Getting the best out of a Distribution Transformer and the Distribution Network depends on how well maintained they are. Good maintenance improves reliability, lowers risk of failure, saves costs and much more. Alok Gupta, Member, MPERC gives us an in-depth look at what it takes.
Health monitoring is not only essential for living organisms but equally important for equipment used in various industries. One of these key equipments is the Distribution Transformer (DT) used in electricity distribution which is utilised in the final stage for the delivery of electricity to end users. Typically, a distribution system comprises of power lines, substations and pole-mounted transformers, low-voltage distribution wiring and meters.
In our country, the distribution system is calculated from 66 kV and below, however distribution voltages are not defined in Indian context anywhere because of wide range of customers taking supply on different voltages. Health monitoring of the Distribution Transformer (DT) improves reliability.
Presently, the Central Control Room does not have any data about the health of DTs except for energy transactions. Though this is an important piece of equipment, ironically itÆs the most neglected one. Often these fail and sometimes even catch fire resulting in considerable loss and heavy expenditure on replacement.
The primary voltage of the DT is generally 11 kV,22 kV,33 kV and may be 66 kV while the secondary voltage is 0.4 kV,11 kV,33 kV etc. Only large consumers are fed directly from distribution voltages. Most of the customers are connected to a transformer, which reduces the distribution voltage to the relatively low voltage used for power, lighting and interior wiring systems. The transformer may be pole-mounted or placed on the ground in a protective enclosure as per CEA's Technical Standards for Construction of Electrical Plants and Electric lines. Different countries follow different practices with regard to the transformer size, to be mounted on pole and so on. In fact, even a 1000 KVA DT can be mounted on pole.
Earthing is Critical
A ground connection to local earth is normally provided for the customer's system as well as for the equipment owned by the utility. Earthing is critical to avoid over voltages in the system. In conditions where there is a loose neutral, one or two phases of voltage rise to abnormal values and the third phase to an extremely low value thus damaging customer equipment like refrigerators, AC, TV etc. These problems may be difficult to resolve since they often originate from places other than the customer's premises. The customer cannot claim the damages from the distribution licensee as it is very difficult to prove the same.
There are two strategies being followed about sizing the DTs. In USA and Canada small transformers of 25 KVA are mounted on a pole feeding 3 or 4 customers. This reduces the length of LT line and is called High Voltage Distribution System (HVDS). In U.K., large size three phase transformers are used for feeding a higher number of customers. Small transformers have the advantages of ease in transportation and increase in reliability as only few customers are affected by transformer failure Replacement time is reduced. In India, few states have not responded well to HVDS mainly since connections are provided in a haphazard fashion and no systematic approach is adopted.
Major Maintenance Strategies
There are four kind of maintenance strategies:
Challenges of Maintenance
As far as maintenance of the distribution network is connected, the DISCOM has two options; either letting their internal department carry out this or outsourcing to a third party. Both have inherent deficiencies due to :
The DT is the backbone of the distribution network loading of distribution transformers and the lines have a direct bearing on poor reliability of distribution network and consumer dissatisfaction. The average monthly interruptions were as high as 60 in some cases and a duration of around 800 minutes. This could be still be worse if annual figures are taken. Though some of the DISCOMs have improved reliability through measures such as using trouble cell management system, improving workflow strategies etc. the rest continue to be poorly maintained. In real conditions, the transformer load varies during the day according to consumer's need and season. Besides this, the load that a transformer can withstand is also affected due to atmospheric conditions, speed and direction of the wind, solar incidence etc. There is a relationship between the aging of the insulation of the transformer and the operating temperature due to loading. The analysis of transformer loading and hot spot temperatures or operating temperatures help to calculate the life of the transformer using a mathematical model. I understand no Discom would be doing that.
The intervention of the political segment in the electricity sector has put DISCOMs under heavy and increasing cost pressure and to meet the performance standards notified by the regulators. By and large, all of them are either in the red or running with Government support. They are faced with the herculean task of keeping the network up despite reduced revenue while at the same time making efforts for appropriate and generally accepted performance standards.
High demands are made on maintenance due to the aging of assets as maintenance reduced for mere cost reasons leads to higher technical and economical risks. A distinction is required to be made between inspections, servicing and repair measures but this systematic approach either does not exist or vanishesdue to paucity of funds and aneed based maintenance is dominating the scene. A lowest tender approach also sacrifices the quality of transformer being purchased.
The following parameters/criteria decide the condition of the transformer:
Empowering Maintenance Through Online Monitoring
The solution lies in online monitoring of transformer parameters and choosing the best maintenance strategy before the outage of the transformer, Online monitoring of DT (OTMS) can be done by a Transformer Information Monitoring System(TIMS) or Distribution Transformer Information Management System (DTIMS) or Distribution Transformer Monitoring System (DTMS) or Transformer Monitoring System(TMS), which are different names given by various stakeholders.
We are now entering into arena of 4 G. The communication network, which is the backbone of information management, has become stronger and enough robust to capture data in seconds. This small component of the monitoring system with dimensions of about 40x60x20 cm collects information from the DT over the communication network in real time mode.
The major parameters captured are:
The benefits of such monitoring is prevention of fire in the transformer, improved customer service, stable power supply though optimum operation, carrying out energy audit, fault analysis reduction on cost of maintenance, longer life, better load management and many more.
Proactive Enhancement of Service
Some keys to bettering service are:
Cost Benefit Analysis Installing DTMS on DTs (>400kVA)
In India, 40000 MVA is added every year. Assuming 50% capacity is provided with DTs having a rated capacity of 400 KVA and above, the total capacity of would be 20000 MVA. The number of such DTs would be 50000 and the cost of such DTs would be about Rs 2000 Crs (Rs 10 lakh per MVA). If the failure rate is 10% then Rs 200 Crs would go towards loss which could be avoided by installing DTMS. Assuming the cost of one DTMS to be Rs 50000, the total cost for 20000 MVA shall be Rs 250 Crs. The payback period for installing DTMS would be about one year. The benefits in terms of avoiding failure of DTs by diagnosing its health and taking timely action would be enormous..
The Next Step The ground realities of the maintenance done in most of the DISCOMs does not enhance the customer satisfaction and this can be achieved only when IT can fetch real time data and is envisaged along with analytic software at the main server. The quantum of maintenance staff is also grossly inadequate. The reliability of operation and maintenance can be brought to a higher level by using a Real Time Distribution Monitoring System for DT in phases. Those DTs with a greater potential must be tapped in the first phase and gradually cover other DTs.
On The Anvil
Many countries have installed such devices efficiently and have brought down System Average Interruption Duration Index (SAIDI) and (SAIFI).Basically energy metering functions is a subset of such device. This device has additional features of monitoring transformer life related parameters and can be called a compact and comprehensive device of health monitoring. The system measures and generates reports to the operator, which includes overload conditions, power outages, voltage disturbances, earth faults, current and voltage imbalance and reactive power. The operator can also set alarm limits for parameters such as current, voltage, temperature, reactive power unbalance, voltage peaks and drops, and if these are breached, the alarm system will get activated. One can detect smoke inside the transformer tank also. A planned program of maintenance, inspection and testing can significantly reduce the number of transformer failures, and the unexpected interruption of power. In short, maintenance is key to uplift the organisation in a competitive environment when in future carriage and content could be segregated.
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