Management of sub-transmission and distribution system in power utilities

Management of sub-transmission and distribution system in power utilities

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Girish Sidana
Hope Technlogies Ltd., New Delhi

Introduction
Electric utilities in India are facing the pressure of reducing costs and improving the quality and reliability of supply. Though the Generation and transmission systems have seen considerable technical development and capital investment, the distribution and to some extent the sub-transmission systems have been neglected and have suffered due to poor operating efficiencies leading to financial losses and cynical customer perception. The distribution systems have suffered mainly from the following maladies:

  • Unbalanced Load Flow
  • Poor voltage regulation
  • High level of Technical (Peak power/Energy losses)
  • Conductor Heating / Equipment Damages
  • Very high unaccounted energy losses [20-40% against international standards of 8-10%]

Acute need for a consistent and Long lasting solution aimed at Improving & Strengthening of the Power Distribution network with minimum losses in the long run through Integrated planning for Power Distribution system has been felt for a long time now for it to be ignored any longer.

Improving the performance of distribution systems to meet required targets is a matter of selecting the most cost-effective technologies and operating practices. The distribution systems tend to be very extensive with a long life span for conductors and plant. It is not sufficient to analyse how a particular portion of the network may be modified to improve its performance today: it is a matter of determining what would be the optimal solution when allowance has also to be made for the uncertainties in the prediction of the future scenario of customer demand. It is valuable to investigate long-term solutions specially so when the implementation of the solutions will also entail decisions involving large-scale investments.

Integrated software solution for Power distribution
Arising from these issues, the realization by the Utilities and the increasing reliance on having accurate up-to-date information for decisions on increasing revenues, improving customer service and development is now setting in. Thankfully, there is now the awareness that this approach will be driven by modern technology and not merely a continuation of the thumb-rule approach. No doubt, the vast field and organizational experience of the power utilities will continue to provide the required inputs into the total process.


Fig. 1

For fast, accurate and reliable data management, a power distribution software solution based on an integration of dedicated GIS + AM/FM and Engineering analysis (viz., network analysis) solution (See Fig. 1) will provide the right kind of approach for addressing the issues that have been raised above. The advantages of using a dedicated GIS + AM/FM approach are manifold:

  • Visualisation: Can visualise the actual network, as it will be laid out on ground.
  • Convenience: User friendly and ease of data entry.
  • Flexibility: Flexibility of choosing what other systems to share or exchang data with (i.e. Network Analysis, Inventory, Trouble call analysis)
  • Better comprehension.

Since the sub-transmission and the distribution network of a power utility have a geographical reference, it will be beneficial to create the network also on the computer in a geographical context. This will provide useful reference for setting up of new facilities, provide necessary information on land use pattern for planning optimum expansion of network and enable more systematic network operation and maintenance.
It would be ideal to model low voltage network emanating from each and every distribution transformer. However, the network data would be acquired in phases due to the large number of network entities and the large number of low voltage consumers. In the first phase, data collection of sub-transmission network (33kV or 66kV) and 11kV distribution and sample low voltage (0.4 kV) system including consumers should only be taken up. The sample low voltage network would be from high density, medium density and low-density pockets and covering different categories of consumers. This will serve the basis for determining the overall losses and improvements required in the entire low voltage network. In the subsequent phase, complete low voltage network could be taken up, if required.

Data collection and mapping of the network would require a survey of the entire geographical area over which the network is spread out. This survey would consist of the following tasks:

  • Create a digitised background map of the area, which could be one or both of the following:
  • Geo-reference satellite imagery procured from NRSA
  • Survey of India Maps
  • Survey entire area locating and collecting attribute data of each and every entity, viz.,
  • Pole
  • Overhead Conductor segment
  • Underground Conductor segment
  • 11/0.4kV distribution transformer sub-station

The poles, consumer location/service poles, sub-stations etc. shall be located to within an accuracy of ± 100meters relative to the base map. This could be done either with a Differential Geographical Positioning System (DGPS) or other techniques using Electronic Distance /Angle measuring Equipment based on lasers or combination of these techniques.

Plot the above points of entities on the digitised maps. Model the network using a network mapping software and build customised queries on network database.


