Chapter-1

 TRANSMISSION PLANNING AND SECURITY STANDARDS

1.              OBJECTIVE

Transmission system planning shall be aimed at the system being capable of receiving power from the generating plants, CTU’s and interconnecting points with the systems of neighbouring States, and transmitting the same up to the outgoing terminals of the transmission licensee's grid sub-stations, under established criteria, for operating the power system as an integrated whole.

2.      TRANSMISSION PLANNING

2.1     Load Forecast and the power procurement Plan of the Licensee for the period under consideration shall form the basis of long term Transmission system planning for 10 years or above. The transmission system would be planned on the basis of regional self-sufficiency with an objective of dovetailing the Licensee’s system into the regional power grid. The Licensee's system would operate in synchronism with generators, which may also include some captive power plants within the State and NREB system. All these elements shall, therefore, be included in the system modeling.

        Keeping in view the future long term perspective plan, a detailed mid term transmission planning for a period of 5 years shall be carried out to ensure efficient investment plan and stating the objective of each of the project modules during the period under consideration.

2.2    System Modeling

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

2.2.2  For modeling purposes, the interconnections with NREB at EHT levels shall be considered. An appropriate electrical equivalent shall be used to take into account the fault level at these interconnection points. Since these Buses will be represented as Generator Buses, generation and respective loads connected at these Buses shall be included in the modeling.

2.3    System Studies

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

i.               Load Flow Studies

ii.             Short Circuit Studies

iii.            Transient Stability Studies

These studies shall be carried out by suitable computer aided programme so that change in system performance could be studied in relation to system parameters on dynamic basis.

2.4                Load and Generation Despatches

2.4.1     Load

• All loads shall be modeled at 220kV or 132kV or 66 kV buses. The load for each bus would be obtained for any year within the plan period from the Load Forecast and a reasonable estimate of transmission losses shall be made to arrive at peak generation requirement. The annual minimum load shall be taken as a percentage of annual peak demand as prevailed in the base year.

• 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.

• Studies shall be carried out for Annual Peak Load, seasonal variation in peak loads and Minimum Load conditions.

2.4.2            Generation

• Generation availability in the state, its share in Joint /Central sector power stations and from any other reliable sources shall be considered for dispatches.

• 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.

• Special dispatches corresponding to high agriculture load with low power factor wherever applicable.

• 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 Manual on "Transmission Planning Criteria" issued by Central Electricity Authority (CEA) 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".

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. Application of series capacitors in existing transmission line shall be considered to increase its power transfer capability.

4. Reconductoring of the existing AC transmission line with higher size conductors or with AAAC (All Aluminum Alloy Conductor) where ever applicable shall be considered.

5. Adoption of multiciruit and multi voltage level transmission lines.

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

2.5.4 Normally Double Circuit Towers shall be used for construction of all future lines. However, where only 2 circuit are planned for evacuation of power from a generating station, particularly thermal plants, these shall be 2x S/C lines.

2.5.5  Reactive power flow through ICTs shall be minimal and normally should not exceed 10% of rating of ICTs.

2.5.6  Steady State Voltage Limits

The transmission System shall be so planned as to maintain the steady State Voltage within the limits stated below.

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 necessitating load shedding or rescheduling of generation output as also to maintain voltage profile.

a)                     Outage of one transmission circuit or

b)                Outage of double circuit line (in case of generating station)

c)             Outage of one Interconnecting Transformer or

d)                 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 :

a.             The outage of the single largest unit in the NREB system.

b.           A permanent single line to ground (SLG) fault on a 400 kV/220 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).

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.

c.             The system shall be capable of withstanding a permanent fault on one of the circuits of a 400 kV D/C line when both circuits are in service and a transient fault when the system is already depleted with one circuit under maintenance/outage. Accordingly, 3-pole opening (100 msec) of the faulted circuit shall be considered when both circuits are assumed in operation and single pole opening (100 msec) of the faulted phase with successful reclosure (dead time 1 sec) when only one circuit is in service.

d.         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.

e.            No stability studies need 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:

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

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                Reactive Power Compensation

4.4.1    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.

4.4.2         Shunt Reactors

Switchable shunt reactors shall be provided at 400 kV/220 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.

5.                   General

   i)   While planning, the transmission lines for power supply in urban area and other areas where the Right –Of –Way (R-O-W) constraint could be anticipated, long term requirement may be taken into account and all the lines shall be planned as D/C lines using twin conductor of higher than the conventional size. This approach would help in avoiding the Right –Of –Way problems later on.

  ii)    In urban (high population density) areas, availability of land and possibility to its encroachment are the major problem being faced by most of the power utilities. In order to tackle this problem, it is suggested that land might be acquired for sub-station requirements anticipated in long term and few approach towers from the sub-station may be constructed as D/C towers to prevent encroachment and overcome difficulty in obtaining R.O.W. in future.