Download Interconnect Protection of IPP Generators Using Digital Technology

Interconnect Protection of
Dispersed Generators
Chuck Mozina
Beckwith Electric Co., Inc.
The Utility Industry
Today: Acquisition and Consolidation
The Utility Industry
Today: Acquisition and Consolidation
The Utility Industry
Today: Acquisition and Consolidation
The Utility Industry
Future: Distribution Generation
What Is Interconnection
Protection?
 Protection that allows the Independent
Power Producer (IPP) to operate in
parallel with the utility.
 Large non-utility generators do not
require specific interconnection
protection.
 Smaller dispersed DG generators do
require specific interconnection
protection.
Typical Interconnection Protection
To Utility System
Interconnection
Transformer
Interconnection
Relay
Utility System
IPP System
Local Loads
 Disconnects the generator when it is no longer operating in
parallel with the utility.
 Protects the utility system from damage caused by
connection of the generator (fault current and overvoltage).
 Protects the DG generator from damage from the utility
system, especially through automatic reclosing.
Typical Generator Protection
 Generator internal short
circuits.
 Abnormal operating conditions
(loss of field, reverse power,
overexcitation and unbalance
currents).
Local Loads
Challenges for the
Protection Engineer
 Seamless integration of DG’S into the
utility protection system despite:
– ownership boundaries
– conflicting objectives of DG owners vs. utility
 Making sure protection is operational
over the life of the installation
Utilities Generally Specify
Interconnection Protection
Requirements
They typically Include:
 Winding configuration of interconnection
transformers
 Utility grade interconnection relays
 CT and VT requirements
 Functional protection
– 81U/O, 27, 59, etc. (required speed of operation)
 Settings of some interconnection functions
Impact of Interconnection
Transformer Configuration on
Interconnect Protection
 Overvoltage caused by ungrounded primary
windings
 Ground fault current caused by grounded
primary windings
 Source feeder relaying responding to secondary
faults at the DG facility
Typical 4-Wire Distribution Feeder Circuit
Typical 4-Wire Distribution Feeder Circuit
Pole-top transformer rated for line-to-neutral voltages
example: 13.2 KV 3  7.6 KV
Ungrounded Interconnection
Transformers
Low
Voltage
(SEC.)
High
Voltage
(PRI.)
Ungrounded Interconnection
Transformers
Low
Voltage
(SEC.)
High
Voltage
(PRI.)
Problems
Can supply the feeder
circuit from an
underground source
after substation breaker A
trips causing overvoltage
Ungrounded Interconnection
Transformers
Low
Voltage
(SEC.)
High
Voltage
(PRI.)
Problems
Can supply the feeder
circuit from an
underground source
after substation breaker A
trips causing overvoltage
Advantages
Provide no ground fault
backfeed for fault at F1 &
F2. No ground current from
breaker A for a fault at F3.
Saturation Curve of Pole-Top Transformer
 Many utilities use ungrounded primary windings only if
IPP sustains at least a 200% overload on islanding.
Grounded Primary
Interconnection Transformers
Low
Voltage
(SEC.)
High
Voltage
(PRI.)
Grounded Primary
Interconnection Transformers
Low
Voltage
(SEC.)
High
Voltage
(PRI.)
Problems
Provides an unwanted ground
current for supply circuit faults
at F1 and F2.
Allows source feeder relaying at
A to respond to a secondary
ground fault at F3.
Grounded Primary
Interconnection Transformers
Low
Voltage
(SEC.)
High
Voltage
(PRI.)
Problems
Provides an unwanted ground
current for supply circuit faults
at F1 and F2.
Allows source feeder relaying at
A to respond to a secondary
ground fault at F3.
Advantages
No ground current from breaker
A for faults at F3. No overvoltage
for ground fault at F1.
No overvoltage for ground
fault at F1.
Interconnection Protection of
Dispersed Generators
Protection Objectives
 Loss of parallel operation
 Fault backfeed detection
 Detection of damaging system conditions
– Open phase condition
– Phase sequence reversal
 Abnormal power flow
 Restoration
Typical Interconnection Protection for
Wye-Ground (PRI.) Interconnection Transformer
LOAD
Typical Interconnection Protection for
Wye-Ground (PRI.) Interconnection Transformer
Loss of
Parallel
LOAD
Loss of Parallel Detection
 Over/under frequency & over/under voltage “window”
 In some applications rater of change of frequency (81R) is used.
Typical Interconnection Protection for
Wye-Ground (PRI.) Interconnection Transformer
Loss of
Parallel
LOAD
Loss of Parallel Detection
 Over/under frequency & over/under voltage “window”
 In some applications rater of change of frequency (81R) is used.
 If feeder load and IPP generation are near a match, transfer trip (TT) maybe
required.
Typical Interconnection Protection for
Wye-Ground (PRI.) Interconnection Transformer
Fault Backfeed
Removal
Loss of
Parallel
LOAD
Fault Backfeed Removal Detection
 Not Required for induction generator of small synchronous IPP’s
–

Can rely on loss of parallel protection
Moderate to large IPP’s
–
–
Phase faults: 51V, 67, 21
Ground faults: 51N, 67N
Typical Interconnection Protection for
Wye-Ground (PRI.) Interconnection Transformer
Fault Backfeed Damaging
Conditions
Removal
LOAD
Damaging System Conditions
 Unbalanced currents
–
–
Open conductor/single phase (46)
Phase reversal (47)
Loss of
Parallel
Typical Interconnection Protection for
Wye-Ground (PRI.) Interconnection Transformer
Fault Backfeed Damaging Abnormal
Conditions Power Flow
Removal
Loss of
Parallel
LOAD
Abnormal Power Flow
 Enforces interconnect contract
–
Prohibits IPP from providing power to utility in violation of interconnect
contract
Typical Interconnection Protection for
Wye-Ground (PRI.) Interconnection Transformer
Supervises
reclosing of A
Typical Interconnection Protection for
Wye-Ground (PRI.) Interconnection Transformer
Supervises
reclosing of A
CASE #1 - Local Load Exceeds Generation
Restoration Practice
 Interconnection relays trip IPP generator breakers (B&C)
 When utility restores IPP generator auto synchronize to return to service
Typical Interconnection Protection for
Wye-Ground (PRI.) Interconnection Transformer
A
Supervises
reclosing of A
CASE #2 - Local Load Matches Generation
Restoration Practice
 Interconnection relays trip main incoming breaker (A)
 When utility restores 25 function (with , F, and V) recloses A to
restore parallel with utility.
Typical Interconnection Protection for
Wye-Ground (PRI.) Interconnection Transformer
Fault Backfeed
Removal
Damaging Abnormal
Conditions Power Flow
Loss of
Parallel
Restoration
25
VT
LOCAL
LOAD
Total Interconnect Package
 Loss of Parallel
 Fault backfeed removal
 Damaging conditions
 Abnormal power flow
 Restoration
Typical Interconnection Protection for
Ungrounded (PRI.) Interconnection Transformer
Fault Backfeed
Removal
Damaging
Conditions
Abnormal
Power Flow
Loss of
Parallel
Restoration
25
VT
LOCAL
LOAD
Conclusions
 Interconnection protection will have renewed
importance in the next 10 years.
 Properly designed interconnection protection
addresses concerns of both DG owners and
utility.
 This presentation outlines salient points
utility and DG owners need to consider when
developing protection requirements.
 Interconnection transformer configuration
plays a pivotal role in interconnection
protection.
 Digital multifunction relays are an ideal
technology for interconnection protection.