by Planning Engineer
Some of the denizens have requested an introduction to transmission planning and a discussion of how the transmission system is impacted by renewable resources.
This post complements a previous post which addressed renewable resources and generation planning. The considerations here are not of major importance when renewable resources only make up a miniscule portion of the generation mix but they become significant as renewable generation begins to make up larger portions of the resource mix.
On the grid power does not flow downhill, take the shortest path or move from areas of high to low pressure. The grid cannot be well understood as a highway system or a set of pipelines. Energy simultaneously takes every possible interconnected path from source to destination. For the most part in normal operating ranges the flows between a generation addition and a new load are unaffected by the flows that are already on the lines. Energy flows on every possible interconnected path based on the inverse ratio of that paths impedance (resistance).
As can be seen below, the US has three major grids (two shared with Canada). The grids have to be built so that the flows serve planned loads from planned generation without overstressing any part of the system. This is true not just for the major lines shown below but for all the lower voltage lines and interconnected portions of sub-transmission systems as well. The graphic below does not include voltages below 230 kV, however 115 kV and 161 kV lines make up a large part of these interconnected transmission networks.
If a plant is added in Nevada to serve a new load in southern California, some of the power will take the most direct routes, some will flow northward to Canada and then come back through Washington and Oregon and some will go west and south before circling back towards California. This flow of energy on the less direct interconnected paths is referred to as loop flow or inadvertent flow. Every possible interconnected path will be impacted somewhat by any changes in the locations of loads or generation. Loop flow can and frequently does impose burdens and stresses on the components they pass through. When the loop flows are small or non-consequential they can be ignored, but when they cause problems – they must be addressed through improvements or by imposing operational limits.
A little extra background on how power flows work. Altering the previous example, if the generation is now located in southern California and the load is in Arizona the results will be pretty much the opposite so that whatever power flows were increased by an additional X, will now be decreased by X and whatever increased by Y now decreases by Y. (Note – I have framed this discussion in terms of simultaneous changes of both generation and matching load so that the resulting flows can be described as delta changes. Alternatively you can conceptualize every generator as serving a portion of every load on the interconnected system.)
Bottom line is that any time you change generation source locations or add new generation sources, they will to some degree stress some parts of the system and unload others. When the “new” flows go in the same direction as the already existing flows, this can cause potential overloading and voltage problems. When the flows run against the existing flows they serve to net out and reduce the total flows, thus providing benefits. Every new generation resource can be seen as supporting some parts of the system and stressing other parts of the system. Similarly, the loss of any generation source can be seen as supporting some parts of the system and stressing other parts of the system. From a transmission planner’s perspective system additions and system retirements both require careful attention, as either can create problems (or provide relief) depending on how and where they stress or unburden the system.
Introduction to Stability
Modern power grids are complex machines that require a near instantaneous balancing of various electro-mechanical properties. In the US, traditional generators provide three phase voltage and current sinusoidal waveforms that alternate 60 times per second. Every rotating generator within each of the “Interconnections” shown in the map above must be in synchronism with every other generator within that same Interconnection. While the voltage or current wave forms can lag or lead each other by a little bit, they can’t get us much as a whole cycle (1/60th of a second) behind or ahead of any other generator without causing a major system problem. A major problem would involve serious events which would include generation tripping off line and possibly including a collapse of at least some portion of the grid. Generators in Miami, Ontario, Kansas and New Orleans remain in synchronism around the clock with each other and over the years they don’t deviate in the number of turns by as much as a single1/60th of a second cycle. The power input from generators coupled with load characteristics and disturbance conditions makes it possible (likely) for electromechanical forces to begin oscillating and grow to destructive levels if the system is not carefully designed and operated.
Understanding this phenomenon involves challenging math, engineering and computer modelling that are hard to summarize. If you want to get more into the details you might check out these Lectures (part 2 and part 3) which unfortunately are about as good as any discussion on the topic that I’ve sat through.
The grid is built upon and supported by heavy rotating machinery. Synchronous spinning generators combine with power lines and loads to make up complex electro-mechanical machine that must maintains stability. Stability refers to the ability of the system to stay in synchronism, balance loads and generation and maintain voltages following system disturbances. Intermittent generation (wind/PV solar) does not rotate in synchronism with the grid. As such they do not have performance characteristics that support the grid as well as synchronously rotating generators (hydro, coal, gas, nuclear plants) do. The system must be able to ride out power imbalances caused by faults and outages. Greater penetrations of non-synchronous generators (inverters used for PV Solar and Wind) tend to make the system, all else equal, less stable. Without expensive additional equipment and the wasting of some power output, inverter control delivers power based on the performance of the PV solar or wind resource, not the needs of the grid. Synchronous generators on the other hand can naturally respond to grid conditions and work to support stability. This report by a NERC Task Force provides more detail.
