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January 20, 2021
Dehar Power Plant of Beas Sutlej link Project commissioned in 1976 to 1980, has six (four first stage and two second stage) semi-umbrella type vertical generators of 0.95 pf of 165 MW each. A longitudinal section of the scheme is shown in Figure 9.4. The 300 rpm generators are coupled to 282 m (925 ft) rated head Francis turbines having a turbine runaway speed of 516 rpm. Unit size of 174 MVA selected for the power plant was the largest unit size in the country at that time and is perhaps a machine with highest speed with semi umbrella construction. The power plant is to be interconnected with the Northern Regional Grid of India in the first stage by a 280 km long single circuit 420 kV line at Panipat and by a 60 km long double circuit 245 kV line at Ganguwal. A second 420 kV line was added alongwith two second stage units. 420kV was being introduced in the region for the first time.
The main interconnected transmission system to which Dehar power plant was to be interconnected (1st stage) is shown in Figure 9.5and interconnection (1st stage) of the power plant.Considerations of economy dictated the size and type of generating unit to be installed and transmission outlets to be provided at a hydro generating site. Further in the initial stage of development of an EHV system problems of stability of hydro generators are liable to be critical because of weak system, large transmission angle and operation of generators at leading power factor. There is also risk of self-excitation and problem of voltage stability due to capacitive loading of long EHV lines. Further, smaller inertia constants of modern machines of advanced design may also jeopardize the stability of turbine governor gear. These problems are brought out and the system and other considerations involved in fixing the economical size, type and principal electrical and mechanical characteristics of Dehar hydro generators and excitation system keeping in view advancement in material technology, availability of modern fast acting static excitation equipment and methods used for analysis of system problems are discussed.
Dehar Power Station is located in the Himalayan region of India. This region and many other hilly areas where hydro stations are located are seismic zones. Provisions made in the generator for safeguarding against seismic forces have also been outlined.
Megawatt Rating and Number of generating Units
Substantial economies in the cost of equipment and civil structure are obtained by the installation of lesser machines of bigger size especially in a high head power plant. Further, higher efficiency is associated with larger generating units. Limitation on the size of the unit is placed on the one hand by water turbine and on the other hand by system considerations. With the availability of better techniques and materials, hydraulic turbine design has undergone rapid advancement and very large sized medium and high head Francisturbine hydraulic turbines have been made or proposed, e.g., 8,20,000 HP hydraulic turbines for third power plant at Grand Coulee in USA. Accordingly the major constraint on unit size for medium and high head Francis turbine driven generators is system characteristics and other limitations. As the system grows bigger, larger size generating units can be installed. In general maximum economic unit size that can be installed in a system can be found out by evaluating the increase in system spare generating capacity required as a result of increase in unit size. Evaluation of system spare generating capacity for various unit sizes is dependent upon size and characteristics of the power plants in the system and spare capacity already available in the grid. Other considerations involved are part load operation and transportation of heavy and big single piece packages,
The economic unit size at Dehar Power Plant was fixed keeping in view the maximum available working capacity of 583 MW as determined by water and power studies, the system spare generating capacity required to be provided at the power plant to meet forced outages in the system and the capacity outage required for scheduled maintenance of power units. Seasonal variation of load and available generating capacities was taken into consideration for fixing spare capacity. The spare generating capacity required for forced outages was worked out by probability methods.Transportation was also a consideration in fixing the maximum size of the units. Details are given in chapter 2 (Para 2.5). Accordingly the optimum number and kilowatt rating of generating units was fixed as four units of 165 MW each of the first stage. Two additional second stage units of equal capacity were proposed in the power house to take care of peaking requirements of future thermal power plants in the grid.
Power Factor and MVA Rating
The required MVA rating of generators is determined from the system reactive power requirements.Growth of system network and a better understanding of its behaviour has resulted in a definite trend towards specifying higher power factor rating especially for remotely located hydro generators, interconnected with the grid by EHV lines, so that the improvement in performance associated with the operation of generating unit nearer to its rated power factor can be realized. In case of Dehar Power Plant detailed load flow studies were carried out so as to find out the VARS(reactive power) fed into the system from Dehar generators for various interconnection alternatives. The reactive flow, megawatt load and consequently the actual power factor at which the machines operate in the study were found out. In the study total VARS of the system load were balanced by VARS from generators, capacitors already installed in the system and additional capacitors at load end adjusted so as to ensure tail end voltages at proper level.
