Not too long ago power distribution in India referred to a low voltage network connecting to the residential consumer’s house. Even cables rated 11 KV were referred to as “High Voltage Cables”. Today, the term ‘High Voltage’ is reserved for cables at 66 KV and higher, while 11 and 33 KV cables are referred to as ‘Medium Voltage’ (MV) Cables. And in this change in nomenclature lie the roots of the challenges that face MV Cable distribution systems that have not yet received the attention they deserve, with the exception of a few well-managed and progressive urban utilities.
Growth of MV Distribution systems
There are two trends that characterise the changes taking place. Firstly, there is a demand of larger quantity of power by the domestic consumers, and secondly, the move away from uninsulated systems to safer and more reliable insulated systems. Domestic consumers form the largest number of connection points for any power distribution system. With increasing urbanisation in India, the second category that demands high amounts of power is commercial establishments.
Domestic network planning was, and continues to be, based on very frugal estimates of power consumption by domestic users. Many rural connections are still estimated at 1 KW per dwelling unit. This does not adequately consider that rising living standards indicate a high probability of the rural household using a room heater or geyser for hot water for bathing in winter, or an iron for ironing clothes, in addition to lights, fans, TV and in all probability a refrigerator. In urban areas, the planning is also frequently inadequate or outdated. In DLF Gurgaon, an upcoming residential suburb outside Delhi, the system was originally conceived as being required to cater to a load of 5KW per plot. This did not envision the possibility of multiple dwelling units on one plot, or that each dwelling unit may have two air-conditioners running simultaneously in two bedrooms during summer. Nor did it take into account that bare LT lines running along the streets conflicted with the announced intention of having “green” colonies with massive tree plantation plans that would foul with the uninsulated lines. In light of these developments, LT distribution networks are no longer adequate to cater to the needs of the distribution systems. These LT systems are being supplemented, and in many cases replaced, by medium voltage networks. Old 11 KV substations are being replaced by 33 KV or 66 KV substations. At the next level the distribution is being done at 11KV for most low density residential areas. Furthermore, the coming of small guest houses with one or two dozen air-conditioned rooms on offer has led to many of them having small 11 KV Transformers to feed the load. For higher density residential and commercial high-rises, the supply is often at 33 KV to a dedicated sub-station on the premises. All this points to the inevitable proliferation and growth of Medium Voltage distribution systems.
Requirements and Expectations from Distribution Systems
The demand for delivering increased power is accompanied by the requirement to deliver it in a reliable and unobtrusive and environment friendly manner. Power outages for any reason are no longer acceptable to consumers and frequently make news headlines. Excess availability of power on the grid which is forcing the backing down of some generating stations and being publicised by the Power Ministry is a game changer. Apart from consumers questioning the deficiency of service quality, even regulatory agencies have begun to study the model of penalties on utilities for such deficiencies as are already in place in many parts of the developed world. Under this concept, consumers would be entitled to a deduction from their electricity bills for outages. With system voltages increasing from LT to 11 and 33 KV the number of customers being affected and the consequential liability to utilities will increase tremendously.
The other expectation is for the system to be environment friendly and aesthetically unobtrusive. Cutting of trees to clear the way for power lines is already an issue between utilities and forest and environment departments. In one state the utility confessed that it sends out its men on weekends when the Forest Department is expected to be dormant. But environment activists are not. They protest such tree cutting. The problem will only become more acute as the voltage level rise from 1.1 KV to 11 KV will require larger clearances.
The uncontrolled use of 11 KV cable strung haphazardly along roads in ugly festoons may be the norm in Govt PWD departments and highway authorities but is unacceptable to private developers who suffer loss of saleability and price if aesthetics are not taken into consideration. It is only a matter of time before the government departments are forced to also respond to similar expectations that the general public will have from them, just as they will no longer quietly accept poor quality roads.
Installation, Maintenance and Operation of Cable Networks
Since MV cable distribution networks appear to be inevitable, it becomes necessary to ensure that they are given due attention at the stages of planning, installation, maintenance and operation. Unfortunately, this is not the current state of affairs. The concept of a distribution design department is absent in most discoms. The challenge is to increase the knowledge about MV Cables and their use beginning from design concept and extending through Specification, Execution, and System Operation. Each of these areas gets less attention than it merits.
