A DC-link voltage balancing algorithm for 3-level converter using the zero sequence current

Detection of high resistance earth faults in medium voltage networks is an important problem due to ineffectiveness of traditional earth fault protections. Such short circuits can be detected by the criterion of a reactive power of higher harmonics for zero sequence current and voltage. The main problem is determination of the power setting value in the protection, which depends on the asymmetry of phase-to-earth capacitances and higher harmonics in supply voltages, which are generated by non-linear loads. The intensive tests of the asymmetry of the zero sequence currents and voltages for harmonics and their reactive power have been carried out in 15 kV compensated network as a function of all relevant parameters, i.e.: maximum capacitance deviation of the network and protected line, percentage content of harmonics in supply voltages, capacitive current of the network and the line. It has been shown that third harmonics of the zero sequence voltage and current are the best suited for practical use, since the asymmetry reactive power of these components is the smallest among the considered harmonics and the protection sensitivity will be the highest.

This paper investigates fault line selection in small current grounding systems, focusing on the zero sequence current phase characteristics. Small current grounding systems, prevalent in medium voltage distribution networks, face challenges in fault detection due to minimal fault currents during single status phase grounding faults. Traditional methods relying on manual line switching and zero sequence voltage monitoring are inefficient and can induce overvoltage. This study categorizes fault line selection techniques into three approaches: steady-state fault information, transient information, and modern signal processing technology. The analysis of zero sequence currents in non-grounded systems reveals that fault line zero sequence currents equal the sum of system capacitive currents, flowing opposite to those in non-fault lines. Additionally, transient signal analysis shows that these signals provide more robust information for fault detection. MATLAB simulations validate the theoretical models, demonstrating that zero sequence current direction and amplitude effectively identify fault lines. The results confirm the efficacy of combining steady-state and transient signal analyses for accurate fault line selection, contributing to enhanced safety and reliability in power distribution networks. This research offers practical insights and tools for engineers, improving fault management and reducing potential economic losses in small current grounding systems.

Zero sequence voltage suppression control for a modular multilevel matrix converter is investigated, which is required for applications to power systems and high power motor drives. The schemes of power flow control and capacitor voltage balancing control with voltage space vector modulation have been proposed. However, these control schemes have the problem they cause zero sequence current that affects protection for power systems and motors. To reduce the zero sequence current, the control scheme that suppresses zero sequence voltage by compensating voltage difference between input and output systems is proposed in this paper. Numerical simulation of a 9-level modular matrix converter and experiments using a 2-kW 5-level modular matrix converter are conducted to verify the control scheme.

Direct torque control (DTC) systems for open-end winding permanent magnet synchronous motors (OEW-PMSM) with common DC bus generate zero sequence currents (ZSC) and large torque ripple, resulting in increased system loss and poor operating characteristics. In this paper, a DTC based on five level torque hysteresis (FLTH) is proposed to improve the system performance. Firstly, an online calculation and selection strategy based on switch table is designed to suppress ZSC. By analyzing the composition of zero sequence voltage and using the double vector method to suppress it, the ZSC can totally be suppressed in theory, without affecting the system performance. On this basis, this paper proposes to add FLTH to DTC to reduce torque ripple. FLTH can not only effectively use many voltage vectors, but also reduce torque ripple. In the case study, the torque ripple rate has been reduced by 53%. Through simulation and experiment, the conventional DTC and the FLTH DTC are compared, and the results prove the validity of the proposed strategy.

Residual current compensation for single-phase grounding fault in coal mine power network

This paper describes an adaptive protective relaying scheme for parallel-line distance protection. The scheme adjusts its operation based on the availability of input signals to achieve an optimal distance protection performance on parallel transmission lines influenced by the mutual coupling effect. When available, zero sequence current from the parallel line is used to fully compensate the mutual coupling effect. The zero sequence current ratio is used to avoid possible false operations on healthy lines, which may result from such compensation. When the parallel line's zero sequence current is not available, the line operating status is used instead to select the proper zero sequence current compensation factors in impedance calculation. Default zero sequence current compensation factor is used when both the parallel line's zero sequence current and line operating status signals are not available. The performance of the scheme has been verified on a simulated network and the results are presented.