Type of Generators
Savings in the cost of generators, overhead traveling crane capacities and civil structures can be made by adopting umbrella type of construction with a combined thrust and guide bearing below the rotor. Major conditions required to be fulfilled before umbrella type of construction can be adopted for large sized high speed hydro generators may be summarized as below:
a)The ratio of core length to rotor diameter be kept as low as possible and at the same time ensuring that a reasonably high output co-efficient is obtained consistent with the required winding temperature rises and transient reactances.
b)The required flywheel effect is incorporated in the rotor rim and poles with the stresses in the rotor at turbine runaway speed not exceeding two-third of the yield point of material.
c)The rotor overhang above the guide bearing to be reduced sufficiently, to ensure that the calculated critical speed of the combined generator and turbine shaft system is higher than the runaway speed by an adequate margin.
d)The radial width of air gap to be as high as possible so as to minimize the unbalanced magnetic pull on the rotor and thereby reducing the over-turning moment on the bearing.
(e) Ample and adequate design of bearing and easy accessibility to the thrust bearing.
In order to satisfy the above conditions for umbrella construction for Dehar generators, it was necessary that the length of the rotor core (and hence rotor overhang above the guide bearing) be reduced. It is, therefore, obvious that umbrella generator construction would require large diameter rotors with ratio of core length to core diameter less than about 0.29 so that critical speeds are well above turbine runaway speeds. Large diameter rotors would mean higher speeds and consequently higher stresses. Quality of steel available for rotor fabrication was, therefore, one of the main factors which hitherto restricted umbrella construction to smaller sized medium and low speed Francis and Propeller or Kaplan turbine generators. Similarly the limit curve for hydro-generator of above 100MVA rating did not exceed 200 rpm in Japan. It was now possible to adopt umbrella type construction for large high speed generators due to the advancement in material technology with special reference to higher tensile sheet steel for rotor rim punchings so as to obtain a minimum factor of safety of 1.5 on the yield point at turbine runaway speed.For Dehar Power plant 173.8 MVA 300 rpm generators, high tensile sheet steel with a yield strength of 56kg/sq mm (36 tons/sq in.) was used which permits its operation on a turbine runaway speed of 510 rpm at a peripheral speed of 176 m/sec for its large diameter (6.8 m) rotors with the specified factors of safety. With the use of this high tensile sheet steel, it was possible to reduce the length of the core (and hence the rotor overhang above the guide bearing) sufficiently to satisfy the conditions for umbrella construction. A second guide bearing at the top, although not considered necessary by suppliers of generators, was got provided so as to provide better stability for the unit taking into consideration seismic zone of location. Manufacturers intimated that the cost of the umbrella machines is about 12 to 15 percent lower than that of conventional top thrust bearing and two guide bearing arrangement. Semi-umbrella generator actually provided was about 10 percent cheaper. The calculated first critical speed for the combined generator and shaft system with the proposed semi-umbrella arrangement for the generator was about 20 percent above turbine runaway speed.
Generator Flywheel Effect and Stability of turbine Governor System
Large modern hydro generators have smaller inertia constant and may face problems concerning stability of turbine governing system. This is due to the behaviour of the turbine water, which because of its inertia gives rise to water hammer in pressure pipes when control devices are operated. This is in general characterized by the hydraulic acceleration time constants. In isolated operation, when frequency of the whole system is determined by turbine governor the water hammer affects the speed governing and instability appears as hunting or frequency swinging. For interconnected operation with a large system the frequency is essentially held constant by the later. The water hammer then effects the power fed to the system and stability problem only arises when the power is controlled in a closed loop, i.e., in case of those hydro generators which take part in frequency regulation.
The stability of turbine governor gear is greatly affected by the ratio of the mechanical acceleration time constant due to the hydraulic acceleration time constant of the water masses and by the gain of the governor. A reduction of the above ratio has a destabilizing effect and necessitates a reduction of the governor gain, which adversely affects frequency stabilization. Accordingly a minimum flywheel effect for rotating parts of a hydro unit is necessary which can normally only be provided in the generator.
