Outage Scheduling Considerations Outage Scheduling Considerations • • • • • • • • • Equipment outage needs Design criteria, N-1, N-2, N-1-1 Load pocket concepts and analysis Loading considerations System considerations, load flow analysis Seasonal considerations for work Risk vs reward discussion Outage coordination with others Outage contingency planning Equipment Outage Needs • Outage scheduling is the process of removing facilities from service to perform maintenance or modifications. • Electrical equipment require a variety of maintenance and periodic inspection and testing to insure it is operating as required. • Removing facilities from service does make them unavailable to support reliability needs, but can be done in a manner that does not force the system below its design basis. • Equipment that is not maintained will become unreliable over time. • A balance has to be struck between reliability needs now and equipment needs to insure reliability going forward Equipment Outage Needs • Equipment maintenance varies between different types of equipment, age of equipment, voltage classification, etc. • Engineering departments and manufacturers specify various maintenance schedules for equipment either based on time, number of operations or based on periodic test results. • In addition to maintenance, equipment may have to come out of service for repairs. • Outages are often required to replace dielectric fluids, SF-6 gas or other insulating medium. Equipment Outage Specific’s • Transformers require outages to – replace dielectric fluid, – perform tap changer inspections, – address impurities in the dielectric fluid • Breakers require outages for – diagnostic testing, – trip checks, – periodic maintenance • All equipment is equipped with relay protection systems and those systems require periodic calibration and operational tests • Feeder systems also have outage needs for inspection and repair of various components – Underground cables require work on the cooling and pressurizing plants – Overhead feeders require inspection of the string insulators and other components • Various repairs often are required when equipment is damaged by use, failure or weather Equipment Outage Planned Vs Emergency • The process of scheduling equipment maintenance means that the work can be planned in an orderly manner at a time when system conditions permit. • Often, problems develop with equipment that cannot wait and these problems may result in hazards to the equipment or personnel. • These type of issue must be addressed immediately, and then equipment is removed rapidly on an emergency basis. • Different classifications of emergency can exist with associated time frames for removal from service based on the magnitude of the problem that exists. – It is better to remove hazardous equipment from service before it fails and results in an extended outage – Often the hazard can be corrected with much less effort if it is caught before failure Equipment Outage Planning • Bulk Power electrical equipment operate at very high current and voltage levels and move very high energy levels • Failure of this equipment can be quite significant, resulting in explosions, fires, shrapnel, injury, and system disturbances that threaten reliability • For these reasons a detailed outage scheduling process that addresses maintenance needs and emergency response is critical to a successful operation Design Criteria, N-1, N-2, N-1-1 • Understanding the design criteria of your system is important in evaluating when and how to grant equipment outages. • Most companies in the US are required to design and build their system to withstand the single largest loss without impact to customers, N-1 – The system is designed to meet peak load conditions, with the largest capacity loss, and still serve all customer load – This review is done to ensure that no part of the system is exposed due to transmission limitations • Other systems may be designed to meet an N-2 criteria, such as New York City due to the critical nature of the load served • Committing to N-1-1 means we put adequate resources on line to enable us to survive one contingency at a time Design Criteria, N-1, N-2, N-1-1 • Planning and building the system to these design criteria is the first step. • Operating the system to the design criteria requires a daily review of your system and available resources. • You must know your load level and commit the required generation and transmission resources to meet your load and reserve requirements to ensure the design criteria is being met. • These needs change from day to day as loads change. You don’t put all resources on every day, because it would cost the customer too much to do so. • Careful review of generation needed must be done each day to operate reliably first and economically second • Outage scheduling fits right in, as we need to know when and how to allow facilities to be removed for planned work. Design Criteria, N-1, N-2, N-1-1 • Generation commitment is the process of assigning generators to run the next day. • It is done to meet the forecasted load with the reserve requirements, design criteria and planned outages in mind. • If feeders are scheduled out of service, generation requirements may change as different parts of the system my need more or less generators on to support local needs. • A feeder coming out of service may require more generations to support load or less generation because you cannot access the energy without the feeder. • A careful review of all load pockets must be performed Load Pocket Concepts and Analysis • When any transmission system or area is viewed as a whole there is a lot of detail that could be missed • A basic capacity and reserve review for any area would require adequate capacity to meet the expected load demand and adequate reserve to meet the largest loss, plus margin. • However, there is a lot of transmission imbedded in most control areas and power has to be able to move through the area to get to the load. • The basic capacity/reserve review will not find those restricted spots or bottled spots where load or generation becomes restricted. • That means the system has to be dissected into pockets so that a more detailed review can be conducted. Load