How to Improve Reliability of Distribution System
Down time for power network is extremely costly, cost is bear by consumer and as well as distribution utility. Distribution company bear loss in term of loosing revenue for not supplied power, increase in SAIFI (System Average Interruption Frequency Index) & SAIDI (System Average Interruption Duration Index), where as consumer cost / loss depend on type of load / system. Manufacturing industry may be suffer huge losses due to power failure, healthcare may also have losses & risk to human life. Companies listed in Fortune, downtime cost could be high as 1 Million dollar per hour.
The IEC defines?reliability?as “the ability to perform as required, without failure, for a given time interval, under given conditions.”
Methods to improve System Reliability:
Utilities in the world are now moving to this approach of standardization of design, design of various electrical equipment i.e. Switchgear, CB, Protection Panels, ATS ( Auto transfer Switches), RMUs ( Ring Main Units) etc. is submitted by vendors/supplier/OEMs & reviewed by utility team, this has following benefits
2. Fast & Smart Circuit Breaker
Selection of circuit breaker is of extreme importance, CB should have low operating time, better selection & flexible range of settings enabling engineers to select proper relay coordination. That will increase selectivity & hence reduce SAIFI & SAIDI. For example if withdrawable type CBs are used that will need minimum time if require to be replaced. CBs should be robust & ideally needing minimum maintenance. Time tolerance of CBs should be as minimum as possible for better relay setting coordination. Utilities companies should review and approve list of equipment.
3. Power System Monitoring & Management (PSMM)
Distribution systems are usually very large, consist of thousands of feeders, relays, transformer, RMUs, PMTs etc. Therefore now modern distribution system are deploying remote monitoring & remote controls. Monitoring of temperature of transformer from remote will prevent transformer fire hazard if excessive temperature is monitored, further relays are also connection to engineering PC centrally placed, where all relay settings, fault records can be retrieved, settings can be changed. This will enable fast fault rectification, while expert is sitting in center covering all utility. Further control supplies, trip coil supervision etc. can be monitored and urgent calls / message can be send to respect divisions automatically.
Furthermore all maintenance schedule & status can be uploaded to this system. A central dash board / distribution network map diagram can give detail of maintenance due date, status of maintenance done, status of healthy and defective relays, CBs, transformers etc. In this way automatic reminders will be issued by system to relevant department for schedule maintenance, automatic material request may be send to procurement based on defective materials, dashboard may also show failure reports of different equipment manufacturer wise , enabling to monitor different vendor performance, expected life of equipment based on condition monitoring may also been added to system. Record of RCA, indiscriminate tripping can be added in this system, all the data of relays, CTs, transformers, location, conductor , fuse types, PMTs, tower, RMUs in map format will be available with SAP number & this data base will be linked to load flow, short circuit & relay coordination department for fast central access.
4.Remote Online services for fault rectification & Maintenance.
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Online expert central operation room can be developed that can support / monitor during routine maintenance, support & analyze fault rectification, review relay settings from remote through engineering PC. This will reduce number of engineers required, fast response to fault so reduce down time.
5.Selectivity & Fast Recovery
In case of fault on the system, only the faulted portion of the system should be isolated, that is called selective tripping.
Fig 1.1 Selective Tripping Example
Refer to fig 1.1 substation consist of two outgoing feeder and one incoming feeder, if outgoing feeder K11 is faulted, K11 should only trip that is selective trip but if incomer K1 also trip with or before outgoing feeder that is called indiscriminate tripping that must be avoided.
Selectivity can be achieved via proper relay setting coordination studies. Now a days specialized software applications are used to perform this study. Load & fault current is also calculated using computer based applications. Correct fault level must be know in order to perform relay coordination, portable fault recorders may be installed to confirm short circuit level actual verses calculated & also check protection system performance.
Fast recovery also mean less SAIDI, auto re-closer may be installed, further disconnectors may be added to long feeder that will help to isolate faulted portion and restore power in remaining loop. Further fault locators may also installed that will provide type of fault and fault location and that will help in reducing trouble shooting /restoration time.
6. Condition Monitoring & Calculating Expected Failure time.
Equipment / cable / System condition monitoring / yearly test result can be plotted on diagram to forecast replacement time / expected failure date. Find fig1.2 where insulation test results are plotted vs time & expected failure date is calculated. The equipment can be ordered / procure in advance / repair or maintenance can be planned . This data will be placed in power system monitoring and management. Frequency of testing is also important to calculate failure accurately. In this way effective / remaining life of each equipment can be updated in system, that will help for better procurement, planning shutdowns, reduction in cost & increase in system reliability. If more than one maintenance are required as per system by different department same can be automatically combine by PSMM & shutdown is arranged.
Distribution system reliability is complex topic, new dynamics like distributed generation, domestic / local solar system, electronic load, increase power availability and reliability requirements, load diversity, increase in harmonics, micro grid etc. making it really challenging to design a reliable system. New software are being developed & us of PMSS is being increased by various utilities.
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