Ac-side Current Research Articles

Arm Current Control Strategy for MMC-HVDC Under Unbalanced Conditions

Circulating current can be decomposed into double line-frequency positive-, negative- and zero-sequence components under unbalanced voltage conditions. First, in order to decrease peak/rms value of arm current, double line-frequency circulating current should be eliminated. Second, ac-side current that consists of positive- and negative-sequence components under unbalanced voltage conditions should be regulated to track the reference signal. Furthermore, fundamental frequency circulating current caused by asymmetry of the upper and lower arm should be also removed. To effectively control ac-side current and eliminate all circulating currents, an arm current control strategy based on a decoupler and proportional-integral controller plus the vector proportional integral (VPI) controller is proposed in this paper. The proposed control strategy works well under balanced and unbalanced voltage conditions as well as asymmetry of the upper and lower arm. In addition, this paper proposes a design procedure for the VPI controller to reduce the uncertainty of this process. A three-phase modular multilevel converter simulation model is set up on PSCAD/EMTDC to verify the effectiveness of the proposed control strategy.

智能电网三相H桥级联SVG的矢量控制及仿真

为发挥智能电网的巨大优势，简便有效的动态无功功率补偿至关重要。无辅助电源的三相H桥级联SVG以其优越的性能成为无功补偿的首选装置。本文对其控制原理做了详细解析，设计了较为实用的直流侧电压和交流侧电流双闭环的矢量控制系统，并深入浅出地描述无功电流的动态补偿过程。仿真结果证明：此系统可根据电网的无功补偿需求，对输出无功电流实现无静差调节，其稳态和动态性能均能符合要求。 Simple and effective dynamic var power compensation is very important to the smart grid. Cascade H bridge SVG without auxiliary power supply has become the preferred device in var compensation because of its good performance. So this thesis analyzes its control theory in detail. Then a vector control system with double closed loop of DC side voltage and AC side current is designed. Based on it, the dynamic compensation process of var current is also described. The simulation results verify that this system can adjust output var current without static difference according to the grid’s requirement of compensating var power. Both its stable and dynamic performance can meet the requirements.

Control of Three-Phase Bidirectional Current-Source Converter to Inject Balanced Three-Phase Currents Under Unbalanced Grid Voltage Condition

A single-stage, bidirectional, current-source converter (CSC) topology to interface a dc microgrid with an ac grid is reported in the literature. In this topology, under a balanced grid voltage condition, the dc-link inductor current can be regulated over a wide range—from zero to rated value—while the ac-side current has low harmonic distortion. However, unbalanced grid voltages result in second-harmonic pulsation in the current and power on the dc side of the converter. In addition, the ac-side currents will be unbalanced due to the presence of a negative-sequence component. This would result in undesired tripping of the converter if one of the phase currents exceeded its rated value. Various control loop structures for the operation of voltage-source converter under unbalanced grid voltage conditions are reported in the literature. However, use of similar control loop structures for CSC may lead to unstable operation. Therefore, a control scheme to inject balanced three-phase currents into the ac grid under an unbalanced grid voltage condition is proposed in this paper. The stability of the proposed control scheme is studied using a small-signal model of the converter. Performance of the proposed control scheme is studied using MATLAB/Simulink and is experimentally validated.

Decoupled Control of Modular Multilevel Converter Based on Intermediate Controllable Voltages

The decoupled control of the ac-side, dc-link and circulating current of a modular multilevel converter (MMC) can significantly improve its operation and control performances. This paper presents a new mathematical model for MMC, in which six intermediate controllable voltages (ICVs) corresponding to the ac-side currents, the dc-link current, and the circulating currents are introduced. Using the ICVs as the intermediate control variables, the MMC can be decoupled into six single phase circuits, and the current in each circuit responds to its corresponding ICV. Based on the ICV controls, the direct control of dc-link voltage and circulating current and the decoupling of dc side and ac side can be achieved. These conditions can improve the performances and stability of the MMC-based high-voltage direct current (HVDC) systems. Simulations and experiments were implemented to verify the proposed method.

DC‐link current estimation for load‐side converter of brushless doubly‐fed generator in the current feed‐forward control

In the stand-alone brushless doubly-fed generator (BDFG) control system, there are two back-to-back converters composed of the machine-side converter (MSC) and the load-side converter (LSC). The LSC is a three-phase AC-DC pulse width modulation converter which usually adopts the double-loop control strategy with outer voltage loop and inner current loop. The introduction of the DC-link current feed-forward for improving the dynamic performance of the LSC can reduce the effect of the load disturbance to the system. However, the DC-link current has a complex harmonic spectrum with the MSC connected to the DC side, so its average value is not convenient to measure. This study presents a DC-link current estimation algorithm for the current feed-forward control in the LSC of the stand-alone BDFG. With the help of the equivalent circuit of the BDFG and the power balance, the rapid calculation of the average DC-link current can be estimated as well. With this estimation of the current instead of the actual DC-link current, the DC-link voltage and AC side current of the LSC can achieve stability quickly. The effectiveness and correction is validated by the simulation and experiments.

