Gumti Hydel Power Project::By:En. Sudip Das
GUMTI HYDEL POWER PROJECT
By: En. Sudip Das
Gumti Hydel Power Project is the first generating station of the state & only hydro power projects of Tripura. It is situated at Tirthamukh, Jatanbari, South Tripura. It was inaugurated on 10th June, 197 Its installed capacity is 3 x 5 MW(BHEL make). The generating voltage of this hydro projects is 6.6KV, and then the generating voltage is transform to 66KV by 5.88MVA power transformer, which is fed to 66KV bus. Gumti generating station is connected through 2(two) nos. of transmission lines. These are:
1. Gumti to Udaipur 66KV line(45KM).
2. Gumti to Teliamura via Amarpur 66KV line(59 KM).
According to Faraday’s law of electro-magnetic induction whenever flux is cut by the conductor, motion electro-motive force(emf) is induced in it and if the circuit is closed, flow of electric current will be caused.
In hydro power station, water is the main resource which acts as a fuel because the potential and kinetic energy of water head is converted to mechanical energy through the turbine(prime-mover) at a certain speed. This turbine is directly coupled with the rotating part of the generator called rotor. The rotor has field windings, which are provided to give cyclic north-south space distribution of poles. This poles produce a rotating magnetic flux(as the rotor rotates) and the armature(or stator) conductors cut these magnetic flux, therefore, an emf is induced in the armature conductors, which will cause a flow of current in the conductor if the circuit is closed. In this way electrical energy is produced in the generating stations.
It is the basic requirement of a hydro-electric power plant and is used to store water for running the turbines to produce electric power. At GHP it is of natural type and total catchment area of reservoir is about 45 sq. km. Water held in upstream reservoir is called storage whereas water behind the dam at the plant is called pondage. The pondage level of the dam is 92.05 mtr.(above sea level). Dead level of reservoir is 82 mtr.
The dam has been constructed to stock the water and it also makes a passage to flow the water from the reservoir to the channel by controlling the gates.GHP dam is of masonry type and this type of dam resist the pressure by its weight. The construction material used for this dam is solid masonry or concrete.
The flood level of the dam is about 93.4 mtr.
Height of the dam is 95 mtr. and the gross height is 97 mtr.
Water is allowed into the channel, penstock by gates. At GHP vertical sliding gates are used and hoisting rope lifts them. In the dam, there are 2(two) nos. intake gate, through which the amount of flow & volume of water is controlled. In front of the each intake gate there is a emergency gate (for maintenance purpose) & trash rack gate(for blocking the debris). If the water level reaches the flood level, then the superfluous water starts to spill through the 3(three) nos. of manual spillway gate at the dam.
A waterway is used to carry water from the dam to the forebay(upto powerhouse). It includes channel and siphons (closed pipes) or tunnel. The length of the channel at Gumti Hydel Projects is about 2.424 Km.
A spillway acts as safety valve for a dam as well as for the channel as it helps in the passage of flood water without any damage to the dam or channel.At GHP it is Ogee type spillway. At GHP automatic spill water level is 79.25 mtr. maximum.
It act’s as a type of regulating reservoir temporarily. Water is temporarily stored in the forebay in the event of rejection of load by the turbine and there is withdrawal of water from it when load is increased. Minimum water level at forebay at GHP is 75.25 mtr.
It is provided for preventing the debris from getting entry into the intakes from dam or from the forebay. Trash rack is made up of steel bars and manual cleaning is used to remove the debris at GHP.
A hydro-electric plant uses a number of turbines, which are to be supplied water through penstock. Anchor blocks act as support for penstock and they are massive concrete blocks encasing the penstocks at intervals to anchor down the pipe to the ground securely.The penstocks are joined together by riveting & are of 2.15 mtr. diameter each at GHP.
Surge tank is a storage reservoir fitted to the penstock at a point near to the turbine and works as in such a way that when the load on the turbine decreases the guide-vane of the turbine closed partly by the governor to adjust the rate of flow and the water is moving to the turbine has to move backward. This backward moving water is stored in the surge tank. Similarly, when the loads on the turbine increases, the turbine guide-vane are opened by the governor and increased demands of water is partly met by the water stored in the surge tank and thus control the pressure variations resulting from rapid changes in water flow in penstock and thereby prevents water hammer effects.
In hydro-electric power station water turbines are designed to work well under the condition that the head does not fall below a certain minimum level and at GHP, Francis (vertical shaft) type reaction turbine of speed 333.33 r.p.m is used to meet the standard frequency 50 hertz. Francis turbine is used for medium heads, at GHP it is 40.15 mtr .It is an inward radial flow reaction turbine consist of runner provided with guide vanes, guide mechanism, draft tube and spiral casing.In a reaction turbine water enters all round the periphery of runner and the runner remains full of water every time and water acting on wheel is under pressure, which is greater than atmospheric. Before entering the turbine, water has both pressure energy and kinetic energy, which forces on the rotating parts are due to the change of both potential and kinetic energy of water.
The essential parts of reaction turbine is as follows-
Governing of turbine means speed regulation. In normal condition the turbine should be run at constant speed irrespective of changes in load and it is achieved by means of a governor called oil pressure governor (Hart Nell type)
The various parts of the governor are as follows:
Tail race is a water way to lead the water discharge from the turbine to the river. The water after doing work on turbine passes through the draft tube to tail race and water held in the tail race is called the tail race water level. There are 3(three) nos. of tailrace gates, which are used at the time of shut down of units accordingly.
COUPLING OF TURBINE AND GENERATOR:
At GHP hydraulic turbine and AC generator shaft both are directly coupled and run at same speed as it comparatively a low speed turbine of 333.33 rpm.
