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Voltage rise caused by high levels of distributed generation is manifesting as voltage regulation challenges for many electricity network service providers. In this environment it would be ideal to reduce supply voltage magnitudes, however, many network operators are hesitant to do so due to concerns related to consumer appliance performance at reduced supply voltage magnitudes. Voltage regulation requirements are defined by network standards and network service providers must ensure voltages remain within specified limits. Through an evaluation of domestic appliance performance when supplied at various voltage magnitudes, this paper examines the impact of varying voltage levels on residential appliances. Equipment energy demand, operation and actuation were monitored for each applied voltage magnitude. While no equipment failures were recorded, appliance behaviour varied significantly with applied voltage magnitude. Individual appliance conservation voltage reduction (CVR) factors have also been established. The results highlight the importance of good voltage regulation and provide substantiated appliance performance figures for future studies. The outcomes of this paper allow electricity network service providers to understand the implications of supply voltage magnitude on domestic appliance performance, whether it be understating of the impact of higher voltage magnitudes caused by distributed generation or implications of reducing voltage magnitudes to provide headroom for distributed generation integration. [ABSTRACT FROM AUTHOR]

Negative-sequence current is required to be intentionally injected to support the power grid or compensate the imbalanced load in asymmetrical faults utilizing STATCOM. However, the extreme compensation range of negative-sequence current injection becomes complicated when grid voltage becomes asymmetrical. This article presents a novel off-line strategy for negative-sequence current compensation under two typical kinds of asymmetrical grid faults — single-phase voltage drop and double-phase voltage drop, with the following two benefits simultaneously: maximizing negative current compensation capacity; avoiding iterations and reducing computation burdens. In this paper, the mechanism of DC voltage balancing among three phases in STATCOM is analyzed and a new identification method of negative-sequence current compensation range is explored out. To simplify the calculation, this paper takes the ratio of negative-sequence reactive (Iq-) and active (Id-) current as a variable (s). In this way, the relationship between the extreme compensation range, asymmetrical voltage sag depth of power grid h is drawn with MATLAB. Different from the previous iterative method, the off-line identification method is applied to avoid huge iterations. By fitting the curve, the precise ranges of negative-sequence current compensation under different asymmetrical grid faults are approximated as simple polynomials, thus significantly reducing the computation burdens. Simulation and experimental results verify the practicability of the proposed compensation method. [ABSTRACT FROM AUTHOR]

This paper presents the three-phase impulse overvoltage measured at the terminal of a 10 kV distribution line when the impulse current was injected into the phase conductor of the distribution line. The overvoltage observation station was installed at the terminal of the line to record the three-phase overvoltage waveforms. It is shown that the struck phase overvoltage consists of three components including the negative pulse, the stage of residual voltage maintenance and the overshoot oscillation part. The observed negative peak values of the struck phase overvoltage range from −85.4 kV to −96.0 kV in three different injected impulse levels at 90,120,150 kV charging voltage. The duration of residual voltage depends on the duration time of injected current. Furthermore, there is a strong positive relationship between the oscillation amplitude of the struck phase overvoltage and the injected current amplitude. The peak amplitudes of three-phase overvoltage always have the following magnitude relationship: struck phase C > induced phase B > induced phase A. The voltage coupling coefficients of induced phases will not exceed 0.4. The test method proposed in this paper will be helpful to the research on the lightning overvoltage response characteristics of the distribution line and the protection analysis of arresters. [ABSTRACT FROM AUTHOR]

The paper addresses a real-time optimization-based reference calculation integrated with a control structure for Modular Multilevel Converters (MMC) operating under normal and constrained situations (where it has reached current and/or voltage limitations, as it may occur during system faults). Firstly, a nonlinear optimization problem has been developed in which it prioritizes to satisfy the AC grid current set-points imposed by the transmission System Operator (TSO). The constrained nonlinear optimization problem is formulated based on the steady-state model of the MMC, whereby the prioritization is achieved through distinct weights defined in the Objective Function’s (OF) terms. The resultant optimization problem, however, is highly nonlinear requiring high computation burden to be solved in real-time. To cope with this issue, this paper applies a Linear Time-Varying (LTV) approximation, which permits to represent the nonlinear dynamics of the system as constant parameters, while a Linear Time-Invariant (LTI) system is used to formulate the optimization constraints. The converter’s current references are determined in real-time by solving a constrained linearized optimization problem at each control time step, which considers the TSO’s demands, the current MMC operating point and its physical limitations. Theoretical analyses comparing the responses of the linear and nonlinear optimization problems are performed to validate the accuracy of the LTV approximation. Finally, the linearized-optimization problem is integrated with the MMC controllers, evaluated under different AC and DC network conditions and compared with conventional control strategies, where it is shown that the presented method can be potentially employed to obtain the MMC current references for distinct network scenarios. [ABSTRACT FROM AUTHOR]

This paper presents extensive verification, sensitivity, and application studies for an online substation grounding grid condition monitoring method proposed in a companion paper. The studies are conducted using a realistic substation grounding grid and associated feeder configurations commonly encountered in North America. The verification studies confirm that the proposed method is able to estimate the substation grounding impedances with acceptable accuracy. Main factors impacting measurement accuracy are identified and quantified through sensitivity studies. The findings lead to a guide for the implementation and operation of the proposed method. An application case is presented to demonstrate how the gradual deterioration of substation grounding grid can be monitored using the proposed method. [ABSTRACT FROM AUTHOR]

