Abstract: Electric power distribution networks are a direct link of power system to the consumers. The bottleneck weaknesses of a distribution network can be countered using Mobile Service Station (MSS). This paper describes the various components used in the MSS and it explains about the important aspects that need to be considered while designing and installing the MSS. Result and outcomes found while making a road trail on MSS is also mentioned.

Introduction: Electricity has become an essential element of the infrastructure of contemporary society. Planning the strategies to maintain system reliability is a key task for network operators in order to ensure that the frequency and duration of outage experienced by their customers are within the targets imposed by both Energy Regulators (ERs) and Security of Supply (SoS) necessities. But internal inspection and maintenance outages interrupt power supply to the customers and require an alternate transformer to take loads. Failure due to vegetation, ageing, excessive load, lack of maintenance, earthquake or weather conditions, may seriously affect the power networks as shown in Table 1.

Table 1: Reliability Parameters of Distribution Line

 Distribution Category Average Failure Rate (failures/year) Repair Time (hours/year) LV Distribution Network Urban 2.51 17.5 Sub-Urban 1.21 18 Rural 0.87 22 MV Distribution Network Urban 2.26 45.9 Sub-Urban 4.09 67.9 Rural 4.33 74.4

The Monte-Carlo method of reliability assessment says that: fault rate(λ), equivalent failure rate(λeq) and mean repair time(deq) considering total number of power components in each part of the system(N) can be given by Eq. (1) to Eq. (3),

λ = (Number of Failures)/(Number of Components considered*Number of Years of Recorded Data)      (1)

$\lambda_e_q=\sum_{i=1}^{N}\lambda_i\dots(2)$

$d_e_q=\frac{1}{N}\sum_{i=1}^{N}d_i\dots(3)$

The Fig. A characterizes general failure bathtub curve of power distribution assets. The outage rate is high at the initial phase due to manufacturing defect and the incompleteness of establishment. At normal operating stage, the failure rate is uniform and at the aging stage, the failure rate is mounting because of the aging.

Mobile substations can quickly restore power supply following an outage caused by natural disasters. It provides temporary service until permanent facilities are restored. The MSS is equipped with all substation hardware accessories along with protective devices.

It is provided with modular flanged pole which work as tension poles. Stay sets are used to anchor the poles to the ground. MSS contains all the anchor components for all kind of soil and rocks. Since all the components are on trolley, they can be transported anywhere easily. Quick integration with the network and portability are the most important advantages of these mobile service substations. Power can be tapped by using hot line stick from the live line without any shutdown.

The design and installation of a mobile substation have certain things that are need to be considered, which are not present when dealing with conventional substations, such as components safety during transportation, weights on the trolley, electrical clearance during installation.

Mobile Service Substation: A MSS of 11/0.44 kV is designed considering the difficulties in transportation and weight. This MSS has a Completely Self Protected (CSP) transformer that has an inbuilt protective system for the LV side. HV side has an Air Break (AB) switch and Drop Out (DO) fuse for protection. H-type pole is used for mounting the HV side protective equipment with MSS. In order to reduce the weight on the poles, the transformer is fixed with the trolley. All the equipments, other than the transformer, are stored in a compartment under the base of the trolley. Four-wheel tandem axle is used for distributing the trolley load evenly. If the load is not leveled, it can cause extra wear and tear on one set of axles and reduce the life of the tires.

Initially, bottom parts of the pole are fixed with the trolley using nut-bolt arrangement.  Belting angles are used to distribute the load on the pole. The top sections of the pole have the tension fittings and the HV side protective elements such as AB Switch, DO Fuse, Lightning Arrester (LA). They are assembled on the ground and lifted using a gin pole. Gin pole is basically a pole with a pulley or block and tackle arrangement that is used for lifting purpose. The gin pole's free end extends above the object that has to be lifted. Once the pole bases are fixed, gin pole is assembled and it lifts the other two sections of the pole. After complete assembly, gin poles are dropped down.

Design Considerations: The major difference between a conventional and proposed model is the top-entry approach, where incoming line is located at top and outgoing is at bottom.

All the HV side components are mounted at a suitable height thereby maintaining the safety norms and are at a distance bit away from human reach. Fully insulated cables are dropped down and terminated in an enclosed chamber within the transformer. This approach reduces the required ground clearance. The assembly and mounting of the HV component on top of the poles are eased with the help of gin pole. The complete setup is faster and requires less effort.

The incoming HV line is taken from the top using the tension arrangements. It is then passed through LA, AB switch and DO fuse. Transformer steps down the voltage and outgoing line transfer it to bus. Fig. 1 depicts the Single Line Diagram (SLD) of the MSS. The entire substation may be by-passed by making connections to line as they enter and leave the permanent substation. When the load is transferred to the mobile unit, the permanent substation will be de-energized. Alternative way is to by-pass only a part (e.g., HV and LV bus) of the permanent substation by making connections to some parts of the equipment, thus de-energizing only the transformer.

Fencing can be done around MSS to prevent the entry of non-utility persons near it. Fence can be of lower standard since it is a temporary installation.

