The document compares two auction formats - descending clock and sealed-bid - for Colombia's firm energy auctions. The descending clock format currently used asks generators in rounds if they will supply at lower prices until supply is less than demand, paying all winners the clearing price. The document argues the sealed-bid format may be preferable, as it is simpler and prevents strategic supply reduction in later rounds of descending clock auctions. Overall, the sealed-bid approach better balances efficiency, transparency, simplicity and fairness for Colombia's auction objectives and market features.

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Colombia Firm Energy Auction: Descending Clock or Sealed-Bid?
Peter Cramton1
19 July 2015
Abstract
Colombia conducts periodic firm energy auctions to assure reliable electricity supply and
coordinate long-term investment in resources. To date, the auctions have followed a descending
clock auction format. Generators are asked in a series of rounds whether they are willing to
supply at progressively lower prices. The auction continues as long as supply exceeds demand.
All winning generators are paid the resulting clearing price—the reliability charge—for taking on
the firm energy obligation, which is structured as a reliability option. This paper compares this
descending clock auction with a closely related sealed-bid format. I conclude, after weighing the
pros and cons of each format, that the sealed-bid method is preferable in the Colombia setting.
Other issues such as lumpy resources and auction frequency are also discussed.
Introduction
To assure reliable electricity supply and coordinate long-term investment, Colombia periodically conducts
a firm energy auction. The auction assigns the firm energy obligation among generators three or more
years in advance of the commitment period. The firm energy obligation is a reliability option consisting of
two components: (1) the physical capability to supply energy during periods of scarcity, and (2) the
financial obligation to deliver energy consistent with the generator’s load share at the scarcity price
whenever the spot price exceeds the scarcity price. The total firm energy obligation is set by CREG to equal
the anticipated energy need. The firm energy auction establishes the reliability charge that is paid to
resources that take on the firm energy obligation by submitting winning bids.
CREG established the firm energy auction in 2006. The first two auctions were held in 2008 and 2012.
Consistent with the auction rules, the auctions to date have used the descending clock auction format,
which is a simple price discovery mechanism that asks generators in a series of rounds whether they are
willing to supply at progressively lower prices until the point where supply is less than demand. Then the
auctioneer identifies the set of winning generators to maximize the gains from trade as reflected by the
administrative demand curve and the as-bid supply curve. Each winner is paid the reliability charge—the
clearing price in the auction that best balances supply and demand—for the period of commitment.
In this paper, I examine whether the descending clock auction is the best auction format for the Colombian
market or whether a sealed-bid auction would be preferable. My analysis is informed by Colombia’s
1
Peter Cramton is Professor of Economics at the University of Maryland; since 1983, he has conducted widely-cited
research on market design; he has applied that research to design auction-based markets of radio spectrum,
electricity, financial securities, and other products. I am grateful to CREG for funding this research and to CREG
Commissioners and staff, especially Javier Diaz Velasco and Camilo Torres Trujillo, for helpful comments. The views
expressed are my own.

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experience to date with the descending clock auction (Harbord and Pagnozzi 2008, 2012), my expectations
about the Colombia setting going forward, and the experience in similar international markets.
I begin with a brief discussion of objectives. Then I review the key features of the market. Next I compare
the two auction formats: the descending clock auction and the sealed-bid auction. I then make my
recommendations. I conclude with a brief discussion of other issues that CREG should consider in its
efforts to improve the firm energy market and encourage efficient energy investment in Colombia.
Objectives and market principles
In any auction design application, it is best to start with the objectives of the market. The auction designer
evaluates alternative auction designs with respect to these objectives, choosing the rules of market
interaction to best meet the objectives given the details of the setting. The most common objectives in
government auctions are efficiency, transparency, simplicity, and fairness. I describe each.
Efficiency
Efficiency is the most basic objective for economists. An auction design is efficient if it yields outcomes
that maximize social welfare. In a trading environment, this means that all gains from trade are realized.
In the context of the firm energy auction, it means the auction identifies the least-cost resources that
satisfy the reliability requirement.
Simplicity
The auction should be as simple as possible, but not simpler. The setting of the auction is complex; hence,
it should not be surprising that some level of complexity is needed in an efficient design. Nonetheless, it
is important that the market be made as simple as possible to solve the economic problem of the setting.
Simpler designs tend to promote efficiency by letting the bidder express preferences more simply and
effectively.
Transparency
A first requirement of transparent auctions is clear and unambiguous rules that map bids into outcomes.
With a transparent design bidders know why they won or lost and understand why their payments are
what they are. Bidders are able—at least after the event—to confirm that the auction rules were followed.
Fairness
Equal opportunity is a basic requirement of fairness. All potential participants have access to the market
rules and the rules do not inappropriately discriminate among parties. In the context of auctions, this
means that identical bids are treated the same way.
In comparing alternatives, an auction design that scores well with respect to all of these objectives is
desired. We will see that both design alternatives, the descending clock and the sealed-bid auctions,
perform well with respect to these four objectives.

