Showing papers by "Seung-Ki Sul published in 2019"

Analysis on Position Estimation Error in Position-Sensorless Operation of IPMSM Using Pulsating Square Wave Signal Injection

Abstract: In this paper, the theoretical analysis of the position estimation error in position-sensorless operation using pulsating square wave signal injection is presented. The purpose of this paper is to analyze the phenomenon that the position estimation performance varies depending on the injection frequency. Mathematical derivation supported by simulation and experimental results shows that, in no-load condition, the voltage distortion induces position estimation error with 3rd-order harmonics if the frequency of the injected square wave is the same to the switching frequency. While, if the frequency is a half of the switching frequency or heavier load condition in either injection frequency, the effect of nonideal characteristics of inverter disappears and there is no 3rd harmonics in the position estimation error. Therefore, only the effect of cross-coupling inductance is reflected in the estimation error. As the result, the position estimation error appears in the form of 6th-order harmonics due to the spatial harmonics of inductances, and the error is relatively small compared with the switching frequency injection case. The theoretical analysis was verified by simulation and experimental results.

Extending Operational Limit of IPMSM in Signal-Injection Sensorless Control by Manipulation of Convergence Point

Abstract: This paper analyzes the convergence of signal-injection sensorless control (SISC) especially for heavily saturated interior permanent-magnet synchronous motors (IPMSMs). In the analysis, it is revealed that the harmonic inductance and the operating current variation are critical factors in determining the operational limit of SISC. The clear boundary of SISC for an IPMSM can be obtained through the proposed convergence analysis. Based on this analysis, a control algorithm is proposed to extend the operational limit of SISC. With the proposed method, the torque capability under SISC can be maximized. Simulations and experiments are performed to verify the proposed method.

Online MTPA Control of IPMSM Based on Robust Numerical Optimization Technique

Abstract: This paper presents an online maximum torque per ampere (MTPA) control that considers both magnetic saturation and cross-magnetization effects of interior permanent-magnet synchronous machines (IPMSMs). For IPMSM drives, especially in torque-controlled applications, torque accuracy and high-efficiency operations are important issues. They can be dealt as a constrained optimization problem to satisfy both torque reference tracking and loss-minimizing operation. In this paper, nonlinear simultaneous equations are derived from Lagrange multiplier method, which could be solved by numerical algorithms. Among them, Levenberg–Marquardt algorithm (LMA) is employed to guarantee a robust calculation of optimal current references. It is optimized to alleviate calculation burden while maintaining the stability of the proposed algorithm. In addition, a torque reference limiter is implemented to satisfy a current limit in real time. The feasibility of the proposed method is verified under various operating conditions by simulation and experimental results. Through the proposed algorithms, accurate MTPA control is achieved under not only unsaturated but also highly saturated operating conditions.

Discrete-Time Voltage Controller for Voltage Source Converters With LC Filter Based on State-Space Models

Abstract: State-space control could provide both high dynamic performance and sufficient stability margin to voltage source converters with LC filter However, an inherent digital delay induced by a digital control system deteriorates performance and even induces instability In this paper, a discrete-time voltage controller is proposed based on a discrete state-space model First, a state feedback control with a reference feedforward path is designed in the discrete-time domain with consideration for the digital delay Second, an output current decoupling path is augmented to minimize the effects of an output current disturbance Controller gains are derived as the functions of system parameters and design specifications, which is based on a direct pole placement and pole-zero cancellation method in a rotating reference frame Moreover, a parameter sensitivity and digital implementation are discussed to improve the performance and stability of the proposed controller The effectiveness of the proposed controller is verified with various experimental results It shows that a filter resonance is well damped out while maintaining wide voltage control bandwidth

Balancing of Submodule Capacitor Voltage of Hybrid Modular Multilevel Converter Under DC-Bus Voltage Variation of HVDC System

