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 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.

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 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.

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

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.

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.

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

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)

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

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

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

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.

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

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

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.