In this paper, a reliable methodology is proposed in order to implement and validate a Model Predictive Control (MPC) scheme on an actual Voltage Source Converter (VSC) integrated in a scale-down multi-terminal DC grid. The objective of the investigated MPC controller is to enable AC frequency support among two asynchronous AC areas through a High Voltage Direct Current (HVDC) grid, while considering physical constraints, such as maximum and minimum DC voltage. A systematic and accurate implementation strategy is proposed, based mainly on the Hardware In the Loop (HIL) and Power Hardware In the Loop (PHIL), leading to the real-life testing on VSC, controlled by a classical microcontroller. The technical problems during the implementation process, as well as the proposed solutions, are described in detail through this paper. This procedure is deemed valuable to bridge the gap between offline simulation and the actual implementation of such advanced control scheme on experimental test rig. [ABSTRACT FROM AUTHOR]

This paper proposes a novel hybrid AC/DC microgrid architecture incorporating a central energy storage system (ESS) for both the AC and the DC sub-grids. To ensure effective operation of the proposed hybrid AC/DC microgrid architecture, a coordinated control strategy between the central ESS and the inter-linking converter (ILC) is proposed in the paper. The proposed central ESS provides voltage and frequency control to the AC sub-grid by measuring voltage and frequency at the point of common coupling (PCC) of the AC sub-grid, while providing DC voltage support to the DC sub-grid by measuring DC sub-grid’s PCC voltage. Furthermore, per-unit frequency and the DC sub-grid PCC voltage are utilized to transfer power between the two sub-grids via the ILC. Therefore, two control strategies are coordinating with each other and maintain stable operation. The central ESS eliminates the use of multiple ILCs and complex control schemes for charge and discharge control to maintain same state of charge levels at each ESS. Effectiveness of the proposed coordinated controller is verified by applying various load disturbances, faults, and under different modes of operation. [ABSTRACT FROM AUTHOR]

The neutral grounding method is one of the crucial technologies used to ensure the safe and stable operation of distribution networks. With the development of power electronics-based devices, the application of highly controllable inverters to grounding systems has become a new research direction. In this review paper, based on the summary and analysis for the research status, the flexible grounding system is systematically classified according to its control strategy and structure. And then, the following core technologies are summarized: faulted line selection, current component detection, high-resistance ground identification, ground parameter measurement, neutral point voltage control, and fault location. Subsequently, deficiencies of flexible grounding systems are summarized from the following four perspectives: reliability, accuracy, speed, and stability. Finally, some discussions and recommendations for future development and research direction are presented. Especially, the reliability-enhanced hybrid grounding system that combines active inverters and low resistance for high-resistance ground faults is recommended. This paper provides a systematic literature review of the crucial technologies for flexible grounding systems and can be used as a technical reference for future research. [ABSTRACT FROM AUTHOR]

This paper analyzes the unsymmetrical operation of the dc/dc MMC caused by parametric uncertainty. The unsymmetrical operation is referred to as the condition in which parameters involved in ac power generation are not equal in the phase-legs of the converter. Because existing models in the literature cannot exhibit the unsymmetrical operation of the dc/dc MMC, new detailed and simplified steady-state models are developed in this paper. In the developed models, the connection between the operation of different phase-legs shows itself when the dc-link impedance is considered. Using the developed models, the parameters that can force the converter to operate in unsymmetrical conditions are identified, and the maximum variation that the converter can tolerate is found. It is shown that even a small parameter variation leads to the leakage of the ac circulating current into the dc-link, which is not acceptable. By conducting various case studies, the impacts of the dc-link length, the transmitted power, and the conversion ratio on the unsymmetrical operation of the dc/dc MMC are characterized. The results showed that the converter would be more sensitive to the parameter variation as these parameters increase. Comparative analysis and simulation results confirm the validity and effectiveness of the presented models and analysis method. [ABSTRACT FROM AUTHOR]

The paper addresses an improved inner current reference calculation to be employed in the control of modular multilevel converters operating during either balanced or unbalanced conditions. The suggested reference calculation is derived based on the AC and DC additive and differential voltage components applied to the upper and lower arms of the converter. In addition, the impacts caused not only by the AC network’s impedances but also by the MMC’s arm impedances are also considered during the derivation of the AC additive current reference expressions. Another issue discussed in this article regards that singular voltage conditions, where the positive-sequence component is equal to the negative one, may occur not only in the AC network but also internally (within the converter’s applied voltages). Several different inner current reference calculation methods are compared and their applicability during the former fault conditions is analyzed. The paper presents a detailed formulation of the inner current reference calculation and applies it to different unbalanced AC grid faults where it is shown that the presented approach can be potentially used to maintain the internal energy of the converter balanced during normal and fault conditions. [ABSTRACT FROM AUTHOR]