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Key features of the firm energy market
With these objectives in mind, in 2006, CREG adopted a firm energy market to:
• Induce just enough investment to maintain adequate resources;
• Induce an efficient mix of resources;
• Reduce market risk;
• Avoid market power in the firm energy market;
• Reduce market power in the energy market; and
• Pay no more than necessary.
Three key elements of the firm energy market are forward procurement, a long-term commitment to new
resources, and a sloped demand curve. I discuss each in turn. For more details of the Colombia firm energy
market see Cramton and Stoft (2007); for a broader discussion of reliability markets see Cramton and Stoft
(2008) and Cramton and Ockenfels (2012).
Forward procurement—conducting the auction several years in advance of the commitment period—
allows new projects to compete in advance of entry. This makes the market contestable and allows the
cost of new entry to be properly reflected in the clearing price. New projects are bid before major
investment costs are sunk. In this way, forward procurement improves competition in the market and the
pricing process. Forward procurement also coordinates entry. This results in less uncertainty in achieving
the clearing target. The tendency for a pronounced boom/bust cycle is reduced. Finally forward
procurement can offer a long-term commitment for new resources. This reduces investor risk and sends
a better price signal for new investment.
The Colombia market lets generators select a commitment period of up to 20 years at the time of
qualification. This long-term commitment lets new resources lock-in the firm energy price (capacity
charge), thereby reducing risk and encouraging investment. The price is indexed to adjust for inflation. In
contrast existing resources get a one-year commitment. Existing resources do not need a long-term
commitment, since their costs are already sunk. Indeed, the short commitment period reduces risk, since
it allows existing resources more draws from the price distribution. Importantly, both existing and new
resources receive the same prices in the long run. There is no discrimination against existing resources.

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Figure 1: Demand curve
The quantity procured is determine from the demand curve shown in Figure 1. The demand curve depends
on two administratively set parameters: (1) the target quantity—the amount of firm energy needed to
satisfy the reliability criterion, and (2) the cost of new entry (CONE), an engineering-economic estimate of
the cost of an efficient thermal resource. Importantly, the clearing price is determined from the bids of
the generators. CONE plays only a supporting role in price determination. Quantity on the other hand is
primarily determined by the administratively set target. This is as it should be as the target is the key
planning parameter. To address the unilateral exercise of market power, discussed in the next section, a
modest amount of uncertainty (±1.5 percent) is added to the target by the auctioneer at the time of the
descending clock auction. Thus, bidders know the planned target, but not the actual target with the added
uncertainty. The demand curve is defined by a price ceiling of two times CONE for all quantities less than
96 percent of the target and a price floor of ½ CONE at quantities greater than 104 percent of the target.
The price ceiling provides protection to rate payers in the event of insufficient supply. The price floor
prevents the price from falling excessively in response to surplus. Between the floor and the ceiling, the
price is set by the offer of the marginal generator. The sloped demand curve reflects the marginal benefit
of supply around the target quantity. To mitigate market power of existing generators, existing supply can
only exit during the clock stage at prices below 80 percent of CONE.
Comparison between descending clock and sealed-bid formats
I now compare the two auction formats. Both are based on the same uniform-price auction methodology
in which all winners are paid the clearing price at which supply and demand balance. The difference is in
how the bids are collected.

