Single-phase-to-ground fault protection based on zero-sequence current ratio coefficient for low-resistance grounding distribution network

Definite-time zero-sequence over-current protection is presently used in systems whose neutral point is grounded by a low resistance(low-resistance grounding systems).These systems frequently malfunction owing to their high settings of the action value when a high-impedance grounding fault occurs.In this study,the relationship between the zero-sequence currents of each feeder and the neutral branch was analyzed.Then,a grounding protection method was proposed on the basis of the zero-sequence current ratio coefficient.It is defined as the ratio of the zero-sequence current of the feeder to that of the neutral branch.Nonetheless,both zero-sequence voltage and zero-sequence current are affected by the transition resistance,The influence of transition resistance can be eliminated by calculating this coefficient.Therefore,a method based on the zero-sequence current ratio coefficient was proposed considering the significant difference between the faulty feeder and healthy feeder.Furthermore,unbalanced current can be prevented by setting the starting current.PSCAD simulation results reveal that the proposed method shows high reliability and sensitivity when a high-resistance grounding fault occurs.

Parameter Design of Current Double Closed Loop for T-Type Three-Level Grid-Connected Inverter

To reduce current harmonics caused by switching frequency,T-type grid-connected inverter topology with LCL filter is adopted.In view of the disadvantages of the slow response speed of the traditional current control and the failure to eliminate the influence of the LCL filter on the grid-connected current by using current PI control alone,a current double closed loop PI current tracking control is proposed.Through the theoretical analysis of the grid-connected inverter control principle,the grid-connected inverter control model is designed,and the transfer functionmodel of each control link is deduced,and the current loop PI regulator is designed at last.The simulation results show that the control strategy is feasible.

Zero-sequence Current Suppressing Strategy for Dual Three-phase Permanent Magnet Synchronous Machines Connected with Single Neutral Point

Dual three-phase permanent-magnet synchronous machines(DTP-PMSM)connected with a single neutral point provide a loop for zero-sequence current(ZSC).This paper proposes a novel space vector pulse width modulation(SVPWM)strategy to suppress the ZSC.Five vectors are selected as basic voltage vectors in one switching period.The fundamental and harmonic planes and the zero-sequence plane are taken into consideration to synthesis the reference voltage vector.To suppress the ZSC,a non-zero zero-sequence voltage(ZSV)is generated to compensate the third harmonic back-EMF.Rather than triangular carrier modulation,the sawtooth carrier modulation strategy is used to generate asymmetric PWM signals.The modulation range is investigated to explore the variation of modulation range caused by considering the zero-sequence plane.With the proposed method,the ZSC can be considerably reduced.The simulated and experimental results are presented to validate the effectiveness of the proposed modulation strategy.

Zero-Sequence Current Suppression Strategy for Open-End Winding Permanent Magnet Synchronous Motor Based on Model Predictive Control

Compared with the traditional three-phase star connection winding,the open-end winding permanent magnet synchronous motor(OW-PMSM)system with a common direct current(DC)bus has a zero-sequence circuit,which makes the common-mode voltage and the back electromotive force(EMF)harmonic generated by the inverters produce the zero-sequence current in the zero-sequence circuit,and the zero-sequence current has great influence on the operation efficiency and stability of the motor control system.A zero-sequence current suppression strategy is presented based on model predictive current control for OW-PMSM.Through the mathematical model of OW-PMSM to establish the predictive model and the zero-sequence circuit model,the common-mode voltage under different voltage vector combinations is fully considered during vector selection and action time calculation.Then zero-sequence loop constraints are established,so as to suppress the zero-sequence current.In the end,the control strategy proposed in this paper is verified by simulation experiments.

