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State of Tamilnadu - Section

Section 7 in Tamil Nadu Electricity Grid Code, 2005

7. Introduction.

(1)In order to ensure the integrated operation of the State Electricity Grid and to enhance the overall operational economy and reliability of the electric power network spread over the entire State, the following operation guidelines have been specified. It covers the real time operation of the State Electricity Grid with all its interconnections to the neighboring State Grids. It applies to all the constituents of the State Electricity System, STU, SLDC, CTU, RLDC, Generators, IPPs and other agencies who are connected to the Grid. It also applies to the agencies who intent / plan to generate, transmit and distribute the electrical energy and become a constituent of the State Electricity Grid.
(2)Basic Policy: The primary objective of integrated operation of the State Grid is to enhance the overall operational economy and reliability of the entire electric power network spread over the geographical area of the State. The real time operation of the State Grid shall be supervised from the State Load Despatch Centre (SLDC). The roles of SLDC and STU shall be in accordance with the provisions of Act 2003. All State entities shall comply with these operation guidelines and coordinate with each other, for deriving maximum benefits from the integrated operation and for equitable sharing of obligations. A set of detailed internal operating procedures for the State Grid shall be developed and maintained by the SLDC in consultation with the STU and other State entities and shall be consistent with TNEGC and IEGC. The control rooms of the SLDC, all SSLDCs, Power Plants, EHV and HV sub-stations and any other control centers of all State entities shall be controled round the clock and manned by trained and qualified personnel wherever necessary. The SLDC shall develop methodologies / mechanisms for daily / weekly / monthly / yearly demand estimation (MW, MVAr and MWh) for operational purposes. The data for the estimation shall also include load shedding, power cuts etc., SLDCs shall also maintain historical database for demand estimation. The demand estimates are to enable the SLDC to conduct system studies for operational planning purposes.
(3)System Security Aspects: (i) The Tamil Nadu Electricity Grid shall be operated as a synchronized system. All State entities shall operate their respective power systems and power stations in synchronism with the Grid. Appropriate security standard will be adopted in the operation of the system. Reliability criteria will also be adopted.
(ii)No element/part of the Grid shall be deliberately isolated from the rest of the State Grid or islanded except:
• under an emergency, and conditions in which such isolation would prevent a total Grid collapse and / or enable early restoration of power supply,• when serious damage to a costly equipment is imminent and such isolation would prevent it,• when such isolation is specifically directed by SLDC or with specific prior approval from SLDC Complete re-synchronization of the Grid shall be restored as soon as the normal conditions are restored. The restoration process shall be supervised by SLDC, as per the approved operating procedures separately formulated.
(iii)The list of such important Grid link/elements on which the above stipulations apply shall be prepared and be available at SLDC / SSLDCs. In case of opening / removal of any important element of the Grid under an emergency situation, the same shall be communicated to SLDC at the earliest possible time after the event.
(iv)Any tripping, whether manual or automatic, of any of the above elements of State Grid shall be precisely intimated by the concerned entities to SLDC as soon as possible, say within ten minutes of the event. The reason (to the extent determined) and the likely time of restoration shall also be intimated. All reasonable attempts shall be made for the elements' restoration as soon as possible.
(v)All generating units, which are synchronized with the Grid, irrespective of their ownership, type and size, shall have their governors in normal, preferably at free-governor mode at all times. If any generator of over 50 MW rating is required to be operated without its governor in normal operation, the SLDC shall be immediately advised about the reason and duration of such operation. All governors shall have a droop between 3 %. and 6 % for thermal and 0 to 10 % for hydro generators.
(vi)Facilities available with load limiters, Automatic Turbine Run up System (ATRS), turbine supervisory control, coordinated control system, etc., shall not be used to suppress the normal governor action in any manner. No dead bands and / or time delays shall be deliberately introduced.