Fig. 2

Figures 2 and 3 show the views of a distribution network with attached Information and distribution network with Land base Map


Fig. 3

GIS for day-to-day Operation & Maintenance
Interfaces that use the querying facility available with the RDBMS in the network-mapping environment will provide the accurate and reliable information to the utility operational staff on the spatial and non-spatial attribute data of the network created in the GIS. Customised interfaces can be delivered as also the staff can be easily trained to design and create their own interfaces. The information can be in the nature of an inventory report of any type of network entity (poles, conductors, underground cable segments etc.) or location information etc. As a real world example, the facility could be used to point out all the poles and the conductor segments located along a road, which would be effected due to say, widening of road or construction of an over bridge. The actual topology around the identified area could help in planning an alternative permanent route for the affected section or planning a temporary section during the period the construction work is going on.


Fig. 4

Figures 4 and 5 show the views of a query showing all the through poles of a particular type.


Fig. 5

However, monitoring of the distribution system on a real time basis and also to introduce a certain measure of automation in to the distribution system will mean investing in a Supervisory Control and Data Acquisition system (SCADA). Integration of the network mapping and the network analysis software with SCADA will prove to be a tool of immense benefit to the power distribution system utility in improving the operating efficiency and consequently customer satisfaction.
Network Analysis for technical loss reductions and Network planning
Once the Electrical database of the network is imported from the GIS/AM/FM into an Electrical Engineering Analysis platform, the resulting network model can be subjected to various analysis runs for carrying out studies that will be of interest to a distribution engineer. These will include the following but not limited to,

  • Modelling Load for different consumer categories.
  • Modelling unbalanced Load.
  • Voltage drop/ Load flow Analysis.
  • Fault current & Fault flow analysis.
  • Automatic capacitor Placement.
  • Load Balancing
  • Contingency analysis etc


Fig. 6

Figures 6 and 7 show the view of the typical voltage drop analysis and capacitor placement in representative network:


Fig. 7

Real time and historical demand and energy data at source and also at consumer end will also be needed to simulate the network loading conditions. The source for the historical data will usually be the log registers maintained at the distribution sub-stations. Some data augmentation may have to be done either by installing logging type meters at strategic points in the network or by taking one time current measurement using tong-testers.

In the short term, the results of the analysis would be used for estimation of base level technical losses, and for segregating the total unaccounted losses of the system, available from the energy balance into technical losses and the non-technical i.e. commercial losses (occurring mainly due to faulty /tampered metered supplies, due to the illegal connections, due to power delivery at flat rate to the subsidised category of consumers). Besides, extrapolation of the results of the analysis of a sample low voltage (0.4 kV) network for the complete low voltage network will also be done. The technical power loss reduction will be accomplished by the following measures:

  • Network reconfiguration including installation of new Primary and secondary sub-stations/up-gradation of existing sub-stations
  • New distribution transformer sub-stations/up-gradation of distribution sub-stations.
  • Re-conductoring
  • Capacitor placement
  • Load balancing in the three-phase system
  • Refurbishment/replacement of old and obsolete equipment.

The measures will follow certain system design philosophy that would be decided in consultation with the customer e.g., priority level of consumer in terms of service availability, requirement of short-circuit levels, level of redundancy required etc. The system improvement measures will be applied to meet the specified common requirements, which would generally be followed viz., Loading limits of conductors and transformers, supply voltage variations within the limits specified by the Indian Electricity rules, targets for economic loss levels as per CEA norms etc.

In the long-term, due to the need to identify and predict the customer demands that will decide the system loading conditions prevailing in future, use of other techniques like the trend analysis of load growth and load forecasting will be required. The process of load forecasting based on the trend analysis of past load growth, though complex, is invaluable in optimising the planning approach for the network expansion on a long-term basis. As stated above, having a geographical reference for the network will provide necessary information on land use pattern for planning optimum expansion of network and for setting up of new facilities. The spatial load forecast method, which divides the total area into number of small areas, with the specific growth rates and the load characteristics applied to each small area, is the ideal method for optimal planning of the distribution system of the area. For optimal location of new sub-stations and augmentation of the existing facilities, several alternatives, subject to their meeting the agreed planning criteria, will be evaluated on basis of the capital cost of equipment and work and net worth present worth of the energy losses over the total horizon period. The plan objective will be the minimization of losses while maximizing the net benefit i.e., the present worth of loss reduction less the annual cost of capital investment. The cost estimates will be based on the prevailing market rates for each item of equipment and work.

