Distributed Energy Resource Saturation: Page 5 of 7
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Increased research. IOUs need to devote greater resources to understanding new problems that arise from expanded transmission network penetration. One example of a relatively new, and often misunderstood, problem is islanding. With the implementation of UL 1741, which requires all certified inverters to shut down within 2 seconds of grid de-energization, reliability engineers generally agree that there is no need to worry about an island forming and continuing in perpetuity. What is less clear is the extent to which they need to worry about the impacts of a temporary short-duration island.
Overcurrent protective devices manage fault conditions by sensing when a sharp surge in current hits a circuit over a brief period. The problem is that if a fault occurs during a temporary island, the circuit has already been interrupted; therefore, there is no device available to clear the excess current. While a few extra seconds of inverter operation might not seem like a long time, it is more than enough time to generate overcurrent conditions that could damage electrical equipment and negatively affect power quality.
The safety and reliability tests in Screen P of the Rule 21 engineering review analyze the potential for temporary islanding. (See “Rule 21 Engineering Review Process,” pp. 32–33.) These tests are relevant to the interconnection discussion because many projects that fail Screen P face longer interconnection time lines. This is so the IOUs can design and implement DTT, SCADA visibility reclosers or other mitigation techniques to negate the threat of extended run-ons at the substation or transmission network level. The issue of temporary islanding begs further analysis. We need to understand not only the conditions required for a temporary island to occur, but also the potential implications.
In an August 2016 project report for the California Solar Initiative (see Resources), General Electric Energy Consulting recommends five updates to the PG&E interconnection process:
- In initial review: Raise the screening limit from 15% peak load to 60% of estimated simultaneous load; the estimated simultaneous load will be based on conversion factors as was defined and implemented in Task 2 Report: Statistical Analysis of PV Generation and Load Balance.
- In supplemental review: Keep the existing minimum daytime load screen when SCADA data is available, and allow 80% of estimated simultaneous load by maintaining the power factor of the section below 0.98 inductive.
- In detailed review: Allow up to 105% of simultaneous load by detuning circuits to maintain the power factor between 0.95 and 0.98 inductive, to address islanding concerns if needed.
- In protection requirements: Modify the Direct Transfer Trip exemption bulletin to enable the quick interconnection of certified inverters rated less than 1 MW if there are no significant machine-based generators on the island.
- In protection requirements: Eliminate reclose blocking for all certified inverters by lengthening reclose time on high-penetration feeders to 10 seconds.
To date, PG&E has only implemented items 4 and 5 at any level. This situation highlights a larger problem: IOUs are mandating major upgrades based on an incomplete understanding of new phenomena. In addition, the IOUs mandate these upgrades with no oversight from the CPUC. For its part, the CPUC has no technical personnel on staff and therefore lacks the resources to adequately assess and review such issues.
A California bill signed into law in September 2016 establishes an expedited review process for such circumstances that includes an independent engineering expert. However, the state legislature has not approved the funds needed to enact this bill, so the IOUs will continue to have unchecked power to implement such policies.
For their part, IOUs say that they make many of their policies, such as anti-islanding, with an abundance of caution. However, the CPUC should encourage IOUs to take a closer look and analyze whether their determinations are appropriate or whether they have gone overboard because they lack the data for an informed decision. In addition, the solar industry must request, encourage and support technical policy review when new data supports a revised approach.
Most UL 1741–certified inverters are capable of much more than just producing kilowatts. Technicians can program them to absorb or create reactive power and to play an active role in power-factor correction. In a more traditional sense, DER projects can also supplement existing power plants during periods of high demand. The missing piece of the puzzle in the current interconnection model is a lack of understanding as to where we need these services and under what circumstances we should implement them.