1. NERC PRC-024-2 Drafting Team Voltage Ride-Through (VRT) Curve Genesis
NERC Generation Standards Drafting Team September 15-17, 2008 Dallas, TX
NERC PRC Drafting Team Voltage 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 and distributed on
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 drafting team concerning voltage duration at specific locations on the curve.
As PRC 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 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. 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.
HVRT Reduction
Relay Failure
FERC 9 Cycle Limit

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 , 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 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
Chart Source: Brendan Kirby (AWEA)

10. Questions?