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Report on bus transfer. I. Assessment and application
Abstract
The initial assessment phase of bus transfer of station service
loads in power generating stations during emergency shutdowns in the
Southern electric system is reported. The application philosophy
developed for evaluating bus transfer performance is presented. A brief
overview of the types of transfer schemes available, the motivating
factors, and the criteria for transferring loads are given. The
techniques for evaluating transfer performance and general results for a
sample evaluation are presented
... Most of the existing publications regarding Automatic Bus Transfer Scheme (ABTS) is for motor bus load applications [8][9][10][11][12][13][14][15]. The existing automatic transfer methods for primary auxiliary buses that supply major rotating machinery loads can be classified to include Fast, Slow, Parallel, Residual Voltage, and In-Phase Transfer methods [12,13]. ...
... Automatic transfer restores power to either "A" side or "B" side substation main buses. Manual make-before-break operation is used to retransfer to the normal open tie configuration or manually transfer the substation (closed main, closed tie, and open main) for maintenance, repair, or modification [8,10]. Fig 1. illustrates a typical secondary selective system with automatic bus transfer controlling the operation of the Circuit Breakers (CB). ...
Automatic bus transfer scheme (ABTS) is the practice of transferring a load bus to an alternate source when the normal power supply fails or is tripped thus ensuring continuity of supply. This paper comprehensively reviews existing schemes and implementations of ABTS especially for motor bus. To limit the fault levels, during certain situations, the transformers supplying a primary distribution substation can be run in split instead of parallel operation. This is because during outages if one transformer is lost, overloading of remaining transformers, if it occurs, can be managed. This paper proposes an ABTS for a primary distribution substation for a utility facing such a situation and present details of its implementation. In the proposed scheme which is enabled by digital communications, if a transformer is lost, the bus section circuit breaker (CB) will be closed automatically after the incomer CB trips. The proposed ABTS has been implemented in the bus section relay for a new 11 kV switchboard where inter-relay communication is based on the IEC 61850 suites of standard. The contribution of this paper is that it shows how to use a standard automation scheme smartly to defer network reinforcements and manage fault levels in primary distribution substation.
... Bus configurations vary with requirements in different plants. The most widely used three configurations are explained in this section [2]. These configurations are used in thermal power plants, process plants and in nuclear power plants. ...
The proliferation of technology has made global conduction of business increasingly dependent upon the availability of reliable power. As a result, alternate power systems are being installed and expanded to protect the broadening scope of critical electrical loads. Bus transfer restores designated critical loads to an alternate source when utility derived service becomes inadequate or goes out of service due to any contingency. This paper describes the practices, requirements and implementation of bus transfer of motor loads to an alternate source of power. A new high-speed automatic bus transfer scheme is proposed which includes the development of a new algorithm for determining the type of bus transfer required and the realization of the scheme by using modern protection devices and intra-substation communication facilities.
... The automatic bus-transfer scheme (ABTS) is preferred mostly because it keeps the dead time of the motors to a minimum. Dead time is the time where the motor is not connected to any source of power [2]. On a thermal power plant auxiliary system of the type shown in Fig. 1, the loads are mainly induction motors. ...
Motor bus transfer involves the process of transferring a bus that has several critical motors to an alternate source of power when the main power source feeding them is interrupted. Bus transfer is a time-critical application in which the transfer process depends on various parameters such as the type of motor, load on the motor at the time of transfer, the inertia of the motor, and the combined open-circuit time constant of the various motors present in the bus at the time of transfer. A new high-speed bus-transfer scheme is presented in this paper, which determines the type of bus transfer possible based on the first one cycle of voltage information available from the motor bus after it is interrupted. A new algorithm is proposed to predict the open-circuit time constant of the combined motors within one cycle of interruption of the motor bus. Based on the predicted open-circuit time constant and the alternate source circuit breaker-closing time, the algorithm determines if a high-speed bus transfer is possible for that bus. If a high-speed bus transfer is not possible, the scheme determines effectively the next possible type of bus transfer, which is an in-phase transfer or residual voltage transfer based on user settings. The performance of the scheme for a typical thermal power plant, a cogeneration plant, and a nuclear power plant auxiliary is shown in this paper.