The load flow analysis study would provide a graphical display of the results of the network strengthening measures described above in meeting the specified requirements, as well as the losses for the various alternatives. The user can take decision, looking at the graphical display provided by the analysis software, for utilising the different network strengthening measures at his disposal, available as ‘edit’, ‘network sketching’ and specific ‘network optimisation’ features in the software.
Overall energy loss reduction
The investment to be made to upgrade the distribution system can be meaningful only if simultaneous steps are taken to render the power utility financially viable. It is very well understood that capital expenditure on technical improvement or system augmentation/up-gradation alone can not be financially justified merely because it will improve quality and reliability of power supply and benefit customer. Ultimately, consumers individually or as a group have to pay for the services delivered. The most common causes for the commercial losses i.e. loss of revenue to the utility arising out of energy consumed but not paid for are: due to faulty /tampered metered supplies, due to illegal connections, due to power delivery at flat rate to the subsidised category of consumers.

Now, it is unfortunate but true that to day there is not even a correct assessment of the quantum of the total Energy losses i.e. commercial plus technical losses prevailing in a sub-transmission and distribution system of say, a distribution circle or a discom. It is now imperative and a strategic need to establish Energy accounting system for assessment of the commercial (unaccounted) losses in the given system on continuous basis. For proper energy accounting, implementation of an energy metering plan which will ensure generation of accurate metering data at strategic points i.e. at points where energy is input into the system or its part will be the first step. Review of the existing metering practice in the utility and replacement of defective meters plus calibration check of working meters will be taken up on priority. Thereafter, the following steps can be adopted for achieving a reduction of energy losses and an improvement in realisation of revenue in the short-term:

  • Energy balance is to be prepared from the Billed energy input and the actual energy input from the energy meters.

Ideally, energy audit should be carried out over smaller area so that the areas suffering from high-energy losses can be localized. This would however require providing a very large number of energy input meters at strategic points and check meters. While it would certainly be desirable and justifiable to install check meters for high value consumers, installing energy meters on all distribution feeders (at 11 kV) in 33/11kV substations should be a considered as a minimum investment by the utilities and distribution companies for protecting the revenue earning. Energy losses in Pockets known to be having high losses could also be checked by additionally installing energy meters on the distribution transformers feeding these high loss pockets.

  • Estimate the total energy losses from the above. Segregate the technical and commercial losses by computing the technical losses from the results of the network analysis. Once feeder-wise energy losses are established, feeders having high-energy losses should be further investigated for localizing pockets of high-energy losses by installing energy meters after distribution transformers.
  • After performing spatial analysis of the commercial losses to identify high loss areas or consumer category responsible for the loss, implement appropriate energy metering and billing in these areas. In some cases, use of certain technological measures like aerial bunched cables LT lines in theft prone areas and conversion of LT into HT lines i.e. less LT are suggested for reduction of the commercial losses.
  • Computerised billing of energy sales in a pilot area could be taken up to investigate the scope of loss reduction.
  • Simultaneously to the above, prepare a plan for liquidation of revenue arrears from the data/information collected during the energy audit and study of the Billed-energy data and the accounting information. The existing billing and revenue realisation system should be also evaluated for improvement of revenue collection efficiency. Implementation of on-line payment system in a pilot area in the short term could pave the way for offering the system to a larger number of consumers.

The medium and long-term plan would introduce higher levels of automation and remote-monitoring systems, as by then the utility could have started benefiting from the short-term plans in controlling the energy losses and increasing revenues. Gradual introduction of electronic energy meters to replace the outdated electro-mechanical energy meters will be inevitable as then it would permit monitoring. Installation of Computerised customer billing, payment collection, customer complaint registering system and continuous loss monitoring are the key to efficient and financially strong utility.

The above approach is by no means sufficient to eliminate the commercial losses totally. As long as the energy consumed is not being charged to the consumer in accordance with the actual cost of energy being delivered the losses will remain. Issues of tariff cross-subsidisation and rationalisation of the tariff, legislative and legal issues and issues relating to the surveillance and vigilance for revenue protection still remain inadequately addressed.