About Us
    Board of Directors
    Deptt. Of Power
    Statutory Reports & RTI
    WB Projects
Subject:           Methodology for calculation of Transmission System Availability Factor of a Month (TFAM).
            HVPNL has been calculating Transmission System Availability on monthly basis w. e. f. April 2006 on a common reporting format, as per CERC formula, which is given as under:-
% System Availability for AC system
         (o X AVo + q X AVq)
=    --------------------------------- X 100
                 (o + q )
‘o’ signifies total number of AC lines and ‘AVo’ signifies Availability of ‘o’ number of AC lines and is calculated as under:
AVo (Availability of o no. of AC lines) = ∑ Wi (Ti -TNAi) / ∑ Wi
‘q’ signifies Total number of ICTs and ‘AVq’ Availability of ‘q’ number of ICTs and is calculated as under:
AVq (Availability of q no. of ICTs) = ∑ Wk(Tk - TNAk) / ∑ Wk
Where Wi, = Weightage factor for ith transmission line which is equal to Surge Impedance Loading for Uncompensated line (SIL) multiplied by Circuit Km.
Wk =  Weightage factor for ICTs which is equal to the rated MVA capacity of ICTs.
 Ti, Tk,= The total available hours of ith AC line, the total available hours of ICTs excluding time period for outages not attributable to HVPNL i.e. Outage of elements due to acts of natural events beyond the control of the HVPNL, Outage caused by grid incident/disturbance.
TNAi, TNAk, = The non-availability hours for ith AC line and the non-availability hours for ICTs excluding time period for outages not attributable to HVPNL like Shut down of transmission elements availed by other agency/agencies for maintenance or construction of their transmission system & Manual tripping of line due to over voltage as per the directions of SLDC/RLDC.
                        Now, CERC vide notification dated 19.01.2009 has revised these regulations called as the Central Electricity Regulatory Commission (Terms and Conditions of Tariff) Regulations, 2009. These regulations came into force from 1.4.2009, and shall remain in force for a period of 5 years from the date of commencement. As per these regulations, the fixed cost of the transmission system shall be computed on annual basis, in accordance with norms contained in these regulations, aggregated as appropriate, and recovered on monthly basis as transmission charge from the users. Transmission system availability plays an important role in deciding the transmission charges payable for a calendar month for a transmission system. The transmission charge (inclusive of incentive) payable for a calendar month for a transmission system or part thereof shall be:-
AFC x ( NDM / NDY ) x ( TAFM / NATAF )
AFC = Annual fixed cost specified for the year, in Rupees
NATAF = Normative annual transmission availability factor, in per cent
NDM = Number of days in the month
NDY = Number of days in the year
TAFM = Transmission system Availability Factor for the Month, in Percent
            The transmission licensee shall raise the bill for the transmission charge (inclusive of incentive) for a month based on its estimate of TAFM. Adjustments, if any, shall be made on the basis of the TAFM to be certified by the Member-Secretary of the Regional Power Committee of the concerned region within 30 days from the last day of the relevant month.
            TAFM, in percent, shall be equal to (100 – 100 x NAFM), where NAFM is the Non-availability Factor in per unit for the Month, for the transmission system / sub-system NAFM for A.C. systems / sub-systems shall be calculated as follows :
[ ∑ ( OHl x Cktkm l x NSCl)
∑ ( OHt x MVAt x 2.5 ) 
NAFM      =
i = 1
t = 1
[ ∑ (Cktkm l x NSCl)
∑ (MVAt x 2.5 ) +   ]
identifies a transmission line circuit
identifies a transformer / ICT
total number of line circuits
total number of transformers and ICTs R
Outage hours or hours of non-availability in the month, excluding the duration of outages not attributable to the transmission licensee.
Length of a transmission line circuit in km
Number of sub-conductors per phase
MVA rating of a transformer / ICT
Total hours in the month
            The transmission elements under outage due to following reasons shall be deemed to be available:
         I.      Shut down availed for maintenance or construction of elements of another transmission scheme, if the other transmission scheme belongs to the other transmission licensee.
       II.      Switching off of a transmission line to restrict over voltage and manual tripping of switched reactors as per the directions of NRLDC/SLDC.
                        Outage time of transmission elements for the following contingencies shall be excluded from the total time of the element under period of consideration.
               I.      Outage of elements due to acts of God and force majeure events beyond the control of the transmission licensee. 
             II.      Outage caused by grid incident/disturbance not attributable to the transmission licensee, e.g. faults in substation or bays owned by other agency causing outage of the transmission licensee’s elements, and tripping of lines, ICTs, etc. due to grid disturbance. However, if the element is not restored on receipt of direction from NRLDC/SLDC while normalizing the system following grid incident/disturbance within reasonable time, the element will be considered not available for the period of outage after issuance of NRLDC/SLDC’s direction for restoration.
  The data displayed is for general information of Public/Government/Other Users. HVPNL shall not be responsible for any discrepancy in the data. Please contact Chief Engineer Administration, Shakti Bhawan, Sector-6, Panchkula, for further information.  
  Triggered by :: TRIGMA © 2008