Project Proposal
Sectionalizing study Of 132/33 KV Grid Sub-Station
Team Member Md. Siddique Hosssain Master of Electrical Power Engineering Student Kathmandu University Nepal. Supervisor: Local Supervisor: Mr. Roshan Bhattarai
Collaboration With Bangladesh Power Grid Company Limited (An Enterprise of Bangladesh Power Development Board)
July 5, 2005
Title: “Sectionalizing Study of 132/33 KV Grid Sub-Station”
Project Description:
Electric power transmission is the second process in the delivery of electricity to consumers. Transmission lines carry large quantities of electric energy from one point to another in an electric power system over large distances. Even with adequate electric generation, bottlenecks in the transmission system interfere with the reliable, efficient, and affordable delivery of electric power. One of the most important requirements of electric power system operation is to isolate and disconnect faulted parts of the system selectively and quickly. As a side benefit of a coordination study the interrupting ratings of all protective equipment, conductors, and switches are checked for adequacy. Inadequate equipment ratings can result in either extensive damage to the equipment during faults and system operation and may introduce hazards to plant operating personnel. Where there are two or more series protective devices between the fault point and the power supply, these devices must be coordinated to insure that the device nearest the fault point will operate first. Sectionalizing Study (Short circuit analysis, Coordination and reliability analysis) is critical for the safe, efficient, and economical operation of any electrical Transmission and distribution system. A Coordination Study maximizes power system selectivity by isolating faults to the nearest protective device, as well as helping to avoid nuisance operations. A Short Circuit Study will help to ensure that personnel and equipment are protected by establishing proper interrupting ratings. The reliability of electricity supply is a national concern. Reliability of the system is more important for quality supply of power. Traditional reliability statistics are based on the frequency and duration of outages on the power system. The ultimate goal in reliability analysis is the evaluation of the entire system so that overall reliability measures can be obtained. It is more prudent to calculate reliability measures separately for the generation, transmission and distribution systems. For transmission businesses, performance indicators such as Transmission Circuit Service Availability to supply minutes off and customer connection point interruptions to track service reliability and quality of performance. Therefore proper coordination of protective devices and reliability analysis should be made by an electrical engineer. It is the responsibility of the electrical engineer to design for optimum coordination and protection and to prepare reliability improvements plans. This is sometimes more art than science. In this circumstance, the sectionalizing study (coordination and reliability study) of grid sub -station is an important and potential thing for the student and/or organization involved (in this case, Collaboration Company is Bangladesh Power Development Board). The scope of the project involve with – Ø Maximizes power system selectivity by isolating faults to the nearest protective devices. Ø Identification of maximum and minimum momentary short-circuit current. Ø Identification of ground fault current at major buses. Ø Identification of existing coordination problem of the system Ø Identification of influencing parameters in the reduction of the reliability of the system. Ø Cost analysis of power outages.
Background:
With only around 30% of the population connected to the electricity grid in Bangladesh, and with power demand growing rapidly (10% annually from 1974-1994; 7% annually from 1995-1997), Bangladesh's Power System Master
Plan (PSMP) projects a required doubling of electric generating capacity by 2010. In accordance with, the load has been increasing rapidly in the grid system through distribution line. Most electrical power transmission and distribution systems are not planned with protective device coordination in mind. A supply system can be designed for minimum losses and minimum upfront investment and yet fail miserably in the proper coordination of the protective devices. As a result equipment failures within the system can easily result in the shutdown of the entire plant or substation. The objective of this collaborative project is to develop a maximum protection of equipment and a consistence statistical framework for evaluating year-to-year variation of transmission service quality and reliability performance indicators. A coordination study is the examination of the electrical system and available documentation with the goal of ensuring that over-current protection devices are properly designed and coordinated. Over-current protective devices are rated, selected and adjusted so only the fault current carrying device nearest the fault opens to isolate a faulted circuit from the system. This permits the rest of the system to remain in operation, providing maximum service continuity. Electrical systems commonly use fuses, relays and circuit breakers to protect electrical equipment such as conductors, transformers, motors, and other components. If a failure occurs within this equipment, usually a short circuit results. It would be desirable that this short circuit would affect only that portion of the system where the failure occurs. In a properly coordinated system the protective devices are selected and adjusted to minimize the impact of equipment failures within a system. A coordination study analyzes the characteristic curves of the fuses and breakers a compares them against one another on log-log plots. Any areas of miscoordination will be nd apparent by overlapping of curves from the various devices. A coordination study consists of the selection or setting of all series protective devices from the load upstream to the power supply. In selecting or setting these protective devices, a comparison is made of the operating times of all the devices in response to various levels of overcurrent. A new or revised coordination study should be made when a) The available short circuit current from the power supply is increased b) New large loads are added or existing equipment is replaced with larger equipment c) A fault shuts down a large part of the system d) Protective devices are upgrade The utility shall be contacted for information including the minimum and maximum fault currents that can be expected at the entrance to the facility. Once the data has been collected, a short circuit analysis followed by a coordination study should be performed. The resultant data can then be fed into the equations described by either NFPA 70E-2000 or IEEE Standard 1584-2002. Short circuit analysis is based upon a number of assumptions; any or all may change over time; 1. Available short circuit current from the utility may vary, particularly in areas where there has been a significant expansion of, or change to, the electrical systems. 2. The number of motors running at the time of a fault affects the amount of short circuit current and arc fault current available (motor contribution). 3. The facility voltage often varies as a function of time of day. The utility is often more loaded during the day. Similarly, the arc fault may also be affected by variations in any of the following: 1. The available short circuit current.