In order to suppress the inrush problems during the power switching of induction motor as much as possible under the different alternate power sources, it is necessary to select the optimal power switching method and determine the best switching moment with the consideration of the action time of the switch. Therefore, the stator residual voltage after the power off, the voltage difference and the phase-angle difference between the residual voltage and the voltage of the different alternate sources should be quantificationally researched more accurately. Firstly, it is found that there are two operation stages of motor based on the test wave of stator currents after the power off, that is, the short stator two-phase asymmetrical transient process and the rotor freedom operation. And the result approves inevitably the accurate quantitative calculation of the stator residual voltage. Then, the analytical derivation of two stages of the power-off process is made respectively in depth based on the symmetrical and asymmetrical vector models of motor. And the analytical expression of the stator residual voltage is obtained. Finally, the stator residual voltage, the voltage difference and the phase-angle difference between the residual voltage and the voltage of the different alternate sources are studied with emphasis. As a result, the optimal power switching strategy and the best switching moment are determined. And the drawed conclusions are verified to be correct and feasible by the test results of the power switching of induction motor.
Rapid response time and four-quadrant control make static compensators (StatComs) one of the most promising technology to improve control bus voltage; performances are improved when they are coupled to an energy storage system, such as a superconducting magnetic energy storage (SMES). This paper presents a new application for StatCom+SMES systems to support fast bus transfer (FBT) for loads requiring a high reliability power source. FBT supported by StatCom and SMES can be performed with standard electromechanical circuit breakers rather than more expensive static switches, even with passive loads, that cannot support bus voltage when the main source is going out of service.
This paper addresses the design of a high-speed motor bus transfer system for power generating plants and industrial facilities where motor loads require comprehensive source transfer strategies during transfer of the load from one source to another source. Motor bus transfer schemes are needed not only to maintain process continuity but also to transfer sources in such a manner to prevent damage to the motors and connected loads. The motor bus frequency and voltage decay rapidly upon disconnection from the main source. This paper proposes a digital signal processing algorithm which can measure the magnitude and phase angle of the decaying bus voltage accurately, while measuring the auxiliary source voltage magnitude and phase angle at rated frequency. The paper details an algorithm to predict the phase coincidence between the motor bus voltage and the auxiliary source voltage. The algorithm uses delta frequency, the rate of change of delta frequency and breaker closing time to predict the phase coincidence. The paper also details the implementation of the motor bus transfer scheme which includes fast, in-phase and residual transfer methods. Results of some real-time transfer cases are also included.
In some important industrial applications, motor bus transfer often takes place in order to guarantee the reliability and stability of the power supply. However, in the adverse condition, the stator transient currents of motor are very considerable during bus transfer. A thyristor-based control method, which can suppress the stator inrush currents, is presented for the motor soft transfer between sources. Under the implementation of the method, the phase-sequence of the stator windings connected to power source would be BC-ABC-AB-ABC-CA-ABC-. In order to study the complicated transition process between two-phase and three-phase connection, an analytical method is employed in this paper since it is more rapid and efficient than the numerical simulation when the rotor speed is nearly invariant. Then, a group of consistent mathematic models based on space vector is proposed for the alternative transient, especially for three different asymmetrical transient of motor with two-phase connection. Thereafter, the analytical method is detailed for the asymmetrical transient. The results of analytical calculation for two cases are shown and compared with field test and numerical simulation respectively. And the accordance of results verifies the validity and effectiveness of the proposed analytical method and transient models.
Nuclear power plants have Normal and Emergency power supply systems. Both the power supply systems are to be reliable for safe operation of the plant. Prototype Fast Breeder Reactor (PFBR) coming up at Kalpakkam is a single unit 500MWe project. DGs (Diesel generators) are the main on site power sources in the Nuclear Power Plants (NPP).Their capability to pick up the emergency loads in desired sequence is paramount for the plant safety. Extensive factory testing and commissioning tests are normally carried out to assure DG performance. Modeling and simulation tools are used to predict the expected responses in advance, which will be helpful during commissioning test.This work covers the modeling and simulation of DG sets, loads connected to the emergency bus and sequential starting of loads to predict the performance characteristics to meet the plant requirements.