In view of the discussions carried out in the above sections, the major components of MSS are portrayed in Fig. 2 and Fig. 3. Design aspects of MSS are discussed in the following sections:

Distribution Transformer:

CSP transformers are used to eliminate the need of separate protective system and thereby making the system compact. It has three inbuilt protective system.

1. LV feeder system protection along with relevant protection.
2. Circuit Breaker (CB) for overload and secondary fault protection.
3. Magnetic strip to increase the opening speed of CB during high fault condition.

A CSP installation takes half the time of non-CSP installation, which makes it the most convenient for MSS. In case of oil cooled transformer, the oil should be drained in a barrel before transportation and it has to be again filled during installation. Oil cooled transformers are heavier and this affects the mobility of the system. Hence, Air Natural (AN) cooled transformer is preferred to reduce the weight and time of installation. A brief specification of CSP transformer is presented in Table 2 as per IS:1180 (Part-I).

Table 2: Specification of CSP Transformer

 Parameter Specification Cooling Type Dry Type Tapping Off Circuit Link Insulation Class 'F' Temperature Rise Winding up to 1300 C Temperature Protection RTD Rating 63 to 500 kVA Voltage 6.6 to 11/0.44 kV Vector Group Dyn1 or Dyn5 or Dyn11 or any specific

AB Switch & DO Fuse Set:

AB switches are suitable for operation under off load conditions only and are intended to use on distribution substations and tapping sectionalizing points of 11kV lines. AB switch used in this MSS confirms to IS:9920 (Part-I to IV). This switch is used to connect or disconnect incoming cable to transformer sections from the main overhead power distribution line. It is made of galvanized steel base, and having 11kV accessories such as pin insulator, copper alloy contacts, for power connect / disconnect operation. These are usually found in group of three switches connected in parallel and presented in Table 3, essential electrical parameters are given.

Table 3: Electrical Parameters for AB Switch

 Rated Voltage 12 kV Rated Normal Current 200 A Rated Lightning Impulse Withstand Voltage: (i) To earth and between poles (ii) Across the terminals of open switch 75 kV (Peak) 85 kV (Peak) Rated One Minute Power Frequency Withstand Voltage: (i) To earth and between poles (ii) Across the terminal of open switch 28 kV (rms) 32 kV (rms) Temperature Rise: (i) Copper Contact (silver faced) in air (ii) Terminal of the switch intended to be connected in extended conductors by bolts 65 deg C   50 deg C Rated Short Time Current 16 kA for one sec Rated Peak Withstand Current 40 kA Rated Active Load Breaking Capacity 10 A Rated Transformer Off-Load Breaking Capacity 6.3 A (rms) Rated Line Charging Breaking Capacity 2.5 A (rms)

Expulsion type DO fuse is used to point out the fault quickly. In the event of fault, the fuse carrier blows and then hangs down vertically from the bottom contact and indicates faulty phase. Circuit can be restored again by lifting out the fuse carrier with the help of insulated operating rod. IS:9385 (Part-I to III) specification covers outdoor, open, drop-out expulsion type fuse cutouts suitable for installation in 50 Hz, 11 kV distribution system.

Lightning Arrester & Insulator: In general, the probability of direct lightning strike on a mobile substation is lower than a permanent substation. This is due to shorter working period and smaller footprint However, in areas of high keraunic activity, direct strike shielding for the installation is required between phase and earth to improve the lightning performance and reduce failure rate. LA used in MSS conforms to IS:3070 (Part-III) and is mounted on each phase of the incoming line, to protect the transformer and associated line equipment from the occasional high voltage surges resulting from lightning or switching operations. Most of the 11kV equipment have a Basic Impulse Level (BIL) rating of 75kV. The earthing terminal of the LA is connected solidly to the transformer tank earthing terminal. The important characteristics of LA are presented in Table 4.

Table 4: Characteristics of Lightning Arrester

 Rated Voltage 11 kV MCOV 9.35 kV Discharge Current 5 kA Creepage Distance 300 mm

The insulators used in MSS compiles with IS:731 (1963) and the insulator fittings comply with IS:2486 (Part-I) (1971) and IS:2486 (Part-II) (1963). These are type tested as per IS:196 (1966) and IS:731 (1971) before installing. Electrical and mechanical characteristics of insulators are given in Table 5:

Table 5: Test Characteristics of Pin Insulator

 Highest System Voltage 12 kV(rms) Wet Power Frequency Withstand Voltage 35 kV(rms) Power Frequency Puncture Withstand Voltage 105 kV(rms) Impulse Withstand Voltage 75 kV(Peak) Minimum Failing Loads 5 kN Creepage Distance 230 mm

Stay Set & Anchoring Arrangement: Depending upon the soil condition, Anchors have to be chosen. Table 6 provides information on what type of anchor should be used. Four sets of all anchor types are stored in the trolley. An eye nut is connected with one end of the anchor. This is connected with a guy wire with the help of guy grips.