This paper addresses the interactions among the control loops of voltage-sourced converters (VSCs) embedded in high-voltage direct current (HVDC) systems and the impact of these interactions on the performance of individual control loops. Furthermore, this paper proposes a method to design the individual control loops, rather than the interconnected control system, such that the overall system stability is ensured in the presence of interactions. This paper addresses three aspects of the impact of control loop interactions on the overall control system of the VSC including stability, transient response, and steady-state response. To illustrate the impact of interactions on the VSC small-signal stability, stability regions of the interconnected control loops are obtained and compared against those of the various individual control loops. Then, a novel formulation of the VSC stability criterion is presented, and the small-gain theorem is utilized to determine a range of stabilizing gains for the interconnected control system. Moreover, new indices using the infinity norm are introduced to evaluate the impact of interactions on the transient response of the VSC. Finally, the DC gain of a certain set of transfer functions is used to assess the impact of control loops interactions on the set-point tracking capability of the VSC. The introduced indices along with time-domain simulations show that the transient response of the d-axis control loops is not impacted by the interactions, whereas that of the q-axis control loops is significantly impacted by the interactions. The set-point tracking capability is proved to be independent of interactions. [ABSTRACT FROM AUTHOR]

The short-circuit response of inverter-interfaced distributed generators (IIDGs) is not adequately represented in many conventional protection studies. This paper presents an in-depth analysis of IIDG behavior during grid faults and proposes a more accurate reduced-order parameterized short-circuit current (RPSC) model of inverters. Typical inverter components are thoroughly investigated to identify those that play dominant roles during faults. The paper shows that the current limiter is the dominant factor for the steady-state fault-current of inverter; while the inverter filter along with the severity of the voltage disturbance largely determine the initial transient spike of inverter fault current. The proposed model is low-order and can be used in large scale simulations. The parameters of the proposed RPSC model can be extracted from laboratory experiments without requiring proprietary manufacturer information. The proposed fault-current model is analogous to the well-known synchronous machine model that segregates the inverter fault current into subtransient, transient, and steady-state fault currents. Experimental and simulation tests are presented to validate the model. [ABSTRACT FROM AUTHOR]

Autonomous power systems with a single power station and a high-voltage (HV) network may be susceptible to overvoltage instability, especially during light-load conditions. Moreover, in low inertia systems, considerable frequency excursions may be observed after disturbances. To deal with voltage and frequency stability problems in such isolated power systems, several measures should be taken, such as rules for adequate automatic voltage control of the synchronous generators and load-tap-changers (LTCs), and/or adequate design of actions applied during emergency system conditions such as reactors switching, and redesigning underfrequency load shedding taking into account overvoltage problems. The effectiveness of the proposed countermeasures against overvoltage instability in autonomous power systems is illustrated in this paper. In 2016, the autonomous power system of the Greek island of Rhodes experienced similar voltage/frequency problems leading to a blackout. This event has been accurately reconstructed in this paper. The application of the suggested countermeasures during the simulation of the 2016 blackout event in Rhodes, further highlighted the effectiveness of these measures, providing a guideline for similar cases. [ABSTRACT FROM AUTHOR]

The presence of overvoltage can cause the insulation deterioration and excite the partial discharges (PD) in gas-insulated switchgear (GIS), and it is necessary to detect the overvoltage and PDs simultaneously to evaluate the insulation performance. A novel GIS internal composite sensor for PD detection and overvoltage measurement is proposed in this paper. The proposed sensor is mainly formed with a disc-type monopole UHF sensor and an overvoltage sensor. The effectiveness and sensitivity of the sensor are tested on a designed platform under power frequency voltage, switching impulse, lightning impulse, and oscillating lightning impulse, respectively. Experiment results show that the proposed sensor can be used to detect overvoltage and UHF signals effectively and is expected to help in the GIS condition evaluation. [ABSTRACT FROM AUTHOR]

With the advancements of the battery energy storage systems (BESSs), reduction of their manufacturing costs and government subsidies, the BESS uptake is likely to increase rapidly in power distribution networks. This paper investigates the effects of residential BESSs on low-voltage (LV) networks using the actual household load profiles equipped with BESS and solar-photovoltaic (PV) systems. The electricity consumption data collected via smart meters (2200 households with PV/BESS, 1950 households with PV only and 1000 households without a PV or a BESS) at different solar-PV penetration levels and network types are used to simulate real network operating scenarios. A real LV distribution network in Australia is analysed in DIgSILENT PowerFactory under different scenarios, such as, customers with and without a solar-PV/BESS, with a solar-PV but without a BESS, and without a solar-PV, by using both the power flow and quasi-dynamic simulation studies under balanced and the unbalanced network loading conditions. According to the study, customers experience large voltage excursions from solar-PV power exports, which could be resolved by the household BESS, provided that the BESS charging is coordinated with the solar-PV production. Moreover, quasi-dynamic simulation shows that the BESS could reduce the violation of the over-voltage limit during the solar peak hours (midday) by lowering the worst-case feeder voltage by 3%. Finally, extreme-event (high solar PV generation scenario) simulation shows that the implementation of the BESS controller to facilitate charging BESS during afternoon solar-PV export may reduce the negative grid impact and will assist to avoid network upgrades. [ABSTRACT FROM AUTHOR]

Electrical equipment of the floating nuclear power plant operates in a long-term vibration environment. Ground faults occur frequently and are often accompanied by arc, a reliable arc suppression algorithm for the generator stator winding is necessary. In this paper, an active arc suppression method is proposed by connecting a controllable voltage source to the generator neutral point. Through regulating the neutral point voltage to be equal in amplitude and opposite in phase to the fault potential, the fault point voltage can be clamped to a low level to achieve arc suppression. Since the generator's phase potential contains certain harmonic components, the controllable voltage source includes fundamental and third-order harmonic components. To accurately calculate the output voltage, online calculation methods for the fault location, fault potential and grounding capacitance are proposed. The accuracy of the fault location method is verified by simulation and dynamic test. And simulation results based on PSCAD/EMTDC software show that the proposed method can achieve effective arc suppression at different ground fault conditions. [ABSTRACT FROM AUTHOR]

