Generators for Miranda16 October 2000
The generators for the Miranda hydro power plant in Brazil have been operating satisfactorily since completing a fierce regime of tests and checks. Their success means that the possibility of increasing power output from the generators could soon become a reality
In December 1994 the Brazilian authority CEMIG (Companhia Energética de Minas Gerais), appointed IMPSA as turnkey contractor for electromechanical equipment at the Miranda hydroelectric power plant. Miranda is located on the Araguari river, about 440km northwest of Belo Horizonte, the capital city of Minas Gerais State.
IMPSA supplied all the electro-mechanical equipment for the project, including: generators, excitors, turbines, governors, penstocks, gates, cranes, transformers, and electrical and mechanical auxiliaries and controls.
The generators are vertical shaft, three- phase, salient-pole synchronous machines, each with an output of 137MVA, 128.6rpm and a rated voltage of 16.5kV. The three generators are driven by directly coupled Francis turbines.
The contract, which took effect in February 1995, specified that the first unit should start generating within 40 months. In fact, the three generators began commercial operation in May 1998, July 1998 and September 1998.
The electromagnetic requirements of the client are shown in the table on p25. The client gave the manufacturer a range of voltages so the electromagnetic load of the generator could be selected more easily. Apart from the usual considerations for stator winding bars to select the amps per slots, parallel circuits, and so on, the standard insulation levels for range I (iec 71-1) were also taken into account so that a higher level of switchgear insulation was not required.
The choice of the electromagnetic load was based first on the established relationships between apparent power and electromagnetic load and then on full dimensioning, which was selected using the ARGEN program. A voltage of 16.5kV was selected because it produces a current that can maintain the required reactions and supplementary losses at an adequate level to avoid overheating and assure efficiency.
A wave-type stator winding was selected to reduce the connections. During the calculations fractional numbers of slots per pole and per phase were defined, to give a low level of sub-harmonics. This was considered as a standard quality requirement for the voltage wave.
While testing generator vibrations, the possibility of resonance during the magnetisation test (loop test) was also checked in order to avoid the use of costly variable frequency equipment. It was decided that the air-gap should be increased to give better stability and larger reactive power generation. A series of studies were then conducted which included the following: •Verification of flux density over the magnetic circuit. Interpole leakage flux was determined using a finite element method (FEM).
•Losses and heating. Ventilation of the moving parts during operation was calculated, taking into account the more rigorous requirement for increases in temperature requested for this generator.
•Efficiency at different loads and power factors. The losses of the thrust bearing corresponding to the generator load were included in the mechanical losses.
•Generator reactions, time constants and capacities.
•Short circuit forces and torques, out-of-phase synchronising forces, magnetic pulls in different conditions. This included load rejection up to a short circuit of 50% of the poles.
•Rotor oscillation calculation and analysis against turbine torque oscillations.
During these studies a library of more than 100 magnetic, electric and mech-anical computer sub-routines were run to ensure that all aspects of the client’s technical specifications were met. One of the most exacting tests for the magnetic stator core is the magnetisation test, also known as the loop test. The temperature of different core points, currents and voltages, reactive and active power and vibrations were measured at an induction of 1T over five hours, to verify the absence of hot spots.
The wave-type stator winding was provided with two Roebel bars per slot. The insulation consisted of mica tape and epoxy resin, of class F, impregnated by vacuum pressure. This system provides good dimensions and stability. The calibrated bars are firmly wedged in the slots by semi-conductive wedges (as side packing) and ripple spring wedges (for radial compression).
The end windings are tied securely to the support rings on which they rest, with elastic wedging placed in between. This assembly is designed to withstand the electrodynamic forces which could occur in the worst cases of external short-circuit or out-of-phase synchronisation.
The stator rests on 16 feet, with eight upper pillars to support the upper bracket. The stator frame is a welded structure in the shape of a polygon, with 16 sides and eight windows for air coolers. It was constructed from high strength structural steel plate, specially intended for applications where very good weldability and mechanical properties are needed. The stator frame has a strong rigidity in tangential directions and high flexibility in radial directions.
Since the critical point of the stator design is to provide flexibility in the frame in order to avoid buckling in the stator core and unacceptable vibrations, this component was studied using FEM and analytical calculations. These analyses proved that neither inadmissible levels of tangential stresses nor undesirable vibration levels would be present with the adopted design. In fact, the mechanical stresses in abnormal conditions turned out to be irrelevant with respect to the allowable ones.
The stator core was made of 0.5mm thick high quality electrical steel sheets with low specific losses. Due to transport limitations and the size of the stator the core had to be stacked on site. The frame was manufactured in four sections which were welded together in the generator pit.
The core sheets were insulated by applying two different layers of varnish, providing good electrical insulation with good mechanical properties. In addition, the laminations at the end were bonded in packets at the workshop. This prevents vibration and eventual separation of the sheets at the ends of the stator core as a result of fringing flux.
The rim of the rotor was shrunk; a condition was imposed to maintain shrinkage at overspeeds up to 1.1.
As the rotor rim is an active part of the ventilation system, other criteria had to be met. Special attention was therefore devoted to this component at the design stage and studies were carried out using both FEM and traditional methods. These tools made it possible to determine the stresses and deformations caused by the magnetic, thermal, centrifugal, shrinking forces and weights. It was then possible to establish a model, taking into account the stiffness of the various components involved and the acting loads on the rotor in order to study the behaviour of the system and calculate the remaining net positive interference or shrinkage at the required speed. These studies were carried out for normal and maximum operating conditions.
