Transient in Electrical Power System under Large Induction Motor Starting Condition

The on-line monitoring techniques which have normally been used to assess the condition of large HV squirrel-cage induction motors have been dedicated to detecting rotor dynamic problems and bearing wear [1]. This is usually achieved by sensing the shaft movement using eddy current displacement probes in the bearing housings. The technique has proved to be successful for detecting drive misalignment, a bent shaft, oil whirl, bearing eccentricity and wear [2]. Unfortunately, an HV induction motor can fail due to other fault mechanisms which are not simply mechanical or electrical but are often interrelated and complex [3]. For example, frequent direct-on-line (DOL) starting can cause broken rotor bars or end rings due to the large starting currents producing high mechanical and thermal stresses on the cage winding [4]. DOL starting also produces large end winding forces and core vibration during the initial transient. This can cause mechanical damage to the end winding insulation and electrical failure.

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.

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.

In this paper, the authors describe the application of SVC (static VAr compensator) to AC electric railway system, outlines SVC installed at Shin-syo sectioning post on San-yo Shinkansen and report on the result of verifiable site investigations. Single-phase SVC can reduce the voltage drop in the feeding circuit and improve the power factor at the substation by compensating the reactive power. In the case of compensating the voltage drop, it is more effective to locate the SVC at a sectioning post than at a substation. The site investigation has revealed that SVC can reliably suppress the voltage drop.

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.

Induction motors are the most widely used for loads requiring rotational force because they are self-starting, very simple construction, stronger and easier to maintain than other motors. In spite of such various advantages, a large current is generated at the time of direct-on line starting, and a voltage drop is generated. Since the voltage drop may be adversely affected by other facilities connected to the same distribution system, a start-up method is required to reduce the voltage drop as much as possible. The induction motor can select several starting methods by the capacity. Recently, a soft-start starting method using thyristors has been introduced. Induction motors have a lower power factor because the proportion of reactive power for magnetization at startup is relatively higher than the active power, but as the rotation speed increases, the power factor is gradually increased if the active power is higher than the reactive power. That is, when the power factor becomes high in a low state, the phase angle is developed from a high value to a low value. When the change of this phase angle is used at the firing angle of the thyristor, the operation change of the motor can be used as it is to reduce the starting current. In order to reduce the starting current and lower the voltage drop, the power factor change of the induction motor varying from starting to normal speed during operation is used at the firing angle of the thyristor. As a result of analysis using the power factor change of the motor as the firing angle, it is confirmed that the voltage drop is extremely effectively reduced by the reduction of the starting current.

Induction generator is easy to durability and maintenance than the synchronous generator. So, recently Induction generator has been widely applied to small-scale hydroelectric power plant. When the rotor is operating faster than synchronous speed, induction machine can generate electric power. Induction generator has a large inrush currents, such as the starting current of the induction motor. Induction motor has been designed a variety of rotor shape in order to reduce starting current. Since the occurrence of high inrush current cause a voltage drop to the system, it will need to reduce possible. Because the starting current of the squirrel-cage induction motor varies in accordance with the rotor shape, it is necessary to analyze the magnitude of inrush current in order to apply to the generator. In this study, we analyzed the inrush current and the voltage drop caused in accordance with the rotor shape of 1500kw induction generator.

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.

Among a myriad of techniques in motor current analysis for fault detection in transient regime, the discrete wavelet transform (DWT) was widely implemented in the embedded online condition monitoring system (CMS) of the direct online (DOL)-fed induction motor (IM). However, the use of DWT-based CMS faces practical problems with variable frequency drive (VFD)-fed IM due to the required continuous redefinition of the decomposition tree to cope with the variable spectral content. To overcome this major limitation, an approach based on a novel DWT called multiple-windowed harmonic wavelet packet transform (MWHWPT) is then presented in this article. The originality of the proposed approach lies in the use of drive’s data to continuously tune the programmable MWHWPT in order to adapt to the varying spectral content of the motor current. The new approach has been validated experimentally under transient conditions covering both random variations of load and fundamental frequency with broken rotor bar fault on a VFD-fed 3-kW double-cage IM. Fault feature extraction results show the robustness and flexibility of the proposed approach through a clear distinction between healthy and faulty motor conditions for both steady and transient modes.

Electric arc furnace (EAF) is one of the largest loads in electric power systems. It is highly time varying and nonlinear. Its reactive power variations cause voltage fluctuations in nearby system which is known as flicker. On the other hand the nonlinear voltage-current characteristic causes strong voltage and current harmonics in EAF. To this end Flexible AC Transmission Systems (FACTS) technologies are used to mitigate the flicker and harmonics caused by EAF. The mostly used FACTS device in EAFss is Static VAr compensator (SVC) which can reduce the flicker and harmonics to some levels. Delays relevant to reactive power measurement and thyristor ignition reduce compensation capabilities of SVC. In order to prevent these delays, one can use faster compensation devices. Static Synchronous Compensator (STATCOM) may be considered as a high performance candidate for this purpose due to its fast and flexible response. In this article the performance of STATCOM is compared to SVC in EAFs plants. For this purpose the system is simulated with SVC and STATCOM in PSCAD/EMTDC using the practical EAF data records. Data records are the instantaneous EAF voltage and current recorded from EAFs in Mobarakeh steel company in Esfahan/Iran.

Evaluating voltage drop snapshot and time motor starting study methodologies—An offshore platform case study

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.

Enhancing voltage stability and LVRT capability of a wind-integrated power system using a fuzzy-based SVC

A novel, hybrid converter system employing combination of a load-commutated inverter (LCI), a DC-DC buck converter, and a voltage-source inverter is proposed for the large induction motor drives. By avoiding the use of output capacitors and a forced decommutation circuit, this system can eliminate all disadvantages related to these circuits in the conventional LCI based induction motor drives. Improved quality of output current waveforms and faster dynamic response can be achieved. In addition, the use of the buck converter enables reduction of the delink inductor size and eliminating requirement for the controlled rectifier. The proposed hybrid scheme features the following tasks: 1) safe commutation angle for the LCI controlled by the voltage source inverter over all speed regions, and 2) a delink current control loop to ensure minimum voltage source inverter rating. Advantages of the proposed solution over the conventional LCI-based induction motor include the followings: 1) sinusoidal motor phase current and voltage based en the instantaneous motor speed control; 2) fast dynamic response through the operation of voltage source inverter and buck converter; 3) elimination of motor circuit resonance and motor torque pulsation; 4) compact and simple system implementation due to small size of the delink inductor. The feasibility of the proposed hybrid circuit for the high power drive system is verified by computer simulation for a 500 hp induction motor. Experimental results to support the use of the proposed system are also included for a 1 hp induction motor laboratory setup.