Abstract: A hybrid modular multilevel converter (MMC) consists of both half-bridge submodule (HBSM) and full-bridge submodule (FBSM) in each arm. It has several advantages over other MMC topologies, such as low system loss, reduced cost of the system, but still maintains fault ride-through capability against a solid dc-side fault. However, the hybrid MMC may have a difficulty of submodule (SM) voltage balancing under variation of its dc-bus voltage. Since the voltage unbalance of SMs results in SM capacitor overvoltage and low reliability of the system, the voltage of SMs, which can be also represented as energy of SMs, should be balanced at all time. In this paper, to extend the narrow adjustable range of dc-bus voltage due to the voltage/energy unbalance, new feedback and feedforward energy balancing controllers with revised sorting algorithm are proposed. The feedback control ensures the zero steady-state error, while the feedforward control provides fast dynamic response in the energy balancing control. By applying the proposed control scheme, the hybrid MMC can keep the voltage balance between HBSMs and FBSMs in wider range of dc-bus voltage. The full-scale computer simulation results and down-scale experimental results demonstrate the validity of the proposed control scheme.

A Modular Multilevel Converter with a Zigzag Transformer for Bipolar MVDC Distribution Systems

Abstract: This letter presents a modular multilevel converter (MMC) for bipolar medium-voltage direct-current (MVDC)) distribution systems. By employing a zigzag transformer as a grid interface transformer with a dedicated operation method, the operation of two dc poles of an MMC can be fully decoupled. Consequently, the MMC can provide full bipolar-operation capability. No additional component is required, and no penalty of increased power rating is imposed on the grid interface transformer. The scheme based on the proposed topology is competitive and cost-effective for emerging MVDC distribution systems. Feasibility of the proposed scheme is verified by both simulations and down-scale experiments.

Non-Invasive Magnet Temperature Estimation of IPMSM Based on High-Frequency Inductance With a Pulsating High-Frequency Voltage Signal Injection

Abstract: In this paper, a method to estimate the temperature of a permanent magnet of a traction motor for automotive applications is derived by analyzing the high-frequency inductance of an interior permanent-magnet synchronous motor. The derived method exploits the high-frequency inductance as an indicator of the magnet temperature, and it does not require a temperature sensor not only on the rotor side but also on the stator side. Hence, the method is truly noninvasive. The proposed method has been verified in a small-scale experimental setup. The performance of the method has been evaluated at various torques, speeds, and magnet temperatures ranging from −24 to 82 °C. Through testing at various operating conditions, it has been confirmed that the maximum error is less than 2.9 °C even when the speed and torque vary.

Overvoltage Suppressing Snubber Circuit for Solid State Circuit Breaker considering System Inductances

Abstract: Because of nature of the DC power distribution system, such as existence of power electronic converters with DC link capacitors and low system inductance, it has been known that the protection of DC system against short circuit fault would be difficult compared to AC system. The solid state circuit breaker (SSCB) is one of candidates for solution of DC protection thanks to its faster response and higher controllability and reusability compared to mechanical breaker or fuse. However, without proper overvoltage suppressing circuit, the overvoltage due to system inductance would collapse semiconductor switches in SSCB. In this paper, by the position of the inductance in the DC power distribution system, the effect of the inductance related to the SSCB overvoltage is analyzed. Also, the design of an overvoltage suppressing circuit absorbing the energy stored in the system inductance is addressed. And, a maximum operating range of the designed overvoltage suppressing circuit regrading system inductances is investigated. The effectiveness and performance of the overvoltage suppressing circuit is verified by the simulation and experimental results.

Improved Signal-Injection Sensorless Control Robust to Inverter Nonlinearity Effects by Prediction of Voltage Disturbance

Abstract: In signal-injection sensorless control, the estimated rotor position includes considerable error induced by the inverter nonlinearity effects. It would degrade overall performances of the sensorless drive, and it could even result in the failure of the position tracking. In this paper, an improved signal-injection sensorless control thanks to the compensation of the voltage error is proposed. The compensation depends on a precise calculation method of the voltage error induced by the inverter nonlinearity. The proposed method effectively reduces the voltage disturbance especially under the switching frequency injection for sensorless control. The performance of the proposed method is verified by computer simulations and experimental results.