2. Dirt buildup in the equipment that may affect the conductive path. 3. Moisture (humidity). 4. Circuit supply voltage. Coordination time intervals shall be done by following ways1) When coordinating inverse time overcurrent relays, the time interval is usually 0.3-0.4 seconds. This interval is measured between relays in series either at the instantaneous setting of the load side feeder circuit breaker relay or the maximum short circuit current, which can flow through both devices simultaneously, whichever is the lower value of current. The interval consists of the following components: (a) Circuit breaker opening 0.08 seconds time (5 cycles). (b) Relay overtravel . . . . . . . 0.10 seconds (c) Safety factor for CT satu- 0.22 seconds ration, setting errors, contact gap etc. 2) This safety factor may be decreased by field testing relays to eliminate setting errors. This involves calibrating the relays to the coordination curves and adjusting time dials to achieve specific operating times. 3) When solid-state relays are used, overtravel is eliminated and the time may be reduced by the amount included for overtravel. 4) When coordinating relays with downstream fuses, the circuit opening time does not exist for the fuse and the interval may be reduced accordingly. The total clearing time of the fuse shall be used for coordination purposes. 5) When coordinating circuit breakers, equipped with direct-acting trip units, the characteristics curves shall not be overlaped. Coordination shall be done for a) Primary and secondary voltage coordination b) Ground Fault Coordination – There are three methods for ground fault protection 1) Non selective single zone method 2) Selective time coordinated method 3) Selective zone coordinated method. c) Phase coordination Coordination requirements The primary purpose of the coordination procedure is to select the proper ratings and settings for the protective devices on an electrical supply system. These ratings and settings should be selected so that pick-up currents and time delays allow the system to ignore transient overloads, but operate the protective device closest to the fault when a fault does occur. Proper selection of ratings and settings of protective devices requires knowledge of NFPA 70 requirements for protection of motors, transformers, and cables, knowledge of Magnetizing inrush as well as knowledge of ANSI C57.12 requirements for transformer withstand limits. Reliability is the probability of a system performing its function adequately for the intended period of time under specified operating conditions. The availability of power depends on the proper coordination of protective devices. Technically, reliability is considered under the two main aspects of adequacy and security. Adequacy refers to the ability of the electricity system to provide and transport energy to meet the requirements of customers. Security, on the other hand
Series System: For a series system, the failure rate is given by, l = l1 + l2 + l3 + …………………… failures per year The repair or down time is given by the equation r = (l1 × r1 + l2 × r2 + l 3 × r3 + ………….)/ (l1 + l2 + l 3 + …………) hours The hours/year of unavailability is given by: u = u1 + u1 + u1 + ………… = l1 × r1 + l2 × r2 + l3 × r3 + …………. Parallel System: For parallel configurations, the equations arel = l1 × l2 (r1 + r2 )/8760 failures per year r = r1 × r2 / (r1 + r2 ) hours per failure The hours/year of unavailability is given by: u = u1 × u1 ……… = l1 × r1 × l2 × r2 …………….. The performance of a system can be assessed and compared against that of other systems by collecting data on individual components. It must be noted that the comparison is on a probability basis rather than an absolute basis. A Case Study: Kulshi Grid sub -station is situated in the west-north part of Chittagong city, Bangladesh. The incoming source of this substation is coming from Madanghat Grid sub-station. This substation is very important for supply of power in Chittagong city area and national grid as well. Therefore, the proper coordination is very essential for reliable power supply in Chittagong. Kulshi-Halishahar: Line length -13 km : Circuit – Single : Conductor Name & Size – Grosbeak & 636 MCM Madanghat-Kulshi: Line length -13 km : Circuit – Single : Conductor Name & Size – Grosbeak & 636 MCM Kulshi-Baraulia: Line length -13.5 km : Circuit – Single : Conductor Name & Size – Grosbeak & 636 MCM Transformer Capacity: 2x44.1/63 MVA Or, Barishal Grid substation is situated in the southern part of Bangladesh. Almost six districts are getting power through this substation. Therefore, proper coordination is very essential for reliable power supply among these districts. Barisal-Patuakhali: Line length -37 km : Circuit – Single : Conductor Name & Size – Grosbeak & 636 MCM Bheramara-Faridpur-Barisal: Line length -225 km : Circuit – Double : Conductor Name & Size – HAWK & 477 MCM
Transformer Capacity: 2x25/41 MVA
Goals & Objectives of Project:
