Transmission System Impacts of Drawing Down John Day Dam
As the Pacific Northwest explores lowering reservoir elevations as a potential option to improve salmon survival, it is imperative that all costs and impacts be assessed. Many scenarios have been discussed, ranging from complete bypass of the four lower Snake River and John Day dams year round to a partial drawdown implemented only during the migration season (April to August). The potential impacts of such actions are far reaching, affecting irrigation, navigation, industrial river users, recreation, flood control, cultural resources, and of course, power production and the transmission system. Many parties in the Northwest are in the process of analyzing the biological benefits and societal costs of various drawdown scenarios. This paper addresses one such scenario in which the John Day Dam is bypassed or breached, permanently returning the river to a more natural state. This option eliminates all power generation at the John Day Dam.
What is not widely appreciated is the role that the generation at John Day plays in maintaining the transfer capability and reliability of the transmission system. Previous cost estimates for similar drawdown scenarios may be understated due to the omission of costs for actions that would have to be taken to compensate for transmission impacts due to loss of generation. This paper focuses only on the costs associated with power generation and, in particular, the impacts to the transmission system.
Total power system cost falls into three categories: 1) energy losses, 2) capacity losses, and 3) transmission impacts. Assessing the cost of energy loss is perhaps the most intuitive. On average, John Day generates about 10.5 million megawatt-hours of energy per year (about 12 percent of Bonneville's average annual energy production). The value of that energy depends on the price, which varies by month and by time of day. Current estimates indicate the cost to be between $100 and $200 million per year.
The cost of capacity loss is a little more complicated to evaluate. The ability of John Day's generators to swing with fluctuations in demand saves the region money and contributes to the stability of the Northwest's power system. John Day generators provide up to 2,500 megawatts of peaking capacity (about 11 percent of Bonneville's total generating capacity). The value of that capacity is currently estimated to be in the range of $25 to $50 million per year.
A better estimate of capacity loss is much more difficult to assess. Hydroelectric projects contribute greatly to system reliability through the Automatic Generation Control (AGC) system that adjusts the generation, second by second, to match changes in demand. The dams also fulfill part of the Western Systems Coordinating Council (WSCC) reserve requirements and provide backup generation in the event of an unexpected outage. In addition, they provide extra energy during extreme cold weather periods and help maintain transmission stability during system disturbances. The U.S. Army Corps of Engineers and other federal agencies are examining capacity losses and their cost in more detail in a study to be completed in 1999.
The impact to the transmission system is even more complicated to evaluate. Due to John Day's proximity to the California-Oregon Intertie (COI), a loss of generation at John Day would affect both exports and imports. In either case, energy would have to be transmitted over greater distances. The further energy is transferred, the harder it is to maintain constant voltage on the transmission system and the higher the losses. Even though sufficient generation may be available in the West to make up for the loss at John Day, actions would have to be taken to upgrade the transfer capability of transmission paths in order to maintain the same level of reliability to Northwest customers. While at this time no estimate is available, it would be safe to say that the added cost to offset transmission impacts is significant. (See Appendix A for background information on electrical power systems.)
The Issue at John Day Dam
Removing the generation at John Day Dam without taking any compensating actions means that replacement energy would have to come from further away. But, delivering energy over greater distances requires greater reactive support and could lead to voltage instability. So, without additional reactive support, it would not be possible to replace all of the John Day generation from existing resources.
Because of John Day's proximity to the California-Oregon Intertie, transactions with California utilities would be affected. The COI's transfer capability would be significantly reduced, thereby also significantly reducing the Northwest's ability to import and export energy. So, even if out-of-region suppliers could make up all of the lost generation from John Day, it is not likely that all of it could be delivered to demand centers in the Northwest. The same problem exists for exports of Northwest surplus energy to California markets.
The loss of John Day generation would not only put additional pressure on existing transmission bottlenecks in the Northwest but also in California. It may be difficult to route additional energy through California subsystems to the interties. Likewise, energy generated north of the John Day area may have trouble getting to the intertie for export to the Southwest.
Based on a recent WSCC operating study , the approved north-to-south operating transfer limit for both the COI and the Pacific Direct Current Intertie (PDCI) is 6,900 megawatts for both winter and summer. The export transfer capability is limited by the loading on six 500 kilovolt transmission lines that pass through the North of John Day (NJD) cutplane illustrated in Figure 1. Studies show that high north-to-south flows across these six transmission lines stress the Northwest transmission system. So, depending on the loading of the NJD cutplane, additional generation from the north may not be able to make up for the loss at John Day.
