1. Interconnect Protection of Dispersed Generators
Chuck Mozina
Beckwith Electric Co., Inc.

2. The Utility Industry Today: Acquisition and Consolidation

3. The Utility Industry Today: Acquisition and Consolidation

4. The Utility Industry Today: Acquisition and Consolidation

5. The Utility Industry Future: Distribution Generation

6. 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.

7. 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.

8. Typical Generator Protection
Generator internal short circuits.
Abnormal operating conditions (loss of field, reverse power, overexcitation and unbalance currents).
Local Loads

9. 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

10. 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

11. 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

12. Typical 4-Wire Distribution Feeder Circuit

13. Typical 4-Wire Distribution Feeder Circuit
Pole-top transformer rated for line-to-neutral voltages example: KV 3  7.6 KV

14. Ungrounded Interconnection Transformers
Low
Voltage
(SEC.)
High
Voltage
(PRI.)

15. 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

16. Ungrounded Interconnection Transformers
Low
Voltage
(SEC.)
High
Voltage
(PRI.)
Problems
Advantages
Can supply the feeder circuit from an underground source
after substation breaker A
trips causing overvoltage
Provide no ground fault
backfeed for fault at F1 &
F2. No ground current from breaker A for a fault at F3.

17. Saturation Curve of Pole-Top Transformer
Many utilities use ungrounded primary windings only if IPP sustains at least a 200% overload on islanding.

18. Grounded Primary Interconnection Transformers
Low
Voltage
(SEC.)
High
Voltage
(PRI.)

19. 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.

20. Grounded Primary Interconnection Transformers
Low
Voltage
(SEC.)
High
Voltage
(PRI.)
Problems
Advantages
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.
No ground current from breaker
A for faults at F3. No overvoltage
for ground fault at F1.
No overvoltage for ground
fault at F1.

21. 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

22. Typical Interconnection Protection for Wye-Ground (PRI
Typical Interconnection Protection for Wye-Ground (PRI.) Interconnection Transformer
LOAD

23. Typical Interconnection Protection for Wye-Ground (PRI
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.

24. Typical Interconnection Protection for Wye-Ground (PRI
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.

25. Typical Interconnection Protection for Wye-Ground (PRI
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

26. Typical Interconnection Protection for Wye-Ground (PRI
Typical Interconnection Protection for Wye-Ground (PRI.) Interconnection Transformer
Fault Backfeed
Removal
Damaging
Conditions
Loss of
Parallel
LOAD
Damaging System Conditions
Unbalanced currents
Open conductor/single phase (46)
Phase reversal (47)

27. Typical Interconnection Protection for Wye-Ground (PRI
Typical Interconnection Protection for Wye-Ground (PRI.) Interconnection Transformer
Fault Backfeed
Removal
Damaging
Conditions
Abnormal
Power Flow
Loss of
Parallel
LOAD
Abnormal Power Flow
Enforces interconnect contract
Prohibits IPP from providing power to utility in violation of interconnect contract

28. Typical Interconnection Protection for Wye-Ground (PRI
Typical Interconnection Protection for Wye-Ground (PRI.) Interconnection Transformer
Supervises
reclosing of A

29. Typical Interconnection Protection for Wye-Ground (PRI
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

30. Typical Interconnection Protection for Wye-Ground (PRI
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.

31. Typical Interconnection Protection for Wye-Ground (PRI
Typical Interconnection Protection for Wye-Ground (PRI.) Interconnection Transformer
Fault Backfeed
Removal
Damaging
Conditions
Abnormal
Power Flow
Loss of
Parallel
Restoration 25 VT
Total Interconnect Package
Loss of Parallel
Fault backfeed removal
Damaging conditions
Abnormal power flow
Restoration
LOCAL LOAD

32. Typical Interconnection Protection for Ungrounded (PRI
Typical Interconnection Protection for Ungrounded (PRI.) Interconnection Transformer
Fault Backfeed
Removal
Damaging
Conditions
Abnormal
Power Flow
Loss of
Parallel
Restoration 25 VT
LOCAL LOAD

33. 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.