Effective UI in ABT
Concept of
Effective Unscheduled Interchange Rate
in
Availability Base Tariff
Introduction: Availability Base Tariff is tripartite tariff. Third part frequency
linked unscheduled interchange rate indirectly provides open market for sale /
purchase of power at desired rates without any agreement, consent or
interaction with any agency. UI rate is variable from Rs 0.00 at 50.50 Hz to Rs
4.20 at 40.00 Hz. Any beneficiary can dump or draw power at prevailing rate. Consideration
of effective UI rate was useful to Gujarat to
fetch optimum benefit.
Preface: 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.
ABT: The salient Features of ABT scheme. Availability base
tariff is has three part tariff.
Fix Charge:
Capital cost charges of generator is
claimed as Fix 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 unfavorable condition of high frequency, they had tendency to maximize
generation up to target and beyond to get full fix 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 beneficiary to manage required power on merit order. Earlier
beneficiary were liable for payment of allotted share of generation including
unwarranted unused power.
Unscheduled
Interchange (UI): UI is 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 others 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 schedule import or exporting power
less than schedule export is consider as UI import. Similarly importing power
less than schedule import or exporting power more than schedule export is consider
as UI export. Generator injecting power more than schedule injection is UI
export and injecting power less than schedule 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 their
power system for least energy cost with optimal security. Available power from
own power plant, IPP and ISGS may have different energy cost 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. 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 of system. Costliest power is 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, system
operator reduce intake from it and draw 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 drop with drop in generation and UI rate
increase. This increased UI rate is now applicable not only to incremental
import but 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 table below.
|
|
Total
Grid |
System
A |
Rest
of Grid |
|
System
Size MW |
20000 |
4000 |
16000 |
|
System
Bias MW/Hz |
800 |
160 |
640 |
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 derive 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.
Judgment in the matter is rather complex for real time operation. Chart may be useful as ready beckoner to system manager for the purpose. Chart may be different for different system.
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.