Figure 1. Major Transmission Lines in the Northwest
The graph in Figure 2 illustrates how the maximum combined COI and PDCI north-to-south transfer capability is reduced due to loading on the NJD cutplane. The vertical axis represents the total combined transfer capability of the COI/PDCI in megawatts. The horizontal axis represents the total loading on the NJD cutplane, also in megawatts. The normal operating range is below and to the left of the curve in Figure 2. Up to a loading of about 7,300 megawatts (as measured on the horizontal axis), the transfer capability stays constant at the recommended limit of 6,900 megawatts (point 1 on the graph). The transfer capability falls to 5,900 megawatts when the NJD cutplane is loaded to 7,900 megawatts (point 2 on the graph).
Figure 2. North to South Summer Intertie Limit
Here is an example of how to use this chart. Let's assume that the intertie is fully loaded at 6,900 megawatts and that John Day is generating power at a rate of 1,200 megawatts . Let's also assume that the NJD cutplane is loaded to 6,700 megawatts. What happens if John Day generation is not available? If replacement energy were to come from generators north of the NJD cutplane, the loading on those lines would increase to 7,900 megawatts. Unfortunately, at that level, the combined transfer capability of the COI/PDCU interties is only 5,900 megawatts (point 2 in Figure 2), in other words, attempting to replace all of the John Day exports with generation from the north is not possible.
So, in our example above, how much can we transfer from the north? A point on the graph in Figure 2 has to be found where the increase in NJD cutplane loading equals the allowable increase in transfer capability. For this example, that value is about 800 megawatts. The loading on the NJD cutplane would increase from 6,700 to 7,500 megawatts. At that level, the transfer capability of the intertie is about 6,500 megawatts (point 3 on the graph) or 800 megawatts higher than 5,700 megawatts being transferred without John Day generation. Thus, even if generation in the Northwest is available for export, conditions on the transmission system may prevent all of it from getting to the COI/PDCI interties.
Before the region can seriously consider bypassing the John Day Dam, a much more comprehensive analysis of potential transmission impacts would have to be done. The mitigation measures employed to relieve the energy, capacity and transmission constraints due to the removal of John Day generation are interrelated. The discussion below represents a very preliminary and superficial approach to this problem. Council staff would be hard pressed to provide a definitive solution at this point in time. However, to stimulate regional discussion, a guess as to the set of actions required to compensate for the loss of John Day generation is provided below. This guess is vague by design and is solely based on discussions with Bonneville engineers who did not have the benefit of running detailed transmission studies.
Do nothing alternative
The first question might be "Why do anything?" What happens if no actions (i.e. no new generation or transmission) are taken to compensate for the loss of John Day generation? The Northwest would find itself more likely to be short on resources during high demand hours. Also, the same level of transmission capability would not available, meaning that the same level of demand could not be served with the same level of reliability. During certain conditions this option would leave customers without service. Thus, doing nothing is not an option.
Replacement generation alternative
Jumping to the other extreme, what if all of John Day's generating capacity were replaced? Building 2,500 megawatts of combined cycle combustion turbine capacity at the John Day site would be very costly. Ignoring the problem of fuel supply, the capital cost would be on the order of $1.25 billion, which when amortized and added to operating costs yields an annual cost of about $280 million per year. On average, the turbines would run about 50 percent of the time to produce the same annual energy as the John Day generators.
Combustion turbines, however, are lighter machines with less rotational inertia and they do not respond to system disturbances the same way that heavier hydroelectric machines do. This means that additional measures may have to be taken. Thus, a "one-for-one" replacement of generating capacity is not a viable option, by itself.
Transmission only approach
Let's take another approach. What if no new generation were planned? What if the region simply relied on the existing West Coast surplus to compensate for the loss of John Day? In this case, the region would depend more heavily on out-of-region suppliers. This means that some actions would have to be taken to maintain or increase the transfer capability of existing interties. In addition, some new transmission lines may have to be built outside of the Northwest to ensure that surplus energy could be delivered to the interties. Also, devices such as capacitors or other reactive sources would have to be added to the network at strategic locations to maintain system stability. While this alternative might work, it may not be the most cost effective. In some cases, it may be cheaper to simply build new replacement generation closer to the demand centers than to import the energy from far away.