Three-dimensional surface current loops in broadband responsive negative refractive metamaterial with isotropy

We propose a bulk negative refractive index （NRI） metamaterial composed of periodic array of tightly coupled metallic cross-pairs printed oR the six sides of a cube for applications of superlenses. The structural characteristics of the three-dimensional （3D） metamaterial consist in the high symmetry and the superposition of metallic cross-pairs, which can increase the magnetic inductive coupling between adjacent cross-pairs and realize a broadband and isotropic NRI. The proposed 3D structure is simulated using the CS~ Microwave Studio 2006 to verify the design validity. The simulation results show that the proposed structure can not only realize simultaneously an electric and magnetic response to an incident electromagnetic （EM） wave, but also exhibit a broadband NRI whose relative bandwidth can reach up to 56.7%. In addition, the NRI band is insensitive to tile polarization and the incident angle of the incident EM wave. Therefore, the proposed metamaterial is a good candidate material as three-dimensional broadband isotropic NRI metamaterial.

Symbolic Computation and Graphic Presentation of Magneto-Static Field and Its Associated Vector Potential for a Steady Looping Current

With the advent of Computer Algebra System (CAS) such as Mathematica [1], challenging symbolic longhand calcula-tions can effectively be performed free of error and at ease. Mathematica’s integrated features allow the investigator to combine the needed symbolic, numeric and graphic modules all in one interactive environment. This assists the author to focus on interpreting the output rather than exerting the efforts of relating the scattered separate modules. In this note the author, utilizing these three features, explores the magneto-static field and its associated vector potential of a steady looping current. In particular by deploying the numeric features of Mathematica the exact value of the vector potential of the looping current conducive to its 3D graph is presented.

Voltage Generation by Rotating an Arbitrary-Shaped Metal Loop around Arbitrary Axis in the Presence of an Axial Current Distribution

A thin metallic wire loop of arbitrary curvature is rotated with respect to an arbitrary axis of its plane. The device is excited by an electric dipole of infinite length and constant current. The resistance of the loop is computed rigorously as function of the position of the source. In this way, the induced voltage along the wire, under any kind of axial excitation, is given in the form of a superposition integral. The measured response is represented for various shapes of the coil, with respect to the time, the rotation angle and the position of the source. These diagrams lead to several technically applicable conclusions which are presented, discussed and justified.

某型地面发控点火电流异常分析和电路改进

针对某型地面发控系统在使用过程中出现的点火电流异常问题,采用数学方法研究点火电流与元器件参数的变化规律,发现地面发控点火电流在某些环境下不稳定的根本原因。最后提出一种闭环控制方案,增加电流反馈环节,解决点火电流异常问题。实际应用验证表明,改进后的点火电路能适应更大的点火负载,有效提高了点火的可靠性。

抑制采样白噪声干扰的PWM整流器无差拍控制

无差拍控制(DBC)是一种应用于整流器电流内环的预测控制,但该算法对系统参数敏感,当采样受到持续噪声干扰时波形畸变严重。为此,此处提出了一种基于DBC的白噪声抑制算法,实验结果表明,该算法在较大电感参数偏差下能保持稳定,在白噪声干扰下能够显著降低电流畸变率。

一种进行环路隔离的大电流高电源抑制比LDO设计

针对传统带有电荷泵、以NMOS作为功率管的LDO驱动能力低下和输出纹波偏高的问题,基于Huahong 0.35μmBCD工艺,设计了一种隔离交直流环路的大电流LDO。该LDO通过将直流环路和交流环路进行隔离,降低了对电荷泵驱动能力的需求,从而保证NMOS功率管栅极驱动电压的较低纹波并实现大电流输出。通过加入纹波电流吸收电路,增强了LDO的PSRR。结果表明,在3.41~5.5V的输入电压范围内,LDO的输出电压为3.3V,输出电流最高达到3 A,压差为110 mV。LDO在轻负载下的PSRR为:111.261dB@DC,86.9005dB@1kHz,78.9472dB@1MHz;重负载下的PSRR为:111.280dB@DC,84.1231dB@1kHz,39.2638dB@1MHz。

