Large induction motors in distributed wind power generation

Starting an induction motor causes voltage sag in the industrial grid that may disturb the operation of grid equipment. Direct-on-line starting is the simplest and most cost-effective method for starting induction motors, but also the most problematic. Large industries often use internal power plants with synchronous generators and starting powerful motors may impact the generator operations. The synchronous generators could be operated with the automatic or manual mode of voltage control. As the operation experience proves, the generator voltage control mode has a significant impact on the transient behaviours in the industrial power grid when starting a large induction motor. This article presents a case study of the synchronous generator tripping within a true medium-voltage industrial grid during the direct-on-line starting of a large induction motor driving the feed water pump. An analysis of the generator protection logs after the tripping showed that the synchronous generator was controlled in manual mode and its protection relay settings were exceeded. The transients initiated by induction motor starts were studied for possible configurations and operating conditions of the grid using a model developed on the Matlab/Simulink Software platform. The simulations have shown that concern about starting large motors in industrial grids containing internal synchronous generators needs to be solved considering the grid configurations and the coordination of generator protection and control devices.

The main aim of this paper is to study effects of different voltage sags and their intensity on the behavior of induction motors using a simulation approach. For this purpose, necessary simulation was done in Matlab - Simulink first. Then, to ensure the reasonable accuracy of the simulation, some results of simulation were compared with the corresponding results measured in the laboratory. Then effects of intensity of symmetric voltage sag (three phase) and its starting point on the behavior of small and large induction motors was studied using simulation and subsequently, the effects of asymmetric voltage sags (single phase and two phase) has also been studied and finally the conclusion of the study were presented.

Recent power quality (PQ) research shows that the most common types of power quality disturbances are voltage sags and swells in medium and low-voltage distribution grids. This paper shows how to improve two significant power quality disturbances: sags and swells voltage. To find out what effect these two PQ issues have in real life, a study case based on real nonlinear loads data from large induction motors at Beshai Company in Sadat City in Egypt is investigated. The dynamic voltage restorer (DVR) has been suggested as a solution to the voltage swell and voltage sag issues. The point of common coupling (PCC) is linked with the dynamic voltage restorer to mitigate the PQ problems that have been found. The power network, loads, and DVR may all be modeled using the MATLAB/Simulink platform. The jellyfish search optimizer (JFS) is used to get the gain settings of the proportional and integral (PI) controller for the proposed DVR. MATLAB/Simulink's results show that the proposed device is effective, reliable, and has low latency.

Large induction motors can have a high inrush and run-up current during starting, often up to ten times the rated current. In weak supplies, this could be a problem, causing system stability issues and the voltage to dip below acceptable levels. In islanded systems, the capacity could be pulled below its maximum. There are several different starting methods possible, but they are often only suitable for smaller machines. One method not investigated is the use of parallel capacitor compensation during the starting because large induction motors are very inductive during the starting sequence, so that supplying reactive power may be more effective than supplying energy. This paper first investigates several different induction motors with increasing size and assesses their compensation requirements in terms of the reactive power requirement during starting. It is shown that they do generally become more inductive as they increase in size. It is illustrated, using simulations, that using parallel capacitors during starting can significantly reduce the starting current. The concept is tested using a small experimental motor. Series inductors are added to the motor to make it more inductive and look more like a larger machine. The experimental results confirm that the method works. It is also found that the parallel capacitors require series filters to prevent harmonic current, and these can be realized using inductors in series with the compensation capacitors. Point-on switching using solid-state relays was tested, and these can reduce the transient switch-on current over the first voltage cycle.

Voltage support in industrial distribution systems in presence of induction generator-based wind turbines and large motors

Motor stator winding temperature rise limits and test requirements are established by industry standards, customer specifications or the materials used in the insulation system. NEMA, IEC, and CSA standards describe how stator winding temperature rise is measured based on certain criteria, such as the rating or frame size of the motor. This paper details the differences in these standards and how the tested motor temperature rise varies depending on the standard being used. The value of temperature rise measured by embedded detectors placed in the stator slot is investigated and compared to the results from thermal stabilization tests where the temperature is determined from a rise by resistance calculation. Test data for Medium and Large motors is provided.

