Short Term Course On Hydropower Development Engineering (Electrical) for Teachers of Polytechnics in Uttarakhand ( July 14-18, 2007) Lecture on L33-2 By S.N.Singh Senior Scientific officer ALTERNATE HYDRO ENERGY CENTRE INDIAN INSTITUTE OF TECHNOLOGY, ROORKEE ROORKEE-247 667
ALTERNATING CURRENT (AC) HYDRO GENERATORS OR HYDRO ELECTRIC GENERATORS AC hydroelectric generators are electrical machines which convert mechanical energy which is obtained by hydro-turbine into electrical energy. This energy conversion is based on the principle of dynamically (or motionally) induced e.m.f. According to Faraday s law of electromagnetic induction, if there is relative motion between magnetic flux and a conductor then induced e.m.f. is produced in the conductor. This e.m.f. causes a current to flow if the conductor circuit is closed. Hence the basic essential parts of an electrical generator are: A magnetic field Conductor (winding or stator) & Relative motion between magnetic field and conductor. These are illustrated in Fig. 1 given below.
Fig. 1: Synchronous generator
In case of low capacity (up to 10 kw) hydro-electric generators the conductor (winding) rotates system and magnetic field is stationary, in case of higher capacity hydro-electric generators the magnetic field rotates and conductor (winding) system is stationary. These are shown in Fig 2. If its stator has single winding, it is called single phase hydroelectric generator and it is called three phase if stator has three windings. CONSTRUCTION The hydroelectric synchronous generator consists of armature (stator) windings mounted on a stationary element called stator, field windings on a rotating element called rotor. The rotor is like a cylindrical wheel having North (N) and South (S) magnetic poles fixed to its outer rim. These magnetic poles are excited or magnetized from DC (exciting current) supplied to the field windings by a rectifier and automatic voltage regulator (AVR) unit.
Fig. 2: Synchronous generator (alternator) of the rotating field type
Type of the Synchronous Generator Hydroelectric synchronous generators are classified as Separately excited hydroelectric synchronous generators : In this case d.c. supply is given to the field winding from other external source as shown in Fig.3: (a) and (b). Fig. 3:(a) manual voltage control
Fig. 3:(b) Automatic Voltage Regulator (AVR) principal elements. Separately excited generator with automatic voltage control Self excited hydroelectric synchronous generators: In this case d.c. supply is given to field winding from generator out put itself as shown in Fig.4..
Self excited hydroelectric synchronous generators are most commonly used in small hydro electric power stations. There are following two types of self excited hydroelectric synchronous generators used in Small Hydropower (SHP) development. Fig 4: Self excited synchronous generator
With carbon brushes generator The brushed generator (also called synchronous generator with static excitation) employes a rectifier unit and an AVR on its stator frame. AVR and rectifier unit deliver d.c. to field winding through two slip rings and carbon brushes as shown in Fig. 5. The carbon brushes need to be replaced periodically and the brushed type generator requires a continuous maintenance.
Fig 5: Schematic representation of a bushed type synchronous generator
Without carbon brushes i.e. the brushless generator They differ from each other mainly in the way they generate and supply the excitation current. The brushless generator is free from slip rings and carbon brushes and uses a small A.C. generator (Exciter) mounted on the rotor shaft to generate the excitation current. This AC is rectified n the rotor itself by rotating rectifier so as to supply the rotor windings with the required d.c. (field current) as shown in Fig. 6 (a) and (b). The absence of brushes and slip rings makes the generator more reliable and virtually maintenance free. However, brushless excitation system require on additional shaft extension for their physical location.
Fig 6 (a) : Schematic representation of Brushless synchronous generator with automatic regulator (AVR)
Fig 6 (b) : Electrical circuit diagram of Brushless synchronous generator with automatic regulator (AVR)
OPERATIONAL PRINCIPLE When rotor of hydro electric generator is driven by hydroturbine then some voltage is produced at stator winding terminals due to residual magnetic field (flux) and relative motion between field and stator winding. After that D.C. is delivered to field winding through rectifier unit and magnetic field is produced by field windings and in the stator winding voltages are produced upto rated value.
