Transcript Slide 1

Concept of Congestion in
power system & Congestion
Management Procedure
What is the nature of the problem?
• Generation, Distribution and Transmission of
Electricity are monopolistic in nature
• Competitive markets provide the lowest price.
• Transmission is the enabler of competition in
the electricity sector.
• For generators to compete it is essential that
lowest cost generators are scheduled on the
basis of merit order.
• Congestion limits the possibilities of
competitive markets
Power sector unable to utilise full capacity
due to poor transmission
Mitul Thakkar, ET Bureau Dec 15, 2014NEW DELHI
The power sector is unable to use at least 10 per cent of its
capacity due to the choked transmission network in the country,
hurting projects of Tata Power,Essar Power, Jindal Power, JSW
Energy, CLP, DB Power,MB Power, Emco Energy, GMR and
Adhunik Power.
Inadequate transmission has kept about 25,000 Mw of
generation capacity idle, making the problem comparable to the
issue of acute fuel scarcity that has hit about 30,000 Mw of new
capacity, industry executives say. In addition to the inadequate
capacity, regulatory authorities impose strict restrictions on the
utilisation of existing networks and keep a large amount of
transmission capacity idle as a safeguard against grid collapse.
IEX sees higher demand for electricity in
October ET 7 Nov, 2014, 1838 hrs IST, PTI
•
About ten per cent fall in the average area clearing price was seen across all bid areas while
in the Southern states, the area price increased by almost 50-70 per cent on account of
congestion in the transmission network.
"Almost 286 million units were constrained (in October) due
to unavailability of transmission corridor as compared to only
42 MUs lost in September.Persistent transmission congestion
towards the Southern region and between the Southern
states is a key reason for increased price of electricity in the
South," the statement said.
Meanwhile, in October, IEX's spot power market traded 2.8 BUs of electricity. The amount is
around ten per cent higher than 2.5 BUs seen in September.
On October 11, the day-ahead market witnessed trading of 131 MUs -- "highest ever on a
single day since the inception of the exchange in 2008".
Congestion & Congestion
Management
• Transmission congestion occurs when there is
insufficient transmission capacity to simultaneously
accommodate all requests for transmission service
within a region
• It 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 be considered any systematic
approach used in scheduling and matching
generation and loads in order to manage congestion
How do we measure transfer
capability
• ATC—Available Transfer Capability
How do we measure transfer
capability
• The ATC of a transmission system is a measure of
the transfer capability (transmission capacity)
remaining in the physical transmission network
for further commercial activity over and above
already committed uses . It is a measure at a
given time and depends on a number of factors
such as a system generation dispatch, system
load level, load distribution in the network,
network topology and the limits imposed on the
transmission network.
Why is TTC/ATC calculation required?
• Restructuring of electric power industry to an
open-accessed and market-based system
introduced high level of power transactions to
large power system networks operation
• Deregulation of power systems has increased
the need for defensible calculations of transfer
capability and related quantities such as the
transmission reliability margin
Impact of Congestion—efficiency
in markets
G
400 MW
Rs. 2/unit
G
400 MW
Rs. 4/unit
ATC >200
G
200 MW
200 MW
400 MW
Rs. 2/unit
G
400 MW
Rs. 4/unit
Total cost= Rs.800/ATC =100
200 MW
Total cost= Rs.1000/-
200 MW
The Weakest Link
1015 MW
750 MW
630 MVA
Transfer Capability depends on
load-generation
scenario
900 MW
600 MW (max)
300 MW
900 MW
300 MW
0 MW
0 MW
600 MW (max)
600 MW
900 MW
1200 MW
1800 MW
Procedure for calculation of ATC
• Base case LF
• Identify the buses involved in transfer
• Base case ATC-calculate PDTFs for monitored
lines
• Identify credible contingencies
• Calculate LODF, GODF and recalculate ATC
Determinants of Transmission
Capability of Individual Line
•
•
•
•
Thermal Limits
SIL
Stability-angle limits
Voltage limits
• The capability of a high voltage power line is usually set by thermal limit
for shorter lines and transmission distances (up to 80 km), and longer (80
to 320 km) lines by voltage regulation, and very long lines (> 320 km) by
stability issues The capability of a high voltage power line is usually set by
thermal limit for shorter lines and transmission distances (up to 80 km),
and longer (80 to 320 km) lines by voltage regulation, and very long lines
(> 320 km) by stability issues
Transmission Capacity vs Tansfer
capability
• Why Transfer Capability is less than Transmission
Capacity?