Nuclear power plants have Normal and Emergency power supply systems. Both the power supply systems are to be reliable for safe operation of the plant. Prototype Fast Breeder Reactor (PFBR) coming up at Kalpakkam is a single unit 500MWe project. The normal power supply system consists of two Station Buses and two Unit Buses. The station buses are fed from grid through a Station Transformer (ST). The Unit buses are fed through Unit Auxiliary Transformers(UATs) either from plant generator or grid .The UATs are normally fed by generator while plant is in operation .When plant generator is not available the UATs are fed from grid through generator transformer. Normally unit buses and station buses supply the loads connected to them. In the event of a need on supply interruption to one unit bus, the loads supplied from this bus can be transferred to a designated station bus and vice-versa for continuity of power supply to the loads. The ST and UAT are adequately sized for this. This paper covers the modeling of turbine generator, ST, UAT and the loads connected to the station and unit buses of this system to analyze the bus transfer of loads from a unit bus to station bus and vice-versa which keeps the plant loads running on fast transfer mode.
The main objective of this work is to develop a cost effective off-line method for determination of induction motor equivalent circuit parameters by conducting a single load test on the motor. The proposed scheme is an alternative viable method to conventional means of no-load and blocked rotor tests. The identification of motor parameters is redrafted as a multi-objective optimization problem and solution is sought through conventional optimization method as well as genetic algorithm (GA). The conventional method employed is the well known Rosenbrock's (RB) rotating coordinates method. When the results of the two methods are analyzed, it is observed that while GA offers near optimal solution to the problem, the method of RB always results in global optima, provided initial values are chosen judiciously. Hence, it is proposed to combine these two methods to gain the advantages of both the methods. In such a hybrid optimization method, the task of global search is carried out by GA, while Rosenbrock's method is devoted to local search. Comparison of these two techniques are discussed and presented in conjunction with computed and practical results. It is shown that combination of GA with conventional method yields improved results.
This paper addresses the design of a high-speed motor bus transfer system for power-generating plants and industrial facilities where motor loads require comprehensive source transfer strategies during transfer of the load from one source to another. Motor bus transfer schemes are needed not only to maintain process continuity but also to transfer sources in such a manner to prevent damage to the motors and connected loads. The motor bus frequency and voltage decay rapidly upon disconnection from the main source. This paper proposes a digital signal processing algorithm that can measure the magnitude and phase angle of the decaying bus voltage accurately while measuring the auxiliary-source voltage magnitude and phase angle at rated frequency. This paper details an algorithm to predict the phase coincidence between the motor bus voltage and the auxiliary-source voltage. The algorithm uses delta frequency, the rate of change of delta frequency, and breaker closing time to predict the phase coincidence. This paper also details the implementation of the motor bus transfer scheme that includes fast, in-phase, and residual transfer methods. The results of some real-time transfer cases are also included.
Bus transfer restores designated critical loads to an alternate source when utility derived service becomes inadequate or goes out of service due to any contingency. The proliferation of technology has made global conduction of business increasingly dependent upon the availability of reliable power. As a result, alternate power systems are being installed and expanded to protect the broadening scope of critical electrical loads. This paper describes the design, implementation and testing of a new bus transfer relay which decides the type of transfer to alternate source based on the motor bus open circuit voltage. The algorithm detects the fast bus transfer, in-phase transfer and the residual voltage transfer. A new high-speed automatic bus transfer scheme is proposed which includes the development of a new algorithm for determining the type of bus transfer required and the realization of the scheme by using modern suitable hardware
The DQ/dq model has been widely used to study single machine
transients but it is difficult to use for multimachines during
unbalanced faults when the effects of source impedance must be included.
