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TRANSMISSION PLANNING AND SECURITY STANDARDS

  1. OBJECTIVE

    2. Transmission system planning shall be aimed at the system being capable of delivering power from the generating plants and interconnecting points with the systems of neighbouring States, and PGCIL to the load centres ie., the outgoing terminals of the E. H. T. grid sub-stations, under established criteria, while operating the power system as an integrated whole.

  2. TRANSMISSION PLANNING

    2.1 Long Term Transmission Planning shall be originated from Load Forecast and Least Cost Generation Expansion Plan of the Licensee for the period under consideration. Since, the Licensee's system operates in synchronism with generators and captive power plants inside the State and EREB system all these elements shall be included in the system, modelling. Any interconnection existing with the neighbouring State in radial mode shall not be included in the modelling.

    2.2 System Modelling

    2.2.1 Separate system models shall be developed for each year of a Plan Period to assess probable year of commissioning of particular lines, based on the network, obtaining for the year in question, with the generation and load buses properly located.

    2.2.2 For modelling purposes, the interconnections with EREB at 400 kV and 220 kV voltage levels shall be considered. An appropriate electrical equivalent shall be used to take into account the fault level at those interconnection points. Since those Buses will be represented as Generator Buses, generation and respective loads connected at these Buses shall be included in the modelling. Interconnection with the Southern Regional Grid and the Western Regional Grid shall be modeled as they exist.

    2.3 System Studies

    2.3.1 The system shall be evolved based on detailed power system studies which shall include;

    1. Load Flow Studies

    2. Short Circuit Studies

    3. Transient Stability Studies

    2.3.2 Computer Programmes
    The studies shall be carried out by suitable computer aided programmes.

    2.3.3 System Data
    The Licensee shall use updated system data, referred to in Transmission Operating Standards, in carrying out system studies.

    2.3.4 Active and Reactive Load allocation
    2.3.4.1 All loads shall be modeled at 220kV or 132kV Buses. The load for each Load Bus is obtained for any year within the Plan period from the Load Forecast and a reasonable estimate of transmission loses shall be made to arrive at peak generation. The annual minimum load shall be taken as a percentage of annual peak demand as prevailed in the base year.
    2.3.4.2 The MVAR loading at each Load Bus shall be assumed to be 50% of the MW loading taking average Power Factor of 0.90 Lag for peak and 0.95 lag for minimum load condition.

    2.4 Load and Generation Despatches

    2.4.1 Load
    Studies shall be carried out for Peak Load and Minimum Load conditions.

    2.4.2 Generation
    For peak load conditions two generator despatches shall be used i.e., Maximum Hydro Generation and Maximum Thermal Generation. For the minimum load the 'must-run' generation shall be used in conjunction with the most economical thermal generation. The generation despatch for purpose of carrying out sensitivity studies corresponding to complete closure of a generating station close to a major load centre shall be worked out by increasing generation at other stations to the extent possible keeping in view the maximum likely availability at those stations, cost of power, etc. Transmission constraints will be brought out and addressed.

    2.4.3 Studies shall be repeated for Normal and Contingency conditions as specified under security standards.

    2.5 Planning Criteria

    2.5.1 The Central Electricity Authority (CEA) "Manual on Transmission Planning Criteria" shall be adopted with modification as stated below, particularly with reference to steady state voltage limits and security standards for withstanding outages.

    2.5.2 Line Loading Limits
    The permissible line loading limits shall conform to CEA's "Manual on Transmission Planning Criteria". The over loading and under loading of lines shall be decided accordingly.

    2.5.3 Options for Strengthening of Transmission Network

    1. Addition of new Transmission lines to avoid over loading of existing system (wherever three or more circuits of the same voltage class are envisaged between two sub-stations, the next higher transmission voltage may be considered).

    2. Upgradation of the existing transmission lines.

    3. Reconductoring of the existing transmission line with higher size of conductors or with AAAC (All Aluminium Alloy) Conductor.