(vii)All generating units, operating at / up to 100% of their Maximum Continuous Rating (MCR) shall normally be capable of (and shall not in any way be prevented from) instantaneously picking up 5% of extra load and sustain the increase for a minimum of 5 minutes when frequency falls due to a system contingency. The generating units operating at above 100% of their MCR shall be capable of (and shall not be prevented from) going at least up to 105% of their MCR when frequency falls suddenly. After an increase in generation as above, a generating unit may ramp back to the original level at a rate of about one percent (1%) per minute, in case continued operation at the increased level is nol sustainable. Any generating unit of over fifty (50) MW size, not complying with the above requirement, shall be kept in operation (synchronized with the State Grid) only after obtaining the permission of SLDC. However, the entity can make up the corresponding shortfall in spinning reserve by maintaining an extra spinning reserve on the other generating units of the entity.
(viii)The recommended rate for changing the governor setting, i.e. supplementary control for increasing or decreasing the output (generation level) for all generating units, irrespective of their type and size, would be one (1.0) per cent per minute or as per manufacturer's limits. However, if frequency falls below 49.5 Hz, all partly loaded generating units shall pick up additional load at a faster rate, according to their capability.
(ix)Except under an emergency, or to prevent an imminent damage to personnel and equipment, no entity shall suddenly reduce his generating unit output by more than one hundred (100) MW without prior intimation to and consent of the SLDC, particularly when frequency is falling or is below 49.0. Similarly, no entity shall cause sudden increase in its load by more than one hundred (100) MW without prior intimation to and consent of the SLDC.
(x)All generating units shall normally have their Automatic Vqltage Regulators (AVR) in operation, with appropriate settings. *In particular, if a generating unit of over fifty (50) MW size is required to be opera ted without its AVR in service, the SLDC shall be immediately intimated about the reason and duration, and its permission obtained. Power System Stabilizers (PSS) in AVRs. of generating units (wherever provided), shall be got properly tuned by the respective generating unit owner as per a plan prepared for the purpose by the STU from time to time. STU will be allowed to carry out checking of PSS and suggests further tuning, wherever considered necessary.
(xi)Provision of protections and relay settings shall be coordinated periodically throughout the State Grid, as per a plan to be separately finalized by the STU in coordination with all entities.
(xii)All entities shall put in all possible efforts to ensure that the Grid is operated within the frequency limits prescribed by the implementation of Availability Based Tariff (ABT) i.e. frequency always remains within the 49.0 - 50.5 Hz band.
(xiii)All State entities shall provide automatic under-frequency load shedding, based on the operation of df/dt or constant frequency setting relay in their respective systems, to arrest frequency decline that could result in a collapse / disintegration of the Grid, as per the plan separately finalised by SLDC in consultation with STU, and shall ensure its effective application to prevent cascaded tripping of generating units in case of any contingency. All entities shall ensure that the under-frequency load shedding / islanding schemes are functional and no under-frequency relay is bv-passed or removed without prior consent of SLDC.
(xiv)Procedures shall be developed by SLDC in consultation with STU, to recover from partial / total collapse of the Grid and periodically updated in accordance with the requirements given under Code 7 (5) (f). These procedures shall be followed by all the entities to ensure consistent, reliable and quick restoration.
(xv)Each entity shall provide adequate and reliable communication facility internally and with other entities/ SLDC to ensure exchange of data / information necessary to maintain reliability and security of the Grid. Wherever possible, redundancy and alternate path shall be maintained for communication along important routes, e.g. SLDCs to SSLDCs and generating stations.
(xvi)The entities shall send information / data including disturbance recorder / sequential even recorder output etc., to SLDC for purpose of analysis of any Grid disturbance / event. No State entity shall block any data/ information required by the SLDC for maintaining reliability and security of the Grid and for analysis of an event.
(xvii)Only authorized persons as certified by STU can give/take line clear on Grid equipment and lines for maintenance/fault rectification works. For carrying out erection/ repair in the circuit above 230 kV the competent person should have a minimum operating experience of one year.
