MRO Under-Frequency Load Shedding (UFLS) Program NOTE: The following is an MRO Guideline to be used until a mandory standard is developed per the NERC or MRO Regional Standards development process and approved by the NERC Board of Trustees and applicable regulators. Introduction An automatic under-frequency load shedding (UFLS) program is required to be developed, coordinated, and documented by the Regional Reliability Organizations (RRO) as described in NERC Standard PRC-006-0. Presently, this Standard is not regulatory-approved, and therefore the Load Serving Entities (LSE) within the MRO Region are obligated to participate in the MRO UFLS program according to good utility practice, and not through mandatory compliance at this time. The MRO, which is a Regional Entity as defined by NERC, is assuming the role of RRO for the purposes of developing, coordinating and documenting the UFLS program within the MRO footprint. This MRO Guideline for PRC-006-0 describes the existing UFLS programs within the MRO footprint. A review of the existing program was completed in March, 2007. The results and recommendations of this review are posted at: http://www.midwestreliability.org/REL_asr.html The results and recommendations in the March, 2007 MRO review can be considered for implementation within the MRO-US if they are included as requirements in the NERC and/or MRO UFLS Standards that are presently under development and if/when these Standards receive regulatory-approval. MRO UFLS Program The intent of the MRO UFLS Program is to provide a last resort system preservation measure during severe frequency declines that can result from an extreme disturbance or an islanding condition where load exceeds generation. A UFLS program can minimize the risk of total or partial system collapse, prevent damage to generation and transmission facilities, provide for equitable load shedding, and, improve the overall reliability of the interconnection. The MRO UFLS Program shall be coordinated between all sub-systems within the region and with other UFLS neighboring regional programs, as necessary. The MRO UFLS Program shall be coordinated with generation protection, any automatic load restoration programs, and any tie-tripping schemes or intentional islanding schemes. All appropriate entities within the MRO footprint shall keep their Reliability Coordinator advised of their capacity situation and of actions taken or contemplated to supplement capacity. The Reliability Coordinators within the MRO region will monitor the system frequency of the bulk power system in the upper Midwest and will coordinate any actions needed to maintain scheduled frequency levels. A significant decline in frequency may require the shedding of load in order to avoid widespread system outages and to minimize the risk of damage to equipment. If a gradual frequency decline is caused by an imbalance within the intact eastern interconnection, time

will usually allow for manual intervention to occur before the frequency declines to the first step setting of UFLS relays. If an islanding condition were to occur suddenly, the frequency decline may be rapid, depending upon the size of the island and the relative imbalance between generation and load. The operation of UFLS relays may occur immediately in an attempt to quickly rebalance the generation and load within the island. The MRO Region consists of three coordinated UFLS programs: MRO-Canada: Manitoba Hydro

MRO-Canada: SaskPower Step Step Step Step Step

1: 2: 3: 4: 5:

59.3 59.0 58.7 58.5 58.3

Hz, Hz, Hz, Hz, Hz,

6% 9% 7% 7% 5%

Step 1: Step 2: Step 3: Step 4: Step 5: Step 6:

Load Load Load Load Load

59.3 Hz, 20.6% Load 59.0 Hz, 12.2% Load 58.7 Hz, 16.6% Load 58.5 Hz, 7.2% Load 58.3 Hz, 7.5% Load 58.0 Hz, 8.9% Load

MRO-US Step 1: 59.3 Hz, 10% Load Step 2: 59.0 Hz, 10% Load Step 3: 58.7 Hz, 10% Load Figure 1: MRO UFLS Programs The load percentages shown in Figure 1 are what would be expected to be shed at the corresponding frequency during summer peak load conditions. Actual load shed (as a percentage of total system load) will vary slightly during off-peak conditions. The first three frequency set points (59.3 Hz, 59.0 Hz, 58.7 Hz) are the same for the entire region so that all entities would participate during a region-wide or multi-region islanding condition. However, the Canadian provinces have additional load-shedding steps internal to their provinces since they are at a higher risk of islanding on their own. These additional load-shedding requirements are required to help maintain stability within the province and to reduce the risk of a complete province system shutdown/blackout. Because of the need for these additional load-shedding steps within Canada, the two Canadian provinces within the MRO region can be considered as regional variances with respect to the MRO-US UFLS program because of their unique stand-alone needs. Adjacent Regional Entities: SPP utilizes the same three frequency set points (59.3 Hz, 59.0 Hz, 58.7 Hz) with a minimum of 10% load shed per step. SERC and RFC utilize the same three frequency set points (59.3 Hz, 59.0 Hz, 58.7 Hz) with a minimum of 10% load shed per step for the portions of their system that are adjacent the MRO Region. To enhance the effectiveness of automatic under-frequency load shedding, all entities that own UFLS relays are encouraged to: 1. Use solid state relays for all new installations and relay replacements. These relays should be applied to take advantage of their improved stability and speed over electromechanical relays and their constant response time which is not significantly affected by voltage magnitude or frequency decay rate.