考虑MAF延时和前馈补偿的高压直流快速锁相环

同步参考坐标系锁相环是高压直流(high voltage direct current,HVDC)同步触发控制系统中广泛应用的一种窄带宽锁相环,在交流系统故障引起相位跳变情况下,其动态响应缓慢。为增大锁相环的带宽,一种滑动平均滤波器(moving average filter,MAF)被前置于锁相环路,然而MAF本身存在响应延迟,制约了锁相环的同步速度。为了缓解响应延迟问题,文中提出一种考虑MAF延时和前馈补偿的HVDC快速锁相环。首先,利用MAF线性暂态特征预测相位变化,并分别针对故障接入和切除引起的相位跳变问题提出不同的补偿策略;接着,利用不变性原理对锁相环路进行前馈补偿,在负反馈控制和前馈补偿共同构成的复合校正控制系统的作用下,锁相环能够在较小PI参数下实现快速响应;最后,将所提快速锁相环在CIGRE HVDC标准模型和三峡—上海直流工程模型中进行仿真验证。结果表明,该快速锁相环能够有效缓解滤波器响应延迟的制约,缩短失锁时间,进而提高高压直流逆变侧抵御换相失败的能力。

双闭环控制的三相电压型PWM整流器的研究

针对传统的不控整流电路中网侧电流谐波很大、晶闸管相控整流电路中深控时网侧功率因数低和闭环控制时动态响应慢等问题,设计出一种引入脉冲宽度调制技术的三相电压型整流器。在完成三相电压型整流器的电压外环电流内环的双闭环控制系统设计的基础上,对其进行了仿真实验。仿真结果表明,与不控或相控整流电路相比,该三相电压型PWM整流电路中,不仅输入电流的谐波大大减小,电能可双向流动,能够实现整流和逆变状态下的单位功率因数运行,还可较好地抑制负载突变时直流侧电压的波动,从而显著提高系统的抗扰能力,真正实现绿色环保和高效节能的高度结合。

极端气象灾害下考虑MESS主动调控的配电网故障恢复策略

针对极端灾害天气下配电网的故障恢复问题,计及极端灾害的时空演变特性,提出了一种利用移动储能(mobile energy storage system,MESS)的灵活性主动参与抑制合环冲击电流,辅助配电网故障恢复的策略。首先,建立了暴风雨等不同气象灾害条件下配电线路的故障率统计模型。其次,将复杂交通网与配电网进行耦合,建立了以最小化MESS调度时空成本的上层预布局模型。然后,分析了MESS出力对冲击电流的影响,以抑制合环冲击电流、最大化负荷恢复率和最小化MESS调度成本为目标,构建了MESS参与网络重构的下层调度模型。最后,在改进的IEEE 123节点系统中进行测试,验证了MESS对配网故障恢复安全性的提升效果。结果表明,采用MESS主动参与调控能够有效应对移动的极端气象灾害引发的配电网动态故障恢复问题,并抑制合解环过程中电流的波动,提升网络重构方案的可行性和增强配电网的韧性。

CMG框架超低速无电流环矢量控制研究

针对控制力矩陀螺框架在超低速工况下存在转速测量精度低、多源扰动力矩及非线性摩擦等问题,提出了基于无电流环矢量控制的超低速驱动控制方法。首先,建立超低速工况下的框架电机的机电模型,设计了无电流环磁场矢量控制器(field-oriented control,简称FOC);其次,针对转速测量精度的问题,设计了转速观测器作为反馈环节。结果表明,与方波驱动法相比,该方法无换相转矩,提高了转速测量精度,且在面对多源扰动、非线性摩擦时具有鲁棒性。仿真和实验结果验证了该方法的可行性和有效性。

基于实际触发角响应的连续换相失败抑制方法

针对逆变侧交流故障下高压直流(line commutated converter-high voltage direct current,LCC-HVDC)输电系统易引发连续换相失败问题,该文分阶段分析了连续换相失败的产生机理。明确了当产生触发角偏差后,直流电流调节效果与实际触发角不匹配是连续换相失败的重要诱因。同时不对称故障后,负序分量通过极控、阀控两级控制系统对实际触发角造成的影响,增加连续换相失败的风险。因此提出了一种基于实际触发角响应的连续换相失败抑制方法,通过实际触发角修正整流侧电流指令值。与此同时,该方法通过陷波器降低负序电流对极控层触发角指令值的影响,通过相序解耦控制器和换相电压相位补偿降低负序电压对阀控层触发角实际值的影响。该方法能实现动态、自适应的电流指令值调整,提升恢复过程中直流电流与实际触发角的匹配度,提升直流系统抵御连续换相失败能力。在CIGRE标准测试模型和实际工程模型中,验证了理论分析的正确性和优化方法的有效性。