For the final outline of the rotor, a rotor spider made from 14 radial arms was adopted. The shrunk condition generates large compression efforts in the rotor spider — important to prevent the rotor spider structure from buckling.
The quality of the structural steel plates used in its manufacture was the same as that used for the stator frame. The shrunk rotor rim was built on-site, using steel sheets specifically intended for this application. The 56 poles were anchored to the rim by two ‘T’ keys.
A nominal air-gap of 20mm was used. The air-gap monitoring system has been in operation since the first unit started up and shows excellent air-gap distribution.
The bearing arrangement and the quality of bearings dictates how well the generator will perform over the operating range. For this part of the project it should be emphasised that the hydraulic load of the Francis turbine and weight of the rotating parts produces a significant axial load of 980t on the thrust surface. This factor made the calculation and design of the thrust bearing more difficult.
A tilting pad hydrodynamic bearing was designed. The pads were arranged on a mattress of helical springs, which are not pre-compressed. The bearing is self- adjusting and the load is uniformly distributed over all the pads due to the high axial flexibility.
The sliding surfaces of the pads were made of white metal lining, which has a low coefficient of friction for starting and stopping conditions. Because of the size of the bearing and the pressures and temperatures in normal and extreme operating conditions deformation of the bearing was considered. The thermal and elastic surface distortions of the pads and thrust block were calculated and com-pared with the minimum oil thickness to check that they were within safe values.
The entire range of operating conditions was simulated on a numerical model. The upper guide bearing is of the tilting pad type, with the guide pads mounted around the outer diameter of the thrust block. In both cases, the thrust and guide bearing and the oil-water coolers were assembled outside the generator pit.
A complete study of the shaft line was made using FEM. Stress, deflection and critical speed on the shaft line were calculated, considering the structural properties of the generator rotor and turbine runner, inertia distribution, magnetic pull and stiffness of the supports (oil, bearing, brackets and reinforced concrete structure).
Generator cooling system
Optimising the ventilation system was a key consideration, to meet the guaranteed efficiency and to maximise the service life of the generator’s active components.
A self-pumping ventilation scheme was selected, with the rotor acting as a radial fan. A long series of studies was performed to achieve a reliable ventilation system, optimising the rotor rim duct and stator duct dimensions, arrangement of air baffles, and airflow.
The air pumped by the rotor is uniformly distributed along the entire height of the rotor rim and poles, then through ducts in the stator core and winding ends before finally reaching the air coolers. Airflow control is provided by partially closing the inlet windows in the rotor spider and by adjusting hole sizes in the upper and lower horizontal stator rings. A static digitally controlled stator was provided.
The main excitation circuits are:
•Equipment for de-excitation.
•Equipment for field flashing.
The main circuits provide the field current to the field winding of the synchronous machine, and the de-excitation device absorbs the energy stored in the inductance of the field winding when necessary.
The oil-immersed excitation trans-former supplies the controlled rectifiers. The primary side is connected to the generator terminal bus and the secondary side is linked to the rectifier bridge input. The excitation transformer has a rated power of 1400kVA; rated primary and secondary voltage of 16,500V and 740V, respectively, and a Yd11 connection.
The thyristor convertor has identical ordinary and standby thyristor bridges. During normal operation the stand-by bridge is blocked, and the ordinary bridge conducts all the current. In the event of a failure, the ordinary bridge is blocked and the current is automatically (without interruption) transferred to the standby. The two bridges have their own independent trigger pulse amplifiers.
The voltage regulator is based in a programmable process controller which can operate in automatic, manual or field regulation current mode. The computer’s main functions are:
•Automatic voltage regulation.
•Field current regulation.
•Automatic loading and unloading of reactive power.
•Under excitation limiter.
•Over excitation limiter.
•Stator current limiter.
•Power system stabiliser.
•Power factor regulation.
•Calculation of field winding temperature.
De-excitation is achieved by a field circuit breaker connected on the DC side of the converter, and a controller thyristor discharge circuit. The breaker disconnects the rectifier from the field winding and the discharge thyristor opens a path for the field current during the event of de-excitation.
In order to prevent excessive voltages in the field circuit, there is another thyristor connected anti-parallel to the discharge thyristor and in series with the Crowbar-type discharge resistor.
The field flashing device provides a small current supplied from the auxiliary services to the field winding of the synchronous generator during start-up. After a few seconds the voltage is sufficient to supply the converter. Then the field flashing unit is automatically disconnected.
The Miranda hydro power project units were supplied with an on-line monitoring and integrated system showing different generator parameters in real time, so each unit has a god predictive maintenance programme.
Miranda is the largest hydroelectric power plant in Minais Gerais that is operated by remote control.
To assure effective operation and give dispatchers the capability to optimise operations, the following on-line monitoring systems were supplied:
•Stator bar vibrations.
•Radial vibration in shaft line.
•Axial vibration in shaft line.
•Radial displacement in upper bracket.
•Axial vibration in turbine cover.
•Radial vibration in penstock.
•Pressure in spiral case.
•Pressure in draft tube.
The acceptance tests for the Miranda generators were applied according to the IEEE standard 115 and IEC-34.2/2A. CEMIG specialists performed most of the required routine and acceptance tests, together with IMPSA supervisors and specialists for testing and calculations. The guaranteed data obtained from the tests were comparable with data obtained from calculations, as shown in the table on p26.
Since the commissioning test, the performance of the three units has been very satisfactory and CEMIG is analysing the possibility of increasing the operation power from 137MVA to 150MVA.
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