Decomposed Current Controller for a Paralleled Inverter With a Small Interfaced Inductor

Abstract: In this paper, a current controller for a parallel operation of inverters is described. When inverters operate in parallel, it is a well-known fact that the inverter current is composed of the average current (AVC) and the zero-sequence circulating current (henceforth ZSCC), which inevitably flows between inverters under the parallel operation. Considering this, previous studies concentrated on reducing ZSCC. However, in this paper, it is found that the inverter current has another component, a differential current, in addition to the two components of AVC and ZSCC. Based on these three decomposed current components, three equivalent circuits are derived from n -paralleled inverters. To minimize the differential current and improve the regulation performance of the total output phase current, a decomposed current control scheme is proposed from equivalent circuit models. The proposed algorithm is applied to a system consisting of three paralleled two-level inverters with small shared inductance whose dc links are connected in common. Additionally, synchronous pulsewidth modulation is employed as the PWM method for each inverter. The proposed current controller is compared with two conventional current controllers, one where each inverter controls its own currents and the other where only AVC is controlled. Through experiments, it is shown that these conventional controllers have limitations and the proposed current controller overcomes these limitations. The effectiveness and feasibility of the proposed control scheme are verified through experimental results. Especially, both theoretical analysis and experiment results verify that the proposed method is robust with regard to parameter errors.

Control Strategy of Single-Phase Active Front-End Cascaded H-Bridge Under Cell Fault Condition

Abstract: Cascaded H-bridge (CHB) inverter is the most widely used topology for a medium-voltage drive system due to the high degree of modularity, easier implementation of medium output voltage, and the ability to continuous operation under the cell fault condition Because each power cell of CHB should have isolated dc source, multiwinding input transformer and three-phase active front end (AFE) are generally used for regenerative applications The whole system can be simplified by replacing the three-phase AFE with single-phase AFE However, if the control strategy of normal operation is adopted under the cell fault condition, input power imbalance among three phases inevitably occurs In that situation, not only faulty cells, but also some unscathed cells should be excluded with giving up the maximum capability of the system, not to deteriorate grid current This paper proposes a control scheme against the cell fault condition of the single-phase AFE CHB By applying the proposed control scheme to the system, dc-link voltage of each cell and grid current are well regulated without imbalance even under the cell fault conditions Finally, it can minimize the number of undamaged cells which should be turned off and maximize the capability of the system under the cell fault condition Simulation and experimental results are provided to verify the effectiveness of the proposed scheme

Control Strategy of DC-Link Voltage for Single-Phase Back-to-Back Cascaded H-Bridge Inverter for MV Drive With Interfacing Transformer Having Tertiary Winding

Abstract: This paper describes a dc-link voltage control method of a single-phase back-to-back cascaded H-bridge inverter (SBCI) for a medium-voltage motor drive system. The main advantage of the SBCI topology over the conventional regenerative cascaded H-bridge topology with a three-phase active front-end (AFE) is a simple system structure, which is composed of an input transformer, a power cell, a current sensor, etc. However, the challenging points of the SBCI are larger voltage ripple in the dc-link capacitor and imbalance of dc-link voltages of each phase. The asymmetric dc-link voltage of each power cells could cause unstable operation such as over-modulation due to the lack of the dc-link voltage of a particular phase and result in over-voltage or under-voltage faults. In this paper, the control strategy of the dc-link voltage for the SBCI that uses the negative-sequence voltage of the converter is described. The proposed control method is verified with a computer simulation whose target is a 6.6-kV–1.25-MW medium-voltage drive system. Also, through the experimental setup with the prototype SBCI whose power rating is 16.2 kVA, the dc-link voltage of each AFE has been controlled within a 0.5% error of its reference value at the full load.

Magnet Temperature Estimation of IPMSM by Using Reactive Energy

Abstract: In this paper, a method to estimate the temperature of NdFeB magnet of an Interior Permanent Magnet Synchronous Motor (IPMSM) is derived in medium and high speed operating condition, where the back EMF of the motor is high enough. To estimate the temperature of the magnet, a concept of reactive energy is newly brought out. By using this concept, no temperature sensors are required in neither rotor side nor stator side. Hence, the method is truly noninvasive. Furthermore, as the reactive energy itself is utilized to estimate the temperature, the sensitivity to the variation of the magnet temperature is enhanced compared to the conventional method. The proposed method has been verified in the small-scale experiment set. The performance of the method has been evaluated at various operating conditions of IPMSM.