Relation between rotor speed and frequency of generated e.m.f. (voltages) Let f p = No. of magnetic poles. n = speed of the rotor in reduction per minute (rpm) = speed of the magnetic field (flux) = synchronous speed of generator (alternator) = frequency of generated e.m.f. in Hz (no. of cycle per second or in one second) As shown in Fig.7. if rotation of rotator is moving in clockwise direction then e.m.f. induce in the stator winding (conductor) (x) will be as shown in Fig 7 and explained as below:
3 2 N 1 Interpolar Axis Stator 4 S S 8 Rotor 5 N 6 7 Polar Axis Fig. 7: Motion of rotor in clock wise direction
Fig. 8: Cycle of induce e.m.f. in a for pole generator
THE MECHANICAL ANGLE: The mechanical angle around the machine periphery is always θ m THE ELECTRICAL ANGLE: The electrical angle covered by induce voltage in one Cycle of rotor is called electrical angle and denoted by letter RELATION BETWEEN ELECTRICAL AND MECHANICAL ANGLES. θ e 2π As shown in Fig 8 for 4 poles generator θ e = 2θm 4 θ e = θm 2 In general for p poles machines generator p θ e = θm 2
E.m.f. is zero when stator winding (conductor) lies on interpolar axis (minimum flux density) E.m.f. is max in one direction when stator winding lies on polar axis i.e. top of N pole (max. Flux density) E.m.f. is max but in opposite direction when stator winding lies on polar axis i.e. the top of S pole max. Flux) i.e. one cycle of e.m.f. is induce in a stator winding when one pair of poles passes over it.
p No of cycle of e.m.f. is one revolution = 2 n and no. of revolution of rotor or magnetic poles in one second = 60 No. of cycle of e.m.f. (voltage) in one second p i.e. frequency f = x 2 n 60 or n = 120 f p The equation (!) gives the relationship between speed of rotor and frequency of induced e.m.f.
Run away speed: the run away speed is defined as the speed which the prime mover and hydro electric system generator it is suddenly unloaded when working at its rated load. Thus hydro electric synchronies generator may be designed for the following values of runway speed & with full gets opening Pelton wheel,turgo wheel and cross flow: 1.6 to 1.8 times rated speed. Francis hydro turbine: 2 to 2.2 times rated speed Kaplon turbine: 2.5 to 2.8 times rated speed Propeller: 2.2 to 2.6 The generation voltage preferred for 3-, 50 Hz hydroelectric synchronous generator are: 0.415 RV, 3.3 KV, 6.6 KV and 11KV Voltage / Phase = 4.44 K c K d f T ϕ
PROTECTION AND CONTROL EQUIPMENTS Control: control Philosophy of SHP means operation of the plant or machines. It includes sequential operations like start up of machine, excitation control, synchronization, loading of unit under specified operating conditions, normal and emergency shutdown etc. The mode of control may be locally or from remote location. Plant control usually include monitoring and display of the plant conditions. Protection: This term is referred as disconnection of the of faulted plant or equipment. There are two types of protections which are provided in a SHP station. One is to protect the machines & other equipment from fault and the second is to protect people. The equipments which are needed for protections are as below.
Earthing Earthing of all equipment as well as the non-current carrying metal parts of an electrical installation such as frames, supports panels, fencing etc. is necessary. For the earthing GI pipe or copper plate buried in the ground. Fuse (Fuse Wire) A fuse is a small piece of metal wire connected in between two terminals mounted on installed base which form a series part of circuit. It is taken as one of the simplest protective divice and is used as circuit interrupting device under short circuit conditions symbols for pouse is given in Fig. 9.
