An Adjustable Speed Synchronous Machine For Hydroelectric Power ...

May 1981, p. 2163

An Adjustable Speed Synchronous Machine For

Applications

Hydroelectric

Power

W. B. Gish, J. R. Schurz, B. Milano and F. R. Schleif, Consultant Water and Power Resources Service, Denver, Colorado This paper describes results of study and model test on the potential applicability of the doubly fed machine for relieving constraints inherent in hydropower applications. Although the literature contains many theoretical studies of the machine, this paper reports a more pragmatic examination of its characteristics and potential under a restricted set of conditions. The primary function of the machine in this type of applica¬ tion is to allow a modest range of speed adjustment, sufficient to main¬ tain best turbine efficiency in spite of load and head variations. In so do¬ ing, cavitation and draft tube surging are also relieved. In the mode considered here, the doubly fed machine operates as an adjustable speed synchronous machine with conventional control range for terminal voltage or reactive power either when motoring or generating. The characteristic instability of the double fed machine is suppressed by a stabilizer loop and by rotor current feedback. Besides the capability of convenient adjustment of speed, the double fed machine demonstrates impressive capability for rapid synchronizing, tremendous capacity for damping system swings, and the capability for constant output frequency with isolated variable loads. Application to pumped-storage projects, conventional hydrogeneration projects, and rapid start emergency standby applications appears particularly attrac¬ tive. Full-scale application on a hydroelectric project is being planned. The Operating Model Recognizing that the theory has long been well established and antic¬ ipating that important considerations would be controllability and other practical problems, an examination of the characteristics of an operating model, under conditions of the contemplated application, was considered appropriate. The apparatus for this model study consisted of a doubly fed machine of % -horsepower capacity coupled to a d-c motor/generator to permit driving the doubly fed machine in a generating mode and to permit loading the machine in a motoring mode. Auxiliary apparatus to the motor/generator set included a cycloconverter system for supplying exci¬ tation power to the rotor of the double fed machine, an SCR firing system with a variable frequency oscillator and feedback systems for cycloconverter control, and a tachometer system and stabilizer. The stabilizer system was driven by a tachometer signal and operated to advance or retard rotor frequency. An analog study had indicated that a two-term stabilizer would be more effective than the single term which has been proposed in the literature. The transfer function developed by experimental optimization corresponded very closely to that developed in the analog study. Utilizing this function, optimized to suppress the most unstable condition, operation was also satisfactory at all other conditions. It was also demonstrated that current feedback had a strong stabilizing influence, evidently by producing the effect of high internal resistance in the cycloconverter amplifiers. Test Results and Conclusions Considered from an operator's standpoint, controllability of the model proved to be very good. With current feedback holding field current cons¬ tant, speed could be adjusted smoothly from 70 percent to 130 percent of nominal synchronous speed in either the motoring or generating mode. Transient controllability of the machine proved to be even more im¬ pressive. This derives from the fact that rotor frequency, being under electronic control, can be changed very rapidly. With a conventional synchronous machine, the matching of speed and phase during synchronizing is impeded by the inertia of the rotor. But in the double fed machine no such impediment exists. Both frequency and phase match may be achieved, anywhere within a wide range of rotor speed, by control of rotor frequency which may be very rapid. To explore synchronizing capability, auxiliary circuitry was assembled to accelerate the machine from standstill to rated speed at a rate equivalent to full gate opening for an average hydroelectric generating unit. A phase matching circuit and high-speed synchronizer were enabled