• ‘N-1’ criteria
• Stability Criteria
• Non uniform loading of parallel lines
• Loop flows
• Voltage profile
• Load generation disposition
• Intra-state network configuration
• Law of diminishing returns
Determinants of Transmission
Capability of Individual Line
• The rating is the physical capability of a line or other equipment for a
particular set of conditions, allowing for safe operation.
– Usually in the context of thermal amp limits.
– Voltage and stability constraints may sometimes be more limiting.
1351.5 ASCR on 750 F day = 1627 amps
1351.5 ASCR on 950 F day = 1512 amps
Minimum
Clearance
Rating is
higher in
cooler
weather.
Why?
Ratings Considerations
• Physical Design & condition
• Ambient (weather) conditions
• ROW conditions
Ratings change
constantly due to
weather and ROW
conditions.
Potential Equipment Limits (limiting elements)
Busses
Conductors
Hardware
Jumpers
Lines
Transformers
Fuses
Interrupters
Bushings
Relaying
Switches
CTs
Breakers
Transfer Capability
Think of an elevator with a single 1000 lb cable.
– How much load can it carry?
1000.
This is the cable Rating.
Contingency
– What would be its reliable Transfer Capability?
0.
Due to the N-1 Contingency loss of Cable A.
– Add a second 1000 lb cable. Now what is the Transfer
Capability?
1000.
This is Total Transfer Capability (TTC).
– Now add (4) 200 lb passengers to the elevator. How much
Transfer Capability remains?
1000 – 4(200) = 200.
This is Available Transfer Capability (ATC).
A
B
Transfer Capability
What if Cable B had a 10
minute emergency rating
of 1000 lbs.
Now let’s change some assumptions.
– What if Cable B is only rated for 900 lbs? What is the TTC?
Contingency
900.
Cable B is the Constraint or Limiting Element.
– What is the ATC?
A
900 – 4(200) = 100.
– What is the risk if a fifth 200 lb passenger rides?
If Cable A fails, Cable B would fail also.
This would be a System Operating Limit (SOL)
violation and could lead to a Cascading Event.
– What should the operator do?
Take a Pre-Contingency Action to reduce the load.
A Post-Contingency Action could be used to reduce
the load.
B
Very Simple Circuit Example
Neighbor Blue
Neighbor Green
A
100
Worst Contingency
200
150
Generator
Flow of
100 MW
Load
B
Line Ratings
(MVA) 200
100
C
100
(Offline)
•Home Area connects with two neighboring systems, Blue and Green.
•Power flows split evenly. Ignore impedance, losses, spinning reserves,
etc. We’ll focus on thermal limits (ignore voltage, stability, etc.).
Concept: TTC depends upon direction.
Neighbor Blue
Neighbor Green
TTC can be
higher in
cooler
weather.
Why?
What is the
Contingency
? Constraint?
200
Worst Contingency
A
100
Constraint A-B
150 Base flow 200
100 of 100
100
Assume C Off
B
200
200
Constraint B-A
Off
C
Assume C Off
TTCA-B= ? 50
TTCB-A= ? 200
ATCA-B= ? 50
ATCB-A= ? 200
Concept: Loop Flows impact TTC
(Contract Path)
Point of Receipt (POR) = B2
Point of Delivery (POD) = L2
Neighbor Blue
L2
B2
B1
Long
way
Neighbor Green
Contract path
L1
100
Can a deal
from Blue to
Green impact
our TTC?
Loop Flow = 40
A
200
Base flow
of 100
+ 40 Loop
flow
TTCA-B=
50
TTCA-B=
10
150
B
200
100
Off
What if our
Loop Flow
was 60?
C
150 - 100 -60 = -10
Curtail needed!
w/Loop Flow Redispatch locally or call a
Transmission Loading Relief (TLR).