(xviii)Before undertaking hot line working/hot line washing of the insulators connected with Grid equipment, advance information should be given to SLDC and other operating personnel concerned.
(xix)All entities shall put in all possible efforts to ensure that the Grid voltage always remains within the following operating range.
Nominal Maximum Minimum
  Voltage in (kV rms)  
400 420 360
230 255 210
110 120 100
(xx)The steady state and transient stability limits of the Grid should be communicated periodically.
(xxi)There should be adequate spinning reserve in the system.
(xxii)The STU should provide power for maintenance, short term and emergency to the IPPs and other non utility generators in accordance with the connection agreement entered into.
(4)Demand Control: Demand control is concerned with the provisions to be made by SLDC to ensure the reduction of demand in the event of insufficient generating capacity, and transfers from external interconnections being not available to meet demand, or in the event of breakdown or operating problems (such as frequency, voltage levels or thermal overloads) on any part of the Grid.Towards this end the following requirements shall be complied with:
(i)Power drawing entities shall endeavor to restrict their net drawal from the Grid to within their respective drawal schedules whenever the system frequency is below 49.5 Hz. When the frequency falls below 49.0 Hz., requisite load shedding (manual) shall be carried out to curtail the over drawal. Such load shedding shall be pre-planned for each level of under frequency.
(ii)Further, in case of certain contingencies and / or threat to system security, the SLDC may direct the SSLDCs and other sub stations to decrease its drawal by a certain quantum. Such directions shall immediately be acted upon.
(iii)Each distribution licensee shall make arrangements that will enable manual demand disconnection to take place, as instructed by the SLDC /SSLDC, under normal and / or contingent conditions. The measures taken to reduce the entities' drawal from the Grid shall not be withdrawn as long as the frequency / voltage remains at a low level, unless specifically permitted by the SLDC / SSLDC.
(5)Operational Liaison (a) Operations and events on the State Grid: Before any operation is carried out on the State Grid, the SLDC will inform each participating entity, whose system may, or will, experience an operational effect, and give details of the operation to be carried out. Immediately following an event in the State Grid, the SLDC will inform each State entity, whose system may/ or will, experience an operational effect following the event and give details of what has happened in the event but not the reasons why.
(b)Operations and events on an entity's system: Before any operation is carried out on an entity system, the entity will inform the SLDC, in case the State Grid may, or will, experience an operational effect, and give details of the operation to be carried out. Immediately following an event on an entity system, the entity will inform the SLDC, in case the State Grid may, or will, experience an operational effect following the event, and give details of what has happened in the event but not the reasons why.
(c)Periodic Reporting: A weekly report shall be issued by SLDC to all entities of the State and STU and shall cover the performance of the State Grid for the previous week. The weekly report shall contain the following:
(i)Frequency profile showing the maximum and minimum frequency recorded daily and daily frequency variation index (FVI)
(ii)Voltage profile of selected substations.
(iii)Major Generation and Transmission outages and constraints.
(iv)Instances of persistent/significant non-compliance of TNEGC The SLDC shall also prepare a quarterly report, which shall bring out the system constraints, reasons for not meeting the requirements, if any, of security standards and quality of service, along with details of various actions taken by different agencies, and the agencies responsible for causing the constraints.
(d)Outage Planning: It is assumed that the list of elements of the Grid as per Annexure C-l and Annexure C-2 covered under these requirements shall be prepared and be available with SLDC for the proper implementation of the outage planning exercises. The objective of outage planning are:
• To produce a coordinated generation outage programme for the State Grid, considering all the available resources and taking into account transmission constraints, as well as irrigational requirements.• To minimize surplus or deficits, if any, in the system requirement of power and energy and help operate the system within Security Standards.• To optimize the transmission outages of the elements of the State Grid without adversely affecting the Grid operation but taking into account the generation outage schedule, outages of constituent systems and maintaining system security standards.Outage planning is prepared in advance for the current year and reviewed during the year on quarterly and monthly basis.