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Figure 2: Auctioneer’s perspective of descending clock auction
First consider the descending clock format, shown in Figure 2. In this case, generators are asked in a
sequence of rounds which resources are willing to supply at prices between a start of round price and an
end of round price. The initial start of round price is the price ceiling. At lower prices, a generator may
decide to reduce supply and indicate the resource and price at which the supply reduction occurs—this is
an exit bid. After each round, the auctioneer forms the aggregate supply curve from the start of round
price to the lower end of round price. Exit bids are indicated by the horizontal steps in the supply curve.
Initially, at high prices, supply is apt to exceed demand, as indicated in the red portion of the supply curve.
As the price is reduced with each round, supply can fall. The auction ends when in the last round, the
auctioneer asks for the final interval of prices, down to the price floor. The clearing price is determined as
the highest bid accepted, PC, in the figure. Due to the lumpiness of supply, the quantity, QC, does not
perfectly balance supply and demand, but nearly so. More specifically, the set of winners is chosen to
maximize gains from trade, as described in the final section of this paper. The figure is drawn from the
auctioneer’s perspective in that only the auctioneer observes the actual demand curve and the full supply
curve.
One feature of the auction is that supply can only decrease or stay the same as price falls. A resource that
is “in” at a lower price must be willing to supply at a higher price. This natural requirement is enforced by
the auction rules as it limits gaming behavior and improves price discovery. It also guarantees that the
supply curve is weakly increasing as drawn in the figure.

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Figure 3: Bidder’s perspective of descending clock auction
Figure 3 shows the same descending clock auction from a bidder’s perspective. After each round, the
auctioneer reports the aggregate supply at the end of round price. These are the sequence of blue dots
starting with the highest at P1 at the end of the first round, then at P2, then at P3, and finally at P4. In the
fifth and final round of this example, the auctioneer asks for any exit bids between P4 and P5—the floor
price. At this point the full supply curve has been collected and the auctioneer reports the winning bids
and the clearing price. The uncertainty in demand is illustrated with the multiple lines in the sloped portion
of the supply curve. Actual demand can fall anywhere within this range. This uncertainty is introduced to
mitigate somewhat the exercise of market power, as described below.
Figure 4: Auctioneer’s perspective of sealed-bid auction
Figure 4 shows the same example, but conducted with the sealed-bid format. Rather than conduct a series
of rounds, the auction is conducted in a single round. The auctioneer simply asks each generator to name
exit bids for each resource for prices between the price ceiling and the price floor. As before, a resource
that is “in” for a lower price must be “in” for all higher prices. Notice that Figure 2 (descending clock) and
Figure 4 (sealed-bid) appear identical when drawn from the auctioneer’s perspective. Indeed, they are apt
to be nearly identical in practice. However, differences can arise because of the extra information—the

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supply at the end of round price—that is revealed by the auctioneer in the descending clock auction.
Indeed, the two formats only differ to the extent generators change exit bids in response to the end-of-
round supply information revealed after each clock round.
Figure 5: Unilateral exercise of market power in descending clock auction
Figure 5 shows one reason why the as-bid supply curves may differ between the two formats. The figure
depicts the following possibility. A generator, learning the end of round demand, recognizes that supply
is close to demand and the exit of the generator’s unit likely causes supply to fall below demand and set
the clearing price. As a result, the generator may decide it is better-off raising its exit bid to P’C rather than
PC. The unit is apt to be accepted at either price and the generator would prefer the higher price. This is a
classic example of the unilateral exercise of market power. The generator has adjusted its bid recognizing
its likely impact on price. The difference between the descending clock and the sealed-bid auctions is that
in the descending clock the bidders have some additional information on which to determine their bids.
In this case that information was used in a potentially harmful way to increase prices above competitive
levels and possibly distort the auction outcome. This behavior is not just a theoretical possibility. It
appears to have occurred in the first auction in Colombia (Harbord and Pagnozzi 2008, p. 11).
The extra information, however, may have positive effects (see Cramton 1998 and Ausubel and Cramton
2004). The beneficial source is from a reduction in uncertainty, which allows for improved decision
making. Information about supply can reduce common value uncertainty. For example, stronger bidding
may reveal a general optimism about future electricity prices and thereby lower the investor’s exit bid;
whereas, weaker bidding may reveal market pessimism and raise the investor’s exit bid. In both cases,
uncertainty about the future is reduced because the supply information conveys some valuable market
information. The reduction of uncertainty reduces the winner’s curse—a form of adverse selection.
Bidders can bid with greater confidence with less uncertainty about common value.
The second reason that more supply information may improve decision making is the improved outcome
discovery. The supply information helps the bidder form price expectations and the likelihood of winning.
For a generator with many units, the generator can then make more informed decisions in setting exit