Alternatively mechanical acceleration time constant could be reduced by the provision of a pressure relief valve or a surge tank, etc., but it is generally very costly. An empirical criteria for the speed regulating ability of a hydro generating unit could be based on the speed rise of the unit which may take place on the rejection of the entire rated load of the unit operating independently. For the power units operating in large interconnected systems and which are required to regulate system frequency, the percentage speed rise index as computed above was considered not to exceed 45 percent. For smaller systems smaller speed rise be provide. For Dehar Power Plant, the hydraulic pressure water system connecting the balancing storage with the power unit consisting of water intake, pressure tunnel, differential surge tank and penstock is shown in Figure 9.8. Limiting the maximum pressure rise in the penstocks to 35 percent the estimated maximum speed rise of the unit upon rejection of full load worked out to about 45 percent with a governor closing time of 9.1 seconds at rated head of 282 m (925 ft) with the normal flywheel effect of the rotating parts of the generator (i.e., fixed on temperature rise considerations only). In the first stage of operation the speed rise was found to be not more than 43 percent. It was accordingly considered that normal flywheel effect is adequate for regulating frequency of the system.
Generator Parameters and Electrical Stability
The generator parameters which have a bearing on stability are the flywheel effect, transient reactance and short circuit ratio. In the initial stage of development of 420 kV EHV system as at Dehar problems of stability are liable to be critical because of weak system, lower short circuit level, operation at leading power factor, and need for economy in providing transmission outlets and fixing size and parameters of generating units. Preliminary transient stability studies on network analyzer (using constant voltage behind transient reactance) for Dehar EHV system also indicated that only marginal stability would be obtained. In the early stage of design of Dehar Power Plant it was considered that specifying generators with normal characteristics and achieving requirements of stability by optimizing parameters of other factors involved especially those of excitation system would be economically cheaper alternative. In a study of the British System also it was shown that changing generator parameters have comparatively much less effect on the stability margins. Accordingly normal generator parameters as given in the appendix were specified for the generator. The detailed stability studies carried out are given in chapter 10 in Para 10.12.
Line Charging Capacity and Voltage Stability
Remotely located hydro generators used to charge long unloaded EHV lines whose charging kVA is more than the line charging capacity of the machine, the machine may become self excited and voltage rise beyond control. The condition for self excitation is that xc < xd where, xc is capacitive load reactance and xd the synchronous direct axis reactance. The capacity required for charging one single 420 kV unloaded line E2 /xc up to Panipat (receiving end) was about 150 MVARs at rated voltage. In second stage when a second 420 kV line of equivalent length is installed, the line charging capacity required to charge both the unloaded lines simultaneously at rated voltage would be about 300 MVARs. The line charging capacity available at rated voltage from Dehar generator as intimated by suppliers of the equipment was as follows:
(i)70 percent rated MVA, i.e., 121.8 MVAR line charging is possible with a minimum positive excitation of 10 percent.
(ii)Up to 87 percent of rated MVA, i.e., 139 MVAR line charging capacity is possible with a minimum positive excitation of 1 percent.
(iii)Up to 100 percent of rated MVAR, i.e., 173.8 line charging capacity can be obtained with approximately 5 percent negative excitation and maximum line charging capacity that can be obtained with negative excitation of 10 percent is 110 percent of rated MVA (191 MVAR) according to BSS.
(iv)Further increase in line charging capacities is possible only by increasing the size of the machine. In the case of (ii) and (iii) hand control of excitation is not possible and full reliance has to be placed on continuous operation of quick acting automatic voltage regulators.It is neither economically feasible nor desirable to increase the size of the machine for the purpose of increasing the line charging capacities. Accordingly taking into consideration operating conditions in the first stage of operation it was decided to provide for a line charging capacity of 191 MVARs at rated voltage for the generators by providing negative excitation on the generators. Critical operating condition causing voltage instability may also be caused by the disconnection of load at the receiving end. The phenomenon occurs due to capacitive loading on the machine which is further adversely affected by the speed rise of generator. Self excitation and voltage instability may occur if.
