Transmission System Impacts of Drawing Down John Day Dam
February 25, 1998 | document 98-3
Executive Summary
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.
Potential Solutions
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.
Appendix
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.
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