ELECTRIC POWER SYSTEM RELIABILITY AND ITS UNCERTAINTIES
By Jack Casazza
Power Engineers Supporting Truth
December, 2004
A basic question that has been raised is whether one can determine if an electric power system is reliable or not. This infers there is a sharp line of demarcation that defines reliability vs. unreliability. This approach fails to recognize that the reliability or the risks of system blackouts are a continuous spectrum involving many uncertainties. Standards and analyses can only determine which system state is more or less reliable and which possible contingencies involve the greatest risks.
The reliability of a power system depends on its overall design and how it is operated. It depends not only on the reliability of each piece of equipment but also how the various lines, transformers, circuit breaker, reactive sources, relays, and communication equipment are interconnected to form the transmission grid. It depends on many operating decisions such as safe loading limits, the contingencies for which provisions should be made in advance, and when and how often to schedule equipment outages for maintenance. From time to time operators are also required to decide changes in generation dispatch to reduce power transfers over critical paths safe loading limits, sometimes called "congestion".
A deterministic approach is usually used in developing system plans and operating procedures. Reliability risks are minimized by conforming to established reliability standards that specify the loading or voltage conditions that can not be safely exceeded under various contingencies. These reliability standards have been developed based on past experience and probability analyses. The reliability analyses involved in applying these standards require the estimation of many factors in the studies that are made both off-line in planning and on-line in operations. However uncertainties still exist.
The four major uncertainties that determine the reliability of an electric power system are:
a)
The range of possible loads to be supplied
b)
Range of
possible generator locations, the schedule of generator outputs, generator
maintenance schedules and generator retirements; these depend on many factors
including fuel costs, generator availability and efficiency, and power and
energy purchases and sales.
c)
The possible contingencies
which may occur in generation or in the transmission system, including in fuel
supplies
d)
The probable
capacities of the transmission and delivery system for the above possibilities
Probability distributions can be determined to represent each of these four uncertainties. Through their analysis the relative reliability and potential plans for the future of the transmission system can be determined, and alternate operating arrangements can be evaluated. Sufficient evaluations are not being made recognizing present restructured electric power industry. While some procedures exist for evaluating the probabilities for uncertainties (a), (b), and (c), above, little has been done for uncertainty (d), the capacity of the transmission system.
PROBABLE CAPACITY OF THE TRANSMISSION SYSTEM
The capacity of transmission can be limited by the capacity of the line conductors or any of the devices connected to it including transformers, wave traps, current transformers, etc. The biggest uncertainty, however, are those concerned with the line conductors.
The ratings of transmission conductors are generally determined in advance based on certain estimates including the ambient temperature, wind speed, solar heating, and maximum allowable conductor temperatures. The allowable temperatures for overhead lines depend on the "sag" that will result and the annealing effect on the conductor. The amount of "sag" in the conductors of the line depends on the conductor temperature since the conductor expands as it heats up. The conductor annealing effect depends on the temperature and the periods of time to which the conductor has been exposed to it.
Since the line conductors are metallic, they have some heat storage capability and require a brief period of time to change temperature. Emergency line ratings are therefore assigned for various time periods, e.g., 10 minutes, 30 minutes etc. An important uncertainty is the loading and temperature on the line immediately prior to the condition for which the line capability is being determined. IF the line temperature was high it will reach unsafe temperatures very quickly, if it was low it will take longer to reach unsafe temperatures.
All of these factors lead to uncertainties about the capability of the transmission system and need further research. Newer technologies are being developed that may improve the ability to determine the line capability recognizing some of these variables that need further testing.
PROCEDURES FOR ADJUSTING GENERATION DISPATCH
Procedures that decide on generator adjustments solely based on economic optimization without considering the relative reliability risks are not sufficient, particularly if the economic optimization is based on quoted prices rather than incremental or marginal costs. In the short run, prices can be adjusted to manipulate the market, and in other cases may be subsidized.
PLANNING VS. OOPERATING RELIABILTIY
Planning a power system and operating it have two different restraints. The planner is free to add facilities, rearrange existing facilities or even retire existing facilities. The planner must also anticipate the requirements of the future so as to provide a system that meets initial needs but also recognizes long range requirement. To do this many believe a National Power Survey is needed to provide a framework indicating long range requirements.
The planner's job also is to provide for the operator with a system which can be operated reliably and economically. The planner must decide many key questions with regard to the transmission system, fully recognizing uncertainties (a), (b), (c) and (d) above and the actual responsibilities and capabilities of the operator.
On the other hand, the operator must make many decisions with the restraint he must use the system as it exists. These decisions must be based on information from the extensive and exceedingly complex group of participants in the overall process of providing electric power. They involve such decisions as scheduling generation, scheduling maintenance, controlling voltages, monitoring transmission loadings and handling contingencies and emergencies. These decisions must be made for a system which is continually changing for which conditions even a few minutes ahead can not always be predicted. The operator must make decisions considering reliability and cost. While cost minimization is a requirement, reliability optimization of reliability is also a top priority.
THE FOUR NETWORKS
Electric power systems are made up of a physical network which is inter-linked with three other networks that also effect reliability:
·
The fuel
network
·
The money
network
·
The
communication network
In the "fuel network" the "generators" are the various sources of coal, oil, gas, nuclear fuel, hydro, the sun, wind, hydrogen, etc. A diagram for fuel network can be drawn showing the delivery paths and mechanisms, delivery (or transformation) points, the energy consumed (losses) in the various paths, etc. Such netwks have been developed in the past but have received little attention in the past 20 years.
The availability of various fuel resources should not be ignored in reliability evaluations. For coal and oil the amount of fuel in storage at the plant site determines risks for various potential causes of interruption in fuel supply that include very cold weather and freezing, strikes, oil supply cut-offs, and transportation disruptions. The gas supply arrangements are critical with a single pipeline often supplying a number of generators. The loss of a single gas pipeline can result in the interruption of fuel to a number of generators. Important questions are the power system conditions immediately after the loss of a pipeline and the effect of the lack of fuel for an extended period. The simultaneous loss of a number of generators could trigger a major power disruption and blackout. The outage of many generators for an extended period could cause substantial rationing of the available sources. Adequate recognition of this possibility is needed and plans for handling such a contingency prepared.
The "communication network" consists of everything from the internet to the telephone to mail. It is used to control the electric power system to communicate with customers. There are a vast number of interconnection points between the communication network and the electric power system and its customers.
RELATIONSHIP BETWEEN THE NETWORKS
These four networks have many transformation points connecting them. For example the money network is connected to the electric power, fuel and communication networks. By diagramming each of the networks and determining the transformation points, it is possible to analyze the effect of the total system, i.e., the four networks, on the reliability and cost of electric power.
Such an analysis would be a part of a National Power Survey.