基于扩张状态观测器的并联直流变换器增强控制策略

为进一步改善并联直流变换器在多种内外部扰动下的动态性能,提出一种基于双闭环扩张状态观测器(ESO)的增强控制策略。该策略将比例调节器与ESO相结合,然后分别应用于公共电压外环和各个电流内环,实现各功率单元的动态均流,也强化对输入电压扰动的抑制能力,且无需额外传感器。首先,根据并联变换器的状态方程,推导内环电流ESO控制的表达式。然后,根据电流闭环极点的分布情况,从理论上分析ESO型电流内环在输入电压扰动抑制方面强于比例积分型电流内环的原因。此外,借助伯德图,给出ESO型电流内环对电感失配的鲁棒性更强并能有效消除动态电流偏差的理论依据。最后,使用三相buck变换器进行不同方案的对比验证,实验结果证明了所提策略在改进动态性能方面的有效性和先进性。

高响应交流永磁伺服系统快速电流环带宽扩展方法

交流永磁伺服控制系统中现有电流采样和脉冲宽度调制(Pulse width modulation,PWM)占空比更新的方法延迟时间较长,在一定程度上会影响电机的电流环带宽和动态特性;针对该问题,建立了电流环闭环系统数学模型,分析了电流环的内部延迟和影响电流环带宽的因素,在此基础上提出了一种通过改进电流采样与PWM更新时间的电流环带宽扩展方法。该方法通过提前PWM占空比更新时机来降低电流采样和PWM占空比更新之间的延迟,减小信号传输滞后,从而扩展电流环系统的闭环带宽,提高系统响应速度。仿真与实验结果表明,相比现有电流采样方法,采用新方法的交流永磁伺服控制系统的电流环具有更宽的带宽和更高的动态响应能力,从而提高了交流伺服系统的动态响应能力和稳态精度。该方法可为交流伺服系统在机器人、高端智能装备等对响应速度和定位精度要求较高领域的应用提供参考。

基于频域分析的PMSM双闭环PI控制器整定方法

以永磁同步电机(PMSM)双闭环调速系统为对象,在对其数学模型进行合理简化的基础上,综合考虑了逆变器开关周期、电流和速度滤波等环节的影响,给出了一种在频域进行PI控制器参数设计的整定方法。通过建立双环的数学模型,得到关于开环频域特征量的解析式,根据电流环在中低频段的特性以及转速环中阻尼系数难以精确测定的问题,分析推导了电流环和转速环PI参数简化设计的方法。利用工程经验将闭环时域指标转化开环频域特征量,并结合各类约束条件合理界定其取值范围,由此得到的PI控制器可以兼顾快速性和稳态跟踪能力。仿真和实物实验验证了整定方法的可行性,该方法确立了双环控制器参数与其对应的开环截止频率和相位裕度的解析式,并明确了开环频域特征量与闭环时域指标的关系,具有较强的工程参考和应用价值。

不同锁相环对LCC-HVDC控制回路稳定性的影响

该文研究了电网换相换流器高压直流输电(line commutated converter high voltage direct current,LCC-HVDC)系统逆变侧采用不同锁相环时,锁相环控制回路比例–积分(proportional integral,PI)参数变化对直流控制回路稳定性的影响。首先,分别建立了逆变侧锁相环采用滑动平均滤波(mo ving average filter,MAF)和级联延时消去滤波(cascaded delayed signal cancellation,CDSC)的LCC-HVDC小干扰动态模型,并通过电磁暂态仿真验证了该模型的正确性。其次,基于拉普拉斯变换获得系统定电压控制回路的传递函数,利用奈奎斯特稳定判据以及稳定裕度指标分析不同锁相环对定电压控制回路稳定性的影响,并进行了机理分析,同时在PSCAD/EMTDC的电磁暂态模型上进行了验证。最后,进一步在LCC-HVDC工程模型上对该文所得结论的普适性进行了仿真验证。