Inertia plays a vital role in maintaining the frequency stability of power systems. However, the increase of power electronics-based renewable generation can dramatically reduce the inertia levels of modern power systems. This issue has already challenged the control and stability of small-scale power systems. It will soon be faced by larger power systems as the trend of large-scale renewable integration continues. In view of the urgent demand for addressing the inertia concern, this paper presents a comprehensive review of inertia enhancement methods covering both proven techniques and emerging ones and also studies the effect of inertia on frequency control. Among those proven techniques, the inertia emulation by wind turbines has successfully demonstrated its effectiveness and will receive widespread adoptions. For the emerging techniques, the virtual inertia generated by the dc-link capacitors of power converters has a great potential due to its low cost. The same concept of inertia emulation can also be applied to ultracapacitors. In addition, batteries will serve as an alternative inertia supplier, and the relevant technical challenges as well as the solutions are discussed in this paper. In future power systems where most of the generators and loads are connected via power electronics, virtual synchronous machines will gradually take over the responsibility of inertia support. In general, it is concluded that advances in semiconductors and control promise to make power electronics an enabling technology for inertia control in future power systems.

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On the Inertia of Future More-Electronics Power Systems

The paper discusses load compensation using a distribution static compensator (DSTATCOM). It is assumed that the DSTATCOM is associated with a load that is remote from the supply. It is shown that the operation of a DSTATCOM assuming that it is connected to a stiff source in such situations will result in distortions in source current and voltage at the point of common coupling. To avoid this, the DSTATCOM is connected in parallel with a filter capacitor that allows the high frequency component of the current to pass. However, this generates control issues in tracking, as standard controls such as a hysteresis control are not suitable in these circumstances. This paper proposes a new switching control scheme and demonstrates its suitability for this problem. It also proposes a scheme in which the fundamental sequence components of a three-phase signal can be computed from its samples. The overall performance of the proposed scheme is verified using digital computer simulation studies.

Load compensating DSTATCOM in weak AC systems

In this paper, different Distribution Static Compensators (DSTATCOMs) topologies, state of the art, their performance, design considerations, future developments, and potential applications are investigated for power quality improvement. These DSTATCOMs for three-phase three-wire systems and three-phase four-wire systems are developed and installed in the distribution system for many functions, such as reactive power compensation, harmonics elimination, load balancing, and neutral current compensation. This paper is aimed to explore a broad perspective on DSTATCOMs to researchers, engineers, and the community dealing with the power quality improvement. A classified list of some latest research publications is also provided for quick reference.

Comprehensive Study of DSTATCOM Configurations

Fast and reliable DC fault detection is one of the main challenges for modular multilevel converter (MMC) based DC grid with DC circuit breakers (DCCBs). This paper extracts the high-frequency components in transient voltages by wavelet transform and proposes a fault identification method based on the difference of square of transient voltages to identify the faulted lines for DC grids using overhead lines. Meanwhile, a faulted pole discrimination method based on the difference between the change of positive and negative pole voltages is presented. A line protection scheme including detection activation, fault identification, faulted pole discrimination, and post-fault re-closing is designed. Using only the local measurements, the scheme can realize the protection of the whole line without communication and has the capability of fault resistance endurance and anti-disturbance. The proposed method is tested with a four-terminal MMC-based DC grid in PSCAD/EMTDC. The selection methods of threshold values are presented and the impact of DCCB operation on the reliability of DC fault protection is analyzed. Simulation results verify the fast detection and reliability of the designed DC line protection scheme.

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A Transient Voltage-Based DC Fault Line Protection Scheme for MMC-Based DC Grid Embedding DC Breakers

The transient response of the voltage source converter (VSC) interfaced dc system is significantly different from the ac counterpart. The rapid discharge current from the dc-link capacitors and the vulnerability of the freewheeling diodes during the short circuit in dc grid demand that the transient detection algorithm executes within a few milliseconds. The rapidly rising fault current in the dc grid is expected to have high-frequency components which might be an effective indicator of the transient condition. This paper presents quantitative investigation of the high-frequency components utilizing short-time Fourier transform (STFT) during transient conditions. Detailed operating principles with various factors affecting the STFT operation such as ripple content of the input dc signal and window type and length have been thoroughly investigated. STFT algorithm is able to detect low-impedance faults within 1 ms and high-impedance faults in 2 ms. Moreover, it is able to distinguish between short-circuit fault and less severe transient conditions such as sudden load change. The STFT algorithm is evaluated analytically and subsequently applied to a MATLAB/Simulink based dc test system. It is further validated and substantiated with the real fault current data obtained from a scaled-down experimental testbed. Sensitivity analysis and comparison with the existing frequency-domain-based fault-detection method are done to support the efficacy of the proposed method.

