Download Instantaneous Reclosing

Power Quality Impacts of
Distributed Generation
Roger C. Dugan
Sr. Consultant
Electrotek Concepts, Inc.
Tennessee Tech.
February 9, 2004
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 < 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
Typical Overcurrent Protection
FEED ERS
BREAKER
R EC LOSER
FUSED LATERALS
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Only One Device Has to Operate
Operating
Device
FEED ERS
Fault
Current
BREAKER
R EC LOSER
FUSED LATERALS
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Transmission Fault Protection
Two Devices Must
Operate to Clear Fault
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Multiple Sources
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LV Network Systems
THESE DEVICES
MUST OPERATE
FAULT
SUBSTATION
PRIMARY FEEDERS
TRANSMISSION
SYSTEM
FEEDER BREAKER
OR RECLOSER
LOW-VOLTAGE NETWORK
NETWORK
PROTECTOR
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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
REDUCED REACH DUE TO DG INFEED
X
DG
DG INFEED
FAULT
RESISTANCE
REACH OF SUBSTATION BREAKER
NORMAL REACH OF LINE RECLOSER
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
Reclosing
• Most faults are temporary
• Reclosing is prevalent in North
America
• Reclosing Interval: 0.2 – 5 sec
– “Instantaneous” < 30 cycles
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Reclosing, cont’d
• 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 won’t
• 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 ...
Voltage Drop After Fault
VOLTAGE PROFILE
BEFORE FAULT
VOLTAGE PROFILE
AFTER RECLOSE
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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 …
Ferroresonance
DG Required to have separate Transformer and disconnect at first sign of
trouble
G
1) Fuse blows
or switch opens
3) Ferroresonance
develops
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2) DG breaker detects
negative sequence and
trips leaving transformer
isolated on cable
capacitance with no
load/generation
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Ferroresonance
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Ferroresonance Waveform
4
Voltage, pu
3
2
1
0
-1
-2
-3
0.000
0.050
0.100
0.150
0.200
Time, sec
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Reduced Insulation
Arrester Duty for SLG
ARRESTERS
SUBJECTED TO LINETO-LINE VOLTAGES
G
UTILITY
INTERRUPTING
DEVICE OPEN
PERMANENT LINETO-GROUND FAULT
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UNGROUNDED
TRANSFORMER
CONNECTION
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Varying Generation
Varying Generation Example
10 000 ft
115 kV
12.47 kV
10 000 ft
1000 kW
20 000 ft
Regulator
500 kW
600 kvar
336 MCM ACSR
(Untransposed)
Unbalanced
impedances
Wind Gen
8 MW
Rapidly-varying “load”
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Varying Generation
Wind Farm Voltage
8000
7900
7800
V, volts
7700
Regulator
Operations
7600
7500
7400
7300
7200
7100
7000
1
301
601
901
1201
t, seconds
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1501
1801
2101
2401
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Transformer Connection and
Ground Faults
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 don’t allow this
connection on their system.
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Wye-Delta Connection
FAULT
M AIN FAULT
CONTRIBUTION
FROM EPS
1
(MULTI-GROUNDED NEUTRAL)
CONTRIBUTION
FROM DR
TRANSFORM ER
G
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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, cont’d
• 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 req’d 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
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