2. Trip bulk power capacitor banks along with loads if studies show this to be of benefit. MRO-US Program: The following UFLS setpoints exist within the MRO-US footprint: Step 1: At 59.3 Hz, shed not less than 10% of system load. Step 2: At 59.0 Hz, shed additional load so that the total amount shed from Steps 1 and 2 is not less than 20% of system load prior to Step 1. Step 3: At 58.7 Hz, shed additional load so that the total amount shed from Steps 1, 2 and 3 is not less than 30% of system load prior to Step 1. If the frequency is still below 59.5 Hz even after the 3 steps of UFLS have occurred, all appropriate entities shall coordinate additional manual load shed amounts with their Reliability Coordinator. This additional manual load shed must be in addition to the feeders already identified and used for the three step UFLS program. Frequency decline below 58.7 Hz: If frequency continues to decline below 58.7 Hz, all appropriate entities shall take any action necessary to arrest the frequency decline. The Wisconsin entities within MRO-US that were formerly a part of the legacy MAIN Reliability Council had criteria that prohibited the opening of major transmission lines for frequencies between 58.7 Hz and 58.0 Hz. These Wisconsin entities should continue to adhere to this legacy criterion until it gets superseded by a mandatory NERC and/or Regional standard that is approved by the NERC Board of Trustees and the Commission. If frequency continues to decline below 58.0 Hz, nuclear units and some fossil steam units within the affected area can be expected to trip automatically within a period of a few seconds to a few minutes. Once an island’s frequency has stabilized and is close to the interconnection frequency, resynchronization can be coordinated through the Reliability Coordinator(s). Manual load restoration and resumption of normal voltage levels and interchange schedules shall also be coordinated through the Reliability Coordinator(s) as generation availability and frequency levels allow. Obligation to Shed Load The MRO-US program, under summer peak conditions, requires 3 steps of 10% load shed (30% total) on an aggregated basis, for the entire MRO-US footprint as a whole. This MROUS requirement has been achieved by individual Load Serving Entities (LSE) coordinating their load shedding needs with neighboring utilities. Load shed within an individual company does not necessarily need to total 10% per step since it may make electrical sense for a neighboring utility to shed a portion of their load-shed obligation. In the future, it is anticipated that a mandatory NERC Standard and/or MRO Regional Standard for UFLS will be developed for regulatory-approval. At that time, specific sub-regional or individual entity percentages may be required to meet a specific islanding condition or scenario.

UFLS Coordination with Generation Under-frequency Protection The Wisconsin entities within MRO-US that were formerly a part of the legacy MAIN Reliability Council have implemented the following generator under-frequency setpoints and time delays to coordinate with the 3 step UFLS program within the MRO-US footprint: Generator UF Setting (Hz): > 59.5 < 59.5 to > 59.2 ≤ 59.2 to > 58.5 ≤ 58.5 to > 58.0 ≤ 58.0

Minimum Time Delay (Sec): Automatic tripping not permitted 2700 120 15.0 Owner’s discretion

Table 1: Allowable Generator Automatic Under-frequency Trip Setpoints And Required Time Delays (Applicable to former MAIN companies only) All other MRO-US entities (previously within the legacy MAPP Reliability Council) are also obligated to coordinate their generator under-frequency protection with the UFLS program, however specific time delays are determined by the individual generator owner. It is important that UFLS be allowed to function before generator under-frequency relaying causes tripping of generators. Frequency setpoints and time delays shown in Table 1 should provide sufficient time delay to permit system frequency recovery. In those cases where generators must be tripped for their own protection prior to all three UFLS steps, additional load shedding must be installed within the vicinity of the generator, to compensate for the generators that trip outside the specifications of Table 1. The amount of load shall be in addition to that shed in accordance with Steps 1, 2 and 3. UFLS Timing Requirements Total tripping time (relay + breaker time) at each load shed point shall be set to meet the objectives of the MRO-US UFLS Program but shall not exceed 60 cycles at the indicated frequency set points. Coordination with Tie-line Tripping and Islanding Coordination must include consideration of automatic tie tripping schemes, intentional islanding schemes, automatic load restoration schemes, or any other schemes that are part of or impact the MRO UFLS program. MRO UFLS Program Database A database of the MRO UFLS Program shall be maintained and annually updated. This database shall include sufficient information to model the MRO UFLS Program in dynamic simulations. MRO will provide the following data (based on the Model) in the annual data request: • • • •

Bus # Load ID’s, Area, Zone, and Owner Numbers Peak load at bus Frequency set points

Data that is collected includes: • • •

MW shed for each Step # Breaker Time Pickup time

The data representatives are also requested to assure all load is counted once and only once, and that the most recent UFLS feeder loadings be used for accuracy.