This article presents a cost-effective variable frequency drive for starting and operating large three-phase squirrel cage induction motors. The proposed drive employs the constant V/F control scheme, which regulates the rotor frequency such that it is always kept below a predetermined value. This has the effect of drastically reducing the motor shaft torque oscillations and significantly limiting the maximum value of the motor line current. In addition to its simplicity and hence cost-effectiveness, the proposed scheme is shown to be capable of (i) reducing the shaft torque oscillations to approximately 22% of that of the direct on-line start of the large motor, and (ii) limiting the motor line current to approximately 36% of that of the direct on-line start value.

Voltage sag is considered to be the most serious problem of power quality. It is caused by faults in the power system or by the starting of large induction motors. Voltage sag causes about 80% of the power quality problems in power systems. One of the main reasons for voltage sag is short circuit fault, which can be compensated for by a distribution static compensator (DSTATCOM) as an efficient and economical flexible AC transmission system (FACTS) device. In this paper, compensation of this voltage sag using DSTATCOM is reviewed, in which a sliding mode control (SMC) technique is employed. The results of this control system are compared with a P+Resonant control system. It will be shown that this control system is able to compensate the voltage sag over a broader range compared to other common control systems. Simulation results are obtained using PSCAD/EMTDC software and compared to that of a similar method.

There are applications where low current inrush and or high starting torque are specified for reducing disturbance on the electrical distribution network. There are also situations where it is required to successfully start large AC motors at reduced voltage conditions when only one substation transformer of a double-ended transformer is in service at petrochemical plants. The latter condition requires the motors to draw lower inrush current while producing high starting torque. These requirements can create unfavorable design characteristics where one or both requirements can not be accomplished using standard techniques at final design stage or at final motor test. The authors have accomplished designing and installing magnetic material retaining rings on Induction motors with significant improvement in inrush currents and starting torques. Inrush currents have been reduced from 600% to 508% and starting torques increased up to 14%. Several case studies of using magnetic retaining rings on large induction motors are described in this paper for machines ranging from 6,000 up to 14,000HP and testing as low as 380% inrush for machines with magnetic rings. The application of this concept is not limited to only AC induction motors. The same process can also be applied to synchronous motors. Very limited information is available on the benefits of applying magnetic retaining rings and what the consequences are for large AC motors. The application of magnetic retaining rings is a practical approach to solving inrush and torque problems at the manufacturing or design stage. The theory, test, concepts and consequences along with electrical and mechanical considerations are described in this paper.

There are applications where low-current inrush and/or high starting torque are specified for reducing disturbance on the electrical distribution network. There are also situations where it is required to successfully start large ac motors at reduced voltage conditions when only one substation transformer of a double-ended transformer is in service at petrochemical plants. The latter condition requires the motors to draw lower inrush current while producing high starting torque. These requirements can create unfavorable design characteristics where one or both requirements cannot be accomplished using standard techniques at the final design stage or motor test. The authors have accomplished the design and installation of magnetic material retaining rings on induction motors with significant improvement in inrush currents and starting torques. Inrush currents have been reduced from 600% to 508%, and starting torques have been increased up to 14%. Several case studies of using magnetic retaining rings on large induction motors are described in this paper for machines ranging from 6000 up to 14 000 hp and testing as low as 380% of inrush for machines with magnetic rings. The application of this concept is not limited to only ac induction motors. The same process can also be applied to synchronous motors. Very limited information is available on the benefits of applying magnetic retaining rings and what the consequences are for large ac motors. The application of magnetic retaining rings is a practical approach to solving inrush and torque problems at the manufacturing or design stage. The theory, test, concepts, and consequences along with electrical and mechanical considerations are described in this paper.

Voltage sags is a problem affecting the electrical qualities. In this paper, effects of large induction motors on voltage sag characteristic are discussed. Moreover, the influences of different mechanical loads of the motors on voltage recovery are investigated. The study results show that reactive power contribution owing to the motor's trapped flux linkage prevents fast collapse of the voltage, resulting in non-rectangular shape of sag. The motor's negative torque induced during fault interval causes speed loss. The constant torque of mechanical load gives rise to the most speed loss, leading to a longer reacceleration and a great delay in voltage recovery.