FUSE Fig 9: Symbol of fuse
Isolators These are manually operated switches which have the basic function of isolating the load from the supply. Symbol for isolator is given in Fig.10(a) and (b). SINGLE POLE ISOLATOR 3 POLE ISOLATOR Fig 10(a) : Symbol for single pole isolator Fig 10(b) : Symbol for 3 Pole isolators
Switch fuses or main switches These are again manually operated switches which are similar to the isolator but with an addition of a fuse on the phase conductor. The addition of a fuse allows this switch to be used where current limiting is required. Symbols for main switches are given in Fig. 11(a) and (b). SINGLE POLE MAIN SWITCH 3 POLE MAIN SWITCH Fig. 11(a): Symbol for single main switches Fig. 11 (b) Symbol main for 3 pole main switches
Circuit Breaker It is a mechanical switching device which is used to close and open one or more electric circuits by means of separable contacts. Following circuit breakers are used in SHP Molded cube circuit breaker (MCCB) Oil circuit breaker (OCB) Air circuit breaker (ACB) Vacuum circuit breaker (VCB) Gas circuit breaker (SF6)
Contactor It is an electromechanical device which has two types of contacts Normally: open contacts (NO) Normally closed contacts (NC) as shown in Fig. 12. NO P O NC O N COIL Fig. 12: Symbol for contactor
Relays Relays are the devices that detect abnormal conditions in electrical circuit by constantly measuring the electrical quantities which are different under normal and fault conditions. The basic electrical quantities which may change during fault conditions are voltage, current, phase angle and frequency, the purpose of relay is to trip (disconnect) the circuit breaker. Following relays are used in SHP stations. Reverse power relay (RPR) Reverse power protection is essential for synchronous and induction generators to prevent motoring. There is some times risk of the generator passing over to the motoring mode in the event of prime mover failure due to low flow of water and RPR trips the circuit breaker.
Under voltage Relay (UVR) The under voltage relay trips the circuit breaker if the voltage drops below a pre-set value. Under voltage for long periods could damage electric motors or alternators itself. Under-voltage may occur in SHP installation if: The automatic voltage regulator in the generator is defective. The machine is overloaded. The load has poor power factor. Over-voltage Relay (OVR) The over voltage relay (OVR) trips the circuit breaker if the voltage rises over a preset value. Over-voltage could damage loads such as lamps, heating elements etc. and it is very high could cause a breakdown of certain insulating materials in the circuit. It is generally caused by: A defective AVR A leading power factor Generator over speed
Over Current Relay In this case over current relay will trip the circuit breaker if load is taking current in excess of the pre-set value. Over current can cause damage to the generator windings, switches, cables, and other equipment due to the excess heat generated in the conductors. The simplest form of over current protection is to use a fuse on the current-carrying conductor. The next form of protection is to use an MCCB, and the third form of protection is current limiters will isolate the load from the generator current in excess of the pre-set value. Over current relay (OCR) is used for triping the circuit breaker. Over current is caused by: excess load being connected to the generator faulty equipment being connected to the generator short circuit
Frequency trips In this case speed sensing relay (SSR) will disconnected load if the supply frequency is over or under the preset value. If supply frequency is not the rated value or not within its limits, it could cause damage to the electric motors and also the alternator. Over frequency may be caused by: excess water defective governor defective alternator or AVR
Temperature trips Alternators can be made with temperature sensing probes embedded in the windings. These probes can be wired to trips or to alarms to indicate when the winding temperature has risen over the safe value. The winding temperature may rise due to: overloading the alternator incorrect frequency excess current due to poor power factor waveform distortion due to the load defective bearings ad ventilation ambient temperature too high altitude not within rated value
Various measuring and protection equipment are shown in Fig.13. Fig.13. Single line diagram for SHP station
REFERENCES Manual on Induction motors used as generators by J.M. Chapallaz, J.Dos Ghali, P.Eichenberger, G. Ficher Electrical power by Dr. S.L. Uppal Handbood of electrical Engineering by SL Bhatia Basic Electrical Engineering by IJ Nagrath Micro hydro design manual by Adam Harvey Fundamentals of electric machines by B.R. Gupta and Vandana Singhal.