    4. Raising height of conductor supports and or switch over to insulated cross-arms to facilitate change over to higher voltage, if the tower designs so permit.

    The choice shall be based on cost, reliability, right of way requirements, energy losses, down time, etc.

    2.5.4 Double Circuit Towers shall be used for construction of all future lines.

    2.5.5 Steady State Voltage Limits
    The Licensee shall plan its Transmission System so as to maintain the steady State voltage within the limits stated below.

    Nominal
    Voltage(kv)

    Maximum

    Minimum

    (%)

    (kv)

    (%)

    (kv)

    400

    105(100)*

    420(440)*

    95(90)*

    380(360)*

    220

    110

    245

    90

    198

    132

    110

    145

    90

    119

    3. (* marked figures relate to contingency conditions)

  3. SECURITY STANDARDS

    3.1 Steady State Stability
    The system shall be planned to supply all loads during normal conditions and the following contingency conditions without the need for rescheduling of generation and to maintain voltage and line loading criteria.

    Outage of one transmission circuit or
    Outage of one Interconnecting Transformer or
    Outage of one Generator.

    (Prior to such contingency, all elements shall be considered to be in service)

    3.2 Transient Stability

    3.2.1 The system shall be designed to maintain synchronism and system integrity under the following disturbances :

    1. The outage of the single largest unit in the EREB system. (For this condition the GRIDCO share of spinning reserve shall be considered as 125 MW being 25% of one 500 MW unit).

    2. A permanent single line to ground (SLG) fault on a 400 kV transmission circuit, single pole opening of the faulted phase (100 M.Sec or 5 cycles) with unsuccessful reclosure (dead time 1 sec.) followed by 3 pole opening (100 M.sec) of the faulted line on a 400 kV transmission circuit (subject to note below).

      3. Note: In order to facilitate simulation, a 3 phase fault with 5 cycle duration shall be considered for 400 kV circuit fault. Should the system survive this fault condition, it shall be assumed that system's stability is established. Should the system not survive this fault, then SLG fault criteria shall be applied.

    3. A permanent three phase fault with a duration of 160 M.sec (8 cycles) on a 220 kV or 132 kV Transmission circuit assuming 3-pole opening.

    4. No stability studies shall be made for radial lines.                                                                        

  4. SUBSTATION PLANNING CRITERIA

    4.1 The rated rupturing capacity of the Circuit Breaker in any sub-station shall not be less than 125% of the maximum fault level at that sub-station. (The 25% margin is intended to take care of the increase in short circuit levels as the system grows). The standard rated breaking current capacity of switch gear at different voltage levels are as follows :

    Voltage Level

    Breaking Current(KA)

    132

    25 or 31 *

    120

    31 or 40 *

    400

    40

    (* The higher rupturing capacity shall be adopted for all new sub-stations).

    4.2 The capacity at any single sub-station at different voltage levels shall not normally exceed.

    Voltage Level

    Breaking Current(KA)

    400 kv

    1000 MVA

    220 kv

    320 MVA

    132 kv

    150 MVA

    4.3 Size and number of interconnecting Transformers (ICT's) shall be planned in such a way that the outage of any single unit would not normally over load the remaining Interconnecting Transformers.

    4.4 Size and number of EHT/H.T. Transformers shall be planned in such a way that in the event of outage of any single unit the remaining EHT/H.T. Transformers would still supply 80% of the load.

    4.5 Reactive Power Compensation

    4.5.1 Shunt Capacitors
    Reactive compensation shall be provided as far as possible in the high voltage systems with a view to meet the reactive power requirement of load close to the load points. In the planning study the shunt capacitors required shall be shown at 132/220 kV Buses.

    4.5.2 Shunt Reactors
    Switchable shunt reactors shall be provided at 400 kV sub-stations for controlling voltages within the limits specified. The step changes shall not cause a voltage variation exceeding 5%. Suitable Line Reactors (Switchable/Fixed) shall be provided to enable charging of 400 kV lines without exceeding voltage limits specified.