Best guess alternative
A more likely solution is some combination of new generation, transmission and compensating reactive transmission devices. The existing generators at John Day, even though they would be "out of water", could be used as synchronous condensers to provide some reactive power support. In addition, some new transmission lines could be built, both in California and in the Northwest to remove bottlenecks and improve the transfer capability to demand centers. Some compensating electrical devices such as series or shunt capacitors, could be added at strategic places to improve the efficiency of the transmission network. And, finally some new generation could be built at John Day or closer to demand centers to reduce transmission losses and improve the efficiency of power delivery.
Bonneville is in the process of evaluating the transmission system impacts of losing generation at John Day Dam. As a part of the U.S. Army Corps of Engineer's Drawdown Feasibility Study, Bonneville is also evaluating the impacts of losing the generation at the four lower Snake River dams. This analysis is complex and will take a good deal of effort to complete. At this time no estimate of the impacts or the costs to the transmission system is available. It would be safe to say, however, that the impact and cost would be significant.
Electricity, Alternating Current and Power
To understand the impact that removal of John Day generation would have on the transmission system, it's necessary to review some of the physics of an electrical power system. Let's begin with the basics. In a conducting material, like a wire, negatively charged electrons are easily pulled away from their molecules by applying a positive charge at one end. Connecting a wire to a battery has the effect of creating a "current" of electrons that flows from the negative to the positive terminal. The electron flow will continue until a balance is reached and the net charge at each terminal is zero (thus the term "dead battery").
The flow of electrons is affected by resistance in the wire. As electrons move, they "bump" into other molecules in the wire. These collisions transfer some of the electron's energy to other molecules and cause the wire to heat up. The less the resistance, the greater the current. In fact, it is probably not a good idea to connect a wire directly to a battery because the ensuing current could melt the wire, depending on the voltage of the battery and the size of the wire. This is where the term "short-circuit" comes from.
Unlike our example above, where the electrical current flows in one direction, power systems operate with alternating current (AC). In these systems, the voltage at the terminals changes from positive to negative and back again many times in one second. In the United States, the voltage changes from positive to negative and back again 60 times per second (60 hertz). This means that the flow of current also changes direction 60 times per second. Fortunately, the delivery of energy does not depend on the direction of electrical current. As long as a current is passing through an electrical component, energy can be absorbed.
Components of an electrical power system An electrical power system is made up of many complicated components. For this paper it is only necessary to understand four basic types: 1) generator, 2) resistor, 3) inductor and 4) capacitor. These components are briefly defined below.
- Generator: A machine used to convert mechanical energy to electrical energy.
- Resistor: Any device that absorbs electrical energy and releases heat or light.
- Inductor: A device made up of coils of wire wound on a core of air, iron or other substance. These devices are typically found in transformers (used to change voltage levels) and in motors (used to convert electrical energy to mechanical energy). Transmission lines also behave as inductors. In AC systems, inductors produce a magnetic field whose strength varies with the change in current. The magnetic field stores and releases energy to the circuit and induces a voltage that opposes the flow of current. This "slows down" the electric current and makes delivery of energy less efficient.
- Capacitor: A device made up of two conducting plates separated by a non-conducting material such as glass, paper, air or oil. A capacitor will not pass direct current, but does create an electric field between the plates that stores electrical energy. Similar to an inductor in an AC circuit, a capacitor will absorb and release energy to the circuit. However, unlike an inductor, a capacitor induces a voltage in the direction of the current. This induced voltage "pulls" the current forward and counteracts the "slowing down" effect of an inductor.
In an AC circuit, resistance and inductance are more generally referred to as impedance. Every electrical component has its own characteristic impedance. The impedance of a component indicates how much effort (voltage) is required to produce a response (current) in that component. In an electric circuit, impedance is defined as the voltage divided by the current. The greater the impedance, the harder it is to "push" current through the circuit.
How a power system works
Power is defined as the rate of doing work or the rate of delivering energy. Electrical power is calculated as the product of voltage and current and is measured in joules per second or watts. A joule is a quantity of energy or work. In an electric circuit, a battery or generator produces a voltage difference that forces electrons to move in the wire. This creates a current that passes through the system's components where energy is absorbed. As mentioned earlier, in an AC circuit the voltage at the generator fluctuates from its greatest positive value to its greatest negative value and back 60 times per second. If we were to graph the voltage as a function of time, we would see a "wave" similar to the one in Figure 1. Because voltage is the force that causes current to flow, the electric current in an AC system will also fluctuate in a similar wave-like pattern.