(e)Outage Planning Process:
(i)SLDC shall be responsible for analyzing the outage schedule given by all State constituents, preparing a draft annual outage schedule and finalization of the annual outage plan for the following financial year by 31st January of each year.
(ii)The STU, the IPPs, Generating Companies and Independent Private Transmission Companies (IPTCs) shall provide SLDC their proposed, outage programs in writing for the next financial year by 30th November of each year. These shall contain identification of each generating unit / line / ICT, the preferred date for each outage and its duration and where there is flexibility, the earliest start date and latest finishing date.
(iii)SLDC shall then come out with a draft outage programme for next financial year by 31st December of each year for the State Grid taking into account the available resources in an optimal manner and to maintain security standards. This will be done after carrying out necessary system studies and, if necessary, the outage programmes shall be rescheduled. Adequate balance between generation and load to be ensured while finalizing outage programmes.
(iv)The final outage plan shall be intimated to all entities for implementation latest by 31st January of each year or by such earlier date as may be mutually decided by STU/SLDC.
(v)The above outage plan shall be reviewed by SLDC / STU on quarterly and monthly basis in coordination with all parties concerned.
(vi)In case of emergency in the system viz., loss of generation, break down of transmission line affecting the system, Grid disturbance, system isolation, SLDC, may conduct studies again before clearance of the planned outage.
(vii)SLDC is authorized to defer the planned outage in case of any of the following:
(a)Major Grid disturbance.
(b)System isolation.
(c)Black out in a constituent system.
(d)Any other event in the system that may have an adverse impact on the system security by the proposed outage.
(viii)Each State entity shall obtain the final approval from SLDC prior to availing an outage.
(ix)The detailed generation and transmission outage programmes shall be based on the latest annual outage plan (with all adjustments made to date).
(x)In cases involving non-reconciliation of the planned outages, TNERC shall be consulted for orders.
(f)Recovery Procedures
(i)Restoration system presents unique challenges to the system operators since the transmission system topologies will be quite different from the well integrated system during the normal condition
(ii)Detailed plans and procedures for restoration after partial / total black out of each constituent's system within the State, will be finalised by the concerned entity in coordination with the SLDC. The procedure will be reviewed, confirmed and / or revised once every year.
(iii)List of generating stations with black start facility, inter-state, synchronizing points and essential loads to be restored on priority, should be prepared and be available with SLDC.
(iv)The SLDC is authorized during the restoration process following a black out, to operate with reduced security standards for voltage and frequency as necessary in order to achieve the fastest possible recovery of the Grid.
(g)Event information
(i)This requirement of reporting procedures in writing of reportable events in the system to all State entities and SLDC needs to be specified. The objective is to define the incidents to be reported, the reporting route to be followed and information to be supplied to ensure consistent approach to the reporting of incidents / events. The incident reporting shall be as per format specified in Annexure C-3.
(ii)Reportable Events: Any of the following events require reporting by SLDC / State Entities.
• Violation of security standards• Grid Indiscipline• Non-compliance of SLDC's instructions• System islanding / system black out• State black out / partial system black out• Protection failure in any element of STS or ISTS• System instability• Tripping of any element of the State Grid.
(iii)Reporting Procedure: In the case of an event, which was initially reported by a State entity to SLDC orally, the entity will give a written report to SLDC. In the case of an event, which was initially reported by SLDC to an entity orally, the SLDC will give a written weekly report to the entity in accordance with this section.
(iv)A written report shall be sent to SLDC or a State entity, as the case may be, and will confirm the oral notification together with the following details of the event:
• Time and date of event• Location• Plant and / or equipment directly involved• Narration of event• Demand and / or Generation (MW) interrupted & duration• All relevant system data including copies of records of all recording instruments including Disturbance Recorder, Event Logger, DAS etc.,• Sequence of trippings with time• Details of relay flags• Remedial measures
(h)Accident Reporting: Reporting of accidents shall be in accordance with the Section 161 of Act 2003 in both fatal and non-fatal accidents.