System design as a whole is a vast subject well beyond the scope of a short article or paper. So, let us look at cable selection. Most cable companies put out a reference book of cable data which lists the various parameters of cables, and also often include notes on installation guidelines. In purchase tenders this information is also captured in the “Guaranteed Technical Particulars” (GTP). Unfortunately these generally remain with the Purchase Department and are not accessible to the field users.
In one instance, the author was called to troubleshoot an 11 KV AB cable network. The line was 26 km long, and carried power from a 5 MW Mini Hydel plant to a grid sub-station. The first issue related to the frequent breakdowns. It was found that the electricity inspector had stipulated single end earthing. The contractor had earthed the screen and armour of the entire length of the 26 km at one end only. Naturally, the screen voltage rise at the far end was excessive, and this could be easily established by calculation. It took some convincing to explain that the electrical inspector’s requirement could be met by sectionalising the run into lengths of about two km each & earthing each section at one end. This was easily achieved by using MVT tap off connectors to join the phase conductors of each section to the next. An additional advantage was that in case of a fault, the individual sections could be easily isolated to localise the section that had developed the fault.
The other issue was that the voltage at the receiving end was below acceptable levels, and on this basis the cable was declared to be sub-standard. A simple calculation of the load current and application of ohms law for the length and conductor resistance showed that the receiving end voltage was exactly what was predicted by the calculations! (Surprisingly this led to a charge by the utility, which had designed the system and specified the cable cross-section based only on basis of current carried, that the contractor should have pointed this out earlier!)
In addition, many utilities have faced the problem of improper selection and specification of cable sheaths in MV AB Cables. In Gulbarga the PE Jacket of 11 KV AB Cables cracked within two to three years and the cables had to be abandoned. A CPRI study revealed that the cause was that the PE used for the jacket had not been UV stabilized, and that the specifications had not called for it to be UV Resistant.
Ensuring proper cable installation will be the biggest challenge as the distribution network moves to MV Cables, whether it is an underground or aerial installation. MV Cables are more sensitive, both due to their more complex design and higher voltage levels as compared to LV Cables. In LV Cables, the insulation is more based on mechanical considerations than electrical. In contrast, MV cables have insulation thickness dependent on electrical stress. Hence, the electrical safety margin inherent in MV Cables is much lower than in LV cables. They have to be handled and installed with far greater care than permissible with LT cables.
IS 1255 lays down the recommended Code of Practice for Installation of Cables. Yet, few Field Installation Supervisors are aware of it, and fewer have read it. At a CPRI training session on Good Installation Practices with about 40 Field Supervisors attending, the Author asked how many had read it. Only four hands went up!
Basic recommended practices for cable pulling, laying on sieved sand or soil and at specified depths are often ignored. In several cases cables have been direct buried, and then roads constructed over them without protective ducts or pipes to provide protection during road construction or from subsequent traffic vibration. In Maharashtra a 4 km 33 KV Cable had to be abandoned due to this oversight.
In another incident with the same installation contractor, a cable fault was detected in the region of a straight joint. Suspicion turned to the joint having failed. The repair team found the joint healthy, and the cable had burst because of over-bending nearby. Against a recommended minimum bending radius of 1.8 mtrs based both on IS 1255 and GTP recommendations, the cable had been bent into a loop with a diameter of two metres and had burst at one point on the loop.
In case of MV AB cables, the problems are more frequent as the contractors are frequently those previously installing bare conductor networks. They are neither familiar with nor sensitive to the more careful handling MV cables require in comparison to ACSR conductors. Dragging on the ground or pulling with excessive force that may not damage ACSR can greatly weaken a MV cable and cause frequent faults and significant shortening of the service life of the cable.
Clearly, there is a need to both provide greater design input, and to train the Field Supervisors. Both of these factors will become more important as more MV Cables become part of the network. Cable and accessory manufacturers can play a crucial role. While a few cable accessory manufacturers do offer these services (either gratis or on a paid basis) by and large the cable companies pay scant attention to how the cables are installed and used once they leave the factory.
Importance of Quality of Cable Accessories and Their Installation
In most MV cable networks, the area of greatest concern is the jointing. The cable is made and tested in the factory before despatch. In case of jointing, the installation or fitting is done in the field. However, the reliability of the system is as much dependent on the jointing as it is on the quality of the cable.