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prices for the various units. This is especially the case when the units are complementary or the bidder
faces aggregate constraints such as budget or portfolio constraints.
Both of these factors are good reasons for the adjustment of exit bids. And these adjustments are
consistent with economic efficiency. However, neither of these reasons seem important in the Colombia
setting. First, unlike oil lease auctions where common value uncertainty is of first order importance, the
firm energy auction is one characterized by private values—projects have particular attributes that
warrant different exit bids. Second, there is only one product being procured and a single merit order.
Hence, even generators with multiple resources can manage risk and budget and portfolio constraints
nearly as well with the sealed-bid format.
There are three other factors to consider in comparing the clock and sealed-bid formats: privacy,
transparency, and simplicity.
Privacy. At least in theory clock auctions respect the privacy of infra-marginal bids—exit bids that are
better (lower) than the clearing price. Only the high (rejected) portions of the supply curve are revealed.
The auction ends when supply equals demand. Winning bidders reveal only, “I am willing to supply at the
clearing price.” In practice, however, due to the lumpiness of resources and to mitigate market power, in
the last round of bidding the auctioneer asks for all exit bids between the start of round price and the
price floor. Hence, there is no privacy difference between the descending clock and sealed-bid formats in
Colombia. The full range of exit bids is revealed to the auctioneer in both cases.
Transparency. Higher levels of transparency are achieved in auction designs with excellent outcome
discovery—both with respect to prices and prospects for winning. Outcome discovery is encouraged with
dynamic auctions, such as clock auctions, in which substantial information is provided to bidders to
understand prices and winning prospects during the auction. Still the auction designer must recognize
that the release of some information could potentially be used to foster collusion or improper
coordination among bidders. For this reason it is common to release anonymous information that is
relevant to understanding demand in a forward auction or supply in a reverse auction. Transparent
auctions have an information policy that reveals information that is most helpful in understanding
demand and supply. Such designs promote outcome discovery, which generally promotes auction
participation and competition.
However, in the case of Colombia’s firm energy auction there is little difference in transparency between
a clock and sealed-bid format. Regarding supply, only slightly more information is revealed: the end of
round supply at the end of each round. Regarding demand, the clock auction is less transparent, since
demand uncertainty is introduced in the demand curve to mitigate the exercise of market power. With
the sealed-bid format, it is unnecessary to introduce this additional uncertainty. The true demand curve
can be revealed. This has an efficiency gain, since then the market is cleared against the true demand,
rather than a curve that has been distorted to increase uncertainty and mitigate market power.
Simplicity. Simplicity is best measured in terms of the simplicity of participating in the auction. Clear rules
that make it straightforward to develop an effective bidding strategy get high marks for simplicity. Simpler
auction designs tend to avoid guesswork. For example, the descending clock format facilitates outcome