Pocket Concepts and Analysis • A load pocket is any group of stations that can be enclosed such that several transmission feeders and generators are isolated as supplies to the load within those stations. • The pocket could be one station with load and supplies or many stations. • It can include generation and feeder supplies or feeder supplies only. • A load pocket with no generation resources within it does not necessarily lend itself to commitment options, but it is still good to know if limitations exist Load Pocket Concepts and Analysis • This pocket if isolated from the rest of the system would have four feeders and two generators as supplies to the four loads FDR A GEN-1 Load Load FDR C GEN-2 FDR B FDR D Load Load Load Pocket Concepts and Analysis The feeders internal to the pocket are not supplies and don’t figure into the support of the pocket FDR A GEN-1 Load Load FDR C GEN-2 FDR B FDR D Load Load Load Pocket Concepts and Analysis Supply Vs Load Keep in mind that feeders A, B, C and D connect to other stations and may be the supplies for other load pockets, so you have to understand your system when using this approach FDR A GEN-1 Load Load FDR C GEN-2 FDR B FDR D Load Load Load Pocket Concepts and Analysis Bottled Capacity Sometimes you need to review the pocket to see if you have too much FDR A supply and whether you will be able to get generation out to other parts of the system, especially when FDR B a feeder is out GEN-1 Load Load FDR C GEN-2 FDR D Load Load Load Pocket Analysis • To conduct the load pocket review, you must identify those groups of stations that form pockets and that are known to possibly become limiting. – You must conduct a basic load vs supply review, taking into consideration the capacity of your generators and feeders. – You must discount the facility being considered for outage. – You must outage your largest supply for an N-1 commitment or your two largest for an N-2 commitment. • If the pocket still has excess supply above forecasted load, you are good for outage. If not you must consider committing additional generation resources. • Keep in mind this is a simplistic review and feeders may not load up to their full capacity equally. Relative impedances between feeders and location of other generators can impact loading Example Calculation • Feeder B is scheduled for outage • Feeder A&B are rated 400MW, C & D both 300MW • Gen-1 & Gen-2 each rated 300MW • Load forecast for period is 1100MW • Can an outage be scheduled and meet N-1 design? Resource Capacity Available Feeder A 400 Contingency Feeder B 400 Outage Feeder C 300 300 Feeder D 300 300 Gen-1 300 300 Gen-2 300 300 Load 1100 1200 Difference +100 margin Load Pocket Concepts and Analysis • An outage request must be reviewed before giving approval for the facility to come out of service. • The system conditions have to be right so that the facility can come out of service and not cause you to operate with less than design criteria. • This means you must be able to maintain design criteria with the facility out plus with the next largest loss to meet N-1 design or without the next two largest for N-2 design. • Obviously, this restricts the load conditions that permit outages to be scheduled. • The load pocket review is just a quick review to determine if any obvious problems exists. • A more complete review should be done by performing a load flow analysis of your system Load Pocket Concepts and Analysis • Outages are reviewed on an individual basis to determine the impact they have on specific areas of the system. • However, some outages can impact parts of the system that are quite remote from their location. • Feeders that are part of a major interface connecting your system to other areas are of special concern because they have a wider impact. • They can conflict with other interface feeders that are part of other transmission interfaces. • It is rare that only one facility would be requested out of service for any given day. On a large system there can be many requests each day. • A detailed load flow analysis that considers a variety of N-1 contingencies must be conducted because many outages spread over the system may have some not so obvious negative impacts Loading Considerations • Seasonal load cycles are known and outage planning is usually performed months in advance. • The load ratios throughout the system are also known, so that each load pocket can be referenced as a percentage of overall peak system load. – For example, if load pocket is known to be 10% of system load, we can reference what load levels the outage will be permitted for. – Load pocket percentages vary from time of year and day of week • It is important to know how your load behaves in all parts of the system when reviewing outages and determining impacts. Loading Considerations • Sometimes feeders do not load up equally either due to different impedances or due to phase angle differences around the system. • Sometimes parallel feeders are not identical, and one may tap off to load or another station. • This can result in one feeder reaching its maximum rating while spare capacity exists on other feeders. • This must be taken into consideration, because any feeder overload is not acceptable when scheduling a facility out. • Lets look at our load pocket example again. Example Calculation • Feeder B is scheduled for outage • Feeder A&B are rated 400MW, C & D both 300MW • Feeder C will load up at twice the rate of feeder D • Gen-1 & Gen-2 each rated 300MW • Load forecast for period is 1100MW • Can an outage be scheduled and meet N1 design? Resource Capacity Available Feeder A 400 Contingency Feeder B 400 Outage Feeder C 300 300 Feeder D 300 150 Gen-1 300 300 Gen-2 300 300 Load 1100 1050 Difference -50 margin Example Calculation • Feeder B is scheduled for outage • Feeder A&B are rated 400MW, C & D both 300MW • It is known feeder A will support an adjacent pocket with 150MW • Gen-1 & Gen-2 each rated 300MW • Load forecast for period is 1100MW • Can an outage be scheduled and meet N1 design? Resource Capacity Available Feeder A 400 -150 Feeder B 400 Outage Feeder C 300 Contingency Feeder D 300 300 Gen-1 300 300 Gen-2 300 300 Load 1100 750 Difference -350 margin Load Pocket Vs Load Flow • The point of these