In this paper, a hybrid model (ABC/dq) of an induction motor is used in
order to analyze the reclosing transients of several hundred critical
motors in a refinery. These results are compared with the results from a
reduced order model with a significant reduction in computational
burden. The correlation is satisfactory
This paper shows the design of an electronic prototype, which senses the values for alternating voltage in two different electric distribution systems. The device connects the load to its normal source if the voltage levels are within a permissible range, in order cases the load is connected to an alternate source. The inverse action is accomplished automatically when the voltage returns to its normal value. The main components of the device are: operational amplifiers, timers, logical gates and triacs which gives a low cost alternative. The prototype also has a manual operation stage which permits one to maneuver in permanent fault conditions or when it is necessary to undertake maintenance work
This paper applies genetic algorithms to the problem of induction
motor parameter determination. Generally available manufacturers
published data like starting torque, breakdown torque, full load torque,
full load power factor etc., are used to determine the motor parameters
for subsequent use in studying machine transients. Results from several
versions of the genetic algorithm are presented as well as a comparison
with the Newton-Raphson method
A thyristor-based static transfer switch (STS) provides a continuous supply for a voltage sensitive load through fast transfer between two sources when the main source does not meet load voltage requirements. This paper investigates the impact of phase difference between the corresponding voltages of the two sources on the load transfer performance achieved by an STS. The studies are performed on the IEEE benchmark STS-1 system using the power system computer-aided design/electromagnetic transient dc simulation software. The studies indicate that the impact of phase difference on the STS load transfer time is insignificant when the sensitive load is a passive (e.g., RL) load. However, if the load can operate in a regenerative mode (e.g., a motor load), the effect of phase difference on the transfer time is noticeable. The studies show that a phase difference of 25deg can increase the load transfer time by about 2 ms
The Motor Bus Transfer Working Group of the IEEE Power System
Relaying Committee has performed a survey on industry practices for
implementing automatic motor bus transfers in various power plant
applications and the application of supervisory and control relays for
these transfer schemes. This report summarizes automatic transfer
methods for primary auxiliary buses that supply major rotating machinery
loads. Fast, slow, parallel, residual voltage, and in-phase transfer
methods are covered in this report. Information is also included on
methods and criteria of acceptance testing of the motor bus transfer
scheme
The dominant trend in US nuclear plants is to use sequential
(break-before-make) fast bus transfers. This study indicates that there
have been at least 54 bus transfer failures at US nuclear plants between
1985 and 1989. The authors analyze the lessons learned from these
failures and compare the alternate designs and practices that can
improve bus transfer practices in the United States. Although the
current rate of bus transfer failures is not alarming it can be
substantially reduced by incorporating some of the features that are
described
Induction motors are used extensively in heavy industry. For these
loads, this often requires reclosing on the motor before it reaches zero
speed. However, if the supply voltage is applied before the motor
reaches zero speed, the motor can be damaged due to excessive current
and torque transients. This paper presents a performance analysis of
fast reclosing transients in induction motors. It is shown that the
optimum time instant for reclosing depends upon the supply voltage and
load parameters. The results indicate that the maximum absolute value of
instantaneous torque occurs in the first cycle and for every time cycle
of supply voltage. In fact, there are at least one positive peak and one
negative peak for the torque
Induction motors make up the majority of the load in many
industries. The industry is becoming increasingly aware of the need to
ride through short term faults on the power system to improve the system
reliability, particularly where motors drive critical loads. This often
requires reclosing on the motor before it has reached zero speed. Such
reclosures can damage the motor due to excessive current and torque
transients. This paper examines the reclosing transients in an induction
motor, including the effects of saturation. A practical case study is
also presented with speed and back EMF measurements from a 20 HP
induction motor driving a mixer in an oil refinery
Some commonly used models of induction motors may give erroneous results in bus transfer studies of auxiliary systems which contain induction motors driving low inertia loads. A thorough bus transfer study must consider three time periods: the disconnect period, the period immediately after reenergization, and the longer time period during which bus voltage and motor speeds are restored. This paper provides a report on a current investigation to determine if improved accuracy can be obtained using different induction motor models than presently being employed by the authors [1,2] for bus transfer simulations where the loads have low inertias. Bus transfer simulations are presented using five different models. Based on a comparison of these simulations, a motor model for each of the three time periods of interest is recommended.
The ability to model the entire auxiliary system of large generating stations during emergency shutdown conditions aids the station designer in selecting the proper bus transfer scheme or schemes to implement. The purpose of this paper is to provide one approach to developing such a model. The paper addresses representation of the power sources and the most popular types of transfer schemes. It also provides discussions on the major plant loads and their representations. Finally, results from the model presented are compared with an actual field test for reclosing an auxiliary bus in a fossil-fired generating station.