Online MTPA Operation of IPMSM Based on Dual-Loop Control in Polar Coordinates

Abstract: In this paper, an online MTPA control is proposed for a torque control of IPMSMs. Firstly, torque and MTPA equations are derived based on flux variables where the influence of magnetic saturation and cross-coupling effects is carefully considered. These equations can be described in polar coordinates, where an output torque and MTPA operation are mainly determined by a current magnitude and angle, respectively. Consequently, a dual-loop controller is introduced to solve the torque and MTPA equation in the polar coordinates with less computational burden. It consists of both a current magnitude and a current angle controller, where a torque reference limiter is embedded to satisfy the current limit. The effectiveness of the proposed method is verified by simulation and experimental results. Through the proposed algorithms, both MTPA operation and torque accuracy can be achieved with less computational burden.

Extending Operational Range for Low-Cost Motor Drive Systems by Mitigating Narrow Pulse Effect

Abstract: In this paper, a method to mitigate narrow pulse effect for extending operational range of low-cost motor drive system, where the bootstrap circuit is embedded, is described. Because of the narrow pulse effect, dead zone appears in output of the inverter where voltage cannot be synthesized. To overcome the narrow pulse effect, the proposed method combines the inverter nonlinearity compensation and dead time elimination (DTE) method. By mixing those two properly, the narrow pulse effect can be reduced effectively and the output capability of the inverter increases conspicuously. Simulations and experiments are carried out to verify the effectiveness of the proposed method. The experimental results reveal 6 % enhancement of torque capability at two times of base speed.

Simplified Thermal Model of Semiconductor Fuse for DC Distribution System

Abstract: For last 50 years the semiconductor fuse has been developed and applied to power electronics system to protect the diode and thyristors. The semiconductor fuses are used in DC distribution systems instead of high speed solid state DC circuit breakers, which are too expensive to apply widely. In this study, simple thermal model of the fuse is presented to describe the melting and cooling effect of the fuse and a method is proposed to derive the parameter from the Time-Current-Curve (TCC) without further detailed data from manufacturer. The effectiveness of the model and circuit have been verified by computer simulation and experimental results.

IPMSM Magnet Temperature Estimation by d-axis Flux Linkage

Abstract: An Interior Permanent Magnet Synchronous Machine, IPMSM, has been widely used as a traction motor in automotive industries for last 10 years thanks to the high torque density and a wide operating speed range. The reliability and torque control accuracy strongly depend on a magnet temperature of IPMSM. D-axis flux had been generally exploited to estimate the magnet temperature, in conjunction with the information of the stator winding resistance, which varies according to its temperature. And in the conventional method, temperature sensors should be attached to the stator windings to decouple the stator temperature effect. In this paper by using the d-axis flux in specific currents, no temperature sensor is required to estimate the magnet temperature. Furthermore, the number of look up table can remarkably be reduced. The proposed method has been verified in the small-scaled experiment set. The performance of the method has been evaluated at various currents, speeds and the magnet temperatures from 3.9 °C to 80.7°C.

Analysis of Position Control Stability Affected by Non-ideal Characteristics of IPMSM in Signal-Injection Sensorless Control

Abstract: In this paper, the stability of position control loop under signal-injection sensorless control (SISC) considering the non-ideal characteristics of IPMSMs has been analyzed. To take into account the effects of the torque ripple and the inductance variation due to the non-ideal characteristics in frequency-domain analysis, they are linearized using Taylor series at each rotor position and operating current. Using the small signal model, it is revealed that the non-ideal characteristics make the system unstable and cause oscillation under position control with SISC. It is also revealed that the stability varies according to the rotor position and load torque even if the same controller gains are used. The validity of the proposed analysis is verified through simulations and experiments.

Optimal Current Trajectory Control of IPMSM for Minimized Torque Ripple

Abstract: In practical IPMSMs, there are always undesirable torque ripples due to the spatial harmonics. Most existing methods attempted to reduce torque ripples based on torque modeling, but their performances were not satisfactory due to inaccurate model of an IPMSM itself. This paper analyzes the torque ripple based on an actual torque map, which is more practical and accurate compared to torque-model-based analyses. Based on the analysis, a torque ripple minimizing method implemented with a few resonant current regulators is proposed. Simulation and experimental results are provided for the verification of the proposed method. For the experiments, the proposed method is performed and evaluated through an experimental torque map. Through the experimental result, it is verified that target harmonic order of torque ripple can be reduced to a tenth of the original value by the proposed method.