examples are to show that although a useful tool, the load pocket method is a rough estimate and you need to understand how your system works and responds before filling in the basic calculation • If things are close, you must perform a load flow analysis to determine if an outage should be permitted Load Flow Analysis • A load flow analysis will work off a model of your system to develop a set of equations that represents the voltages, currents, loads and resources that exist. • The software also has access to current system conditions and planned outages for the next day. • The first run of the program will provide you with the steady state flows and voltages on the system. • The program can then be set to initiate the various contingencies that could exist and provide data on what the post contingency conditions look like and where any violations exist. Load Flow Analysis • This process provides a much more detailed and accurate assessment of the potential risks for the outage being planned. • As a rule, these type of load flows are conducted day-ahead to ensure all planned outages are still good to go. • To conduct a load flow for every outage request is time consuming and not needed. An experienced analyst can get by with a load pocket review for most requests. • It’s a judgment call. Usually outages of short duration or quick recall do not require such calculations. • Outages that will render equipment unavailable for extended durations should be looked at in more detail. System Loading Considerations • Loading considerations play a key role in determining when outages can be scheduled. • The higher the load, the less likely an outage can be scheduled. • Outages are generally scheduled outside of the peak season, as the load levels are much lower. • The electric companies basically spend three seasons of the year getting ready for the one season that places the greatest demand on the system. • In most places the summer is the peak season, with the winter having another, yet smaller peak. • The “shoulder months”, spring and fall are prime season for outages to be performed Outage Considerations • The following is a list of items that come into play when considering an outage; – – – – – – – The significance of facility System load levels Duration of the outage Can the outage be recalled How long is the recall Other conflicting outages How critical is the work Outage Considerations • Significance of facility – The larger, more strategically located, more impact on cost, more risk to reliability • Duration – One or two days Vs 2 or 3 months, big difference • Recall – Is the outage recallable in hours, days weeks or at all • Other conflicts – How much redundancy exists, other critical facilities – Most companies develop guidelines on conflicts • How critical is the work – Routine work that can wait? Work needed to avoid a failure? Regulatory impacts?, etc. Outage Considerations • Conflicting facilities should not be scheduled out of service simultaneously. • A facility should be considered conflicting when it either serves a common load pocket or interface or when it serves a common need on the system – Example, taking two feeders from a common interface or two large reactive resources out at the same time should be avoided – Exceptions can be made, if a good reason exists or if there are many other common resources that can serve that same need Risk vs Reward Discussion • A big part of outage scheduling boils down to a risk vs. reward decision • In every aspect of outage scheduling we are trying to evaluate the risk of not having a piece of equipment in service, so we look at what our needs are and compare that to what our resources are. • We define requirements that enable us to meet our goal, keeping service to our customer, while essentially taking only that risk which is associated with an N-1 or N-2 design. • Even with N-1 and/or N-2 there is some risk associated with removing equipment from service. These contingencies do occur on a somewhat regular basis. Risk vs Reward Discussion • The following all contribute to facilities coming out of service as a result of contingencies. – – – – – Equipment failures, Weather, Vehicle accidents, Construction crews digging into feeders, Animal intrusions all contribute to facilities coming out of service • On any given day the right combination of feeders or generators can come off line unexpectedly and present us with the inability to meet customer demand. • The N-1 and N-2 criteria help to keep us ahead of these events but does not completely protect us, so we always have to evaluate risk with our reviews. Risk vs Reward Discussion • How do we evaluate risk and how do we justify risk? – Risk is evaluated by determining how much margin you have above the normal design requirements, – evaluating your recall options, – evaluating duration, – evaluating weather variability and time of the year • Risk is justified by having a greater overall reward to gain – Taking a lot of risk with little reward is a losing proposition and should not be done – Justifying a risky outage because of a real need or a significant reward is exercising good operating judgment • It is very easy to go from a hero to a goat, so risk evaluation is important. Risk vs Reward Discussion • Risk evaluation – Time of year, only take risk when no options exist in peak season. – Recall Time, an outage that can return to service quickly, hours, is less risky if needs arise. One that cannot be recalled at all is the most risky. – Duration, short durations statistically reduce the possibility of exceeding criteria. – Work needs, is something going to fail if work is not done or are there no immediate consequences. Risk vs Reward Discussion • Reward considerations – How reliable is equipment if we wait for outage? – Are there regulatory requirements that must be addressed and associated fines? – Is the outage going to increase system capacity or capability? – Will the work significantly improve reliability? – Will the work address important environmental concerns? Costs and Risk • In the context of operating reliably we do not generally consider costs. • Costs is money, risk is reliability. • That does not mean that costs are not important and do not play a role