The capacitor condition monitoring is an important issue for the reliable operation of modular multilevel converters (MMCs) in high-voltage applications. The numerous capacitors make the monitoring for the capacitor condition complex and computation-consuming. This paper proposes a hierarchic capacitor condition monitoring strategy to assure the reliability of high-voltage MMCs. In the strategy, the switching times of the switching signals are counted to detect the condition of the capacitor in each submodule (SM). Then, the subsequent capacitance calculation method is only conducted to the abnormal capacitors indicated by the above condition detection algorithm. Meanwhile, the variations of the measured and reference capacitor voltages are employed to calculate the capacitance in the proposed method. The condition detection algorithm shrinks the scope where the abnormal capacitors locate, which avoids calculating the total capacitances in MMCs. In the proposed capacitance calculation method, the complex operations are further saved. Both the two measures reduce the computation burden effectively. The implementation of the whole monitoring strategy is simple and fit for high-voltage MMCs. Moreover, the strategy has no adverse influence on the normal operation of MMCs. The effectiveness of the proposed strategy is verified by the simulation studies with the professional tool Matlab/Simulink. [ABSTRACT FROM AUTHOR]

Ground faults are the main cause of electrical failure in transmission and distribution systems. In ungrounded distribution networks, it is difficult to detect where a first ground fault has happened as only capacitive currents are available. Also, it is important to trip only the faulty line in order to ensure the system's reliability. Directional residual overcurrent relays are the standard solution for these systems, but their difficult commissioning and high costs make it not trivial. This paper introduces a new selective and directional protection method to determine the faulty line in ungrounded medium voltage networks with three or more feeders. The method evaluates the residual currents in the feeders coupled to the same busbars in combination with a reference current created by the sum of a combination of residual currents of all feeders. Both simulation and laboratory results validate this new method, which is easy to start up, as it does not need previous configurations for its implementation, and it presents an economical solution to this problem. [ABSTRACT FROM AUTHOR]

This paper presents an injected current sensitivity (ICS) based multi-phase load flow (LF) methodology with minimal Jacobian computation for the three-phase unbalanced distribution system with multiple Distributed Energy Resources (DERs). In the proposed approach, first, a generalized bus admittance and Jacobian matrix have been formulated that include various distribution system components such as lines, shunt capacitors, voltage control devices such as regulators, different transformer connections, and load models. Second, DERs are modeled as a PV bus and incorporated into the proposed LF algorithm by formulating a reactive power injection-based sensitivity matrix that iteratively computes its net power injections. Unlike the traditional Jacobian matrix-based LF methods where an update of Jacobian (size and element) is required for each PV bus, this method does not require any update, hence, making it faster and more flexible to include a large number of DERs. From the various tests performed on the standard IEEE three-phase distribution network and their derivatives, it has been observed that the proposed approach is accurate, efficient, computationally less complex, and scalable. [ABSTRACT FROM AUTHOR]

Geomagneticallyinduced current (GIC) studies usually focus on transformer response to dc but seldom analyze generator excitation systems. A GIC is driven by an electric field which is induced by a very low-frequency geomagnetic field. It is widely acceptable to model GICs with dc, mostly suitable for static voltage stability analysis, but this does not adequately characterize a power system's dynamic response. Considering this, we studied a multi-machine power system on an OPAL-RT digital real-time simulator (DRTS)-MATLAB/Simulink system. We used real GIC measurement data from high and mid-latitude regions. Wavelet analysis related the frequency components of the GIC to the power system's response. A dynamic generator model with excitation voltage control modelled the source. Our main finding is that excitation system control contributes to reducing the voltage drop caused by increased var demand in saturated transformers with GICs. Measured GICs revealed non-negligible short-term dynamics affecting bus voltages that are not captured with dc-modelling assumptions, yet they are important for voltage stability analysis. This study implies that excitation control creates a buffer for high GICs to flow without violating the grid codes for minimum operating voltages and this paper demonstrates the optimization of this novel approach. [ABSTRACT FROM AUTHOR]

The use of power converters as solid state transformers is an attractive solution to modernize the power network, but this solution has not been fully addressed for MV and HV substations. This paper presents a customized and simple control for the Modular Multilevel Matrix Converter (M3C), specially conceived for its operation on synchronous ports, which is the case of AC substations. The control allows bidirectional power transfer, generation/absorption of reactive power and provisions of ancillary services. The converter is compared to the back-to-back Modular Multilevel Converter (B2B-MMC) where the key performance indicators to carry out the comparison are power efficiency, number of semiconductor devices, passive components required, footprint, voltage cell balance, fault blocking capability and stress of components. The simulation results show the features, performance and attractiveness of the M3C topology in a 33/11 kV, 16 MW substation under different operating conditions, including grid faults and dynamic operation. The M3C presents similar efficiency and performance than the B2B-MMC, but it uses fewer semiconductor devices, passive components and total cell capacitor energy than the B2B-MMC, reducing cost and footprint. The experimental results show the performance of the M3C under less ideal conditions including a substation transformer saturation and power step response. [ABSTRACT FROM AUTHOR]