GOVERNOR AC GENERATOR:
It consists of an armature and several permanent magnets encircling the armature. The permanent magnets are used to establishing the flux in the magnetic circuit, so this generator is known as permanent magnet generator(PMG). The field develop by the such machines remains fairly constant. The output power from the PMG is fed to the governor, which controls the speed of the turbine.
Type: APV 26/5
1.0 KVA, 120 Volts, 4.8 Amps.
333 rpm, 50 Hz, 18 poles, 3 phase, 0.25 pf.
Magnetisation- 54 Volts, 619 Amps.
In construction, dc generators consists of four parts mainly:
1. Field magnets, 2. Armature, 3. Commutator and 4. Brushes.
The object of the field magnet is to create a uniform magnetic field, within which the armature rotates. Electro-magnets are preferred in comparison with the permanent magnets on account of their greater magnetic effects and field strength regulation, which can be achieved by controlling the magnetizing currents.
Armature is the cylindrical rotating part of a dc machine. The purpose of armature is to rotate the conductors in the uniform magnetic field. It consists of coils of insulated wires wound around an iron and so arranged that electric currents are induced in these wires when the armature is rotated in a magnetic field.
Commutator provides the electrical connections between the rotating armature coils and the stationary external circuit. And the brushes are used to collect current from the commutator and supply it to the external load circuit.
Since the operation of a dc machine depends upon a fixed polarity of the poles which does not vary with time, the field coils need energization from a dc source. So, the field winding is excited from an external dc source as shown in fig.
The power received from the DC exciter is directly fed to the poles at the rotor.
Type: DRVX 26/17, Spec- IS-4722-1968.
58.5 KW, 80 Volts, 732 Amps, 333 rpm.
Excitation separate- 28 Volts.
It is a synchronous generator, because the rotor moves at a speed which bears a constant relationship to the frequency of currents, in the armature winding.This is a low speed hydro-generator with speed of 333 rpm and it’s a Revolving Field Salient Pole Type Alternator. It consists of: the armature(or stator) and the field magnet system.The armature is an iron ring, formed of laminations of special magnetic iron or steel alloy having slots on its periphery to accommodate armature conductors and is known as stator. Since the field rotates in between the stator, so that flux of the rotating field cuts the core of the stator continuously and cause a eddy current loss in the stator core. To minimize the eddy current loss, the stator core is laminated.
The field system of the alternator is rotated within the armature ring and is known as rotor. The magnetic field required for the generation of alternating voltage is provided by field magnets and these need to be supplied with direct current(dc). This may be had from some external source, but to make the alternator self-contained & independent of other source of supply, a small dc generator is install for this purpose and is known as exciter. The output current from the exciter is supplied to the rotor through the two slip-rings and brushes. The polarities of the field produced is alternately north(N) and south(S).
Type: ADV 348 44.5, Spec- IS-4722-1968.
5.888 MVA, 6600 V, 515 Amps.
333 rpm, 50 Hz, 18 poles, 3-phsae, 0.85 pf.
Excitation- 61 Volts, 681 Amps.
As the demand for electrical energy continues to grow at a steady space, electrical utilities find it necessary to increase the generating capacity at a regular interval. It becomes necessary to either operate alternators in parallel or else subdivide the load and feed it from multiple isolated systems. Of these two alternatives, parallel operation is much preferred because it does not require any major changes in the existing system .It has the following advantages:
The load on an electric power station fluctuates, usually having it peak value sometimes during the day and its minimum at night hours. Since generators operate most efficiently when operated on full load(or nearly full load), it logical to operate several smaller units(Generators) operating in parallel. Then as the load increases, other units are connected to operate in parallel with the existing ones. This keeps the machines loaded upto their rated capacity and increase the efficiency of operation.
b) RELIABILITY OR CONTINUITY OF SERVICE:
Continuity of service is one of the uttermost requirements of electrical power system. Several small units are more reliable than a large single unit., since if one unit fails, the continuity of supply can be maintained by the remaining units. On the other hand if the power system consisted only of a single unit, in the event of breakdown of the prime-mover or the generator itself, there will be a complete shutdown.
c) MAINTENANCE & REPAIR:
To carry out regular inspection and maintenance of the machinery so as to avoid the possibility of failure and it possible only when the unit is out of service, which means the other units which are in parallel will take care of the entire load of the system.
8.1. SYNCHRONISING OF ALTERNATORS:
It is imperative that changing from one unit to another or connecting an additional unit in parallel with the others (already operating in parallel) is carried out with the minimum possible disturbance to the existing system.The process of connecting an alternator in parallel with another alternator or with the common bus(the bus to which a number of alternators are connected) is called synchronising.
8.1.1. CONDITION FOR PROPER SYNCHRONISING:
Those alternators, which are in operation and sharing the load, are called the running machine, while that which is to be introduced in the system is known as the incoming machine. Before the incoming machine is connected in parallel with the running machine/s it is necessary that it meets certain conditions, which are given below:
a) The terminal voltage of the incoming machine must be exactly equal to that of the others, or of the bus-bars connecting them.
b) The speed of the incoming machine must be such that its frequency (being equal to PNs/120) equals bus-bar frequency.
c) The phase of the incoming machine voltage must be the same as that of the bus-bar voltage relative to the load i.e. the phase voltage of the incoming machine and the bus-bar should in phase opposition. This implies that there will be no circulating current between the windings of the alternators already in operation and the incoming machine.
d) The phase sequence of the incoming machine is same as that of the bus-bar.
About the author: Er. Sudip Das is an Electrical Engineer. He has completed his BE from Tripura Engineering college. He works in TSECL as a Manager.
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