系統連系三相電流形インバータにおける交流側電流の時間周波数解析

– Three-phase current-fed inverters for line interactive systems operated by a two-phase PWM scheme offers relatively low switching times, and low switching losses in comparison to that operated by a three-phase PWM scheme.However, the harmonics involved in the ac-side currents produced in the two-phase PWM scheme are distributed over a wide frequency range, which can be a significant drawback depending on its applications. In this paper, compare the AC-side harmonics on the PWM schemes for the three-phase current-fed converter using a wavelet transform.

Control of the Modular Multilevel Converter Based on a Discrete-Time Bilinear Model Using the Sum of Squares Decomposition Method

The modular multilevel converter (MMC) has become one of the most promising candidates for medium-/high-power applications, specifically for high-voltage direct current transmission systems. This paper proposes a control strategy for the MMC based on the sum-of-squares decomposition method. A bilinear mathematical discrete model of the MMC is derived in the rotating reference frame by including the dynamics of ac-side currents, circulating currents, dc-side current, and stored energy of the converter. The coordinates of the developed model are transformed to the origin, and the steady-state operating point of the MMC is calculated based on the desired reference values of the states. A quadratic Lyapunov function is defined and by converting the Lyapunov difference inequality to an SOS problem, a stabilizing controller is designed. The designed controller not only guarantees the Lyapunov stability of the states, but also controls the external and internal dynamics of the MMC with only one controller, instead of using several proportional-integral controllers. To improve the region of stability of the MMC, an optimization problem is formulated and solved, so the controller can stabilize the MMC from an initial operating point far from the steady-state operating point. The performance of the proposed strategy for a 20-level MMC is evaluated based on the time-domain simulation studies and is compared with the existing control methods.

HVDC시스템 적용을 위한 Triple-tuned 필터 설계방법 연구

The ac side current of a line commutated converter(LCC) high voltage direct current (HVDC) is characterized by highly non-sinusoidal waveform. If the harmonic current is allowed to flow in the connected ac network, it may cause unacceptable levels of distortion. Therefore, ac side filters are required as part of the total HVDC converter station, in order to reduce the harmonic distortion of the ac side current and voltage to acceptably low levels. The ac filters are also employed to compensate the requested reactive power because LCC HVDC also consume substantial reactive power. Among different types of filters, triple-tuned filters have been widely utilized for HVDC system. This paper presents two design methods of triple-tuned filter; equivalent method and parametric method. Using a parametric method, in particular this paper proposes a design algorithm for a triple tuned filter. Finally, the performance of the design algorithm is evaluated for a 250kV HVDC system in Jeju island. The results cleary demonstrate the effectiveness of proposed design method in harmonics reduction.

Indirect Finite Control Set Model Predictive Control of Modular Multilevel Converters

The modular multilevel converter (MMC) is a potential candidate for medium/high-power applications, specifically for high-voltage direct current transmission systems. One of the main challenges in the control of an MMC is to eliminate/minimize the circulating currents while the capacitor voltages are maintained balanced. This paper proposes a control strategy for the MMC using finite control set model predictive control (FCS-MPC). A bilinear mathematical model of the MMC is derived and discretized to predict the states of the MMC one step ahead. Within each switching cycle, the best switching state of the MMC is selected based on evaluation and minimization of a defined cost function. The defined cost function is aimed at the elimination of the MMC circulating currents, regulating the arm voltages, and controlling the ac-side currents. To reduce the calculation burden of the MPC, the submodule (SM) capacitor voltage balancing controller based on the conventional sorting method is combined with the proposed FCS-MPC strategy. The proposed FCS-MPC strategy determines the number of inserted/bypassed SMs within each arm of the MMC while the sorting algorithm is used to keep the SM capacitor voltages balanced. Using this strategy, only the summation of SM capacitor voltages of each arm is required for control purposes, which simplifies the communication among the SMs and the central controller. This paper also introduces a modified switching strategy, which not only reduces the calculation burden of the FCS-MPC strategy even more, but also simplifies the SM capacitor voltage balancing algorithm. In addition, this strategy reduces the SM switching frequency and power losses by avoiding the unnecessary switching transitions. The performance of the proposed strategies for a 20-level MMC is evaluated based on the time-domain simulation studies.