A sectionalizing study analyzes the impacts of short circuits and equipment failures within a facility and determines the effects on the facility operation. Informed decisions can then be made as to the changes necessary for the system. The main goal of this project is to make general guidelines from which proper coordination of protection devices and the reliability of transmission system will be improved in Bangladesh. The main objectives are given belowFault calculation Recommendation for protection coordination proposal Coordination of existing systems Coordination of proposed systems Time/current coordination curves Justification of protective devices proposed for line Reliability study of the system (a case study) Proposed single line diagram Tabulation of fault analysis
Methodology:
Based on the objectives, the following works shall carry out the following tasks. Collection of data for case study regarding the following: A. Visit the proposed (Kulshi Grid Substation or Barishal Grid Substation in Bangladesh, 132/33 KV) site. B. DATA COLLECTION: 1) Data required for the coordination study: a) Single Line Diagram of the system under study. b) System voltage levels c) Incoming power supply data: ü Impedance and MVA data ü X/R ratio ü Existing protection including relay device numbers and settings, CT ratios, and time-current characteristics curves. ü Transformer ratings and impedance data d) Data on system under study ü Transformer ratings and impedance data ü Protective devices ratings including momentary and interrupting duty as applicable. ü Time current characteristics curves for protective device ü CT ratios, excitation curve and winding resistance ü Thermal (I2t) curves for cables ü Conductor sizes and lengths e) Fault current and Load current data: ü Maximum and minimum momentary (first cycle) short-circuit currents at major buses. ü Maximum and minimum interrupting duty (5 cycles and above) short- circuit currents at
major buses. The exact value of ground fault current is impossible to calculate. f) Current Coordination Problems 2) Data required for the reliability study: a) Number of failures in a specified period (Forced outage, Maintenance outage, Momentary outage etc.) b) Average time between failures c) Loss in revenue due to failure in a specified time d) Loss in sub-station output due to failure C. COORDINATION PROCEDURE: The following procedure would be followed when conducting a coordination study. 1) A convenient voltage will be chosen as a base, and all values shall be converted to per unit values. 2) Short-circuit current shall be indicated on horizontal axis of the log-log graph. 3) The following information shall be provided on all curve sheets. a. Device identification and associated settings/size. b. Voltage at which curves are plotted. 4) Protection point shall be specified including magnetizing inrush point and NFPA 70 limits for large transformers. 5) Protective relay pick-up ranges shall be indicated. 6) The coordination curves shall be prepared on log-log paper and illustrate adequate clearing times between series devices. The curves shall be created through the use of the study software package, but must reflect actual protective devices to be installed. Adequate timecurrent curves shall be generated to depict coordination. In addition, protective device characteristics shall be suitably determined to reflect calculated short-circuit levels at the location. D. Fault Current Analysis: 1) Systematically fault currents shall be calculated based on the available fault current at the facility service entrance. Study preparer shall obtain the available fault current from the local utility. 2) The short-circuit currents shall be analyzed by preparing a tabulation comparing the fault levels to the device interrupting ratings. Indicate areas in which integrated/series ratings are utilized. The following information shall be included in the tabulation: a. Bus identification number. b. Location identification. c. Voltage d. Manufacturer and type of equipment. e. Device rating. f. Calculated short-circuit current. E. Various Steps, in reliability study area) Failure criteria shall be defined. b) System shall be defined c) Suitable assumptions shall be made. d) Cost of power outages shall be calculated e) System model will be prepared f) Analysis of carry out failure effect
g) Reliability improvement plans will be prepared. Tools: Commercially available software package (Net Bus, PS Cad, SKM etc.) shall be used for short circuit and coordination analysis. Outcome: The outcome shall be in the form of a report regarding line length of 132 KV systems, the detail information of the prototype software used for the short circuit and coordination analysis. Detail report regarding existing coordination problems and proposed coordination settings. Detail report on tabulation of sectionalizing devices, Fault current calculation worksheet for individual feeders and summary of recommendations. Detail report regarding necessary modifications. General guidelines shall be provided to improve the reliability and coordination of protection devices of transmission system in Bangladesh. Work Plan: Duration of the project shall be six months including site visiting, data collection, analyze and finalize the report. The details work plan is given in table:
July, 2005 August, 2005 September, 2005 October, 2005 November, 2005 December, 2005
Site Visit Data collection Short Circuit Analysis Coordination study Reliability Study Finalize report Final Presentation