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 natureCompetitive 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 transmissionMitul Thakkar, ET BureauDec 15, 2014NEW DELHIThe 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 ofTata Power,Essar Power,Jindal Power,JSW Energy,CLP,DB Power,MB Power,Emco Energy,GMRand 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 ManagementTransmission congestion occurs when there is insufficient transmission capacity to simultaneously accommodate all requests for transmission service within a regionIt 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 capabilityATCAvailable Transfer Capability

How do we measure transfer capabilityThe 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 operationDeregulation of power systems has increased the need for defensible calculations of transfer capability and related quantities such as the transmission reliability margin

Impact of Congestionefficiency in marketsTotal cost= Rs.800/-Total cost= Rs.1000/-

The Weakest Link

Transfer Capability depends on load-generation scenario

Procedure for calculation of ATCBase case LFIdentify the buses involved in transferBase case ATC-calculate PDTFs for monitored linesIdentify credible contingenciesCalculate LODF, GODF and recalculate ATC

Determinants of Transmission Capability of Individual LineThermal LimitsSILStability-angle limitsVoltage limitsThe 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 capabilityWhy Transfer Capability is less than Transmission Capacity?N-1 criteriaStability CriteriaNon uniform loading of parallel linesLoop flowsVoltage profileLoad generation dispositionIntra-state network configurationLaw of diminishing returns

Determinants of Transmission Capability of Individual LineThe 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.Rating is higher in cooler weather. Why?

Ratings ConsiderationsPhysical Design & conditionAmbient (weather) conditionsROW conditionsRatings change constantly due to weather and ROW conditions.

Potential Equipment Limits (limiting elements)BussesJumpersBreakersTransformersCTsBushingsConductorsFusesInterruptersRelayingHardwareLinesSwitches

Transfer CapabilityThink of an elevator with a single 1000 lb cable.How much load can it carry?

What would be its reliable Transfer Capability?

Add a second 1000 lb cable. Now what is the Transfer Capability?

Now add (4) 200 lb passengers to the elevator. How much Transfer Capability remains?Contingency1000 4(200) = 200. This is Available Transfer Capability (ATC).0. Due to the N-1 Contingency loss of Cable A.1000. This is Total Transfer Capability (TTC).1000. This is the cable Rating.A

Transfer CapabilityNow lets change some assumptions.What if Cable B is only rated for 900 lbs? What is the TTC?

What is the ATC?

What is the risk if a fifth 200 lb passenger rides?

What should the operator do?ContingencyTake a Pre-Contingency Action to reduce the load.900 4(200) = 100. 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.900. Cable B is the Constraint or Limiting Element.AWhat if Cable B had a 10 minute emergency rating of 1000 lbs.A Post-Contingency Action could be used to reduce the load.

Very Simple Circuit ExampleHome Area connects with two neighboring systems, Blue and Green.Power flows split evenly. Ignore impedance, losses, spinning reserves, etc. Well focus on thermal limits (ignore voltage, stability, etc.).100ACB200150200Neighbor BlueNeighbor GreenGeneratorLine Ratings (MVA)Load100100(Offline)

Concept: TTC depends upon direction.100ACB200150200Neighbor BlueNeighbor Green100OffTTCA-B= ? ATCA-B= ?TTCB-A= ?ATCB-A= ?Assume C Off5050200200Assume C OffWhat is the Contingency? Constraint? TTC can be higher in cooler weather. Why?