PER MAY 1981

by a speed switch at 50 percent speed. Without even allowing one slip cy¬ cle, the phase matching circuit shifted rotor frequency from 7 Hz to 30 Hz in 0.2 second to achieve phase match. Synchronization was completed 0.8 second later. The machine was started from standstill, synchronized, and loaded in 8 secondsl The system thus has evident potential for rapid start, emergency standby. In the double fed machine, the energy stored in the rotor inertia is at the command of rotor frequency control. While stator frequency remains constant, energy may be stored or withdrawn from rotor inertia by the ramping of rotor speed. The energy shift may be sustained as long as the speed ramp is continued. A special doubly fed machine and flywheel set has previously been proposed for peak shaving of industrial loads. However, a doubly fed machine in a hydroelectric installation may serve many other purposes as weil. To demonstrate the improvement of frequency control possible when serving an isolated load with a hydroelectric unit incorporating a doubly fed machine the model was operated first with rotor frequency fixed at zero to yield performance similar to a conventional synchronous machine. The motor drive was speed controlled through an analog of a temporary droop governor and turbine, yielding characteristics of a hydrounit with mechanical starting time, Tm, equal to 8 seconds, and water starting time, Tw, equal to 2 seconds. The response of the system to a sudden change of load equivalent to 0.14 per unit is noted. In this conventional mode of operation, system frequency deviation is proportional to speed deviation. Then, output frequency was phase locked to a 60-Hz standard, and the phase error signal was used to control rotor frequency. The same load increment was then applied. Settings of the governor analog remain¬ ed the same. Shaft speed deviated more, demonstrating how the load in¬ crease is supported by energy withdrawn from the flywheel effect until the shaft power input can be increased and the rotating mass restored to normal speed. During the entire transient of shaft speed, the frequency of power output remains perfectly constant. Within the speed range of the doubly fed machine the effect is that of a unit with infinite inertia.

May 1981, p. 2171

Protection System Modelling in a Probabilistic Assessment of Tran¬ sient Stability P. R. S. Kuruganty, Member IEEE Manitoba Hydro, Winnipeg, Manitoba R. Billinton, Fellow IEEE Power Systems Research Group University of Saskatchewan, Saskatoon, Saskat¬ chewan application of probability methods in the evaluation of power tions. A probabilistic index for transient stability was developed which in addition to considering the random variations in the operating conditions, included the probabilistic aspects associated with the type, location and clearance of the system faults. It was realized during these studies that there was a need to develop stochastic models for the protection system behaviour which include the actual design aspects. The operational behaviour of the protection system is an important component in the high degree of security and service continuity associated with bulk power systems. This paper illustrates an approach used to model the protection system in the probabilistic assessment of transient stability. The probabili¬ ty density functions of the operating times associated with the com¬ ponents of the protection system such as relays, breakers, etc. are used in conjunction with the reliabilities of the main and back-up protection schemes to obtain the probability density functions of the fault clearing time for different fault locations. These density functions are used together with the critical fault clearing time for a given fault type and loca¬ tion to obtain a probability index for transient stability. A simple distance impedance protection system is used to illustrate the procedure. Con¬ tinuous as well as discrete density functions for the operating times of the protection system components are used. This paper illustrates the basic probabilistic nature of the fault clearing phenomenon and its importance in transient stability assessment. The

system transient stability has been demonstrated in recent IEEE publica¬

19

The protection system serves the purpose of sensing the fault and sending the tripping signals to the appropriate circuit breakers, thus en¬ suring the isolation of the faulty section within a few cycles of the onset of the fault condition. The time in which the fault is cleared is an impor¬ tant factor in stability investigations. The more quickly the fault is remov¬ ed the higher is the stability margin. A severe event such as a three phase fault must be cleared in less time than must a single line to ground fault, in order to preserve stability. If the fault is remote from the generator ter¬ minals, it may be possible to utilize a slower clearing device. In Reference 1 a single machine connected by transmission to an infinite bus system was analyzed to obtain the "probability of stability" for different fault types, locations and clearing times. In this work it was assumed that the fault clearing time can be represented by a Gaussian distribution with known parameters. The probability distributions of fault clearing time in this study were chosen arbitrarily. In an existing system, the probability distribution of the fault clearing time depends upon the probability distributions of the operating times associated with the main and back-up protection systems and the circuit breakers. The probability of stability for a given system is very much influenced by the protection scheme. The fault clearing time is the sum of the relay operating time and the breaker tripping time and is a random variable. The probability density function of the fault clearing time is conditional in nature as it is subjected to the con¬ dition of successful operation of the protection equipment. It involves the reliabilities as well as the probability distributions of the operating times associated with this equipment. A conditional probability approach is used to obtain the probability density function of fault clearing time. The following assumptions are made in the development of the basic technique described in this paper: 1. The protection system consists of three major parts; namely; the main protection, the back-up protection and the breaker failure protec¬ tion. 2. The operating times of relays and breakers associated with these ma¬ jor parts are random variables which are stochastically independent with known probability distritutions. 3. The fault is cleared by either of these systems and if the back-up system fails, the resulting multiple switching action will result in a se¬ quence of events which lies outside the domain of first swing transient stability assessment. The probability associated with these sequence of events is relatively small and therefore is ignored.