The Changing Grid
It takes many years to complete a major transmission system improvement even under the best of circumstances. To determine and prepare for future needs, transmission planners simulate the system operation years in advance using computer models with expected generation and load levels. In years past, transmission planners had greater certainty around expected generation resources and the likely stresses they would place on the grid. Dependably large baseload coal and nuclear plants would operate around the clock. Hydro units with storage capability would cluster their operation around peak demand hours. Based on economics other gas plants depending on their efficiencies would cycle in and out at differing load levels. Simulations could do a good job of matching generation to load level while modelling the anticipated stresses to the system. The grid was not intended to handle all possible generation scenarios but rather just “planned” operations and “likely” generation scenarios. Understanding generation patterns on peak and across load levels enabled planners to make cost effective improvements that supported the peak as well as year round grid operation. Planners and operators take the system models and expose them to a host of potential disturbances (called contingency outages). The system must be able to withstand the disturbances and remain stable while not have unacceptable overloads, load shedding or voltage problems. The system is built and operated to allow recovery from “credible” outages.
Generally the grids operate today to allow for the economic and reliable operation of all planned resources. Additionally the grids typically have sufficient robustness to allow for unplanned economic power exchanges on a non-guaranteed basis. Occasionally system loadings in combination with events such as generator or line outages create conditions where the system operators have to call for redispatch of the system or in very rare cases curtail load to keep the system flows within acceptable boundaries. This may be for loadflow or stability reasons. In system redispatch situations the operators can either call for generating units that serve to relieve the stress to increase their output, or force generating units that contributed to the stress to curtail generation. This may involve in turning units off and on, or just moving generating units between their maximum and minimum output levels. When redispatch fails to resolve the situation, as a last resort system operators will call for the shedding of system load.
Renewables impose new challenges. Wind and solar operate intermittently. Their availability for future system peaks is unknown as it is for off-peak hours as well. As they will undoubtedly be both off and on during most hours when the system can be stressed, the system must be built for both their presence and absence. Other plants have to provide them backup service and are cycled on and off. This shifting of generation resources and backups, at high renewable penetration levels will lead to considerable uncertainty around potential grid flows and operating points for stability. The potential set of “likely” generation scenarios will increase exponentially as the penetration level of intermittent resources increases and at the same time the operators have less control over the generating resources.
Real World Challenges
The retiring of large coal plants provides planners with challenges. The system has been built to support these units and at the same time these units have supported the system. Losing major units that the system has been built around should be expected to provide significantly more dis-benefits than benefits. Their retirement will leave some areas with excess transmission capability and introduce stresses in others. Preparing for the retirement of a large coal plant contains the same challenges as preparing for the addition of a new large generating resource. Limiting the operation of a coal plant (as opposed to shutting it down) may impose problems, but at least in such cases they are available for redispatch during peak demand or emergency situations. When regulations require coal plants to be shut down completely it is important that the timeline provides enough advance notice to allow for needed grid improvements.
Preparing for the additions of large intermittent resources requires planning as well. As with all generation sources these additions will at times support the system in some locations and stress it in others. Since the generation is intermittent in nature, planners cannot count on any support it might provide, but must prepare for stresses the resources introduce. Their potential to stress the system requires action, but their ability to support cannot be counted as dependable. Intermittency introduces extra complexity into the study process and results in extra costs for needed improvements.
The power grid does not always operate as planned. Extreme weather, unanticipated outages and a host of other factors can result in the system operating somewhere outside of planned conditions. Generally the system is robust enough to handle most departures without problems. For more severe departures from planned conditions the re-dispatch of generating resources is a major tool for the system operators. Changing the location of contributing generating sources can relieve stresses and strengthen the system. As the amount of intermittent generation increases, this tool will become blunted from a lack of qualifying capable dispatchable resources. The justifying economics require that intermittent resources run all out pretty much whenever they are able. There are suggestions that intermittents could better mimic conventional generation, but it would incur significant costs. Curtailing existing intermittent resources for reliability reasons could be helpful at times, but it adversely impacts the economic performance of such resources and is politically challenging at this time in most places. Building a surplus of renewable resources to sit idle waiting to back each other up and respond as needed is economically implausible at this time.
Greater penetration of renewable resources will limit the options available to operators while at the same time increasing uncertainty around expected generation patterns. To accommodate such uncertainty the choices are to: 1) increase grid costs and infrastructure, 2) limit the operational flexibility of the grid , 3) increase generation costs through backup generation resources or 4) live with increased risks and degraded reliability. Likely all four are and will continue to occur to some extent as the penetration of intermittent resources increases.
As noted at the start, when intermittent resources only make up a small percentage of total system generation, the adverse impacts are masked by the margin and robustness built into the system. The small additional costs they may incur are fairly easily shared by all users of the grid. As penetration levels rise and renewables replace non-intermittent conventional units, they will have major impacts upon grid costs and reliability. These costs have not been accounted for adequately in many studies estimating the costs of renewables.
JC note: As with all guest posts, please keep your comments relevant and civil.