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Short-Time Fourier Transform Based Transient Analysis of VSC Interfaced Point-to-Point DC System

The distribution static compensator (DSTATCOM) is used for load compensation in power distribution network. In this paper, a new topology for DSTATCOM applications with nonstiff source is proposed. The proposed topology enables DSTATCOM to have a reduced dc-link voltage without compromising the compensation capability. It uses a series capacitor along with the interfacing inductor and a shunt filter capacitor. With the reduction in dc-link voltage, the average switching frequency of the insulated gate bipolar transistor switches of the DSTATCOM is also reduced. Consequently, the switching losses in the inverter are reduced. Detailed design aspects of the series and shunt capacitors are discussed in this paper. A simulation study of the proposed topology has been carried out using power systems computer-aided design simulator and the results are presented. Experimental studies are carried out to verify the proposed topology.

A DSTATCOM Topology With Reduced DC-Link Voltage Rating for Load Compensation With Nonstiff Source

DC system has come a long way to shift the direction of power transmission in future. With respect to that, the protection is still one of the hot topics that need to be addressed, particularly the dc fault detection method. This paper presents methods to robustly differentiate between load change and dc fault in a two terminal dc system connected to passive load. In the first method, wavelet transform is used to determine the wavelet coefficient for the system dc line current. The performance of wavelet transform is compared with the rate of change of current ( $di/dt$ ) method. Details regarding synchronous reference frame $dq$ current controller at source terminal, root mean square method controller at load terminal are discussed. Simulation studies are carried out on MATLAB/Simulink model and the corresponding steady state, load change, and fault studies results are presented. Experimental validation of wavelet fault detection method is performed on the developed laboratory dc grid setup interfaced to dSPACE 1103 controller through MATLAB real time toolbox.

Experimental Validation of Fault Identification in VSC-Based DC Grid System

The rapid discharge of a dc-link capacitor of the voltage-source converter (VSC)-based dc system is the primary indication of the fault condition. Apart from the time-domain analysis, frequency-domain analysis of the fault current could also be utilized for dc fault detection, as the rapidly rising fault current is expected to have high-frequency components. This paper proposes two fault detection methods and compares their performances with the wavelet transform. The first method is the time-domain analysis of the dc-link capacitor discharge and is termed as the capacitive discharge technique. The relationship between the dc line current and the behavior of the dc-link capacitor is measured in terms of a correlation coefficient, whose value can be used to establish a fault basis. The second method is the frequency-domain-based short-time Fourier transform, which is used for quantitative analysis of high-frequency components in the fault current. These methods are extensively analyzed and compared using a scaled-down VSC-based dc system experimental test setup. Comparison has been done based on fault detection time, sensitivity to fault parameters, influence of sampling frequency, and computation speed. Furthermore, the selectivity of the fault detection methods is studied on the multiterminal dc systems of two different topologies (ring and radial), modeled in PSCAD/EMTDC. The experimental and simulation results substantiate the applicability of all the methods to the dc system. Brief comparative analysis with the di/dt method is also presented to highlight the advantages of the proposed methods.

Nagesh Geddada

Papers

Short-Time Fourier Transform Based Transient Analysis of VSC Interfaced Point-to-Point DC System

A DSTATCOM Topology With Reduced DC-Link Voltage Rating for Load Compensation With Nonstiff Source

Experimental Validation of Fault Identification in VSC-Based DC Grid System

Time- and Frequency-Domain Fault Detection in a VSC-Interfaced Experimental DC Test System

Analysis and validation of wavelet transform based DC fault detection in HVDC system