Table 6: Anchors Classification

 Cross Plate Anchor Suitable for normal soil (dry) Quickest and easiest type of anchoring Screw Anchor and Ray Anchor Suitable for loose soil Rock Anchoring Suitable for hard rock

Grounding: Neutral conductors of adequate fault-current carrying capacity can be installed from the mobile transformer to the grid. All the metallic parts are earthed properly. LA is earthed with other exposed metallic parts such as poles, transformer body. They are earthed using a copper bar that runs all along the base of the trolley. DT neutral is grounded separately. Earthing kit along with the installation tools are stored in the base of the trolley. It is also provisioned to connect with the existing ground. IEEE Std 80 guide is used for sizing the ground conductor.

Four Wheel Trolley: The four-wheel trolley is attached with the pull bar which can be hooked to any vehicle and transported to desired place. Trolley is designed in such a way that it can accommodate transformer on top and the other components in a storing chamber below. The trolley is provided with jack screw on 4 corners of the trolley which are dropped down and fixed at the destination to support the load of the MSS and avoid any slip. Trolley is designed to carry a maximum load of 4 tons. Since the setup needs to be transported, the entire setup is designed to weigh approximately 2 tons. It is designed in such a way that it can be transported on pitched road as well unconstructed road into the villages. Levelling deviations on MSS should not be more than 50 in any direction.

Road Trial: The MSS is not in regular use and due to that servicing and test drive (i.e., brake tests at initial conditions, and during accelerating & decelerating mode) is needed prior to send it at site. The complete set up for MSS and its tools were tightly packed kept under the platform of the trolley excluding DT. A road test was performed on a mobile substation to know dynamic& standstill behaviour of a mobile substation with mounted component on the trolley. The mobile substation is transported to the site in accordance with the relevant norms of local transportation authority. It is made suitable to drive at speed upto 30 km/hr on pitched road and 10 km/hr on unpaved road. The following inspections of MSS have been performed before transportation:

Precautions before transportation:

• Lock and secure all frame and cabinet doors.
• Carry a set of instruction manual and control drawings.
• If oil type transformer is used, drain oil from the transformer tank.

Precautions after installation:

• Check nut and bolts for any loose connection.
• Check whether poles are properly anchored.
• Tighten the stay wire using turn buckle.
• Check if all the switches are in OFF position.
• Safe positions of expulsion fuses are checked.
• Continuity of the mobile substation’s ground bus and all connections to equipment is verified.

Conclusion: In these days, failures of distribution substation are common and dominant due to operation of the system under stressed conditions. A brief introduction on current energy crises and the importance of MSS is made in this state-of-the-art of literature. Effective design considerations of various components that are used in MSS along with their functions are elaborated. Finally, results of the road trail made on the proposed model are also described. Unavailability or average interruption duration per year decreases from customer’s view point using mobile substation. The MSS could be justified when the annual benefit value using it exceeds the annual service discontinuity cost.

References

• [1] IEEE Standard 1268, “IEEE Guide for Safety in the Installation of Mobile Substation Equipment”, Jan. 2016.
• [2] Central Electricity Authority, “Guideline for Specifications of Energy Efficient Outdoor Type Three Phase and Singe Phase Distribution Transformers", Aug. 2008.
• [3] Paintini, “Lightning protection of overhead power distribution lines”, ICLP proceedings, Jul. 2008.
• [4] B. Vasconcellos, J. R. Carvalho, M. S. C. Carvalho, M. W. Schlischting, T. I. R. C. Malheiro, L. V. Dutra and W. Gentil, "Proposal for development of a fragmented mobile substation for treatment of seasonal loads", Renewable Energy & Power Quality Journal. vol.1 (13), 2015.
• [5] Lopez-Roldan, J. Alfasten, J. Declercq, R. Gijs, P. Mossoux and M. Vandyck, "Technical considerations regarding the design and installation of mobile substations", CIGRE Session, 2004.
• [6] Dehghan, H. Ghaemi, S. M. Shadman and S. A. Khorasani, “Using the Mobile Substations in 132kVnetwork and studying their effects on the losses of network", IEEE proceedings conference on Electrical Power Distribution Networks, 2012.
• [7] Yousefpoor, A. Azidehak, S. Bhattacharya and B. Parkhideh, "Control of active mobile substations under system faults", proceedings of the Energy Conversion Congress and Exposition, pp. 1962-69, Oct. 2013.
• [8] E. Brown, “Electric Power Distribution Reliability”, Section 4.5, pp. 134-141, New York: Marcel Dekker, 2002.

Authors Biography

• Pradip Barua - General Manager (Electro-Mechanical), Supreme & Co. Pvt. Ltd, Kolkata, India
• Harish Agarwal - CEO, Supreme & Co. Pvt. Ltd, Kolkata, India (harish@supreme.in)
• Irfan Khan-Technical Architect, Supreme Gridtech Pvt. Ltd.
• Kumaran Ramadass - Product Development Engineer, Supreme & Co. Pvt. Ltd, Kolkata, India
• Anjan Sinha Roy - General Manager (Power System)
• Dharmbir Prasad - Energy Management & Research Consultant at Supreme & Co. Pvt. Ltd. and Research Scholar at IIT (ISM) Dhanbad.