Voltage harmonics are very likely to cause commutation failure (CF) in multi-infeed LCC-HVDC systems. However, this has not been sufficiently considered in earlier CF detection methods and control ...strategies. In this paper, by incorporating the voltage harmonics into the inverter quasi-steady-state model, an extinction angle predictive algorithm is firstly presented to propose the enhanced CF predictive detection method. Compared to earlier methods, the enhanced method can detect both local and concurrent CFs more accurately. Moreover, the enhanced method is then used as the starting unit of the enhanced CF predictive control strategy to enable its effective activation. To develop the enhanced strategy, a firing angle order predictive algorithm is introduced using the inverter quasi-steady-state model with the voltage harmonics considered. Compared to earlier strategies, the enhanced strategy can mitigate both local and concurrent CFs more effectively. Furthermore, the response speed of the enhanced method and strategy is evaluated by comparing CF characteristics and signal process delay. It is shown that since the timescales of the voltage harmonics induced CFs are longer than signal process delay, the response speed is fast enough to ensure their practicality. Finally, simulation results based on a dual-infeed LCC-HVDC system validate the enhanced method and strategy.
Commutation error degrades the operating performance of a high-speed permanent magnet brushless dc motor (BLDC) sensorless drive significantly. It is caused by different kinds of nonideal factors ...such as low-pass filters (LPFs), digital control delay, and other units in a control loop. This paper analyzes the commutation error caused by different kinds of nonideal factors and points out that the total commutation error is substantially equivalent to the error of an internal power factor (IPF) angle of the BLDC motor. The purpose of the commutation correction in this paper is to regulate the IPF angle to zero. A novel phase-lock loop (PLL)-based commutation correction strategy is proposed and designed in detail in this paper. The proposed PLL consists of a novel PM flux linkage based phase discriminator to detect the IPF angle, an LPF, and an autophase regulator (APR). The function of the proposed PLL is to lock the IPF angle to a reference value. Compared to the conventional scheme, the greatest advantage of the proposed scheme is that the proposed PLL is robust to any kind of phase delay caused by nonideal factors in the control loop, and it is quite important for a high-speed motor drive. Both simulation and experimental results verify the effectiveness and superiority of the proposed correction strategy.
Based on the Z-source inverter, this paper proposes a novel commutation torque ripple reduction strategy for brushless DC motor (BLDCM). The proposed strategy employs the same modulation mode in both ...the normal conduction period and the commutation period, and the commutation torque ripple is reduced by regulating the shoot-through vector and active vector duty cycles. The proposed detection method acquires the end point of commutation by comparing the clamped terminal voltages with reference zero level, and the signal-noise-ratio of the detection is improved by avoiding the attenuation of the terminal voltages. Furthermore, a certain pulse width of the shoot-through vector can not only reduce the commutation torque ripple but also provide a new opportunity to detect the end point of commutation. Moreover, Z-source inverter provides the buck-boost ability for BLDCM drive system, then the dc voltage utilization can be improved, and the safety of the drive system can also be improved. In addition, this paper analyzes the terminal voltages during each vector. The experimental results verify the correctness of the theories and the effectiveness of the proposed approach.
In this paper, a novel drive method, which is different from the traditional motor drive techniques, for high-speed brushless DC (BLDC) motor is proposed and verified by a series of experiments. It ...is well known that the BLDC motor can be driven by either pulse-width modulation (PWM) techniques with a constant dc-link voltage or pulse-amplitude modulation (PAM) techniques with an adjustable dc-link voltage. However, to our best knowledge, there is rare study providing a proper drive method for a high-speed BLDC motor with a large power over a wide speed range. Therefore, the detailed theoretical analysis comparison of the PWM control and the PAM control for high-speed BLDC motor is first given. Then, a conclusion that the PAM control is superior to the PWM control at high speed is obtained because of decreasing the commutation delay and high-frequency harmonic wave. Meanwhile, a new high-speed BLDC motor drive method based on the hybrid approach combining PWM and PAM is proposed. Finally, the feasibility and effectiveness of the performance analysis comparison and the new drive method are verified by several experiments.
In the multi-infeed LCC-HVDC system, the concurrent commutation failure (CCF) caused by the AC fault of the receiving-end system may lead to the blocking of multiple converter stations, which ...challenges power grid security and stability. This paper proposes a novel method to quickly and accurately identify the CF risk areas, considering the CF probability (CFP) during the single-line-to-ground (SLG) fault. First, an AC-DC voltage asymmetry factor (ADVAF) and its theoretical derivation are proposed to quantify the influence of the fault position on the CF. Second, the critical ADVAF (CADVAF) is derived based on the equivalent method. By comparing ADVAF and CADVAF, the CF risk areas can be quickly divided into non-, possible- and inevitable-concurrent CF risk areas. Third, the relationship between local CFP (LCFP) and fault position is established, and the concurrent CFP (CCFP) is calculated. The proposed method has improved the accuracy and computational efficiency of theoretical analysis than earlier methods. Finally, the validity and accuracy of the proposed method are verified in dual-infeed and five-infeed LCC-HVDC systems built based on the IEEE 39-bus and IEEE 118-bus models, respectively.