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discovery, both with respect to clearing prices and the prospects for winning. This is simpler for bidders,
especially when auctioning multiple interrelated products. These designs help bidders avoid substantial
guesswork and speculation in bidding strategy.
Simpler designs also limit risks to bidders. Again dynamic designs with good outcome discovery often let
the bidder better manage budget and portfolio constraints. Executing a particular business plan is often
more straightforward in such designs.
However, in the Colombia setting with just a single product, the outcome discovery is not especially useful.
In terms of simplicity the two designs are quite close. In two respects the sealed-bid design is simpler:
bidders can prepare bids on their own schedule and without the introduction of demand uncertainty.
Recommendations
Both the descending clock and its sealed-bid variant score well with respect to the four objectives:
efficiency, simplicity, transparency, and fairness. First, both auctions are simple methods for determining
winners and prices. The clock variation has the further advantage of improved price discovery, but this
advantage is somewhat offset by features added to mitigate market power: discrete rounds with large
decrements near the end of the auction and the introduction of demand uncertainty. Bidding strategy in
both cases amounts to figuring out the lowest acceptable reliability charge for a resource and then exiting
when that reservation price is reached. Second, both variations are highly transparent. The rules are clear
and it is easy to see why a bidder won or lost at a particular price. The revelation of end of round aggregate
supply promotes price discovery in the clock version, although this is offset by the introduction of demand
uncertainty. Third, the auction is fair. Every potential bidder faces the same rules and all trade takes place
at the market-determined clearing price. And finally, the auction is highly efficient. Given the
straightforward and effective bidding strategy of exiting when reservation values are reached, the auction
is fully efficient, maximizing total surplus.

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Table 1: Summary of pros and cons of auction formats
Table 1 provides a summary of the advantages of the two auction formats. The decisive element is the
descending clock’s greater vulnerability to exercise of market power toward the end of the auction.2
As a
result of this vulnerability, the descending clock uses a large decrement near the end and adds demand
uncertainty. These efforts to limit market power offset the advantage that the descending clock would
otherwise enjoy. The descending clock does hold an advantage in revealing more common value
uncertainty and outcome discovery, but in this setting with a single product the advantage is slight. On
the issues of privacy and transparency it is a draw. For practical reasons, because of lumpiness and market
power, all exit bids are revealed to the auctioneer. And the transparency advantage of the clock auction
is wiped away with the introduction of demand uncertainty to address market power. Finally, with respect
to simplicity, the sealed-bid auction has the edge. It is easy to implement, easy for the bidders, at least in
this single product case, and it avoids the need to have demand uncertainty.
My overall recommendation is that Colombia should switch to the sealed-bid auction format. Further,
CREG should eliminate the demand uncertainty that was introduced to mitigate the exercise of market
power with the descending clock format. Demand uncertainty is no longer needed or desirable.
International experience
Both descending clock and sealed-bid auctions are used in today’s reliability markets. Colombia, New
England, and the United Kingdom currently use a descending clock. Brazil uses a hybrid with descending
clock followed by sealed bids. PJM is a good example of a major market that has used the sealed-bid
format for many years. As in Colombia, both New England and Ireland are currently considering whether
it makes sense to adopt a sealed-bid format.
Other issues
In the course of my review, I identified three other issues worth discussing: lumpy supply, auction
frequency, and performance incentives.
2
It should be noted that the sealed-bid approach does not eliminate market power problems, see Ausubel et al.
2014. However, it does mitigate somewhat the unilateral exercise of market power near the end of the auction.

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Addressing lumpy supply
The sealed-bid format works well with lumpy supply. As a result of lumpy supply it is desirable to collect
all the exit bids between the floor and the ceiling so that the entire aggregate supply curve is in hand.
Then the auctioneer can identify the winning units as the set of resources that maximizes net value—the
gains from trade, which is the area between the demand curve and the as-bid supply curve. The clearing
price typically is set at the price where supply and demand cross, although a common variation is to set
the price at the bid of the highest exit bid accepted.
Figure 6: Three examples illustrating clearing rule to maximize net value
Figure 6 gives three examples of how the clearing rule works to maximize net value. In example 1, the first
two resources are accepted (in green) and the third (in red) is rejected. The price is PC with the standard
rule that sets the price where supply and demand cross; however, in the variation setting the price at the
highest accepted bid, the price is P’C. In example 2, the first two resources are accepted and the price is
PC. Example 3 shows an interesting case where a large lumpy resource is rejected and a smaller resource—
bid at a higher price—is accepted. The smaller resource better matches the residual demand and only
increases the price slightly to PC.

