Distributed Renewable Energy Operating Impacts and Valuation Study

Technical Value – Transmission System
4.4 Potential Detrimental Impacts to Transient Stability and Spinning Reserve
The PV inverters are designed to turn disconnect the PV systems for low voltage events due to
safety reasons. If a failure on the distribution system occurs, causing an outage on the
distribution system, there is a concern that the PVs could “back-feed” to the distribution system
when utility operations personnel are working on the line. Therefore, it is necessary to turn off
the PVs to prevent this back-feed. This necessary design for safety reasons on the distribution
system can have a detrimental impact on the transmission system.
A fault on the 500-kV system can depress voltages throughout the Phoenix area. If the drop in
voltage is significant, and if the duration of the low voltage event is long enough, many PV
inverters may turn off simultaneously (potentially hundreds of megawatts with full adoption).
This may result in two potential concerns:
• Potential impact on spinning reserve requirements
• Potential decreased transient stability performance
The results of the analysis found that there were no impacts to spinning reserve requirements,
and that detrimental impacts to transient stability performance were minor. As a result, no
negative value was quantified for either of these effects. The analyses to support these
conclusions are described below.
4.4.1 Potential Impacts to Spinning Reserve Requirements
To evaluate the potential detrimental impacts on spinning reserve requirements, a scenario where
a 500-kV fault occurred on the high side of the generator step-up transformer on one of the three
Palo Verde units was theorized (Palo Verde is a 3,200-MW nuclear-fueled generating station that
is partially owned by APS). Such an event could not only cause a loss a Palo Verde unit, but
also hundreds of megawatts of PV, thereby potentially increasing spinning reserve requirements.
In order to test the hypothesis, a review of historical events was conducted to determine if those
events caused extensive loss of the existing APS PV installations. Three historical events were
analyzed:
• June 14, 2004 fault at the West Wing 500-kV substation with a failure of a protection
system.
• July 4, 2004 fault at the West Wing 500-kV substation due to a fire.
• July 20, 2004 fault at the Deer Valley 230-kV substation.
The June 14, 2004 fault on the 500-kV system at the West Wing 500-kV substation was a slow
cleared fault, meaning that there were multiple contingencies causing the low voltage event to be
sustained on the 500-kV system for a longer duration than for a single contingency. Figure 4-4
shows the behavior of the existing PV systems at the time of the fault (7:45 AM). The six
facilities in the figures below represent existing APS systems which have sophisticated metering
equipment on them to record 10-minute interval data. The point of the figures below is to
indicate the impact to these systems as a result of fault events.
Distributed Renewable Energy Operating Impacts & Valuation Study R. W. Beck, Inc. 4-15