TTCA-B= TTCA-B= w/Loop FlowConcept: Loop Flows impact TTC (Contract Path)100ACB200150200Neighbor BlueNeighbor Green100OffB1B2L2L1Long wayCan a deal from Blue to Green impact our TTC?What if our Loop Flow was 60?Point of Receipt (POR) = B2Point of Delivery (POD) = L2150 - 100 -60 = -10 Curtail needed! Redispatch locally or call a Transmission Loading Relief (TLR).1050Contract path

TTCA-B= TTCA-B= Concept: Redispatch impacts TTC (Contract Path)ACB200150200Neighbor BlueNeighbor Green100100B1B2L2L1Long wayWhat if generator at bus c is dispatched instead of gen at bus A?150150

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 FlowsImports and Exports from other areas (simultaneous transfers)

ATC changes ContinuouslyPower 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 ContinuouslyThe 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 far-away 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

DefinitionsTotal 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

DefinitionsAvailable 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 ConditionsPower system model to be consideredInput data for base case preparationSelection of Credible ContingenciesViolation of voltages, angular separation, line loadingsConsideration of Transmission reliability Margins

Sample formulas and calculations--ATC

Power Transfer Distribution Factor--PTDFSensitivity of a flow on a monitored line to an injection at a bus w.r.t a reference busPTDFij-mn = [Xim-Xjm-Xin+Xjn]/xij

m is the seller node/bus and n is the buyer node/busLine between node i, and j is the monitored line.,xij is the reactance of the line between bus I and jXim,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 --LODFWhen 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 jLODFij,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-PTDF0.0020.0020.0020.0020.0020.001250.010.01123456710 MW10 MW

Test Case Calculations-PTDF

1000-500-500-5001100-500-100-500-5001800-800-8001800--500-500-5001000-500-500-5001100-100-100-100200

0.00657610.00554010.00561210.00505460.00475730.00445990.00554010.00581880.00526130.00473870.00445990.00418120.00561210.00526130.00596280.00537050.00505460.00473870.00505460.00473870.00537050.00596280.00561210.00526130.00475730.00445990.00505460.00561210.00657610.00554010.00445990.00418120.00473870.00526130.00554010.0058188

Test Case Calculations-PTDFPTDFij-mn = [Xim-Xjm-Xin+Xjn]/xij To find PTDF 34,15

=[0.0056121-0.0050546-0.0050546+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 MWTherefore continuously increasing the transfer the line limit would be reached and that would be the ATC

Transmission reliability margin-UncertainitiesParameters and their uncertaintyThe 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 (pi) and known variance 2(pi). These statistics are obtained from the historical record, statistical analysis and engineering judgment.

Congestion costs-integrated utilityAB7003000100010000600400100 MW (constrained)MwH500500

Congestion costsderegulated marketsUnconstrained caseAB7003000100010000600400MwH5005002200

Congestion Costs under LMPCongestion 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 LMPRs. 2000/MwHRs. 2500/MwHCharges paid to generators = 2000*700 + 2500*300 = Rs. 21,50,000Charges paid by loads= 2000*500 + 2500*500 = Rs. 22,50,000Congestion Revenue = Rs.1,00,000

Congestion costs--LMPAB7003000100010000600400MwH500500Charges paid to generators = 2000*700 + 2500*300 = Rs. 21,50,000Charges paid by loads= 2000*500 + 2500*500 = Rs. 22,50,000Congestion Revenue = Rs.1,00,000

Congestion Management

Congestion ManagementIf 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 ManagementCongestion 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 ManagementPlanning StageScheduling Stage-adjustments to submitted schedules based on ATCReal Time Adjustment-Redispatch/load shedding

Congestion ManagementThe 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 ManagementCERC (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 ManagementUse 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 ManagementRedispatch of generation/load

Congestion ManagementNon Market Methods:Type of contractFirst Come First Serve Pro-rata methodsCurtailment

Market Based MethodsExplicit Auctioning of Network CapacityNodal PricingZonal PricingPrice Area Congestion ManagementRedispatchCounter Trades

Congestion ManagementDynamic Line Ratings (DLR)The static rating indicates the maximum amount of current that the lines 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 ManagementDLR 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 lines actual operating conditions at specific moments, rather than on fixed assumptions. Dynamic ratings are often, but not always, greater than static ratings.

Congestion ManagementGeneralized 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