METHODOLOGY FOR THE EVALUATION OF THE NTC VALUES AT THE GREEK INTERCONNECTIONS
(This methodology is not binding for the north neighboring TSOs)
ADMIE
INTRODUCTION
The calculation of the NTC values at the Greek north interconnections (at the borders Greece-FYROM, Greece-Albania, Greece-Bulgaria, Greece-Turkey) is based on common transmission models which at all times take into account the actual topology and conditions of the South East European (SEE) region.
The calculation forms the basis of the allocation of capacity rights on each interconnection.
This document refers to the calculation of transfer capacity for imports and exports at the north borders of Greece. For the Greece-Italy connection, due to the fact that this is a DC cable, the NTC value is always 500 MW on both directions (same with the maximum capacity of the cable).
1. EVALUATION OF THE TOTAL NTC VALUE FOR IMPORTS AND EXPORTS FOR THE NORTH GREEK INTERCONNECTIONS
1.1NTC DEFINITION AND DETERMINATION
ADMIE is responsible for the operation of the Greek electric power system and its interconnections with other systems in order to guarantee the security of supply and provide the maximum available transfer capacities to the market.
The Net Transfer Capacity (NTC) values are evaluated using always a complete network of the South East European (SEE) region and an agreed maintenance plan.
For the evaluation of the NTC values the papers taken into account are listed in the references [1-5].
The NTC calculation is performed in different timeframes, on yearly, monthly and daily basis.
As a general rule the evaluation of the NTC values takes into account the ENTSO-E rules and definitions as well as the recommendations from the EU CM 1228/2003 guidelines in order to establish a transparent congestion management methodology that will cope efficiently with the interdependency of the loop flows.
At first the Base Case Exchange (BCE) among ADMIE and its neighbouring TSOs is established. The set of cross-border transactions considered in the base case relates to the best forecast for exchanges at the time frame considered. To be realistic the base cases have to rely on historical recorded data, adapted in order to take into account the foreseeable electric system changes. The same procedure follows for the entire countries of the South East European region. After the complete BCE for the entire SEE region is established among TSOs (harmonized) the individual TSOs send the forecasted models that correspond to the previously defined BCE programs.
After merging the forecasted models, the base case exchange (BCE) model for the entire South East European region is established.
In order to calculate the maximum total TTC/ NTC values for imports/exports for the Greek system by taking into account the interdependency of the loop flows, ADMIE performs combined NTC calculation where one area is considered by the north Greek neighbouring countries, Albania, FYROM, Bulgaria and Turkey and the other area is Greece. Similar procedure is followed by other TSOs for the calculation of the total TTC/NTC values [5].
For the calculations, production is increased in one area by (ΔΕ), proportional to its remaining capacity while production in the other area is decreased by the same amount (according to generation remaining capacity again). When under the N-1 criteria the security limit is reached for the entire South East European region, the Total Transfer Capacity between the two areas is established, taken as TTC=BCE+ΔΕ. In case the generation limit is reached in one area before violation of the N-1 criteria then additional generation will be taken into account though transits from neighbouring countries (e.g transits from Serbia and Romania for the evaluation of the Greek imports and transit from Italy for the evaluation of Greek exports).
The above TTC value corresponds to the overall maximum importing/exporting capacity for the Greek north borders.
In order to calculate the overall maximum importing/exporting capacity for the Greek north borders that can be offered to the market players under security conditions the TTC is reduced by a margin for security purposes (TRM). The Net Transfer Capacity can then be taken as NTC= TTC-TRM.
The following graph illustrates the procedure that is followed between ADMIE (system A) and the neighbouring TSOs, OST, MEPSO,,ESO EAD and TEIAS (system B)
NTC = BCE + ΔΕ – TRM (1) where:
BCE: Base Case Exchange
ΔE: Maximum Shift of generation that can be assigned preventing any violation from
appearing with (n-1) criterion.
TRM: Transmission Reliability Margin
1.2GENERATION SHIFT DEFINITION
The NTC determination depends on the power plant profile (location and power), and therefore an important issue is the method to shift generation for the determination of the NTC values.
The Increase/decrease proportional to the power reserve of the modelled power-plants is done at the following way:
The power plants are simultaneously increased/decreased in proportion to the remaining reserve Pmax – Pbase case (for increase), i.e. Pbase case – Pmin (for decrease).