Design of Full-Order Flux Observer for Induction Motor Drive Robust to Rotor Time Constant Variation

Abstract: This paper proposes a novel full-order flux observer for flux estimation of an induction motor robust to rotor time constant variation. The influence of rotor time constant error in the conventional full-order flux observer is analyzed in the frequency domain, and it is verified that the proposed method has better property on the rejection of the rotor time constant error on flux estimation. The proposed observer is designed on the synchronous reference frame with a simple synchronous speed estimator which doesn't require additional control loop such as phase locked loop or position estimator which was commonly used when constructing the observer on the synchronous reference frame. The reduced influence of rotor time constant error to the observer is verified with simulation and experimental results.

Torque Ripple Reduction in Stator Resistance Estimation using DC Current Injection for Induction Motor Sensorless Drives

Abstract: In this paper, the flux and torque ripple of an induction motor due to DC current injection for the stator resistance estimation is analyzed. For the analysis, the fundamental component of rotor flux is used for rotor flux orientation to avoid the perturbation issue in synchronous reference speed. On the reference frame aligned to the rotor flux, flux fluctuation due to arbitrarily injected DC and 2nd harmonic current is calculated. And based on the calculated flux fluctuation, the current reference which nullifies torque ripple is proposed. The proposed method is applicable to the induction motor sensorless control in low-speed region, where the torque ripple cannot be filtered out by the inertia of the drive system. Simulations and experiments are conducted to verify the performance of the proposed method.

Discontinuous PWM Method for Three-Level Inverters Using Offset Voltage

Abstract: 1. 서 론 3레벨 인버터는 기존 2레벨 인버터와 비교하였을 때, 낮은 출력 전압 왜곡, 낮은 스위칭 소자의 전압 스트레스, 높은 효율 등의 장점을 가지고 있고, 점차 계통 연계 및 전동기 구동 분야에서의 활용도가 높아지고 있다. 인버터의 전압 합성 성능 및 효율은 전압 변조 방법(PWM)에 따라 달라진다. PWM 기법은 크게 두 가지 기준으로 구분할 수 있는데, 하나는 구현하는 방식이고, 다른 하나는 변조된 전압의 형태이다. 첫째로, 전압 변조 방법은 구현 방식에 따라, 캐리어 기반 PWM(Carrier-based PWM, CBPWM)과 공간 벡터 PWM(Space vector PWM, SVPWM)으로 분류된다. CBPWM은 전압 지령과 반송파 간의 간단한 비교를 통하여 스위치의 듀티비를 생성한다. 반면, SVPWM은 섹터 구분, 스위칭 순서, 삼각 함수 등이 요구되며, 이로 인해 CBPWM에 비하여 계산량이 크다. 따라서 오프셋 전압을 이용한 CBPWM이 SVPWM에 비하여 계산 부담이 적어 구현이 간단하다. 둘째로, 변조된 전압의 형태에 따라, 연속 PWM(Continuous PWM, CPWM)과 불연속 PWM(Discontinous PWM, DPWM)으로 구분된다. CPWM에서는 변조된 전압이 클램핑 되지 않는 반면, DPWM에서는 변조된 신호가 DC 버스 중 하나(양성단, 중성단, 음성단)에 클램핑된다. CPWM과 비교하여, DPWM은 스위칭 절환이 적기 때문에 스위칭 손실을 최대 50%까지 줄일 수 있다. 기존 2레벨 인버터에서는 네 가지 DPWM 방식이 널리 사용되고 있다. 이 방식들을 확장하여, 역률각에 따라 클램핑 영역을 가변 하는 2레벨 인버터의 최소 스위칭 손실 DPWM(Minimum loss DPWM, MLDPWM)이 제안되었다. 하지만 3레벨 인버터에서는, 2레벨 인버터와는 달리 중성단 클램핑이 가능하여, 낮은 역률 혹은 낮은 전압 변조 지수(MI)에 대하여 2레벨 MLDPWM 보다 효율이 개선될 수 있다. 본 논문은 전압 벡터공간에서 변조지수(MI)에 따른 중성단 클램핑 가능 영역을 분석하며, 이를 바탕으로 중성단 클램핑을 고려한 3레벨 인버터의 DPWM 구현을 위한 옵셋 전압 생성 방식을 제안한다.