Concept: Redispatch impacts TTC
(Contract Path)
Neighbor Blue
Long
way
L2
B2
Neighbor Green
B1
L1
A
What if
generator at
bus c is
dispatched
instead of gen
at bus A?
TTCA-B= 150
TTCA-B= 150
B
200
150
200
100
100
C
TTC and Power Flows are impacted by:
•
•
•
•
•
•
Loads (weather is a major driver)
Outages (both transmission and generation)
Generation Dispatch (fuel volatility is a major driver)
Loop Flows (activities in other markets)
Counter Flows
Imports and Exports from other areas
(simultaneous transfers)
ATC changes Continuously
• Power system transfer capability indicates how much interarea power
transfers can be increased without compromising system security.
Accurate identification of this capability provides vital information for both
planning and operation of the bulk power market. (Planners need to know
the system bottlenecks and system operators must not implement
transfers which exceed the calculated transfer capability.)
• Repeated estimates of transfer capabilities are needed to ensure that the
combined effects of power transfers do not cause an undue risk of system
overloads, equipment damage, or blackouts. However, an overly
conservative estimate of transfer capability unnecessarily limits the power
transfers and is a costly and inefficient use of the network. Power transfers
are increasing both in amount and in variety as deregulation proceeds.
Indeed, such power transfers are necessary for a competitive market for
electric power. There is a very strong economic incentive to improve the
accuracy and effectiveness of transfer capability computations for use by
system operators, planners and power marketers.
ATC changes Continuously
• The pre-calculated ATC based on some previously
determined set of initial conditions (a 'snapshot' [1])
has a limited value as a forcasting tool for some faraway future set of arbitrarily changed conditions. For
example, the network loading pattern could change,
any arbitrary transaction involving any other pair of
grid nodes could come in, and so on. Events of this kind
are erratic by nature. The very nature of the power
flow model is this: one could not know the power flow
pattern of the meshed network before the execution of
the power flow program.
• ATC has a forecating value only if changes of pattern
flows are minor
CERC Definitions
Definitions
• “Total Transfer Capability (TTC)” means the amount of
electric power that can be transferred reliably over the
inter-control area transmission system under a given set
of operating conditions considering the effect of
occurrence of the worst credible contingency.
• “Transmission Reliability Margin (TRM)” means the
amount of margin kept in the total transfer capability
necessary to ensure that the interconnected transmission
network is secure under a reasonable range of
uncertainties in system conditions
Definitions
• “Available Transfer Capability (ATC)” means the transfer
capability of the inter-control area transmission system
available for scheduling commercial transactions
(through long term access, medium term open access
and short term open access) in a specific direction, taking
into account the network security.
• Mathematically ATC is the Total Transfer Capability less
Transmission Reliability Margin
• “Congestion” means a situation where the demand for
transmission capacity exceeds the Available Transmission
Capability
Important Conditions
•
•
•
•
Power system model to be considered
Input data for base case preparation
Selection of Credible Contingencies
Violation of voltages, angular separation, line
loadings
• Consideration of Transmission reliability
Margins
Sample formulas and calculations-ATC
Power Transfer Distribution Factor--PTDF
• Sensitivity of a flow on a monitored line to an
injection at a bus w.r.t a reference bus
• PTDFij-mn
= [Xim-Xjm-Xin+Xjn]/xij
m is the seller node/bus and n is the buyer node/bus
Line between node i, and j is the monitored line.