All chosen injections are modified proportionally to the remaining available capacity.
(2)
: Actual active power generation (MW)
: New increased injection
: New decreased injection
: Shift generation, negative for increasing and positive for decreasing
: Maximum permissible generation (MW)
: Minimum permissible generation (MW)
2. EVALUATION OF THE NTC VALUES FOR IMPORTS/EXPORTS PER INTERCONNECTION FOR THE GREEK NORTH BORDERS
Being active in providing efficient competition on the Internal European electricity market, ADMIE follows Regulation No 1228/2003 and its ANNEX as amended by Commision´s Decision 2006/7707EC, referred to as ``The Guidelines``.
According to the above congestion management guidelines, coordination of electrical borders shall include:
(a) Use of a common transmission model dealing efficiently with
Interdependent physical loop-flows,
(b) Allocation and nomination of capacity to deal efficiently with
Interdependent physical loop-flows
As mentioned previously, the starting point for the NTC evaluation is the establishment of the Base Case Exchange model. The physical flow over the Greek interconnections in the base case prior to any generation shift represents the Notified Transmission Flow (NTF) in each interconnection [1].
A PTDF (power transfer distribution factor) is the share of a power transfer that flows on a considered border. Multiplied by the amount of the power transfer we get the power flow on the considered border. The physical flow is the physical flow over an interconnector i that was induced by a generation shift, (from the base case exchange up to an amount X) can be given from
(3)
It is obvious that and where N is the number of interconnectors.
For the maximum generation shift from the base case exchange up to the total TTC limit, the total transfer flow can be established for each interconnector as:
(4)
For each interconnector the additional exchange program that can be offered due to the maximum generation shift (TTC condition) can be taken from the formula:
(5)
For each interconnector i the final TTC value can be given from
(6)
For all interconnectors the total TTC value can be found as
(7)
The calculation of the TRM according to ETSO is based on empirical figures of the TSOs, or on statistical methods. ADMIE among other TSOs [5] uses the following formula to calculate the total TRM value:
(8)
where N is the number of interconnectors in operation between the control areas of ADMIE and the Greek north borders (the countries, Albania, FYROM, Bulgaria and Turkey). The above formula has proven its worth in practice the last few years and is appropriate for the evaluation of the NTC values at the Greek interconnectors since it can be applied for coordinated NTC calculation (NTC calculation which takes into account the interdependency of the loop flows).
The above total TRM value is split to each interconnector according to the PTDF factors calculated in (3), so
(9)
From the above formula it is obvious that the more the transfer capacity at an interconnector the more the safety limit in this interconnector.
For each interconnector the additional exchange program that can be offered due to the maximum generation shift under secure operating conditions (NTC conditions) can be taken from the formula:
(10)
For all interconnectors the total NTC value can be found as
(11)
3. NTC CALCULATION FOR IMPORTS AND EXPORTS AT YEARLY LEVEL
The yearly transfer capacity between the transmission network of Greece and the transmission network of each neighbouring TSO in both directions is calculated for the period from the 1st of January (00:00 h CET) to the 31st of December (24:00 h CET) of the year of reference.
In determining the guaranteed Yearly transfer capacity between the transmission network of Greece and each neighbouring country`s transmission network, ADMIE takes into account the agreed maintenance plan among the South East European region TSOs. Calculations of the guaranteed yearly values of the transfer capacities between the transmission network of Greece and each neighbouring transmission network are based on winter (January) and summer (July) forecast models of the entire ENTSO-E region. From the two forecasted models, ADMIE considers a ratio (approximately 50%) of the minimum of the two calculated NTC values as yearly right, in order to cope with uncertainties at the transmission network.
ADMIE and each neighboring TSO perform security analysis and calculates the NTC values.After all calculations, NTCs are exchanged and finally harmonized. If no agreement is taken then the lower value sets the NTC.
ADMIE and the TSO of each neighbouring transmission network harmonize the guaranteed yearly transfer capacity values (corresponding to the agreed coordinated maintenance program and the N-1 security criterion) by November 15th of the previous year.
4. NTC CALCULATION FOR IMPORTS AND EXPORTS AT MONTHLY LEVEL
Monthly transfer capacity values are evaluated by ADMIE and the TSOs of each neighbouring transmission network, agreed in written by the 9th calendar day of the previous month, and shall apply as from the first calendar day (00:00 h CET) until the last calendar day (24:00 h CET) of the month. If the 9th calendar day is not a working day, then it will be the first working day following the 9th calendar day.
