NERC PRC-024-2 Drafting Team Voltage Ride-Through (VRT) Curve Genesis

1. NERC Generation Standards Drafting Team September 15-17, 2008Dallas, TX NERC PRC-024-2 Drafting TeamVoltage Ride-Through (VRT) Curve Genesis Craig Quist, PE PacifiCorp, Transmission Planning Salt Lake City, Utah

2. Voltage Ride – Through Standard • Based on an earlier request from the WECC Voltage Ride-Through (VRT) Standard Task Force, the WGTF has been working on a VRT white paper that would be used as the justification for a new VRT Standard. • This white paper discusses the low voltage, low voltage recovery and high voltage period, immediately following a system event. • The white paper was approved by the Technical Studies Subcommittee (TSS) on 5-22-07 and distributed on 6-13-07. • When reviewing the paper, references to generator, wind plant or wind farm should be considered synonymous with each other.

3. WECC VRT Curve vs. Voltage Traces • Disturbance voltages traces (at POI) fit within the defined WECC VRT Curve (SLG Faults added to during NERC evaluation) • Issues were raised by the PRC-024-2 drafting team concerning voltage duration at specific locations on the curve. • As PRC-024-2 is a generator protection system standard (vs. generator performance standard), questions surfaced concerning how such a curve can be applied? • Generator protection relays typically utilize voltage duration

4. Time Duration - Voltage Traces & WECC VRT Std • The approach of the PRC-024-2 drafting team shifted to defining time duration for generator protection • To develop a time duration VRT boundary it was determined that the voltage traces provided in the WECC white paper could be converted to time duration format • Additional SLG fault voltage traces and Southern Company voltage traces were also added

5. HVRT Reduction Relay Failure FERC 9 Cycle Limit Time Duration - Voltage Traces & New NERC Std • Initially, a “zero margin” boundary was defined as an envelope for the time duration curves • However, it was determined that margin would be added along with SLG voltage traces to develop the new time duration boundary • Additionally, voltage traces from Southern Company were entered to form the Pink curve. • The new time duration boundary reshaped both the low voltage and high voltage boundaries from those proposed by WECC white paper.

6. Proposed PRC-24-02 VRT Standard

7. Other PRC-24-02 VRT Participant Inputs • Areva • Provided the European LVRT voltage performance standard • Suggest sticking with a regular scale for criteria.  Mixing criteria between this and the log scale gets confusing. • The voltage prior to the fault should be clarified • Previous curves stopped at 4 seconds.  This curve goes out to 10 seconds • Excel Energy SLG feedback • When fed from a unit-connected transformer or a delta-wye wound transmission-connected transformer, most low voltage AC contactors will probably ride through close-in SLG faults on the transmission system. • However, if fed from a wye-wye wound transmission-connected transformer, many will probably drop out, even with fast clearing.

8. NERC Time Duration Curve vs. WECC VRT Std • Comparison of NERC time duration curve with the WECC white paper curve illustrates areas that the VRT curve could be modified.

9. PRC-024-WECC-1-CR - Generator Low Voltage Ride-Through Criterion • On 9-10-08, WECC distributed the PRC-024-WECC-1-CR - Generator Low Voltage Ride-Through Criterion. • In developing the criterion, the WECC Reliability Subcommittee (RS) decided to let the NERC PRC-024-2 drafting team do the “heavy lifting” in developing a new standard.  • Effectively, changes were made to bring the WECC standard in-line with the current FERC LVRT standard. • Comments are due 10-10-08. Chart Source: Brendan Kirby (AWEA)

10. Questions?