In the conventional sliding mode control, the four-leg distribution static compensator (DSTATCOM) currents are controlled based on a current error in each phase. However, the four currents in a four-wire system can not be independently controlled variables and hence one of the four controllers is redundant in conventional schemes. Further, the current dynamics of a DSTATCOM converter are coupled through converter pole voltages in the natural reference frame. This leads to cross-coupling in the sliding variable equations with respect to four manipulated input variables. Considering the above points, in this paper, the current due to Thevenin’s equivalent load neutral-point voltage is considered as a fourth independent controlled variable and the corresponding system dynamic equations are presented. To get a decoupled feature, a new sliding surface function is structured. The performance of a DSTATCOM with the proposed control scheme under various operating conditions is validated through a detailed simulation study and experimental results, obtained from a laboratory prototype of four-leg DSTATCOM. [ABSTRACT FROM AUTHOR]

Multilevel converters (MLCs) are pioneers in the field of electrical energy conversion. From the classification of MLCs, modular multilevel converter (MMC) is top-rated due to its attractive structure and performance. The modular nature and no need for isolated DC resources are two significant advantages of this converter. However, the need for many sensors to control its performance is a considerable drawback. In addition, the internal circulating current in the MMC causes losses, decreases efficiency, and increases the voltage ripple of capacitors. This paper proposes combining a new capacitor voltage sorting method and an open-loop controller based on arms energy estimation to control the MMC using two voltage sensors in the arms and one current sensor at the output. The proposed method ensures continuous voltage balancing of the capacitors and removes multiple voltage sensors from sub-modules (SMs). The simulation and experimental results verify the accuracy and effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

The voltage rise problem due to the reverse power flow is one of the main obstacles to expanding the photovoltaic systems (PVSs) in distribution networks. In this paper, a decentralized control method for coordinated control of on-load tap changer (OLTC) transformers and PV inverters, is proposed for the voltage regulation of radial distribution networks. In this method, the allowable voltage range is divided into several zones, and appropriate corrective actions are adaptively taken according to each zone. To reduce the tension on the OLTC, first, the reactive power capability of the PV inverters is employed for the voltage regulation, and if it is insufficient, OLTC is used as the last solution. In this regard, the unused capacity of PV inverters is utilized for the reactive power compensation, considering the grid codes reactive power requirements. Furthermore, decentralized control based on a signaling method is used for coordination between OLTC and PVSs, which eliminates the need for communication links. Finally, the effectiveness of the proposed method is validated by the simulation results for the IEEE 33-bus distribution network, and the impact of different reactive power control methods on the number of OLTC operation is investigated. [ABSTRACT FROM AUTHOR]

The unified power flow controller (UPFC) helps to control the power flow, but its outage due to the abnormal state is a threat to power systems. In this paper, based on the proportion of the random failure, the transition rates of the abnormal state are newly derived. The impact of the routine test (RT) on the transition rates is quantified. The state space models of the UPFC based on voltage-source converter (VSC) and modular multilevel converter (MMC) are extended with the states introduced by the RT. The frequency and duration method is used to aggregate the states. The sensitivities of the availability and outage frequency of the UPFC with respect to the failure rates of the components are newly derived to find the vulnerable component and the optimal RT interval. The numerical results show that the RT reduces the probability of the abnormal state, reduces the failure duration, and improves the availability of the UPFC. [ABSTRACT FROM AUTHOR]

The phase-to-ground admittance in distribution networks is asymmetric. This asymmetry is aggravated by the arc suppression coil, resulting in zero-sequence overvoltage. The magnitude of zero-sequence overvoltage can exceed the line-to-ground voltage, endangering the operation of electrical equipment. Moreover, after the occurrence of a single-line-to-ground fault, the change law of the three-phase voltage is affected by the zero-sequence overvoltage, leading to misjudgment of the fault phase. Accordingly, this paper proposes a fault phase selection method based on active current injection that can adapt to asymmetric distribution networks. The fault phase is selected by injecting current and calculating the equivalent power supply voltage of the faulty line without increasing the ground-fault current. The high sensitivity of the proposed fault phase selection method was verified for high impedance faults through software simulations and prototype experiments. Moreover, the zero-sequence overvoltage can be suppressed by a single-phase flexible arc suppression device during the normal operation of distribution networks. The ground-fault current can be quickly suppressed at different ground-fault locations and resistance values, ensuring the extinguishment of the ground-fault arc. [ABSTRACT FROM AUTHOR]

Power flow (PF) flexibility in the multi-terminal HVDC (MT-HVDC) grids is a thought-provoking issue. It leads to employing the active DC power flow controller (DC-PFC)s. Hence, this paper examines the static average model (AM) and power injection model (PIM) of an interline DC-PFC (IDC-PFC). It is to provide a suitable base for DC PF studies and easy embedding of the DC-PFCs into MT-HVDC grids’ PF equations. In this regard, this paper proposes a new DC PF solver (DC-PFS) for the IDC-PFC compensated MT-HVDC grids within the well-accepted Newton-Raphson (NR) framework. It requires a few modifications in the main structure of the system's Jacobin (J) matrix compared to the uncompensated MT-HVDC grid. Also, the system's admittance matrix and its symmetry are preserved. In the proposed concept, the IDC-PFC cooperates with other MT-HVDC grid's state variables to satisfy the predetermined control objective(s). Furthermore, this paper proposes a new solution procedure (SP) to handle various system's limitations during the processes of solving the DC PF problem. Meanwhile, there is no need to modify the related J matrix. The effective and accurate performance of the IDC-PFC's models, as well as presented NR-based DC-PFS and SP, are verified by performing several simulations on the 8-bus CIGRE MT-HVDC grid. [ABSTRACT FROM AUTHOR]

