Availability based tariff was introduced in India first time in western region from July 2002. Prime objective is to enforce self-discipline by fiscal tool for power drawn/dump by beneficiary. Energy cost is linked to (kWh) units but unit rate is determined by supply and demand condition at any time. Hence, energy accounting is necessary in short time block. Therefore, energy accounting in ABT is for 96 blocks of 15 minutes.
The salient features of ABT scheme with three-part tariff
- Fixed Charge: Capital cost charges of generator is claimed as Fixed Charges. This is linked to assured (declared) capacity of generator rather than actual generation. This is to control unwarranted power injection at high frequency by generating agencies. Even during unfavourable condition of high frequency, they had tendency to maximize generation up to target and beyond to get full fixed charges and also incentive.
- Variable Charge: Energy charge is input and operation cost for power is linked to energy indented by beneficiary. This is to facilitate the beneficiary to manage required power on merit order. Earlier beneficiaries were liable for payment of allotted share of generation including unwarranted unused power.
- Unscheduled Interchange (UI): UI is a deviation from schedule power drawal or generation. Energy rate of this power is linked to operating frequency during the block. The rate is zero for operating frequency 50.50 Hz and above. Rate is increasing at Rs 0.056 per unit for each drop of 0.02 Hz in frequency up to Rs 4.20 at 49.00 Hz. This is in the form of indirect penalty/incentive for frequency control by constituents of the grid. This is vital part of ABT because in addition to frequency control, it is useful in other ways also.
Grid operation
Power systems in the area were interconnected to form regional grid for better stability with other advantages. State power systems were having own power plants and independent power producers and sub-systems. Region had central sector power plants known as Interstate Generating Stations. This is pooled generation in region having share to states and union territories. These power plants are located at different area and injecting generation in the respective power system. So, beneficiaries have mutual import or export schedule depending on real time ISGS generation and share in own system and other system. Actual power flow in the grid network depends on configuration and operating parameters. Therefore, bilateral or link-wise power accounting is meaningless. Global accounting is followed wherein net import or export of each beneficiary is derived from all import export schedule with rest of the system. All beneficiaries have import or export schedule for power exchange.
Beneficiary importing power more than scheduled import or exporting power less than scheduled export is considered as UI import. Similarly importing power less than scheduled import or exporting power more than scheduled export is considered as UI export. Generator injecting power more than scheduled injection is UI export and injecting power less than scheduled injection is UI import.
Payment is receivable by power exporters and payable by power importers at UI rate corresponding to average frequency during concern block of fifteen minutes.
During the operation grid frequency may drop while total generation falls short of total demand in the region. Lower the frequency higher is the UI rate. Beneficiary and generators will try for UI export to take advantage of high rate. Such reactions from the constituents arrest the drop of frequency. On the other side grid frequency may shoot up while total generation is in excess to total demand in the region. Higher the frequency lower is the UI rate. Beneficiary and generators will try for UI Import to take advantage of low rate. Such reactions from the constituents control the rise of frequency.
In addition to this auto regulation of frequency as above, it indirectly provides open access platform for sale (export) or purchase (import) power at desired rate without any agreement, consent or interaction with
any agency.
System operators
Each system manager in the grid operates his or her power system for least energy cost with optimal security. Available power from own power plant, IPP and ISGS may have different energy costs depending on plant capacity, type of plant, location, inputs like coal, washed coal, imported coal, lignite, gas, liquid fuels, nuclear, hydro, etc.
Energy required at any time is managed on merit order base i.e., the cheapest power first and costliest power last. Rate and availability of this energy is fairly fixed from power plants but rate of Unscheduled Energy (UI) is ever changing according to system frequency. Cheapest power is drawn fully without variation on priority for base load in the system. Costliest power drawn as per requirement in the system is regulated peaking power. So, it is marginal regulating power input in the system. Hence, practically marginal power and UI power are each other’s substitute for economic operation. So, when UI rate is lower than marginal power rate, the system operator reduces intake from it and draws more power from the grid and vise a versa.
Observation and study of system operation in Gujarat after implementation of ABT revealed that effective UI rate is different from UI rate corresponding to frequency. Hence, action taken for advantage of economic operation may sometimes outcome as harmful. Such situation occurs due to grid operation characteristic as hereunder.
Low UI rate at high frequency attracts system operators to draw more power. Power drawn from the grid can be increased by creating deficit in the system either by decreasing generation or increasing the load. In any case, this results in overall shortage of generation against existing load in the grid. Obviously, grid frequency drops with drop in generation and UI rate increase. This increased UI rate is now applicable not only to incremental import but also to total import including previous import. Here concealed effect is previous import available at lower rate is now chargeable at high rate. This is the cause for alteration of apparent rate to effective rates.
Effective UI rate
Consider power system A in regional synchronous grid. System BIAS can be about 3% to 5% of system size depending upon the load mix in the system. Grid size and system BIAS @ 4% is assumed as per the table below.
Let system A already importing 200 MW at grid frequency of 49.90 Hz with corresponding UI rate Rs 1.68/Unit. System manager decides for economic consideration to draw more power from grid at this rate Rs 1.68/Unit and offset against costliest marginal power at Rs. 2.20/Unit. So, 80 MW is reduced from corresponding source. Effect of this is change of frequency and power exchange.