A Control Method of HVDC-Modular Multilevel Converter Based on Arm Current Under the Unbalanced Voltage Condition

This paper proposes an enhanced control method for a high-voltage direct-current (HVDC) modular multilevel converter (MMC). To control an MMC-HVDC system properly, the ac current, circulating current, and submodule (SM) capacitor voltage are considered. The ac-side current is a fundamental frequency component, and the circulating current is a double-line frequency component. Existing control methods control the ac current and circulating current by separating each component. However, the existing methods have a disadvantage in that the ac-side current must be separated into the positive and negative sequences for control under an unbalanced voltage condition. The circulating current consists of not only negative-sequence components but also positive- and zero-sequence components under an unbalanced voltage condition. Therefore, an additional control method is necessary to consider the positive- and zero-sequence components of the circulating current. The proposed control method has the advantage of controlling not only the ac-side current of the MMC but also the circulating current without separating each of the current components to control each arm current of the MMC. In addition, it can stably control the positive- and zero-sequence components of the circulating current under the unbalanced voltage condition.

불평형 전압 발생 시 유효전력 조절을 통한 전압형 HVDC의 DC전압 제어 방안

Faced with unbalanced grid operation mode, the high voltage direct current (HVDC) based on voltage source converter (VSC) can be properly controlled by a dual current control scheme. For the modular multilevel converter (MMC) controlling the AC side current is able to limit the arm current which flows along the IGBT of submodule (SM) to rated current. However the limitation of the arm current results in leaving the control range of active power at MMC confined to below the rated capacity. As a result, limiting the arm current causes the problem that the DC side voltage of the HVDC can not be controlled to the reference value since MMC HVDC adjusts the DC side voltage through the active power. In this paper, we propose the algorithm adjusting the active powers of both MMCs to resolve the problem. The back-to-back MMC HVDC applying the algorithm is modeled by PSCAD/EMTDC to verify the algorithm.

Bidirectional Current Sensorless Control for the Full-Bridge AC/DC Converter With Considering Both Inductor Resistance and Conduction Voltages

The full-bridge converter is often connected between the ac grid and the dc bus. In the conventional multiloop control, the inner current control loop shapes the ac-side current waveform, and the outer voltage control loop regulates the dc-bus voltage. In this paper, the single-loop bidirectional current sensorless control (BCSC) for the full-bridge ac/dc converter with considering both the inductor resistance and conduction voltage is first proposed. The average-value behavior of the full-bridge ac/dc converter is analyzed and it is simplified to an equivalent single-switch model. Based on this developed equivalent model, BCSC is designed and implemented in the FPGA-based system. There is only voltage control loop in the proposed BCSC (i.e., no current control loop), but BCSC is able to regulate the dc-bus voltage and shape the ac-side current. The provided results in both the rectifier operation and the inverter operation demonstrate the effectiveness of the proposed BCSC.

The Grid-Connected Inverter of Simulation on Direct-Drive Wind Power System Based on MATLAB

Based on working Principle of the direct-drive wind power systems grid-connected inverter was research, the mathematical model for inverter in Three-phase rotating coordinate was established and analyzed. A double-loop control strategy that based on feed forward decoupling were introduced, Meanwhile, combining the implementation effective control of the AC-side output current by the Space Vector Pulse Width Modulation method (SVPWM), it could generate sine wave current that had the same frequency and the same phase with network voltage. To a great degree, reduced harmonic pollution in power system, moreover, improved the Grid-connected wind power systems efficiency and reliability.

HVDC System 적용 Double-tuned 필터의 설계 방법 연구

The ac side current of an high voltage direct current (HVDC) converter is characterized by highly non-sinusoidal waveform. If the harmonic current is allowed to flow in the connected ac system, it may cause unacceptable levels of distortion. Therefore, ac side filters are required as part of the total HVDC converter station, in order to reduce the harmonic distortion of the ac side current and voltage to acceptably low levels. The ac side filters are also employed to compensate network requested reactive power because HVDC converters also consume substantial reactive power. Among different types of filters, double-tuned filters have been widely utilized for HVDC system. This paper presents two design methods of double-tuned filter; equivalent method and parametric method. Using a parametric method, in particular the paper proposes a new design algorithm for a realistic system. Finally, the performance of the design algorithm is evaluated for a 80㎸ HVDC system in Jeju island with PSCAD/EMTDC program. The results cleary demonstrate the effectiveness of proposed design method in harmonics elimination and steady-state stability.

Simulation Study of Harmonics and Reactive Power in Three-Phase Full-Bridge Controlled Rectifier Circuit with Resistance-Inductance Load

In order to evaluate the harm and impact of power electronic devices on the grid and provide numerical basis for specific design of the corresponding compensated devices, the harmonics and reactive power of three-phase full-bridge controlled rectifier circuit with resistance-inductance load are studied in this paper. Firstly, the harmonics and power factor of the ac side current are analyzed theoretically; the harmonics contents and power factor are calculated. Secondly, the model of three-phase full-bridge controlled rectifier circuit is built in Matlab/Simulink, and the simulation analysis of the ac side power factor and harmonics is achieved, the simulated values are similar to the theoretical values. The results show that the complex calculations of harmonics and power factor can be completed simply and intuitively using simulation study, the simulation study is efficient and accurate.

