B12- Electric Power Supply in India- A century back- Part IV

Major Electric Power Supply Companies in India till 1920

The Tata Hydro-Electric Power Supply Company Ltd, Mumbai (1915) (later Tata Power) - Part 2

In the previous part, we covered the background for Tatas setting up a hydroelectric power plant in the Bombay suburbs for reducing the pollution from the steam power plants used by numerous cotton mills, located in the city. We covered the equipment in the power station for generating power and stepping up the voltage to 100 kV for transmitting power to Bombay city. At that time there were only a couple of projects in the world working at such a high voltage of 100-110 kV. In this part, we will cover switchgear, transmission lines, receiving end substation, and mill equipment used for the project.

B11-Electric Power Supply in India- A century back- Part III

https://www.dhirubhai.net/pulse/b11-electric-power-supply-india-century-back-part-iii-ramachandran

Reference: Alfred Dickinson, The Bombay Hydro-Electric Scheme, The Journal of the Institution of Electrical Engineers, Vol53. No.248, Pages 693-721, 1915.

4.????Switchgear – There was a provision for operating each generator with its transformer bank or any other bank through 5 kV transfer busbars. The generator circuit had three numbers single-pole, non-automatic, solenoid-operated oil switches in cell compartments (today’s term MV circuit breaker), disconnecting switch (isolator), and current transformer and potential transformers.?HV side (100 kV) of transformer banks had three single-pole, automatic, solenoid-operated oil switches in the tank (later these were called bulk oil circuit breakers), disconnecting switches, and series relays. Similar equipment was there in each outgoing transmission line also. Additionally, lines had current transformers, choke coils (series reactors) and lightning arresters, HV oil switches (circuit breakers) had a breaking capacity of 40,000 MVA and were automatically tripped by inverse time-limit, overload, and series relays. These relays were mounted on post insulators. A long rod from the relay closed the switch (mounted on the wall) provided in the opening coil circuits of the solenoids, connected to the control wiring of the breaker.?

??Low tension (LV) wiring was done using varnish cambric insulated, fireproof treated cables. 100 kV bus system consisted of copper tubes on post insulators with a phase-to-phase clearance of 5 feet and ground clearance of 3 feet. A 60 A, 10-hour battery set supplied 220 V DC for control circuits with an 18 kW motor-generator charging set. Two 312 kVA 5000/400 V, 3 phase, 50 periods, transformers supplied station power and lighting. All panels were mounted on blue Vermont marble sheets on pipe supports. Three numbers 30 kW 440/220 V 50 period single-phase transformers provided power for lighting. Two numbers Willans-Siemens diesel sets installed in an adjacent building to the power station provided a standby power supply. Each set consisted of a 100 HP 200 rpm diesel engine directly coupled to an 80 kVA 440 V, 50-period three-phase alternator.

?The voltage on the HV bus was automatically maintained between certain limits using a Tirrill (Brown Boveri make) regulator operating from a 100 kV/ 100 V potential transformer. Regulators controlled the exciters. Close control of HV voltage was maintained by hand-operated rheostats and the above regulators. The power station was protected from line surges using choke coils (reactors) and aluminum-cell lightning arresters, complete with horn- gaps, disconnecting switches, discharge alarm, and discharge recorders.

?Generators were star connected with unearthed neutral. Generator over-voltage protection is provided by static dischargers and star-delta horn-gap arresters. Earthed busbars were run throughout the length of the building and copper earth plates were provided for connecting to lightning arresters and grounding other equipment.

?5. Transmission lines

The route of the transmission line (43 miles - 69 km) was from Khopoli power station to Parel (Mumbai) receiving station via Panvel and Vashi, crossing two creeks (one 10,000 feet long and 42 feet deep), a railway line, and several roads. There were four parallel transmission circuits in two parallel towers. Two change-over stations were provided en route, each with air-break isolators to disconnect or interconnect the three sections of the lines so formed.

Conductor - 7-strand, hard drawn, copper wire; area 0.095 square inch; weight 364-378 lb per 1,000feet; Conductivity of copper- 97 percent of Matthiessen’s standard, max resistance 0.255 ohms per 1000 yards at 60F; for creek crossing conductors used were 7-strand, hard drawn, silicon bronze conductor of 0.169 square inch area; weight 648-676 lb per 1000 feet and resistance 0.524 ohms per mile at 60F.Wires were supplied in standard 2,200 yards and strands in them joined by silver brazing.?At the site, twisted sleeve mechanical joints were used for joining the wires.

??Conductors were spaced at 10 ft.6 inches apart on towers. Intermediate towers (height above ground- 62 feet 1.5 inches) accommodated the conductors in delta fashion on either side of the tower, suspended on 6-unit suspension disc insulators. Metal hooded porcelain insulators were of 10-inch diameter and tested for AC voltages of 60 kV for 5 minutes and 70 kV for one minute. Towers were made of Siemens-Martins steel of tensile strength 28-32 tons per square inch and 20 % elongation on standard test pieces. Each circuit had a transmission capacity of 15 MW at 0.8 pf with the regulation of 11 percent and line efficiency of 93 percent. With a load of 20 MW, efficiency was 91.5 percent with regulation of 14 percent.