,xij is the reactance of the line between bus I and j
Xim,Xjn.. are the elements of reactance matrix [X] ; ([X]=[B]-1)
Pmax ij-mn = [Pmax ij - Pij ]/PTDFij-mn
ATC=min[Pmax ij-mn ij belongs to nl)
Line Outage Distribution Factors --LODF
When an outage occurs, the power flowing over the outaged line
is redistributed onto the remaining lines in the system. The LODF
is the measure of this redistribution. Pij ,rs is the fraction of the
power flowing on the line from i to j
LODFij,rs = Nrs.xrs (Xir-Xis-Xjr+Xjs)/ [NijNrs. xijxrs-(Xrr+Xss-2Xrs)]
Pmax ij-mn = [Pmax ij - Pij ]/[PTDFij-mn + LODFij,rs * PTDF rs,mn]
OTDFij,rs,mn = [PTDFij-mn + LODFij,rs * PTDF rs,mn]
ATCmn,rs = min { (Pij max –Pij)/OTDFij,mn,rs}
Test Case Calculations-PTDF
0.002
1
2
10 MW
0.002
0.002
0.01
3
0.00125
7
4
0.002
10 MW
5
0.002
6
0.01
Test Case Calculations-PTDF
1000
-500
-500
-500
1100
-500
-500
-500
1800
-800
-800
1800
--500
-500
-500
1000
-500
-500
-500
1100
-100
-100
200
-100
-100
0.0065761
0.0055401
0.0056121
0.0050546
0.0047573
0.0044599
0.0055401
0.0058188
0.0052613
0.0047387
0.0044599
0.0041812
0.0056121
0.0052613
0.0059628
0.0053705
0.0050546
0.0047387
0.0050546
0.0047387
0.0053705
0.0059628
0.0056121
0.0052613
0.0047573
0.0044599
0.0050546
0.0056121
0.0065761
0.0055401
0.0044599
0.0041812
0.0047387
0.0052613
0.0055401
0.0058188
Test Case Calculations-PTDF
• PTDFij-mn = [Xim-Xjm-Xin+Xjn]/xij
• To find PTDF 34,15
• =[0.0056121-0.00505460.0050546+0.0056121)/0.00125 =0.892
• Therefore for incremental transfer between bus 1
and 5 the flow increases in line 3-4 by 0.892x 10=
8.92 MW
• Therefore continuously increasing the transfer the
line limit would be reached and that would be the
ATC
Transmission reliability marginUncertainities
Parameters and their uncertainty
• The transfer capability is a function A of many parameters p1, p2,...,pm:
transfer capability = A(p1, p2,...,pm) (1)
• Uncertainty in the parameters pi causes uncertainty in the transfer
capability and it is assumed that this uncertainty in the transfer capability
is the uncertainty to be quanti- fied in the transmission reliability margin.
• The uncertain parameters pi can include factors such as generation
dispatch, customer demand, system parameters and system topology. The
parameters are assumed to satisfy the following conditions:
• Each parameter pi is a random variable with known mean µ(pi) and known
variance σ2(pi). These statistics are obtained from the historical record,
statistical analysis and engineering judgment.
Congestion costs-integrated utility
500 MW
700 MW
A
200 MW
500 MW
0 A
1000
MwH
100 MW (constrained)
500 600 700
500 400 300
B
300 MW
1000
B
0
Congestion costs—deregulated
markets
• Unconstrained case
2200
0 A
1000
MwH
500 600 700
500 400 300
1000
B
0
Congestion Costs under LMP
• Congestion Revenues method, is based on the policy common among ISOs
of paying generators a region-specific LMP for their output regardless of
the region where the generation is used to serve load, and, at the same
time, charging all loads within a given region a single, region-specific LMP
500 MW
700 MW
Rs. 2000/MwH
A
100 MW(constrained)
500 MW
B
Rs. 2500/MwH
300 MW
Charges paid to generators = 2000*700 + 2500*300 = Rs. 21,50,000
Charges paid by loads= 2000*500 + 2500*500 = Rs. 22,50,000
Congestion Revenue = Rs.1,00,000
Congestion costs--LMP
0 A
1000
MwH
500 600 700
500 400 300
1000
B
0
Charges paid to generators = 2000*700 + 2500*300 = Rs. 21,50,000
Charges paid by loads= 2000*500 + 2500*500 = Rs. 22,50,000
Congestion Revenue = Rs.1,00,000
Congestion Management
Congestion Management
• If the ATC is over-estimated, then real-time
congestion could result in reducing system
reliability below acceptable levels. On the
other hand, if the ATC is under estimated,
then economic exchanges are potentially
foregone.
Congestion Management
• Congestion can be managed, before the fact,
with information provided from the
scheduling period only if ATC is known.
• However, congestion must be managed in
real-time if ATC is wrong. And real-time
management of congestion is inherently less
economically efficient because dispatch
options are limited.