A hybrid current and voltage protection scheme (HCVPS) using Stockwell transform, alienation coefficient and Wigner distribution function is designed to detect and classify the faults on utility network with high (50%) renewable energy. HCVPS detected faults by comparing peak magnitude of hybrid current and voltage fault index (HCVFI) with a threshold and also discriminated faults from operational events. HCVPS has minimum fault detection time and perform better with variations in fault impedance, fault incidence angle, and faults incident at locations of test network. HCVPS effectively detected all faults in time of (1/16)th cycle, with noise of 20 dB SNR and also on practical utility power network. Faults have been classified using numbers of faulty phases and hybrid current and voltage ground fault index (HCVGFI) computed by processing the zero sequence current (ZSC) and zero sequence voltage (ZSV). HCVPS performs better compared to discrete wavelet transform-based method.

Simple and reliable zero sequence overcurrent protection in distribution 6–10 kV cable networks with an insulated neutral is most widely used for protection against single phase earth faults. However, protection of this type in many cases does not provide the required sensitivity to internal (inside the protected zone) faults as it must be tuned to the response current from surge transients during external faults through an intermittent arc. It is possible to increase the sensitivity if adaptive current protection is applied. However, the known methods for its implementation are only effective for stable faults through transient resistance but do not provide high dynamic stability of operation in transient conditions in case of arc intermittent earth faults that are the most dangerous for the network. Therefore, an urgent problem to be solved now is improving the principles of adaptive current protection against earth faults. To compare the efficiency of the known and proposed principles of adaptive current protection implementation taking into account the complexity of transients during earth faults through an intermittent arc in 6–10 kV cable networks, we used Matlab simulation with the SimPowerSystem and Simulink extension packages. The research into the operation algorithms of adaptive current protection against earth faults was carried out on simulation models of 6–10 kV cable networks with an insulated neutral and with neutral grounding through a high-value resistor. The studies on the simulation models have shown that the known methods of implementation of adaptive current protection against earth faults based on the use of full zero sequence currents and voltage are ineffective during intermittent arc earth faults. The authors propose a method of adaptive current protection against earth faults in 6–10 kV cable networks with an insulated neutral and with neutral grounding through a high-value resistor that provides a significant increase in dynamic stability of transient operation with arc ground faults and allows using only zero sequence current and voltage components of the operating frequency of 50 Hz as the actuating quantities. The proposed method of implementing adaptive current protection against earth faults in 6–10 kV cable networks with an insulated neutral and with neutral grounding through a high-value resistor does not only increase the sensitivity of this protection type to earth faults through transient resistence and dynamic stability of operation in transient condiitons in case of arc intermittent earth faults but also broadens the range of its possible applications

The measurement results of the traditional method of measuring the ground parameters of the distribution network have large errors, because the influence of the damping resistance and the short-circuit impedance of the voltage transformer cannot be eliminated during the measurement process. In addition, the traditional method of measuring ground parameters of the distribution network is greatly affected by the system grounding method, and different neutral point grounding methods are different. Based on this, an accurate measurement method of ground parameters that is not affected by the grounding method of the distribution network is proposed in this paper. The special $Y / \Delta$ wiring of external grounding transformer with winding tap in distribution networks is used and its winding tap is grounded to change the zero sequence voltage measured by the open delta voltage transformer for distribution networks. The zero sequence current of the feeder is measured by the zero sequence current transformer. The ground capacitance current and leakage resistance current of the system are calculated by the measured voltage and current. The proposed ground parameter measurement technology is simulated and analyzed in the PSCAD/EMTDC simulation environment. Simulation results prove that the proposed method has high measurement accuracy, where the parameter measurement process is safe, simple and economical.

After a single-phase-to ground fault, fault voltage and current information should be sampled accurately for automatic equipments to detect the fault line. And zero sequence current is the very key element to detect the fault line after an earth fault happens. The widely used methods of getting zero sequence current are in details described. The common methods are by zoro sequence current transformers, by wye connections of three CTs, and by zero-sequence current transducer. A new method by magnetic field induction and the structure of magnetic induction device are introduced, as well as a new way by electronic current transformer and its application in digital substation.