The development of a revolutionary design for transmission lines was recently introduced by shifting phase configurations and sub-conductors into “unconventional” arrangements that were geometrically optimized to maximize natural power. The objective of this paper is to study the electromagnetic transients that result from the simultaneous and sequential line energization and re-energization of the previously developed unconventional high surge impedance loading (HSIL) lines. Statistical switching overvoltage (SSOV) studies are performed using a frequency domain method based on the inverse numerical Laplace transform. Compared to a conventional line, all unconventional HSIL line designs studied in this paper lead to very similar surges for long line lengths (400 km) and similar values for short line lengths (100 km), indicating additional advantages of these lines. The use of a frequency domain method for SSOV studies offers enhanced accuracy, simulation flexibility, and substantially lower computational burden compared with traditional time-domain methods. [ABSTRACT FROM AUTHOR]

This paper exploits smart inverters of distributed energy resources (DERs) for the mitigation of fault-induced delayed voltage recovery (FIDVR). According to the IEEE Standard 1547-2018, smart inverters with voltage ride-through and var injection capabilities are promising technologies for mitigating FIDVR. This paper quantitatively investigates the effectiveness of smart inverters for FIDVR from a system-wide perspective. It shows that the penetration of smart inverters and induction motors are important factors affecting mitigation success. In addition, the over-voltages at the end of FIDVR due to load disconnections can be prevented with coordinated control of DERs and automated switching capacitors. The control scheme of smart inverters used for FIDVR is extended from the distributed volt/var droop control to include low- and high-voltage ride-through capabilities. The IEEE 13-bus feeder, the 8500-node test feeder, and the IEEE 390-node low-voltage network test system are used to illustrate the impact of DER penetration. For example, one can determine the residential air conditioner penetration level (for a given system at certain DER penetration level) that makes a system susceptible to FIDVR. The effects of solar intermittency are also studied. [ABSTRACT FROM AUTHOR]

This paper presents a small-signal dynamic model for a modular multilevel converter (MMC) based HVDC grid, and then systematically i) identifies the system dominant oscillatory modes, ii) quantifies their propensity to instability phenomena, and iii) devises appropriate countermeasures. Based on the developed model, the eigen structure of an HVDC grid is characterized and the impact of the system parameters, configuration, AC system strength and operating point on the damping ratios of the oscillatory modes, and consequently the HVDC grid stability, are investigated. The model is also applied to systematically determine the sensitive parameters and optimize them to mitigate oscillatory modes and prevent instability. The eigen analysis results are verified based on time-domain simulation studies in the PSCAD/EMTDC platform. [ABSTRACT FROM AUTHOR]

This paper proposes a feedback linearization controller for a grid-following inverter (GFLI) that uses a conventional Phase-Locked Loop (PLL) as the synchronization unit. The proposed controller enhances the GFLI synchronization by expanding the PLL domain of attraction to the whole plane that is limited to a small region around the equilibrium point in a conventional PLL, provided that the grid impedance and voltage are known. Linearizing the overall closed-loop system, the proposed controller provides linear system attributes for the PLL, leading to an infinite domain of attraction and only one equilibrium point. Additionally, a state-feedback controller is integrated within the feedback linearization controller so that it enables the system to have an adjustable dynamic response. Finally, it is shown that the system is robust against parametric uncertainties. The performance of the proposed control design is validated in Matlab/Simulink and experiment. It is verified that the proposed controller expands the domain of attraction and enhances the system dynamic response. [ABSTRACT FROM AUTHOR]

Commutation failures (CFs) including repetitive CFs (RCFs) are frequently occurring faults in line commutated converter based high voltage direct current (LCC-HVDC) systems. Consequently, sending end system will suffer transient voltage disturbances (TVD) with a characteristic of “first reduce then rise and may repeat”. Due to the complex interaction process of TVD, the electromagnetic process behind this phenomenon cannot be easily explained, which imposes great challenges on the development of mitigation strategies. Given this, this paper provides a deep insight into the mechanism and the impact factors of the TVD caused by CFs and RCFs. The numerical relationship between the transmitting power and the TVD is derived. Research results revealed that the transient process after fault can be segmented into five stages. In each stage, the control interactions among HVDC controllers and power interactions at the PCC are analyzed. Besides, we discussed the TVD under RCFs in detail. Several indices are further proposed to facilitate the characterization of the TVD, and main control loops of HVDC that have impacts on the characteristics of TVD are thereby studied. Finally, the mathematical relationship between wind farm power and the TVD is derived to quantify the impact of wind farms on the TVD. [ABSTRACT FROM AUTHOR]

With the emergence of the hybrid multi-infeed high voltage direct current (HMIDC) system, there is complex dynamic interaction between the AC/DC system and converter stations, while the interaction mechanism of the HMIDC system is unclear. For this purpose, this paper establishes the HMIDC linearized model based on the motion equation concept. Then, after decoupling and reconstructing the interaction path of the HMIDC system, the damping coefficients are derived. The AC system strength and voltage source converter (VSC) terminal voltage controller parameters are selected to analyze the impacts on damping coefficients, and the interaction mechanism of the HMIDC system is analyzed. In order to solve the potential instability issue, an additional damping controller in the VSC terminal voltage loop is proposed to enhance the voltage stability of HMIDC system, which is realized by adjusting the reactive power/amplitude coupling damping coefficient. Finally, the RT-LAB-based hardware-in-the-loop experiments verify the accuracy of the interaction mechanism analysis of the HMIDC system and the effectiveness of the proposed controller. [ABSTRACT FROM AUTHOR]

