflicts
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Typical Overcurrent Protection
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Only One Device Has to Operate
OperatingDevice
Fault Current
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Transmission Fault Protection
Multiple Sources
Two Devices Must Operate to Clear Fault
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LV Network Systems
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Utility Fault Clearing

• Due to cost, distribution systems are designed
  expecting only one source of fault current
• Only one device has to operate
• DG must disconnect to allow utility fault
  clearing process to continue
• This is the source of most operating conflicts

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Reduction of Reach
Utility relays and DG relays may not know
anything is wrong.
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Reduction of Reach

• More of a problem at peak load
• When peaking units are likely to be connected !!
• More of a problem for high resistance faults
• Longer fault detection times

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Instantaneous Reclosing
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Reclosing

• Most faults are temporary
• Reclosing is prevalent in North America
• Reclosing Interval 0.2 5 sec
• Instantaneous lt 30 cycles

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Reclosing, contd

• If DG does not get off, two bad things could
  happen
• DG is damaged by the reclose
• Shafts
• Winding damage
• Fault is not cleared
• Reduced reliability
• Damage to utility system

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Reclosing
DG Must Disconnect Here
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Instantaneous Reclosing

• Utilities use this for PQ concerns
• Increases the probability that DG will not
  disconnect in time
• Makes it almost a certainty that some wont
• If 1-2 sec, most operations will be easily
  detected by DG protection
• Some utilities use 5 sec reclose on feeders with
  DG

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Unintended Consequences ...
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Voltage Drop After Fault
OOPS!
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Low Voltage After Interruption

• A problem when there is a fault at peak loading
• When peaking DG is likely to be on !!
• This is often the most limiting condition for how
  much DG can be accommodated without change
• Particularly on rural feeders
• 500 1000 kW

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Ferroresonance

• Sometimes Conflicting Interconnect Requirements
  Collide

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Ferroresonance
DG Required to have separate Transformer and
disconnect at first sign of trouble
G
2) DG breaker detects negative sequence and trips
leaving transformer isolated on cable capacitance
with no load/generation
1) Fuse blows or switch opens
3) Ferroresonance develops
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Ferroresonance
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Ferroresonance Waveform
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Reduced Insulation
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Arrester Duty for SLG
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Varying Generation
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Varying Generation Example
10 000 ft
10 000 ft
20 000 ft
Regulator
115 kV
12.47 kV
500 kW
1000 kW
600 kvar
Wind Gen
336 MCM ACSR
8 MW
(Untransposed)
Unbalanced impedances
Rapidly-varying load
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Varying Generation
1-Phase Regulators
Regulator Operations
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Transformer Connection and Ground Faults
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Grd Wye-Delta Connection

• Some claim this is the best way to interconnect
  DG
• Same way as nearly all central station generation
  is connected
• However, most utilities dont allow this
  connection on their system.

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Wye-Delta Connection
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Problems with Wye-Delta

• Increases ground fault current
• Interferes with coordination
• Faulted Circuit Indicators get fooled
• Transformer subject to failure
• Special design and/or neutral reactor
• Utilities often discourage Specials

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On Wye-Wye

• Most common U.S. connection
• Generally well behaved
• Does not block Triplen Harmonics
• Leads to Harmonic Surprise when many machines
  paralleled with utility system
• Also occurs with Delta-Wye
• 2/3 pitch machines lessens the impact

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Conclusions

• Interconnected DG likely to increase
• DG must adapt to Distribution protection system
• Too much infrastructure already in place to
  overhaul the system to better accommodate DG
• Who would pay for it? Non-participating
  ratepayers?

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Conclusions

• DG impact on Power Quality
• Often good for individual end users
• Largest gainers Those with high-value product
  that suffer sustained interruptions
• But, should not expect power quality improvement
  in all areas
• Maybe slightly negative for utility
• Contributes to operating conflicts
• Can we see it in the Average indices?

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Conclusions, contd

• Revisit Overcurrent Protection
• Utilities may have to change some practices
• Instantaneous reclosing ?
• Use of line fuses
• Tradeoffs for Transformer Connections
• WYE-Delta good for DG but not for ground fault
  coordination
• Special engineering reqd for some DG application.

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Despite These Conflicts

• DG can almost always be successfully engineered
• Larger conflict may be over who pays
• Some long-standing utility practices may have to
  be changed

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How Much DG ?

• What is the No Changes Penetration Limit ?
• Typical 15 of feeder capacity
• 5 - 10 if Rural