Streamlining the System

CEA releases the draft manual on transmission planning

The Central Electricity Authority (CEA) recently issued the Draft Ma­n­u­al on Transmission Planning Cri­teria. It had brought out the first Manual on Transmission Planning Criteria in 1985, which was revised in 1994 and then again in 2013.

The revised manual has been brought out in light of the major changes taking place in the transmission landscape, including large-scale renewable energy integration, load growth, increased fault levels, right-of-way (RoW) issues, technical advancements and notification of transmission rules.

Expected to be finalised this year, the draft covers the planning philosophy, the information required from various en­tities, permissible limits, reliability criteria, broad scope of system studies, modelling and analysis, and provides guidelines for transmission planning. The planning criteria are meant pri­marily for the inter-state transmission system (ISTS) down to the 132 kV level and the intra-state tra­nsmission system (In-STS) down to the 66 kV level, including dedicated transmission lines.

A look at the key points contained in the Draft Manual on Transmission Planning Criteria…

Planning philosophy

  • The principle for planning the ISTS should be to ensure that it is available as per the requirements of the states and the generators, as reflected by their general network access (GNA) re­ques­ts. As far as possible, the transmission system should be planned and developed to match the growth in generation and load, and care should be taken to ensure there is no wasteful investment.
  • Transmission customers and utilities should give their network access re­quirement well in advance, considering the time needed for the implem­e­ntation of the transmission assets. Transmission customers also need to provide a reasonable basis for their tra­nsmission requirement, such as size and completion schedule of their generation facility, demand based on the electric power survey or otherwise, and their commitment to bear the tra­ns­mission service charges.
  • Transmission system planning for evacuation of power from hydro projects should be done river basin-wise considering the identified generation projects and their power potential.
  • In the case of highly constrained areas such as congested urban/semi-urban areas, difficult (hilly) terrain, etc., the transmission corridor may be planned by optimising RoW and cost. This will be done by adopting higher voltage levels for the final system and operating one level below the voltage level at the initial stage, or by using multicircuit towers for stringing circuits in the future, or using new technology such as high voltage direct current (HVDC) and gas-insulated switchgear.
  • For planning new transmission lines or substations, the draft manual states that the PM Gati Shakti National Mas­ter Plan portal launched last year could be used to identify the preliminary feasibility of lines/substations.
  • Critical loads such as railways, me­tro rail, airports, refineries, undergro­und mines, steel plants and smelter plants will plan their interconnection with the grid with 100 per cent redundancy and, as far as possible, from two different sources of supply.

Transmission planning

  • As per the draft manual, the voltage level consideration for ISTS planning in the transmission network may be generally modelled down to the 220 kV level, except for the north-eastern re­gion, Uttarakhand, Himachal Prade­sh and Sikkim, which may be modelled down to the 132 kV level. Generating units that are stepped up at 132 kV or 110 kV may be connected at the nearest 220 kV bus through a 220/132 kV transformer for simulation purposes. Generating units smaller than 50 MW within a plant may be lumped and modelled as a single unit; however, the total lumped installed capacity of a single unit should be limited to 200 MW. Load may be lumped at 220 kV or 132 kV/110 kV as the case may be.
  • The load generation scenarios should be worked out in a pragmatic manner so as to reflect the typical daily and se­a­­sonal variations in load demand and generation availability. A typical load generation scenario may include high renewable energy generation, high hydro generation, high demand, low demand or a combination thereof.
  • In the case of thermal units (including coal, gas, diesel and nuclear), the minimum level of output (ex-generation bus, that is, net of auxiliary consumption) should be taken as not less than 40 per cent of the rated installed ca­pacity. If the thermal units are encouraged to run with oil support, they may be modelled to run for up to 25 per cent of the rated capacity.

Contingency criteria

  • As per the manual, it is required that after a more probable grid disturban­ce, that is, loss of an element (single co­­ntingency condition), all system pa­rameters such as voltage, loading and frequency shall be within permissible normal limits.
  • However, after suffering one contingency, the grid could still be vulnerab­le and experience a second continge­ncy, though less probable, wherein so­me of the equipment may be loaded up to the emergency limits. To bring the system parameters back within th­e­­­ir normal limits, the manual states that load shedding/rescheduling of generation may have to be impleme­nted ei­ther manually or through automatic system protection sche­mes. Such me­a­sures can be applied within one hour after the disturbance.
  • The emergency thermal limits for the purpose of planning will be 120 per cent of the normal thermal limits for one hour and 110 per cent of the normal thermal limits for two hours.

Substation criteria

  • The transmission licensee must provide details of the transmission equipment that needs to be upgraded or for which renovation and modernisation needs to be carried out to the CEA/central transmission utility/STUs.
  • The transmission lines approaching the substations should normally be pe­r­pendicular to the substation boun­dary for a stretch of 2-3 km to avoid cr­ossing of lines with multicircuit towers.
  • The maximum short-circuit level on any new substation bus should not exceed 80 per cent of the rated short circuit capacity of the substation; the 20 per cent margin is intended to take care of the increase in short-circuit le­vels as the system grows.

Addition criteria for renewables, bulk transmission and RoW optimisation

  • As per the manual, all generation projects based on renewable energy sour­ces will be required to comply with the Central Electricity Authority (Te­ch­ni­cal Standards for Connectivity to the Grid) Regulations, 2007 and the amen­d­ment thereof, for which requisite system studies will be carried out by the rene­wable generation project de­­ve­l­oper. Also, the GNA quantum will have to be considered while plan­n­ing the evacuation system, both for im­me­diate connectivity with the ISTS/In-STS and for onward transmission requirements.
  • In the case of a transmission system as­sociated with a nuclear power station, the draft manual states that there will be two independent sources of power supply for the purpose of providing start-up power. Also, the evacuation system should generally be planned so as to terminate it at large load centres to facilitate islanding of the power station in case of a contingency.
  • The manual further states that the option of HVDC bipoles may be considered for transmitting bulk power (more than 2,000 MW) over long distances (preferably over 700 km). HVDC transmission may also be considered in the transmission corridors that have AC lines carrying heavy power flows (over 5,000 MW) to control and supplement the AC transmission network. The ratio of fault levels in MVA at any of the convertor stations (for conventional current source type), to the power flow on the HVDC bipole should not be less than 3.0 under any load generation scenario. Further, in areas where multiple conventional HVDC bi­poles are feeding power (multi-in­feed), appropriate studies may be carried out at the planning stage in order to avoid commutation failure.

Conclusion

By making provisions for large-scale renewable energy integration, load gro­wth, increased fault levels, RoW issues and technical advancements, the Draft Ma­nual on Transmission Planning Cri­te­ria addresses some of the key limitations in the existing regulatory framework. It has, moreover, outlined several grid strengthening measures as well as plans for the adoption of advanced technologies to enable large-scale power evacuation and maintain grid security, which aligns well with the accelerated growth expected in the transmission segment. Going forward, the draft manual’s guidance will help tackle the specific challenges of transmission developers to make the grid larger, more flexible and more resilient.

 

 

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