Xc ≤ n2 (Xq + XT)Where, Xc is capacitive load reactance, Xq is quadrature axis synchronous reactance and n is the maximum relative over speed occurring on load rejection. This condition on the Dehar generator was proposed to be obviated by providing a permanently connected 400 kV EHV shunt reactor (75 MVA) at the receiving end of the line as per detailed studies carried out.
Damper winding
Principal function of a damper winding is its capacity to prevent excessive over-voltages in the event of line to line faults with capacitive loads, thereby reducing over-voltage stress on the equipment. Taking into consideration remote location and long interconnecting transmission lines fully connected damper windings with the ratio of quadrature and direct axis reactances Xnq/ Xnd not exceeding 1.2 was specified.
Generator Characteristic and Excitation System
Generators with normal characteristics having been specified and preliminary studies having indicated only marginal stability, it was decided that high speed static excitation equipment be used to improve stability margins so as to achieve overall most economical arrangement of equipment. Detailed studies were carried out to determine optimum characteristics of the static excitation equipment and discussed in chapter 10.
Seismic Considerations
Dehar Power Plant falls in seismic zone. Following provisions in the hydro generator design at Dehar were proposed in consultation with the manufacturers of equipment and taking into consideration the seismic and geological conditions at site and the report of the Koyna Earthquake Experts Committee constituted by Government of India with the help of UNESCO.
Mechanical Strength
Dehar generators be designed to withstand safely the maximum earthquake acceleration force both in the mvertical and horizontal direction expected at Dehar acting at the centre of machine.
Natural Frequency
Natural frequency of the machine be kept well away (higher) from the magnetic frequency of 100 Hz (twice the generator frequency). This natural frequency will be far removed from the earthquake frequency and be checked for adequate margin against the predominant frequency of earthquake and critical speed of rotating system.
Generator stator support
The generator stator and lower thrust and guide bearing foundations comprise a number of sole plates. The sole plates be tied to foundation laterally in addition to normal vertical direction by foundation bolts.
Guide Bearing Design
Guide bearings to be of segmental type and the guide bearing parts be strengthened to withstand full earthquake force. Manufacturers further recommended to tie up the top bracket laterally with the barrel(generator enclosure) by means of steel girders. This would also mean that the concrete barrel in turn would have to be strengthened.
Vibration Detection of Generators
Installation of vibration detectors or eccentricity meters on turbines and generators were recommended to be installed for initiating shutdown and alarm in case the vibrations due to earthquake exceed a predetermined value. This device may also be used in detecting any unusual vibrations of a unit due to hydraulic conditions affecting the turbine.
Mercury Contacts
Severe shaking due to earthquake is liable to result in false tripping for initiating shutdown of a unit if mercury contacts are used. This can be avoided by either specifying anti-vibration type mercury switches or if found necessary by adding timing relays.
Conclusions
(1) Substantial economies in the cost of equipment and structure at Dehar Power Plant were obtained by adopting large unit size keeping in view size of the grid and its influence on system spare capacity.
(2) Cost of generators was reduced by adopting umbrella design of construction which is now possible for large high speed hydro generators due to the development of high tensile steel for rotor rim punchings.
(3) Procurement of natural high power factor generators after detailed studies resulted in further savings in the cost.
(4) Normal flywheel effect of the rotating parts of the generator at the frequency regulating station at Dehar was considered sufficient for stability of turbine governor system because of the large interconnected system.
(5) Special parameters of remote generators feeding EHV networks for ensuring electrical stability can be met by fast response static excitation systems.
(6) Fast acting static excitation systems can provide necessary stability margins. Such systems,however, require stabilizing feed back signals for achieving post fault stability. Detailed studies should be carried out.
(7) Self-excitation and voltage instability of remote generators interconnected with the grid by long EHV lines can be prevented by increasing line charging capacity of machine by resort to negative excitation and/or by employing permanently connected EHV shunt reactors.
(8) Provisions can be made in the design of generators and its foundations to provide safeguards against seismic forces at small costs.
Main Parameters of Dehar Generators
Short Circuit Ratio = 1.06
Transient Reactance Direct Axis = 0.2
Flywheel Effect = 39.5 x 106 lb ft2
Xnq/Xnd not greater than = 1.2