Figure 1. Voltage as a Function of Time
To create the most efficient power system, the current and voltage "waves" must be closely synchronized (or in phase) so that their product yields the highest value. (Remember power is the product of current and voltage). If the waves are not synchronized, as happens when inductors are added to the system, the efficiency of the system is reduced.
Inductors tend to shift the phase of the current so that it lags behind the voltage. When electricity flows through a wire, it produces a magnetic field. It takes some effort to create that field. In an AC circuit, the current is constantly changing directions, which means that the magnetic field is also constantly changing. In an inductor, because the wire is coiled up, the magnetic field from each coil adds together to create a very strong magnetic field. The current has to do a lot of work to keep changing the relatively strong magnetic field in the coil. In a circuit with a lot of inductance, energy used to drive the current in one part of the cycle is stored in the magnetic field of the inductor and is given back during a later part of the cycle. The inductor induces a voltage that opposes the flow of current and causes the current to lag behind. The amount of lag between the voltage and the current is referred to as the phase angle and is used to calculate the power factor, a measure of the efficiency. Figure 2 illustrates how the voltage and current waves can be out of phase.
Figure 2. Two Waves that are Out of Phase
Perhaps using an analogy will help. Suppose we want to pull a heavy box with a rope. We attach the rope to the box and pull in quick short bursts or cycles. Every time we pull, the box moves a bit and because the rope is tight, the box moves synchronously with each pull. If we add a spring to the middle of the rope, when the energy from our pull gets to the spring, the spring stretches, absorbing some of our energy. After a short time, when the spring contracts, it pulls the second half of the rope and consequently pulls the box. The box, however, no longer moves in phase with our pulls. The spring periodically absorbs and releases energy to the rope and causes the motion of the box to be out of phase with our pulls. This would not be a very efficient method of pulling a box. In a similar way, an inductor absorbs and releases energy to an electrical system and causes the current to be out of phase with the voltage. This makes delivery of energy less efficient.
Every power system in the world is primarily an inductive system; that is, most of the components are inductors. The more inductive the system, the more energy is "trapped" in the magnetic fields of the inductors and the less efficient is the delivery of energy. While it is not meaningful or practical to remove the inductors (transmission lines, motors, transformers, etc.), other actions can be taken to make the system more efficient. Distributing generating resources to keep transmission lines as short as possible helps, as does adding counteracting components (such as capacitors) in strategic locations.
Making an efficient power system
Knowing the relationship between voltage and current for each component of an electrical system allows us to derive a mathematical equation for power for the entire circuit. In an AC system, the solution is comprised of two parts. The first part shows the contribution due to the resistance in the circuit and the second part shows the contribution due to the inductors and capacitors. Generally, the first part reflects real delivered power, which is measured in watts. The second part of the equation is associated with the energy that is "trapped" in the magnetic and electric fields in the system. That part of the solution is referred to as the reactive power component, which is measured in volt amperes reactive (VAR).
Inductors in AC circuits absorb and release energy so that the current is no longer synchronized with the voltage, similar to our example with the rope and spring. Capacitors also absorb and release energy to the circuit, but do so in a way that counteracts the effect of inductors. By sizing the components properly, a circuit can be designed in which the inductors and capacitors are more balanced and the current stays synchronized with the voltage. In such a circuit, the phase angle is zero, and the power factor is one. Energy is delivered without efficiency loss. Unfortunately, for a real power system, the phase angle can never be reduced to zero throughout the entire system. It would not be cost effective to add the counteracting components throughout the system. It is, however, important to add such components at strategic places to allow energy to flow to desired destinations as efficiently as possible.
The transmission system is made up of a number of sub-systems each with its own characteristic voltage level. Bulk power is generally delivered via high-voltage transmission lines (500 kilovolts or more) to minimize thermal losses. The power is then transformed to lower voltage lines (230 kilovolts down to 34.5 kilovolts) that deliver power to smaller distribution systems. Finally, at the consumer level, power is delivered at the 110 volts that we are accustomed to. For each sub-system of transmission lines, voltage is kept as close to constant as possible. This ensures that the maximum amount of energy is delivered with the greatest efficiency.
An electrical power system must balance both real and reactive power flows to maintain stability. When reactive power flows get out of balance, such as during a loss of generation, voltage will drop, causing higher current flows and further instability. Temporary measures can be taken to stabilize the system until generation is back on line. A permanent loss of generation, however, means that the same level of energy transfer could no longer be provided unless permanent compensating actions are taken.