Line-commutated converter based high-voltage direct-current (LCC-HVDC) has been extensively applied in power system. LCC-HVDC is prone to continuous commutation failure (CCF), which consequently ...causes numerous active and reactive impacts, thereby resulting in various power system security and stability issues. Given that there is no CCF predictive method at present, the effect of CCF suppression measure is still limited, and the control margin of DC commutation station and power grid cannot be fully applied, so that the negative impact of CCF is difficult to reduce through advance control. In view of this situation, this paper aims at proposing a predictive method of LCC-HVDC CCF, which occurs again during the first CF recovery process. First, the predictability of CCF is demonstrated, and the idea for predicting CCF by comparing commutation voltage is proposed; Second, the calculation equation of CCF threshold voltage is derived by analyzing the CF process; Third, a predictive method of CCF based on threshold commutation voltage is proposed; Finally, the correctness of the theoretical analysis and predictive method is verified through the CIGRE HVDC standard system. Simulation results show that the proposed method has high accuracy and can provide sufficient time margin for emergency control to suppress the LCC-HVDC CCF.
In view of the sensorless commutation deviation of brushless dc motor with asymmetric three-phase back electromotive force (EMF), this article presents a correction method of commutation deviation ...based on the principle of current consistency before and after commutation to achieve optimal commutation. First, this article analyzes the different factors resulting in the commutation deviation of the motor with asymmetric three-phase back-EMF, and then points out the deviation disadvantage. Based on the analysis, a commutation deviation correction method based on current consistency in ideal optimal commutation is proposed. At the same time, in order to eliminate the influence of waveform distortion caused by winding inductance in the commutation process, dc-link current reconstructor is proposed. Based on the reconstructed current value after commutation, an optimal commutation phase estimator based on commutation current consistency is designed to estimate the position of each commutation point. Finally, experimental results confirm that the proposed method can achieve high-precision deviation correction and optimal commutation at each commutation point in the case of asymmetric three-phase back-EMF.
The commutation torque ripple of a brushless dc motor (BLDCM) in a six-step driving mode generates vibration noise. To minimize the commutation torque ripple of the BLDCM, this article analyzes the ...cause of torque ripple and establishes the model of torque ripple suppression by using the phase current predictive method based on the trapezoidal back electromotive force (EMF). In addition, based on the prediction model of the non-commutation phase current, the pulsewidth modulation model predictive control (PWM-MPC) algorithm is proposed. By changing the duty cycle with prior evaluation, the predictive control avoids commutation current hopping, thereby reducing torque ripple during commutation. This control approach is more accessible to be implemented, because it does not require changing the topology of the motor driving circuit. Simulation models of the proposed control scheme constructed in the MATLAB/Simulink environment are given, compared with the conventional square-wave driving method. Moreover, experiments are performed to verify the feasibility. The output torque during experiments is transmitted to the computer software through the torque transducer. Compared with the traditional driving methods, the simulation and experimental results show that the proposed novel algorithm in this article could suppress the torque ripple efficiently.
Both brushed dc motors and brushless dc motors have a wide range of applications, but mechanical commutating devices have the problem of harmful commutation arcs, and electronic commutating devices ...increase the cost of control systems. The uncontrolled mechanical-electronic hybrid commutating device passively controls the voltage of the freewheeling commutator segment through the capacitor, but the capacitor, inductor and resistor with fixed parameters in the freewheeling circuit are difficult to apply to the entire operating range. Therefore, this article proposes a controlled mechanical-electronic hybrid commutating device and a position sensorless control method. The mechanical component realizes the conduction mode of 120<inline-formula><tex-math notation="LaTeX">^{\circ }</tex-math></inline-formula> square waves through brushes and a mechanical commutator. The electronic component actively controls the voltages of the commutator segments through the power electronic switches in the commutation circuit so that the brush and the newly separated commutator segment remain equipotent, and the gap between them does not bear voltage, which is equivalent to operating in the brush-slip ring state and completely solves the problem of sparking and arcing during the commutation process. The controlled hybrid commutating device requires only two switches to obtain the same commutation effect as the electronic commutating device with six switches and does not require a position sensor or control chip, which reduces the system cost. The validity and feasibility of the controlled mechanical-electronic hybrid commutation theory and device are verified by experiments.
This paper presents a further significant development to the developed flexible LCC HVDC system with controllable capacitors 1, which can provide ac voltage/reactive power control 2. The development ...involves the installations of fixed parallel capacitors at the valve side of converter transformer, which brings the following significant benefits: First, ac filter banks at the ac side of converter transformer are not needed as a better harmonic filtering performance can be achieved; second, significant reduction of the HVDC station land requirement (compared with traditional LCC HVDC), as the ac filters together with the switchgear can occupy over 50% of the HVDC station footprint; third, up to 50% reduction of the required voltage rating and more than 60% reduction of the capacitance of controllable capacitors for commutation failure elimination can be achieved while similar power system dynamic performance (ac voltage/reactive power control) compared with that in 1 can be demonstrated. Detailed analyses are presented to illustrate the effective commutation process and superb harmonic filtering performance with the fixed parallel capacitors. Selections of the component values are presented. Simulation results in real-time digital simulator are presented to verify the effectiveness of commutation failure elimination, power system dynamic performance, harmonic filtering performance, and show voltage/current stress of the fixed parallel capacitors.
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