TSOs agree for monthly reference network models, which are used for the monthly NTC calculation. A general monthly time schedule for making the network model for the 3rd Wednesday 10h30CET of month M:
- till 15th of month M-2: sending of the Base Case Exchanges (table)
- till 17th of month M-2: harmonization of Base Case Exchanges (table)
- till 20th of month M-2: exchange of the network models with agreed totals
- till 25th of month M-2: checking and merging of the regional model
- till 1st of month M-1: NTC calculation for month M
- till 5th of month M-1: harmonization of NTC values for month M
The countries which models are exchanged and merged into a common regional model within this procedure are: Albania, BiH, Bulgaria, Croatia, Greece, Hungary, Austria, FYROM, Romania, Slovenia, Serbia, Montenegro and Ukraine. If any of the above models is not available the last available model/information is used (e.g DACF-Day-Ahead Congestion Forecast files obtained from the EH).
The calculation of the NTC values is performed in a monthly basis with the complete network for 1030 CET (high tariff) as well as an additional monthly NTC calculation and evaluation is performed when one of the critical lines is expected to be out of operation (for example for maintenance purposes) as well as in cases when there is something unexpected in the region.
The parties inform each other for any unplanned disconnection of transmission components in their own grid, provided that these disconnections have an essential impact on the grid security of other party.
Each TSO performs security analysis and calculates the NTC values. After all calculations, NTCs are exchanged and finally harmonized. If no agreement is taken then the lower value sets the NTC.
After all monthly NTC calculations for imports/exports will be completed, then the monthly Available Transfer Capacity for imports/exports (assuming no netting) that will be offered to the market will be taken from the following formula:
ATC MONTHLY = NTCMONTHLY-AACYEARLY RIGHT (12)
where ATCMONTHLYis the Available Trasfer Capacityfor monthly auction, NTCMONTHLY is the Net Transfer Capacity for mothly auction and AACYEARLY RIGHT
is the Already Allocated Capacity from the yearly auction.
The following graph illustrates the procedure that is followed where area A is Greece and area B is the Greek north countries. (Albania, FYROM, Bulgaria, Turkey).
5. NTC CALCULATION FOR IMPORTS AND EXPORTS AT DAILY LEVEL
Calculation of the daily transfer capacity values is based on day-ahead congestion forecast (DACF) models, with the models of the Greek and its neighbouring TSOs transmission systems being planned using the latest data available (generation, load and system data for the auctioning day).
After all daily NTC calculations for imports/exports will be completed, then the daily Available Transfer Capacity for imports/exports is taken from the following formula (assuming that no netting is applied):
ATC DAILY = NTCDAILY-AACNOTIFIED YEARLY RIGHT -AACNOTIFIED MONTHLY RIGHT (13)
Where as ATC DAILY is the Available Trasfer Capacity for daily auction, NTCDAILY is the Net Transfer Capacity for daily auction and AACNOTIFIED YEARLY RIGHT is the Already Allocated Capacity from the yearly auction that has been notified and AACNOTIFIED MONTHLY RIGHT is the Already Allocated Capacity from the monthly auction that has been notified.
6. OTHER ASPECTS
ADMIE and the neighbouring TSOs have agreed to inform each other immediately, i.e on short notice in case of an unplanned disconnection and not later than the fifth working day of the previous month in case of a planned disconnection, of any disconnection of transmission elements in their own grid, provided that such disconnection has an essential impact on other country´s grid (according to the Annual Maintenance program for the SEE region).
If such necessity should arise , ADMIE upon agreement with the neighbouring TSO`s or in its own discretion yet on a short notice, may introduce amendments in the initially evaluated ATC values depending on the specific combination of circumstances that compromise the system operation security.
REFERENCES
[1] Definitions of Transfer Capacities in liberalised Electricity Markets, ETSO,
April 2001
[2] Net Transfer Capacities (NTC) and Available Transfer Capacities (ATC) in the Internal Market of Electricity in Europe (IEM). ETSO, March 2000
[3] Procedures for Cross-Border Transmission Capacity Assessments, ETSO, October 2001
[4 ] Technical guidelines for NTC determination, UCTE, March 2004
[5] General model used by Vattenfall Europe Transmission GmbH for calculating
the TTC and the safety margin,
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