?6.????Receiving-end Substation – The receiving station at Parel (in downtown Mumbai) had provision for eight numbers 9.36 MVA, delta-delta connected, 3 phase step-down transformer banks, HT & LV oil switches, synchronous condensers (to improve power factor), and other accessories. In Stage 1, only 5 banks for two lines (on one tower) were installed. Each bank consisted of 3 numbers GE, USA make, shell type, water-cooled 3.12 MVA at 0.8 pf 85.8/6.6 kV water-cooled transformers of the same design and dimension as step-up transformers at Khopoli PS. Maximum oil temperature rise was limited to 58 F (14.5 C ) above a cooling water temperature of 87 F( 31 C)?with a flow rate of 1200 gallons per hour( 90 LPM). Transformers had 20 % overload capacity for 10 hours without injurious heating, but with a higher cooling water flow of 1450 gallons per hour.

Note the rated primary voltage of the transformer. It was only 85.8 kV compared to 100 kV of sending end transformers. There was provision for cooling the outlet water from transformers in a cooling tower and then again pumping back to transformers. There were duplicate cooling towers with a capacity of 18,000 gallons per hour. ????????????

?There were two synchronous condensers (one as standby) at the receiving station to improve the power factor and reduce the voltage regulation. Each was the synchronous motor of 3 MVA capacity,6.6 kV, 3 phase 50 cycles 500 rpm with 25 kW 125 V, shunt-wound, exciter on the overhang part of the shaft. Synchronous condensers had 20 % overload capacity for 10 hours without injurious heating.

There were remotely controlled HV and LV oil switches (oil circuit breakers) on both sides of the transformers with automatic tripping provision from the relays. Motor-operated single-phase oil switches were in separate oil vessels. MV (6.6 kV) busbars were copper bars mounted on insulators in concrete compartments, sectionalized by disconnecting switches. LV (220 V) connections were by varnish-cambric insulated, fireproof treated cables.

The control panel was a bench board, consisting of blue Vermont marble panels, mounted on pipe supports, with grillwork at the back, and fitted with instruments, control switches, indicating lamps, and a mimic busbar system for operating the two incoming lines, five transformer banks,34 feeder circuits, synchronous condensers, and the station transformer. The secondary relays were fixed on the back of the board. The control supply was at 220 V DC from a 60 A 8-hour battery with an 18 kW charging unit that also operates the series motors of the motor-operated oil switches (circuit breakers).

Station transformers were 42 kVA 6600/220 V 50-period single-phase transformers. An auxiliary switchboard (built of blue Vermont marble panels) controlled the supply from the above transformer bank. It had a transformer panel, four double feeder panels, a lightning panel, a battery, and a motor-generator panel.

MV 6.6 kV busbar room was connected to the feeder distribution system to the cotton mills through varnish-cambric, single-conductor feeder cables to the armored 3-core cables – 0. 1- 0.2 inch square section, paper insulated, lead sheathed type at a current density of 2.4-1.6 A/mm2. This was 3 phase 6.6 kV supply with unearthed neutral.

7.????Mill Equipment

Conversion from the mechanical to electrical drive was done at the Cotton Mills using the existing driving shafts to reduce the disruption to production. Group driving with direct-coupled motors was adopted with standard speeds of 265,290 and 365 rpm. The motors were 30-500 HP, 2 kV 3 phase 50 periods, slip-ring induction motors with brush lifting and short-circuiting gear. The motor temperature rise was limited to 70F (22 C) over an ambient air temperature of 100 F (38C). Motors had 50 % overload capacity for half an hour with efficiencies ranging from 87 to 92.5 % and a full load power factor of 0.74 to 0.84. Motor switches were liquid type, the electrodes being operated through slow-motion gears.

Transformers used at the mills for the drives were 500,600,700 or 900 kVA, 6.6/2.2 kV, 3 phase,50 periods, oil insulated, air-cooled, core type, star-star transformers. Transformers were capable of 50 % overloading for half an hour without injurious heating. Transformers had an efficiency of 98 % and regulation of 2.5 %, at full load, 0.8 PF. Transformers used for lighting were 10,20,30,40,50 kVA 6,600/240-120 V 50 periods single-phase transformers. Full load unity power factor efficiency varied 96-97.5 %, with a regulation ranging from 1.95-1.2 %. Oil temperature rise was limited to 70 F(22 C) over an ambient air temperature of 100 F (38 C).

All electrical equipment at the mill end was subjected to insulation tests at the maker’s factories at a pressure (voltage) of three times over working pressure (voltage).