AC–DC–DC isolated converter with bidirectional power flow capability

A novel AC-DC-DC isolated converter with bidirectional power flow capability is proposed and studied. The proposed structure consists of a three-phase current source converter and a special DC-DC isolated converter. In this DC-DC converter, the primary side of the high-frequency transformer is a current source converter and the secondary side is a voltage source converter. Based on this novel topology, a simple and efficient control strategy using the voltage of the inductor in the DC-side to control the current of the AC-side is presented. The feature of the bidirectional power flow allows the converter to provide energy as a power source or to regenerate power back to the AC power grid with high overall energy efficiency. Another feature of this converter is that the DC-side current and voltage can be regulated in a wide range from zero to the rating value. Furthermore, the AC-side current and voltage have low total harmonic distortion and unit power factor in a wide output power range. The operation of the proposed topology and control scheme are confirmed by the experimental results on a 10-kW design example.

Minimize Low-Order Harmonics in Low-Switching-Frequency Space-Vector-Modulated Current Source Converters With Minimum Harmonic Tracking Technique

Gate turnoffs (GTOs) are usually used in high-power current source converters (CSCs), i.e., rectifiers and inverters. Space vector modulation (SVM) technique for CSC is established by dividing ac-side line current cycle into six sectors. Each sector is divided into a certain number of SV cycles. SV cycle is divided into three states: two active and one zero state. For low switching frequency as required by GTOs, the SVM technique generates fifth and seventh harmonics (HD5-7) in the CSC ac-side current. Minimal reduction in HD5-7 was achieved with certain states sequence inside the SV cycle. Moderate reduction in HD5-7 was obtained by calculating states on -times at once in the middle of each SV cycle. In this paper, larger reduction in HD5-7 at CSC ac-side current is achieved by new techniques for calculating states on-times. First, two straightforward techniques are proposed. One calculates states on-times from SVM equations in the middle of each state on-time. The other calculates all states on -times when the state changes from one active state to the other. Both techniques are effective in reducing HD5-7. Then, minimum harmonics tracking (MHT) technique for calculating states on-times in SVM CSC is proposed. Tracking technique adjusts states on-times once per four ac-side line current cycles to give the least HD5-7. In CSC with a large overlap period, power factor affects HD5-7, so two-variables MHT technique for both active states on-times inside SV cycle is proposed to give the least HD5-7. Also, a variable perturbation tracking technique is proposed to reduce transient time with unperturbed steady-state operation. Finally, experimental investigations and obstacles are introduced.

Diagnostic of a Faulty Induction Motor Drive via Wavelet Decomposition

An approach to the analysis of ac side current is presented for the purpose of identification of faults in the stator phase resistance of an ac induction motor drive. The method relies on the correlation between wavelet decomposition coefficients of the current in healthy and faulty conditions. The findings highlight that the fault causes waveform variations that are localized at specific decomposition levels. The presented approach may open the way to efficient training for fault recognition systems.

A Harmonically Coupled Admittance Matrix Model for AC/DC Converters

This paper proposes a new method to model the harmonic generating characteristics of AC/DC converters. The model transforms the time domain nonlinear characteristic of the converter into a frequency domain linear admittance matrix. The matrix represents the coupling among the converter AC side harmonic voltages and currents accurately and it does not vary with the harmonic conditions of the system. The proposed model opens up a new way to conduct harmonic power flow studies. This paper presents the theoretical foundation and analytical derivation of the admittance model for both single-phase and three-phase bridge rectifiers. Potential applications of the model are discussed.

Control of high-performance switched-mode rectifier system

A novel high-performance switched-mode rectifier system for high-power applications is introduced. The rectifier system comprises a three-level neutral-point-diode clamped (NPC) AC/DC rectifier, which is interfaced to the DC load through a DC/DC buck converter. The NPC converter provides a tightly regulated DC voltage at the input of the buck converter; the buck converter provides a rapid current control for the load. This novel arrangement has the following technical merits: (i) fast transient response, (ii) operation at unity power factor, (iii) low distortion AC-side currents, and (iv) capability to operate at high-voltage levels with commercially available power switches rated at lower voltage levels, (v) elimination of multi-winding transformers, and (vi) decoupling between the DC load and the supply system in terms of rapid transients of AC and DC sides. Dynamic models and control schemes for the proposed rectifier system are presented. The control schemes include the buck converter control, the NPC capacitor voltage balancer, the DC-bus voltage control and the AC-side current control. Superior performance of the proposed rectifier system is validated through digital time-domain simulation studies in the PSCAD/EMTDC environment.