The fluctuation of output parameters of a wind farm due to the randomly varying wind speed is a serious problem especially for fixed speed wind generators, like induction generator (IG). This paper presents a new technique to improve the performance of a fixed speed wind generator by using permanent magnet synchronous generator (PMSG) along with a storage device. A wind farm of a total capacity of 12 MW has been used for this purpose. It is shown that the output power of an IG based wind farm can be made smooth by including PMSG with it. In this concept, the power capacity required for a storage device can also be reduced. A low pass filter has been used to calculate the required capacity of the storage device. Simulations have been done by using the engineering software PSCAD/EMTDC. Real wind speed data have been considered for this study.

With steady increase in the wind power penetration, regulatory standards for grid connection have evolved. The regulatory standards require the wind generation system to ride-through disturbances such as faults and support the grid during such events. This fault ride-through capability is determined by its corresponding critical clearing time (CCT). Conventionally, determination of CCT involves a large number of trial and error iterations that requires repetitive runs of related time-domain simulations. A method is proposed in this paper to compute CCT using non-conventional approach that avoids running related time-domain simulations several times. This work proposes a method to enhance the fault- ride through capability of squirrel cage induction generator based wind farms using Dynamic Voltage Regulator (DVR). The DVR controller is designed to work under fault conditions, which improves the critical clearing time of the system by injecting real power required by the system. The detailed simulation results are presented.

The contribution of wind power in conventional generation is increasing rapidly now-a-days. Hence a better understanding of wind power plants is required. But the wind power plants generally containing induction generators exhibit a rather different behavior than the conventional power plants which use synchronous generators. The purpose of this paper is to analyze and study transient stability of a wind farm based on conventional fixed speed Induction generator when it has been integrated with a weak grid. A comparative analysis of the transient stability improvement of a FSIG wind plant has been done, when it is supported with FACTS like S.V.C and STATCOM of equal ratings. Single case of transient condition i.e. a three phase fault has been studied by simulation. With the increase in demand of power and decrease of fossil fuels, mankind has been forced to search alternative sources for the generation of electricity. Wind power in spite of being stochastic in nature has proved itself as a viable solution to this problem. As the wind turbine technology is developing at a good pace, more and more wind power plants are being integrated with the conventional form of generation. With the increase in the ratio of wind generation to conventional generation, several problems related with integration of wind farms have emerged. These problems are due to distinct properties of the generators used with the conventional form (Thermal & Hydro) of generation and wind based generation (1)-(4). In thermal and hydro power based generation synchronous generators are used while in wind based generation mostly induction generators are used. Induction and synchronous generators do not exhibit similar behavior during power system stresses (transients). Moreover, increase in wind power based generation requires a reliable transmission system for power evacuation to the load centers. As the wind farms are mainly concentrated in far flung and rural areas their interconnection with the grid becomes a problem. This problem is mainly due to the existence weak transmission grids in the rural areas (1)-(4). Earlier when the share of wind power generation as compared with the conventional generation was low, during transient conditions; whenever the power system reached on the verge of instability the wind farm was disconnected from the grid and was reconnected when transients passed away. But as the share of wind power in the conventional generation increased, the practice of disconnecting the wind farms from the rest of the grid led to the drop in frequency of the power system. So now-a- days even if a fault occurs in the power system somewhere near the wind farms, they still remain connected with the grid for a critical time period. This phenomenon is called fault ride through (FRT) in wind farms (5). This paper has studied a transient condition (fault) in a grid connected fixed speed induction generator based wind farm and its fault ride through performance. A comparison of the performance of fact devices like static var compensators (svc) and static synchronous compensators (statcom) in improving the transient stability of wind farms during fault is also done.

An appraisal is made of the general problem of system-design synthesis by digital computers. Published literature on the design of electrical machines by computer is surveyed, and it is concluded that the majority of this work may be categorised as design analysis and partial design synthesis. The development of a pilot program for the design synthesis of large squirrel-cage induction motors is described, with emphasis on the methods adopted in the search for solutions. Particular attention is given to the development of special techniques dealing with the major discrete parameters, e.g. frame size and numbers of slots and conductors per slot. The final program comprises a marriage of methods of solution, including iterative techniques and preknowledge of the system functions, which was considered essential to achieve an economical convergence scheme. Results are presented, illustrating that the program produced satisfactory convergence to meet required performance specifications. A number of alternative solutions are obtained for each case of input data. There is considerable scope for the improvement and refinement of the program.