May 1981, p. 2177

A Computer-Controlled Laboratory Model of a Three-Area, Tieline Power Flow System

A. J. Goetze, H. W. Painchaud and S. R. North Carolina State University, Raleigh, North Carolina

Searcy

A laboratory model of a three-area tieline power flow system is describ¬ ed in this paper. A DEC 11/03 minicomputer and three Motorola M6800 microprocessors are used to acquire data and control tieline power flow. Algorithms are presented that implement data acquisition and permit various control strategies. Student interest in the Power SystemsMachinery Laboratory is greatly enhanced with this model and its related computer-controlled design projects, and this interest continues with fur¬ ther model development of economic dispatch, stability, and fault analysis and other area control problems. Lagging student interest provided the stimulus for introducing small, low-cost computers into the Power Systems-Machinery Laboratory. By taking advantage of most students' natural interest in computers, tradi¬ tional machine experiments can be made to encompass system principles of computers, electronics, power, and control. To this end three, com¬ mercially available, LAB-VOLT Electric Power Transmission Line systems were obtained together with six Motorola MEK 6800-D2 microprocessors, one DEC-PDP 11/03 minicomputer and associated peripheral equipment consisting of a video terminal, floppy disk unit, power supplies and elec¬ tronic data-acquisition components. As a teaching tool in power system analysis, it would be very desirable to model an interconnected system in the laboratory. With this in mind, a

20

model system is set up with the three LAB-VOLT Transmission Line Systems as shown in the Figure. Area Control System

_i_1_ Area Control Lines Motor Field

Controllers

Data

Fig.

Acquisition Systea

Laboratory Model Of A Three Area Interconnection And Com¬

puter Control.

Each LAB-VOLT system consists of a small 1/4 hp, de motor that drives a three-phase synchronous generator. This motor-generator set is used to model the equivalent turboalternator of a power area. The three M-G sets, as power areas, are interconnected with model, three-phase transmission lines or tielines. Tieline power flow and frequency are monitored by sampling the voltage and current of each tieline and by pro¬ cessing this data with three M6800 microprocessor data acquisition systems. The remaining three microprocessors are used in closed-loop excitation systems to keep alternator bus voltages constant. The process¬ ed data is transmitted via serial data lines to the PDP 11/03 computer which computes each area requirement according to tieline bias control^ strategy and returns system frequency to normal for varying area loads. Portions of the model can be used as undergraduate power-laboratory design projects such as, 1. Computer control of de or ac generator terminal voltage. 2. Computer control of de motor speed. 3. Computer control of solid-state motor drives. 4. Automatic synchronization of ac generators to the line. Many design projects associated with this model can be envisioned that combine computers, electronics and control with machines that improve the quality and scope of the power systems laboratory.

May 1981,

p. 2184

Computer-Dedicated Voltage Regula¬ tion Method For Distribution Substa¬ tions

R. Grondin Institut de recherche Quebec, Canada

d'Hydro-Quebec, Varennes,

The paper describes a voltage regulation method for distribution substations equipped with on-load tap changers. The method has been developed with a view to achieving the entire regulation of a substation using small-scale computer-based equipment. This regulation method ap¬ plies to either single or parallel transformers that may or may not have similar characteristics (power rating, impedance, number of taps, dynamic range or regulation). In addition, it can also be applied to com¬ pensate for the load drops on feeders. Besides offering great flexibility for regulation itself, computer-based equipment is very appropriate for remote changes of voltage settings. This remote voltage setting function is especially useful for lowering the peak load and thus avoiding load

shedding.

Since distribution substations represent the lowest level of regulation in its system, Hydro-Quebec finds it logical to control the voltage settings at

PER MAY 1981