1. “Course Name: Restructured Power System”
“Transmission Congestion Management”
Introduction, Classification, Importance ,Effects and Methods involved
to reduce Congestion
Mrs.Pooja Jain,Assistant Professor
Department of Electrical Engineering
Swami Keshvanand Institute of Technology, Management &
Gramothan, Ramnagaria, Jagatpura, Jaipur
2. Introduction
In general, one of the suppositions that define the framework for
perfect competition is free entry and exit to the market in other words, free market access should be
guaranteed in a perfect competition. This prerequisite is not fulfilled in transmission markets therefore it
is a natural monopoly. The first important step of power industry restructuring is the transmission open
access.
Transmission services have been unbundled as separate businesses from generation. However regarded
as a natural monopoly, the transmission sector remains more or less regulated to permit a
competitive environment for generation and retail services. The operating and planning of transmission
network and the pricing of the transmission services are still retained as challenges on both theoretical
and practical aspects in the development of electricity markets.
3. Contd..
Transmission congestion can be defined as the condition where desired transmission line-flows
exceed reliability limits. Following this definition, congestion management can be defined as the
actions taken to avoid or relieve congestion. More broadly, congestion management can beconsidered
any systematic approach used in scheduling and matching generation and loads in order to manage
congestion.
Electricity, unlike many other commodities can’t be stored easily and its delivery is constrained
by some physical transmission limits that have to be satisfied all the time to keep the operating
security of the power system. With transmission limits, the deregulation of the power industry is
more difficult therefore one of the major responsibilities of any type of SOs in any type of electricity
markets is to manage transmission congestion and constraints.
4. Causes for Transmission Congestion
The deregulation of the power industry was much easier without transmission limits. These limits are the
main causes for transmission congestion and can be listed below:-
Thermal Limits
Voltage Magnitude Limits
Stability Limits on Power Lines
Voltage Stability Limits
Contingency Constraints
5. Thermal Limits
Colliding electrons in the AC power line cause electrical resistance and resistance interferes with
current in a wire, producing heat. As a wire heats up, it softens. Since power lines are heavy, their
weight makes them sag as heat builds. Beyond a certain temperature the overloaded line will be
permanently damaged. It is caused not only by real power flow but also by reactive power flow.
6. Voltage Magnitude Limits
Voltage constraints define operating bounds that can limit the amount
of power flowing on transmission lines. Voltage constraints inevitably require
attention to both the real and reactive power loads and transfers in the AC
transmission system. Consumption of reactive power tends to make the voltage sag.
Often this must be corrected by injecting reactive power locally because reactive
power is not easily transmitted over long distances.
7. Stability Limits on Power Lines
Power flows through AC power lines because the voltage at the
generator end reaches its maximum slightly ahead of the voltage at the load
end. The amount by which the generation voltage is ahead is called the
phase angle beyond 90 degrees, power flow decreases become completely
unstable. This is the lines physical stability limit. Angle stability can be
classified into two categories: small-signal stability, which is the ability of the system to
maintain synchronism under small disturbance; transient
stability, which is the ability to maintain synchronism when subjected to a
severe transient disturbance.
8. Voltage Stability Limits
Voltage stability is the ability of a power system to maintain steady acceptable voltages at all buses in
the system under normal condition or after being subjected to a disturbance. The main factor causing
voltage instability is the inability of the power system to meet the demand for reactive power. The
heart of the problem is usually the voltage drop that occurs when active power and reactive power
flow through inductive reactance associated with transmission grid.
9. Contingency Constraints
Transmission system operators leave some unused capacity on power
lines in case an unexpected event (a contingency) occurs somewhere on
the system. If, for example, a large power line drops out of service, the power flows will shift to other
lines at the speed of light. The power system operators’ job is to ensure that none of those power
lines overloads.
Contingency constraints are fundamental element of economy-security control. Contingency analysis
identifies potential emergencies through extensive simulations on the power system network. A more
conservative estimation of transmission capability will be obtained after considering the
post-contingency constraints.
10. Features of Congestion Management
Different market structures and market rules lead to different methods for congestion management.
Basically, a proper approach for resolving transmission congestion in competitive electricity markets
should at least have the following features:-
Each market participant, a consumer or a producer, should be treated equally and the price for a specific
good at a specific place and time should be the same for everybody.
The meth od s h o u l d give incentives to producers, consumers and the network operator to improve the
systems in order to relieve transmission constraints.
The implementation should be well defined and transparent for all participants.
The available resources like information, computer systems need to be capable of producing the
necessary quantitative results in the time frame available.
In a real system the surrounding ISOs and their specific methodologies have to be taken into account.