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Figure 7: To maximize net value, resource A is rejected; then resource B is rejected
Formally, the maximization of net value is a combinatorial optimization problem (see Cramton, et al.
2006). Figure 7 illustrates the tradeoffs in a simple example. The left panel considers whether the resource
A should be accepted. Accepting A causes supply to exceed demand. There is an incremental gain shown
by the green triangle for the portion of A where demand exceeds supply and an incremental loss shown
by the red polygon where supply exceeds demand. Since the green triangle has less area than the red
polygon, net value is improved by rejecting A. The right panel then considers resource B, which is priced
higher than A, but may improve net value. However, since the incremental benefit (the green triangle) is
smaller than the incremental cost (the red triangle), resource B is rejected too.
Auction frequency
In the Colombian firm energy market, auctions are conducted when it is projected that there is a future
need. As a result of lumpy supply, such as large hydro projects, and slower economic growth as a result
of the global financial crisis, this has led to only two auctions, an initial auction in 2008 and a second
auction in 2012. I believe there is a downside to such infrequent auctions. First it causes ratepayers to pay
too high a reliability charge in periods of surplus, such as in the years immediately after the financial crisis.
Second, it prevents entry during these periods without an auction.
The reliability markets in other parts of the world, such as the US and the UK, have annual auctions.
Auctions are held regardless of whether new entry is required to meet demand. These annual auctions
provide a steady opportunity for entry and exit. With the annual auctions, a price is set each year based
on the market fundaments. The price is higher when new entry is required; the price is lower during
periods of surplus and retirements may be desirable. With an annual auction, new and existing generators
compete in all years. This better sustains an ecosystem for new investment in the Colombian market.
I recommend that Colombia consider shifting to an annual market.
Performance incentives
A recent development in reliability markets around the world is the recognition that strong performance
incentives are a requirement of a successful market. Reliability markets other than Colombia’s market

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began with performance incentives there were two weak. The rules had performance requirements to
deliver energy during scarcity periods, but typically there were long lists of excuses for not delivering that
would exempt a resource from any penalty. This led to poor performance and near crisis situations in New
England and PJM, especially during winter cold snaps when there would be insufficient gas to fuel the gas
generators.
Many markets have now adopted strong performance incentives, which like Colombia’s reliability option,
eliminates excuses. In the following markets there is a requirement to deliver during times the system is
short of operating reserves and there is a high shortage price to settle deviations from obligations:
• New England’s pay-for-performance rule with a shortage price of about $5400/MWh.
• PJM’s capacity performance rule with a shortage price of about $2700/MWh.
• Texas’ scarcity pricing rule with a shortage price of about $9000/MWh.
In each of these markets, the high spot price during reserve shortages has proved essential to motivate
operational reliability in these thermal systems. These systems are characterized by short and more
frequent shortage events—on the order of 20 per year. In Colombia’s hydro dominated system, the
shortage events are better characterized as long and rare—on the order of one major event every 10
years. For this reason, the reliability option approach is best in Colombia, where obligations to deliver
energy are triggered by a high scarcity price that is comparable to that of a peaker with a high marginal
cost.
In all markets, it is important that load is properly hedged. This is accomplished in the reliability markets
with a load-following obligation that covers 100% of load. The Colombia market has been an early example
of this successful design.
Conclusion
This paper considered whether Colombia’s firm energy auction should follow a descending clock or sealed-
bid format. The two auction variations are in fact remarkably similar. Indeed the sealed-bid format is
equivalent to a descending clock auction in which in the initial round the bidders are asked to name all
exit bids from the price ceiling to the price floor. The difference between the two formats rests on how
bidders make use of the extra information revealed at the end of each round—the end of round supply.
In many applications, this extra information is beneficial in improving bidder decision making to increase
the efficiency of the auction outcome. However, in the Colombia setting, the beneficial use of the extra
information about supply is not great. Furthermore, the risk of negative use of the extra information to
exercise market power is relatively high in this setting given the lumpiness of resources relative to the size
of the market. For these reasons, I recommend that Colombia in future auctions switch to the sealed-bid
format.
One other change that I believe Colombia should consider is a switch to annual auctions. This will provide
a regular and predictable means of entry into and exit from the market. It also will mean that he reliability
charge will better track market fundamentals. And it is more apt to support a healthy ecosystem for
investing in generation assets.

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