in outage scheduling, as they clearly do. • We just do not associate cost as a risk factor. • Once we have determined that an outage is acceptable to go forward from a risk perspective, we then need to ask whether there are negative cost impacts. Costs and Risk • What are the cost impacts of system outages? – If an efficient and inexpensive generator comes off and gets replaced by an expensive one. – When a key interface feeder comes out and reduces transfer capability, causing congestion and price increases in an area. – When a key facility outage results in post-contingency concerns that require increase in generation levels. Outage Coordination With Others • Very often an outage of a facility in your area can have an impact on facilities in adjacent areas. – Everything is connected so flows and contingencies can be impacted in neighboring areas. – Generation changes alter the flows and phase angle of buses. – Intertie feeders may be relied on for support. – Large reactive components coming out can cause voltage concerns. – Bus section outages may render a generator with no outlet. – Generation outages may conflict with planned transmission outages. – Market Implications Outage Coordination With Others • No system is an island of its own! • We intentionally have large interconnects within North America to be more secure – Eastern interconnect – Western interconnect – Ercot • With this arrangement we must work together to ensure coordination of outages. • Electrons do not know or respect man made jurisdictional boundaries. • Two things that determine the level of independence we have with regard to outage scheduling is electrical distance and control ability. Outage Coordination With Others • Neighboring entities coordinate outages with each other, TOP-TOP, RC to RC, BA-BA as determined by studies and facility lists for notification • Various entities are required to coordinate outages within an RC area, TOP-GOP, TOP-RC, GOP-RC, TOP & GOP-BA • Neighboring RC’s coordinate outages with each other Outage Coordination With Others • BA’s often exchange power between areas and have to know what facilities will be available or limited to address transactions. • GOP’s must coordinate unit availability with the BA as the BA has the responsibility to match generation to load. • TOP’s have to coordinate with everyone as their lines connect everything together and impacts all aspects of operation. Outage Coordination With Others • Many Regional control areas maintain lists of required outage notifications within its region to be made to adjacent RC’s. • Likewise many RC’s maintain lists of required outage notifications between TOP’s in their areas. • Much of this is generated out of necessity and good utility practice, however, NERC standards do spell out requirements for such notifications and coordination. • The southwest blackout in 2011, that shut down San Diego was partly attributed to neighbors not communicating outage information to each other. Outage Contingency Planning • On occasion outages have to be permitted that ordinarily would not be allowed due to reliability concerns. • This can happen because of a pressing issue that develops during a time of year when loading does not support scheduling an outage within design capabilities. • This can result in conditions being such that the next contingency will result in capacity shortages or overloads that cannot be cleared without emergency actions. • Under these conditions a contingency plan must be in place so that operators know what pre-determined actions will be implemented. Outage Contingency Planning • Contingency plans are important as they coordinate actions to be taken among various working groups who have a role to play if things go wrong. • Contingency plans may call for or rely on the following actions; – – – – – – – – Voltage reduction, “brown outs” Load shedding, “black outs” Use of temporary emergency ratings Use of DSM, demand side management Shifting of load via emergency switching Recall of facilities Notification requirement! Alternate cooling options Outage Contingency Planning • Voltage reduction and load shedding are generally the last resort efforts to protect facilities from damage and salvage most of the customer loads. • They directly impact service to the customer, but it is generally better to experience a temporary short outage to service if it means preventing damage and a longer term impact. • Most entities can remotely activate these systems in very short time frames, seconds to minutes, to enact the desired action. Outage Contingency Planning • Very often an improved emergency ratings can be provided by Engineering Departments to allow emergency operation at higher loadings for a short period of time. • These ratings may assume some loss of life and do depend on the pre-load of the feeders. • The temporary emergency ratings provide opportunity and time for restoration of the contingency or the facility causing the issue. Outage Contingency Planning • Demand side management is the use of customer efforts to reduce loading. • Most companies offer customers the opportunity to participate in DSM programs with various prenotification requirements and compensation incentives. • DSM usually has to be requested in advance and can be used when it is known that loads will exceed acceptable levels to withstand certain contingencies. Outage Contingency Planning • Emergency switching options are sometimes available on the distribution system, where load can be transferred to neighboring stations or feeders under emergency conditions. • Alternate cooling methods may involve something as simple as water sprays on transformer radiators or modifications to normal oil circulation facilities. Outage Contingency Planning • Notifications – Notifications may seem like a trivial requirement, but it is far from that. In this day of instant communication people and agencies want to know things immediately • Many organizations have a real need to know as they will be impacted by possible outages to electric service. – – – – Police OEM Hospitals Critical customer facilities • Be sure to get the notifications right, as often you will get more complaints about the notifications than you will about the outage itself!