The implemented system needs to interact with other systems.
12. Contd…..
The Figure shows the sequence of congestion management ,the steps are listed below:-
1. The capability of the network to transmit power must be expressed as a “transmission capacity”. The
first phase of congestion management is thus to determine the amount of TTC and ATC which is very
important system information to be published in any electricity market.
2. The capacity allocation step is required to distribute the ATC among the network users wishing to utilize
it.
3. After the transmission capacity has been allocated and the wholesale energy markets are settled, the
ISOs perform a congestion forecast and determine if the foreseen constellation of power generation and
consumption will be feasible or if the network security limits will be breached.
4. If during phase 3 a violation of network security limits is foreseen, the ISOs must take measures to
relieve the network.
13. Available Transfer Capability (ATC)
In a vertically integrated market, the inter-area tie lines are designed only to address the reliability,
system security and system restoration purposes. This integration of various systems becomes a
market need in the deregulated era. Thus, inter-area tie lines become means of bulk power transfers
on a regular basis from sources of cheap generation to loads. In other words, due to deregulation, the
paradigm of grid integration has shifted from regional self-sufficiency to optimal utilization of
resources across large geographical areas. Thus, it becomes imperative on the part of system operator
to quantify the Available Transfer Capability (ATC) of the network and allocate the same to the
market participants in an efficient manner.
Generally, the non-market based methods rely upon the information about the ATC in order to take a
decision while allowing the next set of transactions. Thus, calculation of ATC gains a lot of
importance under such market structures.
14. Definition of Various Terms related to ATC
AVAILABLE TRANSFER CAPABILITY (ATC):
It is a measure of the transfer capability remaining in the
physical transmission network for further commercial
activity over and above already committed uses.
Mathematically, ATC is defined as the Total Transfer
Capability (TTC) less the Transmission Reliability
Margin (TRM), less the sum of existing transmission
commitments (which includes retail customer service)
and the Capacity Benefit Margin (CBM).
ATC = TTC - TRM - Existing Transmission
Commitments (including CBM)
Fig.2 Basic Definition of ATC
15. Contd…
• Total Transfer Capability (TTC)
It is defined as the amount of electric power that can be transferred over the interconnected
transmission network in a reliable manner while meeting all of the specific set of defined pre and post
contingency system conditions.
• Transmission Reliability Margin (TRM)
It is defined as the amount of transmission transfer capability necessary to ensure that the
interconnected transmission network is secure under a reasonable range of uncertainties in system
conditions.
• Capacity Benefit Margin (CBM)
It is defined as the amount of transmission transfer capability reserved by load serving entities to
ensure that the interconnected systems do meet generation reliability requirements.
16. Types of Congestion Management Methods
The delivery of electrical energy from point to point is partly governed
by the capacity of the transmission lines and transformers. Congestion is said
to occur whenever the system state of the grid is characterized by one or more
violations of the physical operational or policy constraints under which the
grid operates in the normal state or under any one of the contingency cases in
a set of specified contingencies.
In other words congestion occurs when the transmitted power exceeds the capacity or transfer limit of
the transmission line or transformer. The capacity of a transmission line or transformer may
have different values under different conditions. Congestion, needless to say, is undesirable. A system
without congestion will have a uniform price (in nodal pricing).
As soon as we have congestion, prices in some areas willincrease and in others decrease. Congestion
therefore distorts the market. Another disadvantage of congestion is increased risk of market
manipulation by some participants.
17. Contd….
• In the VIC the economic load dispatch was normally formulated as an optimal power flow (OPF)
problem with the objective of minimizing total generation cost subject to, generation lower and
upper limits, bus voltage limits, power flow limits of lines and transformers and etc.
Congestion was therefore intrinsically managed at the dispatch stage.
• In the deregulated market, congestion is likely to occur more often since the market for the selling
and buying of energy may be settled without the constraints of the power system imposed. The
ensuring generation schedules may result in some transmission paths being congested. Congestion
management remains the central issue in transmission management in
deregulated power systems. Congestion management (CM) includes both the congestion relief
actions and the associated pricing mechanisms.
18. Contd….
Congestion may be alleviated through various ways. Among the technical solutions we have outaging
of congested lines, operation of FACTS devices and operation of transformer tap changers. Among
the non technical solutions we have market based and non market based methods of CM. Non market
based methods are those where no form of market mechanism is used to allocate the scarce
transmission capacity but use other reasonable criteria. These include sharing of capacity on a pro
rata basis where users share in proportion to their requirements, first come first serve and preference
for certain types of contracts.