In DC distribution networks, the uncertainties of renewable energies and loads cause frequent voltage fluctuations. In this paper, a voltage regulation strategy based on the combined coordination of centralized optimal control (COC) and adaptive droop control (ADC) is proposed for droop-controlled DC distribution networks. In the COC strategy, the operation reference power of voltage source converters (VSCs) is optimized using the bus injection power measurements to improve the voltage profile and balance the power loading rates of different VSCs. The VSC capacity margin constraint is established to ensure that the capacity margin of each VSC is sufficient to cope with the unexpected fluctuation of loads and renewable energies. Then, an ADC strategy is adopted to handle voltage fluctuation between adjacent two COCs. In the ADC strategy, the operation reference power of the VSCs is further adjusted to recover the voltages. The voltage regulation is achieved based on the relationship among the VSC voltages and the minimum/maximum voltages of the whole network, which does not depend on the information of each nodal voltage and thus real-time network communication is avoided. The simulation results show that the proposed voltage regulation strategies can effectively improve the voltage profile under various uncertain environments. [ABSTRACT FROM AUTHOR]

Fault response of an inverter based resource (IBR) is dictated by its control scheme, making it markedly different from the response of a synchronous source. This has led to misoperation of distance relays when fed by IBRs. Published literature is mostly focused on changing the control design of inverters, but this approach is not practical, as these controls are proprietary and beyond the reach of utilities. To resolve this issue, this paper proposes a method for a source-agnostic distance relay that works in time-domain. Using only single ended measurements, the relay detects, classifies and locates any fault on the line it monitors. The method does not require any polarization technique. It is shown to work for unbalanced lines of any length, for transmission or distribution system with any number (even 100%) of IBRs, is immune to load encroachment, decaying dc offset and prefault currents. With very limited communication, it is also shown to be immune to fault resistance. [ABSTRACT FROM AUTHOR]

The stability problem of low-voltage multi-terminal DC (MTDC) systems has drawn much interest. This paper focuses on the oscillation instability phenomenon of low-voltage MTDC systems and analyzes the interaction behavior between each voltage-source converter and the DC network oscillation mode. The typical structure and control strategy of a low-voltage MTDC system under master-slave control is first described, and its equivalent circuit is established. Then, an interaction behavior analytical model is proposed, and the source-network and network-load interaction characteristics are studied. Furthermore, a system stability analysis method is built based on the modularized modeling, which can solve the problem of how to address the complexity of high-order low-voltage MTDC systems. The transfer function of the whole system is conveniently established, revealing the system instability mechanism. The proposed method can provide effective stability assessment for low-voltage MTDC systems. Finally, the results are verified using electromagnetic transient simulation and experimentally validated using a low-voltage three-terminal DC system test platform with AC 0.4 kV/DC 800 V. This research can be flexibly extended to high-order systems for controller design and performance optimization. [ABSTRACT FROM AUTHOR]

This paper reports the waveform characteristics and parameters of the overvoltage at the terminal of a 1.5 km 10 kV distribution line by direct triggered lightning. The three-phase overvoltage exhibited three kinds of waveforms, all including the initial rising process of the negative peak, the transition from negative peak to the positive peak or the residual voltage, and the subsequent oscillation process. It is observed that the negative peak value of the three-phase overvoltage is between −8.2 kV and −96.1 kV and the rise time of negative peak is between 3.2 μs and 31.7 μs, while the corresponding peak value of the return stroke current is between 3.3 kA and 32.9 kA. When the return stroke current is relatively small, the negative peak value of the direct struck phase overvoltage is twice or more than that of the indirect struck phase. When it is larger, the negative peak value of the indirect struck phase overvoltage is much closer to that of direct struck phase. There is good linear relation between the negative peak value of the indirect struck phase overvoltage and the peak value of the lightning return stroke current, with correlation coefficient being 0.98 and 0.90. [ABSTRACT FROM AUTHOR]

Ultra-high voltage fast vacuum circuit breakers (FVCBs) could benefit a power system by promoting transient stability because their opening and closing times are significantly lower than those of commonly used circuit breakers at transmission voltage. This paper presents the design and verification of a prototype 363-kV/5000-A – 63-kA multiple-break FVCB. The prototype consists of two parallel-connected circuit branches in each phase. Each branch comprises three breaking units. The breaking unit, arranged on a 330-kV-class insulating platform, is a double-break FVCB driven by two independent electromagnetic repulsion actuators. The breaking units include a specially designed 40.5-kV vacuum interrupter (VI) with an extended ceramic envelope and optimized contact contours. A 4.0-nF grading capacitor is connected in parallel with each break to achieve a uniform stressed voltage distribution across the break. The prototype performs controlled switching of short-circuit current with an arcing time of 2.0 ms. The feasibility of the 363-kV FVCB is verified by a dielectric test on one break of the prototype and a series of tests on a single-phase prototype according to the international standard IEC 62271-100, 2017 and the Chinese standard GB 1984-2014. The tests include controlled switching of a short-circuit current in a short arcing time under different transient recovery voltages following test duties T100s(b) and T100a, short-time withstand current and peak withstand current tests, a temperature rise test, and an electromagnetic compatibility test. [ABSTRACT FROM AUTHOR]