Congestion Management
Planning Stage
Scheduling Stage-adjustments to
submitted schedules based on ATC
Real Time Adjustment
-Redispatch/load shedding
Congestion Management
• The fundamental point is that if ATC is wrong during
the scheduling period when there is the attempt to
efficiently allocate transmission capacity, the SO can
still maintain system reliability through real-time
adjustments.
• Reliability is not compromised, however, economics
may suffer. This can happen because the calculation
of ATC depends upon forecasts of load and
generation patterns throughout the system. If the
forecast is wrong, then the ATC calculation can be
wrong
Congestion Management
CERC (Measures to relieve congestion in real time),
Regulations, 2009
• (1) To relieve congestion in the real time, a
congestion charge shall be applied as a commercial
measure. The congestion charge will be payable by a
Regional entity or entities causing congestion in the
inter-regional link or intra-regional link and
receivable by a Regional entity or entities relieving
congestion.
Congestion Management
• Use of FACTS devices such as SVC,
STATCOM(parallel) and TCSC,SSSC, etc to
overcome voltage and angle stability limits
and operate the lines at their thermal limits.
HTLS/MULti Ckt Lines
Congestion Management
• Redispatch of generation/load
Congestion Management
Non –Market Methods:
• Type of contract
• First Come First Serve
• Pro-rata methods
• Curtailment
Market Based Methods
• Explicit Auctioning of Network Capacity
• Nodal Pricing
• Zonal Pricing
• Price Area Congestion Management
• Redispatch
• Counter Trades
Congestion Management
Dynamic Line Ratings (DLR)
• The static rating indicates the maximum amount of current
that the line’s conductors can carry (under a set of assumed
weather conditions) without violating safety codes or
damaging the conductor.
• Static ratings are adjusted infrequently, so they are usually
conservatively based on “worst-case scenario” conditions (i.e.,
low wind speed, high ambient temperature, and high solar
radiation).
• When static ratings are adjusted daily, hourly, or even more
frequently to account for different ambient temperatures,
they are called ambient-adjusted ratings
Congestion Management
• DLR technologies enable transmission owners to
determine capacity and apply line ratings in real
time. This enables system operators to take
advantage of additional capacity when it is available.
Unlike static ratings, dynamic ratings are calculated in
real time based on the transmission line’s actual
operating conditions at specific moments, rather
than on fixed assumptions. Dynamic ratings are
often, but not always, greater than static ratings.
Congestion Management
• Generalized Network Access
--Shifting from a contract based to an access
based paradigm
--Many IPPs are coming up with substantial
generations without any PPAs/firm beneficiaries
and trying to use the existing corridors for
evacuation which leads to congestion.
--cancellation of PPA after availing LTA causes
the generator to pay transmission charges. LTA
cannot be used for transfer to non-target
regions
Thank you
• “Transmission Reliability Margin (TRM)” means the
amount of margin kept in the total transfer capability
necessary to ensure that the interconnected
transmission network is secure under a reasonable
range of uncertainties in system conditions “Available
Transfer Capability (ATC)” means the transfer capability
of the inter-control area transmission system available
for scheduling commercial transactions (through long
term access, medium term open access and short term
open access) in a specific direction, taking into account
the network security. Mathematically ATC is the Total
Transfer Capability less Transmission Reliability Margin
• Without congestion, the marginal cost to supply an
increment of load is determined by the lowest
marginal cost generator on the system and is the
same for any load on the system. However, with
congestion the marginal cost to supply an increment
of load is determined by location. The marginal cost
for a given location is the lowest cost increment of
generation supply with available transmission
capacity between the generation source and the
load.
• Robust transmission network is a pre-requisite
for open access
• – thereby enabling competitive market
• • National Electricity Policy envisions
– network expansion should be planned and
implemented keeping in view the anticipated
transmission needs that would be incident on
the system in the open access regime.
• • 'Congestion' is a situation
– where the demand for transmission capacity
exceeds the Available Transmission Capability
(ATC) as per Grid Code, 2010.
The loss of energy due to Congestion during last
3 years in IEX
• – In terms of time, congestion in IEX : 100.00%
; PXIL : 73.85%.
• Congestion affecting economic operation of
power sector - merit order operation getting
disturbed.
• Delay of line congestion and delay of
generation congestion