The non market based methods for congestion management do not send any signals for investment
and have no measure of the value of the congested line. Market based methods are based on market
mechanisms and hence give an indication of the value of the scarce resource of transmission
capacity.
These methods are briefly discussed below.
20. Nodal Pricing
In the nodal pricing scheme every bus in the grid is treated as a zone.
The locational marginal price (LMP) for each bus is determined by the ISO
by carrying out an economic dispatch with the flow limits. The LMP becomes
the price and payment that buyers pay and the generators receive respectively.
The market is settled with the network constraints hence congestion does not
arise.
21. Zonal Pricing
In Zonal pricing system buses with similar LMPs are aggregated into
zones. The market is first settled constraint free. Each zone will have a price
for energy that buyers can pay and sellers receive. In the case that congestion
occurs the ISO receives supplementary bids for increase and decrease of
generation. The most expensive supplemental bid for increase of generation
becomes the price for that zone and the cheapest supplemental bid for
decrease of generation becomes the price for that zone. In this way the ISO
earns congestion rent over the congested lines. In case that there is no
congestion the zonal prices will be the same. The California market migrated
from this CM mechanism to the zonal pricing method.
22. Re-Scheduling
In this method of CM the market is settled without the constraints of
the transmission system being applied. If congestion occurs the ISO reschedules the generation in
such a way that congestion is get rid of. This will entail the ISO purchasing power from high price
areas. The generators in the low price areas will be commanded to regulate downwards. Since the
ISO in essence is buying power at a high price and selling it at a lower price he
incurs a cost. The net cost incurred by the ISO is an indication of the
congestion charge and is a signal for investment. The ISO directly commands
generators to up regulate or down regulate without the use of the market.
23. Counter Trading
Counter trading is a modified form of re-dispatching the difference
being that up and down regulation power is obtained from the market. The
generators submit bids for up and down regulation on the balancing market.
Similar to the re-dispatch the ISO will incur net cost in the purchase of
regulation power since he has to use more expensive power for up regulation.
Sweden uses this form of CM. Counter trading may be viewed as a special
type of re-dispatching.
24. Market Splitting
In market splitting the market is first settled without constraints applied. If the resulting schedules
cause congestion on some line(s) the market is then split and settled separately with the transfer limit
applied. The ISO purchases power from the low price area and sells it in the high price area. The ISO
thus makes a profit. Norway uses this CM method.
25. Auctioning
In auctioning the available capacity of a normally constrained path is auctioned by the ISO receiving
bids from parties willing to use the path. The lowest marginal bid accepted becomes the price for
transmission on the path. Two forms of auctioning are in use i.e. implicit and explicit.
26. Load Curtailment
By managing load, congestion can also be effectively relieved. The
benefits result from reduced peak demand and reduced pressure on both
electricity generation and distribution systems. The amount of curtailed load
should be as small as possible and the price in the congested area should fall
as much as possible. While there are many different kinds of curtailment
algorithms, a parameter termed as willingness-to-pay-to-avoid-curtailment
was introduced in which is regarded as a highly effective instrument in setting
the transaction curtailment.
27. FACTS
FACTS are a new technology developed in recent two decades, and it has been widely put in practice
in the world. FACTS are defined by the IEEE as a power electronic-based system and other static
equipment that has the ability to enhance controllability, increase power transfer capability.
Nowadays, power producers and system operators all over the world are faced with increasing
demands for bulk power transmission, low-cost power delivery and higher reliability, to some extent;
such issues are being alleviated by the developing technology of FACTS.
28. First Come, First Served
Capacity is allocated according to the order in which the transmission
requests have been received by the ISO. Starting from the earliest request,
all requested amounts of capacity are fully granted until the available
capacity is used up. This method encourages participants to make
longer forecasts. Thus, it allows better and sooner security assessment for the
ISO who knows accurately the volume of exchanges in advance. However,
this method may not leave enough room for short-term trading, which is a
requirement to ensure the success of market dynamics. This method is well
suited for bilateral trades, but fails to provide an efficient priority
mechanism for day-ahead or real-time pool transactions.
29. Pro Rata Rationing
In this method no real priority is defined. All requests are partially
accepted in the way that the ISO curtails them in case of congestion
according to the ratio: existing capacity/requested capacity. This rule is
transparent but brings the participants to an economically inefficient use of
the system.
30. Outaging Congested Lines
In this method congested line is identified by the system operator and
they are decides the possible methods for relieving the congestion by
economical way. If it is not possible the system operator may outage the
congested line for secure operation of the system .This may riser power flows
in the neighboring lines. In this method some of the area isolated from the live
network of the system.