In order to improve the performance of dual inverter drive system, this paper proposes an intermediate hexagonal modulation strategy and improved inverter topology based on solid-state relays. The modulation strategy does not generate a common mode voltage. A composite regulator is utilised in the zero sequence circuit. The zero sequence current can be suppressed. Furthermore, the modulation strategy also provides conditions for fault tolerant control. Improved inverter topology based on solid-state relays is designed. If the switching device fails, the system can achieve fault tolerant operation without additional bridge arms. Finally, an experimental platform based on a dual inverter with a common DC bus is built. The experimental results shown that the method proposed in this paper can not only effectively suppress zero sequence current but also achieve fault tolerant control of switch device failure without additional bridge arms.

This paper presents a closed-loop control strategy for dual inverter-fed, permanent magnet synchronous motors (PMSM) with non-sinusoidal back electromotive force (back-EMF). The inverters are connected to both ends of the machine winding but use only one power source, thus providing a path for zero sequence currents to circulate in the motor windings. Due to the non-sinusoidal back-EMFs and circulation of zero sequence currents, the machine electrical torque is now a function of the rotor position that leads to torque ripple oscillations. Therefore, a closed-loop control strategy to suppress the circulation of zero sequence currents and amplitude of the dominant current harmonics caused by the non-sinusoidal back-EMFs is developed and demonstrated by experiment. The zero sequence component is considered as one additional degree of freedom that has to be managed. Therefore, the control scheme to regulate the ac signals in the regulators is developed and an approach to minimizing the torque ripples is presented.

Because of line parameters, running types, fault grounding impedances and other factors, the veracity of current fault line selection (FLS) method is not high enough and may contain some dead areas for FLS device of non-utility grounding system (NUGS). To overcome this shortage, a novel FLS method is proposed in this paper based on analyzing the fault characters of distribution network, combining merits of prevalent FLS methods, constructing these methods' subjection functions and then synthesizing them based on the fuzzy theory. Experiments show this method can achieve high sensitivity, adjustability and veracity. The dynamic simulation experiment shows in the neutral un-grounding system(NUS), the method which is made of the zero sequence inactive power (ZSIP) method, the base-wave virtual value (BV) method and the base-wave phase (BP) method of zero sequence current (ZSC) can achieve high accuracy; in the neutral resonant grounding system (NES), the method that combines zero sequence active power (ZSAP) method, 5-harmonic phase (HP) method and remaining current increment (RCI) method can even achieve 100% accuracy. Based on the PC104 industry control computer, a new FLS device of NUGS according to this new FLS shows very high accuracy in dynamic simulation experiments, and it should be widely used in China.

Zero-sequence current (ZSC) is inevitable in the structure of an open-end winding permanent-magnet synchronous machine (OW-PMSM) system with a common dc bus due to the simultaneous existence of the ZSC loop and the zero-sequence voltage (ZSV) caused by the inverters and third electromotive force. To suppress the ZSC, a novel strategy with a reduced switching frequency is proposed in this paper. The main contribution of the proposed method is that the ZSC is suppressed totally with a reduced switching frequency. The duration time of the active voltage vectors is redistributed to generate the reference voltage in a zero-sequence loop, which leads to the suppression of the ZSC. Different from the conventional method, only the active voltage vectors instead of the zero voltage vectors are employed to suppress the ZSC. Consequently, the switching frequency in the modulation process can be reduced. In addition, the effective modulation range of the ZSV is analyzed, which further reveals the relationship among the effective modulation of the ZSV, the modulation index, and the ratio of the third flux linkage component to the fundamental flux linkage component. Finally, the effectiveness of the proposed method is validated by an OW-PMSM experimental setup with a common dc bus. Moreover, the proposed method is experimentally compared with the existing methods.

In this paper, the authors present a comparative study between two three-phase boost rectifier topologies. The topologies considered are three-level (18 switches) and two-level (6 switches) converters. The neutral point of the three-phase source is connected to the midpoint of the DC bus to provide a zero sequence current path. The comparison is carried out for both balanced and unbalanced load connected to the centre tap of the DC bus. The control of unbalanced voltage with the injection of the zero sequence current is achieved using the PWM technique. The THD of line-currents, efficiency and power factor are compared for a 25 kW system operating at 3 kHz. Simulation results obtained by the power system blockset (PSB) tool for both steady state and dynamical operations are presented and analysed.