Grid frequency
Frequency drops as generation is reduced. Grid frequency change depends on overall load and generation unbalance in the grid. Therefore, frequency drop = generation drop / grid bias = 80/800 = 0.1 Hz. Hence, frequency drops to (49.90 – 00.10) 49.80 Hz. UI rate at 49.80 Hz frequency is Rs 1.96/Unit (This UI rate is still lower than Rs 2.20/Unit and appears to be lucrative.)
Power exchange
Area Control Error (ACE) is indicative of correction required in the system. It is derived from deviation of frequency and power exchange. Positive value of ACE indicates surplus generation in the system requiring back down. Whereas negative value of ACE indicates deficit of generation in the system requiring pick up (or load drop). Zero value of ACE indicates no correction is necessary in the system. ACE means generation minus load that caused change in frequency and power exchange.
Power exchange between system A and rest of the grid
Area Control Error = ACE = Deviation in export – corresponding system bias × frequency deviation.
For system A
ACE is -80 as 80 MW is dropped in the system.
Therefore -80 = deviation in export – 160 × 0.1
Deviation in export = -64 MW (Negative export means import)
For rest of the grid
ACE is zero as no change in load or generation in the rest of system.
Therefore 0 = deviation in export – 640 × 0.1
Deviation in export = 64 MW (Positive means export)
64 MW power export from the rest of the grid is import to system A.
Revised power import of system A is 200 + 64 = 264 MW.
Scrutiny of change
Comparison of pre and post action conditions:
Pre action condition
Frequency 49.90 Hz.
UI rate Rs 1.68 /Unit.
UI import by A = 200MW.
200 MW means 200 × 1000 Units per hour.
UI payment rate = 200×1000 ×1.68 = Rs 336000 /Hour.
Post action condition
Frequency 49.80 Hz.
UI rate Rs 1.96/Unit.
UI import by A = 264 MW
264 MW means 264 × 1000 Units per hour
UI payment rate = 264 ×1000 ×1.96 = Rs 517440 /Hour
Outcome
Additional import = 264 – 200 = 64 MW.
Additional payment rate = Rs 517440 – Rs 336000 = Rs 181440 /Hour.
64 MW means 64 × 1000 Units per hour.
Unit rate for additional power = 181440 / 64000 = Rs 2.835 /Unit
Summary
UI rate when thought for more power import was Rs 1.68/Unit (authentic beneficial)
UI rate when more power drawn is Rs 1.96/Unit (apparently beneficial)
UI rate effective for additional power = Rs 2.835 /Unit (really harmful)
Effective UI rate is different than frequency linked UI rates before and after the change.
- Factors Affecting the EUI rate: There is no general relationship between FUI and EUI rates in term of difference, percentage or ratio. However all it depends on operating condition.
- Unscheduled Interchange: UI before change because modified UI rate applies to it.
- System Size in Grid: Larger size of system has larger difference between FUI and EUI rate.
- Operating Frequency: Rate difference appears larger in higher frequency regime.
Conclusion
Generally, Frequency-linked Unscheduled Interchange (FUI) rate is compared with Marginal Regulating Power (MRP-costliest power) rate for replacement of power input in the power system for economic operation. But sometimes such action turned out to be uneconomical because of above phenomena. Whereas consideration of effective rate instead of frequency linked rate leave no scope for such incidences.
Typical chart above is plot of FUI rate, EUI rate and MRP rate for one of power system in grid operating at particular frequency. It represents EUI rate variations for different import or export conditions. Relative position of FUI rate, EUI rate ad MRP rate depends on operating frequency and costliest power in the system.
The intersection point N of EUI rate line and MRP rate line can be the operational target for optimum economical system operation. Interaction point N may be anywhere in import or export zone according to inputs in operating condition. Efforts have to be to move operating status towards target as far feasible.
Judgment in the matter is rather complex for real time operation. Chart may be useful as ready beckoner to the system manager for the purpose. Chart may be different for different systems.
A program was developed in computer system of SCADA in Gujarat. The program scheduled to run repetitively every minute. It takes snapshot of relevant parameters from database of SCADA system and calculate EUI rate and update in database. This was displayed on load dispatcher’s console so updated EUI rate was available.
Er. Natvar D. Makwana; Fellow of the Institution of Engineers, India (IE) and Fellow of Society of Power Engineers (CBIP); is a retired Senior Engineer served last 26 years on various positions at state load dispatch centre and post-retirement 7 years as visiting faculty of Parul institute of engineering and technology. He has contributed as a member of state/regional/national level various taskforces/committees related to power system operation.
Paresh R. Modha completed his B.E. from A. D. Patel Institute of Technology, New Vidyanagar, and M.E. in Electrical Engineering specialized in Electrical Power Systems from Birla Vishwakarma Mahavidyalaya (BVM), Gujarat Technological University, Ahmedabad. He has worked as a Junior Engineer in 66 kV Wind farm of Suzlon Substation and then joined academic filed with reputed CHARUSAT University, Changa during his starting career. At present he is working as an Assistant Professor in Department of Electrical Engineering, ADIT, New Vallabh Vidyanagar.