For pt.II see ibid., vol.5, no.3, p.470-6 (1990). Results from
selected full-scale bus transfer tests are presented. The discussion of
each test includes a comparison of the actual test results with computer
simulations. Actual field tests are conducted in a manner designed to
mimic normal operating conditions as closely as possible. High-speed
chart recorders are utilized to gather test data. Typically, the tests
are performed during a major outage or just prior to the initial
start-up of a new unit. After testing, the auxiliary system computer
models are modified to represent as closely as possible actual operating
conditions at the time of the test
This paper applies genetic algorithms to the problem of induction
motor parameter determination. Generally available manufacturers'
published data like starting torque, breakdown torque, full-load torque,
full-load power factor, etc., are used to determine the motor parameters
for subsequent use in studying machine transients. Results from several
versions of the genetic algorithm are presented, as well as a comparison
with the Newton-Raphson method
The paper describes a new static synchronism check relay offering improved performance and characteristics, and discusses its application in transmission line reclosing schemes. Various auxiliaries systems bus transfer methods are reviewed, and the need for synchronism check in these methods is identified. Application examples of the new relay in bus transfer schemes are provided.
Undervoltage relays and starter solenoids respond to motor residual voltage. Test data are presented showing how such response can affect operation of an automatic transfer in a secondary selective system supplying induction motors. Suggestions are given for relay application.
THE PROBLEM of transferring industrial loads from a preferred source to an emergency source has become more important as an increasing number of electric motors are being used to drive critical loads. The previous practice of having turbines drive these critical loads has, as the trend shows, shifted to replacement of turbines by motors. In those cases it is desirable that the continuity of electrical service should be such that an unscheduled shutdown of this type of load be practically eliminated.
Knowing the magnitude, the frequencies and the amplitudes of the air-gap transient torque will definitely help the engineer to steer the design away from any destructive transient situation shown at the early design stage. With this objective in mind, this paper analyzed the simultaneous reclosing (or starting) of an induction motor under constant speed with or without capacitors. The frequencies and amplitudes of reclosing transient torques are very different when the motor is reclosed at different slips. Sample case study showed that as far as simultaneous reclosing is concerned the ANSI C50.41 or NEMA MG-1 on reclosing does not have practical meaning in limiting the magnitude of the simultaneous reclosing transient torque. Reclosing and starting tests conducted on a 2500/1250-HP, 10/12-pole motor agreed with the calculations.
A dynamic model for studying induction motor and power supply electromechanical interaction is presented. The electrical system is represented by conventional three-phase models. The motor-driven equipment shaft dynamics are determined by a lumped mass-torsional spring model. The electrical and torsional models are interconnected thru the motor electrical air gap torque and the speed and position of the motor rotor. Results from the transient simulation of various power supply interruptions on a 250 HP induction motor are presented. The difficulties of predicting the resulting peak shaft torque are discussed.
First Page of the Article
A dynamic model for studying induction motor and power supply electromechanical interaction is presented. The electrical system is represented by conventional three-phase models. The motor-driven equipment shaft dynamics are determined by a lumped mass-torsional spring model. The electrical and torsional models are interconnected thru the motor electrical air gap torque and the speed and position of the motor rotor. Results from the transient simulation of various power supply interruptions on a 250 HP induction motor are presented. The difficulties of predicting the resulting peak shaft torque are discussed.
The ability to model the entire auxiliary system of large generating stations during emergency shutdown conditions aids the station designer in selecting the proper bus transfer scheme or schemes to implement. The purpose of this paper is to provide one approach to developing such a model. The paper addresses representation of the power sources and the most popular types of transfer schemes. It also provides discussions on the major plant loads and their representations. Finally, results from the model presented are compared with an actual field test for reclosing an auxiliary bus in a fossil-fired generating station.
For pt.II see ibid., vol.5, no.3, p.470-6 (1990). Results from
selected full-scale bus transfer tests are presented. The discussion of
each test includes a comparison of the actual test results with computer
simulations. Actual field tests are conducted in a manner designed to
mimic normal operating conditions as closely as possible. High-speed
chart recorders are utilized to gather test data. Typically, the tests
are performed during a major outage or just prior to the initial
start-up of a new unit. After testing, the auxiliary system computer
models are modified to represent as closely as possible actual operating
conditions at the time of the test
Design Philosophy of Station Auxiliary Systems
- W L Snider
- T A Higgins
- B K Patel
Calculating the Transformer Inrush Currents That Result from the Transfer of Power Plant Auxiliary Buses
- S N Talukdar
- H E Sinnot
Problems of Impact Loading on Unit Transformers
- F J Mccann
- R J Ristow