The quality of accessories can be checked prior to allowing despatch to the site through pre-despatch inspection. The pre-despatch tests are generally a few short term tests that test material properties, but time constraints do not allow simulation of performance over the desired 30 years life of the system. For such longer term, evaluation reliance is placed on one-time type tests.
Unfortunately, there is no easy way to correlate the quality of the material submitted for qualifying Type Testing with those actually supplied later. It is always open to unscrupulous manufacturers to source components from a high-quality manufacturer to qualify their product, and later supply inferior materials from a lower priced and lower quality manufacturer. For this reason, it is important to distinguish between assemblers and manufacturers. The Department of Telecom has gone some distance in doing this by restricting purchases to those who manufacture sleeves from granule stage. This can be taken further by insisting that the moulded parts at least must be embossed with the manufacturers name or logo, and easily counterfeited printing of name is not sufficient.
The latest version of the Indian Standard has also addressed some of these issues by recording key properties of track resistant, stress control and insulating materials. It still remains for users to make it a practice to check that the material supplied is not lower than a reasonable margin (say 20 percent?) of the values recorded in the Type Test Report.
Specifications also need to be drawn up with care. In particular, outdoor use materials face an even more severe service condition. Use of certain materials at 11 & 33 KV like EVA which are normally not used for insulators have shown rapid deterioration within two to three years in hot and humid climates compared to their Silicon counterparts and even when compared to the traditional red anti-track heat shrink compounds which are subjected to 1000 hour salt fog tests before adoption in outdoor cable terminations. It is clearly better to specify porcelain, silicon or proven anti-track materials such as are used in outdoor terminations for use in MV Insulators or MV Tee connectors.
Many product warranties which are limited to “twelve months after installation, or eighteen months after supply” allow such products to continue to be used. Longer guarantees which are now becoming more common such as five years help screen out such inferior products. In one major utility in North India the very poor performance of a 33 KV straight joint has resulted in the manufacturer being virtually black-listed, with collateral damage to credibility of other non- multinational manufacturers.
Even when the best of materials are used, the impact of the uncontrolled environment in which the work is to be done and the variability of skill of the installer and quality of workmanship are critical. Unfortunately the abolition of the cadre of Cable Jointers in most Govt utilities has led to the proliferation of self-proclaimed jointers being used by most private Contractors. Part of the responsibility rests with accessory manufacturers who have propagated the myth that installation of heat shrink, cold shrink, or push on joints does not require special skills. This is a dangerous half-truth. The skill required to prepare a cable for jointing is high and requires extensive training and the prospective jointer must be subjected to careful checking. There are high levels of faults in jointing at 11 KV when inadequately skilled jointers are used, and these will only become more the norm as 33 KV distribution cables become more widespread.
On the other hand, careful training and evaluation can dramatically reduce failures. In a major RAPDRP program involving over 5,000 terminations and joints our company decided to carry out extensive checking, evaluation and training of the jointers, as well as the field supervising engineers. As a result, the five-year period passed off with negligible warranty claims. Most significant was a bunch of failures at the tail end of the project from one location. Investigation revealed that the trained jointers had been moved to another project site, and the high number of failures were due to newer unevaluated jointers being used.
This underscores the need for procurement to take into account not just the upfront cost of the product, but the value of the service being provided to ensure the proper use of the product.
Investment in tools and tackle will need to be increased as the requirements of better quality of workmanship and greater volumes increase. Traditional methods of cable stripping using just a knife and a piece of glass will need to be phased out in favour of more sophisticated cable stripping tools. These are available for the crucial stripping of the cable semi-con, as well as for removing insulation without nicking the conductor. Dimension sensitive mechanical crimping tools are also being replaced by hydraulic tools which are based on the force of crimping. Metallic screen connections using binding wire and impractical soldering can be replaced by constant force springs as is common in Europe.
The abandoning of 11 KV MV cable systems in Haryana is entirely due to poor performance not attributable to the concept, but the unplanned and uncontrolled manner in which it was executed. If care is not taken, the entire move towards a more extensively cabled Distribution system can be similarly abandoned in favour of a lower tech but easier to install bare network, which is acknowledged to be less stable that insulated cable networks, provided they are properly installed.
As the MV cable and accessory manufacturers look at the tempting increase in demand that is inevitable they need to be sensitive to their responsibility to provide higher reliability, not just for the benefit of the user, but also in their self-interest.
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