Because of the amount of load shed during winter storm Uri, entities found it difficult to rotate customer outages while maintaining service to critical loads and underfrequency load shed (UFLS) feeders. Simultaneously, the percentage of system load connected to UFLS feeders created a risk of frequency overshoot and instability in the event of UFLS activation.
Underfrequency programs typically require each UFLS entity to shed a predetermined percent of load at specific frequency setpoints. In the event of an underfrequency event, each TOP is required to provide load relief by shedding the required percentage of connected load with automatic underfrequency relays. The percentage of connected load obligation is typically based on “normal” system conditions as a starting point. UFLS entity compliance with the UFLS program is typically verified through periodic surveys of the loads that are armed with UFLS compared to the total system load at the time of the survey. TOPs typically exclude load connected to UFLS relays from manual load plans whenever possible. Additionally, EOP-011-1 R1.2.5 requires that TOPs minimize the overlap between operator-controlled manual load shedding and automatic load shedding. This requirement is intended to ensure that system operators maintain adequate load in their UFLS programs to preserve bulk power system reliability in the event that manual load shed was insufficient to arrest the decline in frequency and relay action was required to further protect the system from a black out. Excluding all load connected to underfrequency relays reduces the amount of load available for manual load shed.
Additionally, system operators must exclude circuits that serve certain critical loads from being used for manual load shed. These critical loads may be public safety facilities, hospitals, military facilities, natural gas facilities, etc. Service should be maintained to these customers because of the direct impacts to public health and safety or security of the bulk power system during load shed events. This further reduces the number of circuits available for load shedding and rotating outages during load shed.
After exhausting the normal processes of public appeals, demand response, and voltage reduction during winter storm Uri, increasing levels of unplanned generating unit outages and de-rates, and increasing electricity demands, the RC had to instruct large amounts of firm load shed to keep the power grid stable. As a direct result of generating unit losses, system operators were forced to order an unprecedented 20,000 MW of firm load shed and to maintain firm load shed for nearly three days. For the reasons described above, the magnitude and duration of the manual load shed limited system operators’ ability to effectively rotate outages during portions of the event, causing some customers to remain out of service for days. Some system operators ultimately ran out of non-UFLS and non-critical load circuits and were forced to use a portion of their UFLS circuits to meet manual load shed directives. During the event, this was deemed an acceptable means to meet the manual load shed directives because the combination of high system loading and the reduced demand due to the prior manual load shed directives resulted in a significant margin of UFLS load above the required levels for TOPs. Several TOPs noted that their UFLS-connected load represented up to 60 percent of its remaining load during the period of February 15–16, 2021.
In cases where a TOP has sufficient monitoring capability during an extreme load shed event to calculate the difference between the UFLS load required to meet its UFLS obligations and the actual load on its UFLS circuits, the TOP may have the option to use some of the UFLS “margin” to include in load shed and rotating outages while still meeting its UFLS obligations. Having this operational flexibility would increase the amount of load available for rotation, spread the burden of outages to a larger pool of load, and reduce customer outage times. Taking this approach of including UFLS circuits during load shed lowers the risk of an overshoot in frequency if UFLS operates when actual UFLS loads substantially exceed the required obligations.
|cookielawinfo-checkbox-analytics||11 months||This cookie is set by GDPR Cookie Consent plugin. The cookie is used to store the user consent for the cookies in the category "Analytics".|
|cookielawinfo-checkbox-functional||11 months||The cookie is set by GDPR cookie consent to record the user consent for the cookies in the category "Functional".|
|cookielawinfo-checkbox-necessary||11 months||This cookie is set by GDPR Cookie Consent plugin. The cookies is used to store the user consent for the cookies in the category "Necessary".|
|cookielawinfo-checkbox-others||11 months||This cookie is set by GDPR Cookie Consent plugin. The cookie is used to store the user consent for the cookies in the category "Other.|
|cookielawinfo-checkbox-performance||11 months||This cookie is set by GDPR Cookie Consent plugin. The cookie is used to store the user consent for the cookies in the category "Performance".|