The distribution system suffers from the problem of low voltage, power theft and high energy losses. The problem of the losses and voltage drop in distribution feeders dependent on each other and varies with the pattern of loading on the feeders.
In case of LT lines, efficiency of the electric gadgets is also affecting and breakdown is also very high. Also there is a tendency of unauthorised connections to hook to the LT lines which results in over loading of the transformers and failure of transformer.
The loads for agriculture are predominantly pump sets used for irrigation purposes. Normally, in the present distribution network, the 11kV HT line goes to the DTC (Distribution Transformer Centre) and from there lengthy LT lines are drawn to give supply to different installations of irrigation pump set. In case of HVDS, the HT lines are run up to the installation premises and there it is stepped down through a suitable capacity transformer before arranging supply through service main. In high voltage distribution system (HVDS), the electricity is distributed to the consumers at higher voltage level (11kV) instead of low voltage (415V).
Adoption of HVDS by converting existing low voltage distribution system (LVDS) to HVDS reduces the technical losses appreciably.
In recent decades, different schemes have been proposed to reduce the losses in the distribution system and hence, to increase the efficiency of electric devices and power distribution networks.
Distribution System
Distribution system with low voltage employs four wires and long low tension lines and multiple loads fed from a bulk power transformer resulting in the increase in system losses affecting voltage profile and performance of distribution system. In existing distribution system, the voltage at buses reduces when moved away from the substation, also the losses are high. The reason for high losses is the use of low voltage for distribution as the current is high in the low voltage system. In the existing system, pilferage is very easy because of lengthy bare LT conductor and thus, many unauthorised connections are tapped from the bare LT conductor which results in over loading of the transformers and failure of the transformers. The prevailing low voltage in the LT line is also affecting the efficiency of the irrigation pump set and breakdown is also very high. Thus, by using high voltage for distribution, we can reduce the losses as current in high voltage distribution system (HVDS) is low. The HVDS employs large three phase 11kV main distribution feeders with three phase lines and three phase distribution transformers transforming 11kV into 415V.
Losses in Distribution Network
Generally, in the process of supplying electricity to the consumers, energy losses occur due to technical and commercial losses. The main reason for high losses is considered to be the use of low voltage for the distribution of power leading to the high current and thus, more resulting in losses.
Technical losses include the losses due to the heat dissipation resulting from current passing through conductors, magnetic losses in transformers, resistive losses in windings and the core losses, resistive losses in service line and losses in energy meter. These losses cannot be eliminated but can be reduced. The commercial (non-technical) losses are the losses which include, power theft by hooking the lines, unauthorised connections from the power line, errors in the meter reading or defective meters and in the estimation of unmetered energy supply, loss at the loose connection ends etc. These losses can be eliminated by taking some precautions. And thus, it is necessary to focus on technical losses as well as on commercial losses and it can be achieved by using HVDS method for distribution. The various reasons for higher losses in the existing system are:
a. Over rated distribution transformers and hence their under utilisation
b. Lengthy distribution lines
c. Inadequate size of conductors
d. Low voltage (less than declared voltage) appearing at transformers and consumers terminals
e. Distribution transformer not located at load center on the secondary distribution system
f. Low power factor
g. Poor quality of equipment
h. Too many stages of transformations
i. Transformer losses
j. Bad workmanship
k. Direct tapping by the non-customers
l. Pilferage by the existing customers
m. Defective metering, billing and collection functions.
To reduce the distribution losses, many techniques are developed and some of the loss reduction approaches are listed below:
a) Network reconfiguration and phase load balance
b) Automatic voltage booster
c) Network re-conductoring
d) Distribution Transformers – Locating and Sizing
e) High efficient Transformer
f) Reactive power compensation
g) Aerial Bunched Cables (ABC)
h) HVDS
High Voltage Distribution System (HVDS)
In the existing system, large capacity transformers are provided at one point and the connections to each load is extended through long LT lines. This long length of LT lines is causing low voltage condition to the majority of the consumers, power theft by hooking the lines, unauthorised connections and high technical losses. To reduce distribution losses, to improve quality of supply and also to prevent theft of electrical energy, high voltage distribution systems (HVDS) are implemented. In HVDS scheme, long length LT lines are converted into 11 kV lines and thereby, installing the appropriate capacity distribution transformer as near as to the end and the supply is provided to the consumer. By converting LT lines to HVDS, the current flowing through the lines shall reduce and will bring down the technical losses in the LT line drastically. The main purpose of using high voltage for distribution is to reduce the theft of energy and decrease in unauthorised connection as the LT lines are virtually eliminated and even short LT lines required will be with insulated aerial bunched cables (ABC). This makes direct tapping very difficult and thus, increases the authorised connection and further faults are totally eliminated which improves the reliability. HVDS is to reconfigure the existing low voltage (LT) network as high voltage distribution system. Each 11kV feeder which emanates from the 33kV substation branches further into several subsidiary 11kV feeders to carry power close to the load points (irrigation pump sets). At these load points, a transformer of suitable capacity further reduces the voltage from 11kV to 415V to provide Low Tension (LT) line to individual customers, either at 240V as single-phase supply or at 415V as three- phase supply.
In irrigation, HVDS provides availability of good quality of motors improving pump set efficiency and providing high yield of water. On each distribution transformer, only two or three pump sets are connected and the problem of frequent failure of power due to failure of distribution transformer will be reduced considerably.
Figure 1 illustrates that only 12 consumers (blue colour) are authorised; remaining 8 consumers (red colour) have unauthorised connections leading to the theft of the power and are making all the consumers to suffer from low voltages and transformer overloading.
Effective implementation of HVDS scheme will have the following benefits:
• Reduction in line losses since HV line is taken almost upto consumer load point and on LV side AB cable is used.
• Failure of agriculture DTRs are minimised as LT overhead line is avoided and also load per DTR is restricted. Hence, there is no failure on account of over load and LT faults.
• Reduction of unauthorised agriculture connections, as one small capacity (25KVA) DTR is erected for two or three agriculture consumers. The agriculture consumers will have a feeling of ownership of transformer due to limited connections on it.
• As 11kV line is taken almost to the load point, improvement in voltage regulation near agriculture pump sets, resulting in good performance of motor.
• Pilferage of electricity is completely avoided as LT AB cable is used from DTR LV upto consumer pump set.
In high voltage distribution system, the authorised consumers do not allow unauthorised tapping by another as their transformer gets overloaded or may get damaged, resulting in outage of power supply for longer durations. The use of HVDS results in reduction in losses and hence, improves quality of supply.
Case Study
Case study of three feeders proposed for conversion from existing LT distribution to HVDS is taken up for the study. The feeders are supplying load for agricultural consumers in which, the 11kV is stepped down to 415V by using distribution transformers. The existing LT distribution system consists of three phase DTR of 100kVA, 63kVA & 25kVA installed at various points and the connections to each load is extended through long LT lines. The long LT lines and many number of load connections with high capacity distribution transformer resulting in the increase in power losses, unauthorised connections, electricity theft losses, overloading and failure of distribution transformers. The existing LT distribution system also affects voltage profile and performance of the distribution system. By converting LT lines to HVDS, the current flowing through the lines will be reduced and bring down the technical losses in the LT line drastically.
The schematic diagrams of the existing LT system (before HVDS) for three feeders supplying load to agriculture consumers are shown in figures.
It is found that, in the above feeder, total no of irrigation pump sets (authorised & unauthorised) connected is 282. The total length of existing 11kV line is about 94 km approx. and total length of existing LT line is about 8 km approx.
It is found that, in the above feeder, total no. of irrigation pump sets (authorized & unauthorized) connected is 281. The total length of existing 11kV line is about 85 km approx. and total length of existing LT line is about 10 km approximately.
It is found that, in the above feeder, total no of irrigation pump sets (authorised & unauthorised) connected is 663. The total length of existing 11kV line is about 185 km approximately and total length of existing LT line is about 14 km approximately. The investigation is carried out to determine the losses in the existing low voltage (LT) distribution system. The technical and commercial losses of the existing LT system for above three feeders for 24 months period are represented in graph 1.
It is found from the above graph, the percentage of losses in feeder-1 varies from 13.70% to 27.51%; in feeder-2 varies from 13.21% to 27.36% and feeder-3 varies from 11.92% to 27.95%.
Implementation of HVDS for the above feeders can result in reduction of technical and commercial losses and also unauthorized connections. The HVDS scheme consists of converting the existing 3 phase 4 wire lines to 11 kV systems using the existing supports and providing intermediate poles wherever necessary and individual transformers of suitable capacity (25kVA) are provided to supply 2 or 3 agriculture consumers. The LT line from the transformer is drawn very close to the agriculture load and hence, the requirement of LT lines is less compared to LT system. The LT lines used is insulated overhead cables like ABC (Aerial Bunched Cables). The authorized consumers do not allow unauthorized tapping by another as their transformer gets overloaded or may get damaged, resulting in outage of power supply for longer durations. As 11kV is extended up to load ends & AB cables are used for transmission from DTR to loads, theft, fault on LT lines and line losses are reduced & voltage profiles are maintained. The typical single line diagram of the HVDS scheme is shown in Figure 5.
The investment on conversion of existing LT distribution system to high voltage distribution system (HVDS) can be easily recovered by the way of loss reduction and annual savings.
Conclusion
Implementation of high voltage distribution system (HVDS) for agriculture consumers will result in reduction in losses, increase in energy saving and improve voltage profile. The adoption of HVDS makes the system more reliable and thus, reduces the number of outages. The chances of unauthorised connections and theft of energy are reduced.
The restructuring of existing low voltage distribution system (LVDS) as HVDS in agricultural sector presents one of the best technically feasible and financially viable method for providing reliable and quality supply to agriculture consumers.
The case study of three feeders implementing HVDS is highlighted. Adoption of HVDS in these feeders reduces both technical as well as commercial losses and improves the commercial and technical performance of the distribution system. HVDS also provides better reliability and results in the increase in annual energy saving to the utility.
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