The advancements in smart grid technologies had brought forth the prospect of self-healing distribution grid through intentional islanding, aided by the steadily rising penetration of inverter-based distributed generations (IBDGs). Adaptive protection is the key enabler for this resilient grid architecture. For cost-effectiveness, overcurrent relays (OCRs) with adaptive features had been proposed, often with the sensitivity problem due to low short-circuit current magnitude from the IBDG taken into consideration and largely addressed. However, apart from this widely discussed sensitivity problem, there is an underlying issue on selectivity of the OCR which is frequently disregarded when designing an adaptive overcurrent protection scheme for IBDG dominated microgrid. This selectivity problem is mainly caused by the current diversion to load during fault, which is often disregarded when performing OCR grading for a conventional grid. In this paper, this selectivity problem of the OCR is discussed in depth through a comparative study of conventional grid and a modern IBDG dominated grid. The coordination time interval margin violations problems are demonstrated through theoretical calculations and real-time hardware-in-the-loop experiments. This research aims to provide better insights into the selectivity problem as a step towards achieving a cost-effective yet reliable overcurrent-based adaptive protection for IBDG dominated microgrid. [ABSTRACT FROM AUTHOR]

Cascading commutation failure (CF) caused by the reactive power interaction between HVDC is one of the new forms of cascading failures in multi-infeed HVDC systems. However, the concept and mechanism of cascading CF have not been explained clearly, and few studies have been devoted to control strategy for cascading CF mitigation considering reactive power interaction. In this paper, the cascading CF is defined and novel coordinated control strategy for cascading CF mitigation is proposed. Firstly, the mechanism of cascading CF in multi-infeed HVDC systems is analyzed. Then influence degree of HVDC on the power system stability is evaluated considering both active power impact and reactive power interaction, based on which novel coordinated commutation failure prevention as well as voltage dependent dc current order limiter is developed to mitigate cascading CF. Simulation results based on CIGRE HVDC benchmark model in PSCAD/EMTDC verify the effectiveness and advancement of proposed strategy on cascading CF mitigation. [ABSTRACT FROM AUTHOR]

Different from typical circuit breakers, breakers used to switch AC filters in convertor stations have to operate frequently according to the real-time operation mode and daily load changes. Therefore, the performance of filter breakers decreases very quickly, which leads to frequent accidents. Both preinsertion resistors and controlled switching devices can suppress switching transients. In this paper, a comparative analysis of whether a controlled switching device and preinsertion resistor should be adopted is carried out using PSCAD. The transient is calculated as the combined result of the switching angle and preinsertion resistor value, and the optimal configurations are proposed. After determining the resistor value and whether a controlled switching device should be included, the reference voltage of the arrester should be reevaluated. [ABSTRACT FROM AUTHOR]

The single-phase-to-ground (SPG) fault at the valve side of the converter transformer is one of the most critical faults threatening the operation of bipolar half-bridge modular-multilevel-converter-based multi-terminal direct-current (HB-MMC MTDC) systems. Following a valve-side SPG fault, the upper-arm submodule (SM) capacitors in the faulty converter may suffer a serious overvoltage, and the grid-side fault currents may experience no zero-crossings. The existing protection methods generally rely on tripping the grid-side alternate-current circuit-breaker (ACCB), which cannot fully isolate the fault. The healthy poles and stations may encounter severe disturbance and be all blocked, which will lead to a power outage of the entire MTDC system. This paper proposes an ACCB-independent scheme based on thyristor-pairs and damping SMs to address these problems. Antiparallel thyristor-pairs are connected in series within the upper and lower arms in each phase. Serial damping SMs are connected within the lower arms to help the lower-arm thyristor pairs cut off the fault currents. Although the proposed additional devices inevitably cause conduction losses, they can effectively protect the faulty converter and healthy circuit in the MTDC system. Simulations in PSCAD/EMTDC have been conducted to verify the effectiveness of the proposed strategy. [ABSTRACT FROM AUTHOR]

This paper presents a combined control scheme that consists of two complementary control strategies, namely hierarchical control and autonomous control to handle contingencies in a multi-terminal high voltage direct current (MTDC) system. The proposed hierarchical control considers three optimal targets, i.e., equal or proportional power sharing, minimizations of transmission loss and total DC voltage variation. The primary control layer implements the droop control for voltage source converters (VSCs). While in the secondary control layer, voltage reference setpoints of the VSCs in droop control scheme are calculated by the proposed optimal power flow method with the equal or proportional power sharing as equality constraints. As the hierarchical control relies on communication system, the autonomous control for power sharing can be activated during communication system malfunctioning or rapid power variation caused by renewable energy sources. The combined control strategy can greatly increase the reliability of the MTDC system. The simulation results of a five-terminal MTDC network conducted in the MATLAB/Simulink software verify the performance of the proposed combined control strategy. [ABSTRACT FROM AUTHOR]

Traditional single-ended fault location method is influenced by the fault resistance, and the remote in feed, which reflects as the two equations built by fault phase have four unknowns. And in the grid-connected converter system, the weak feedback characteristic further reduces the performance of the method. To solve it, a single-ended fault location method based on control and protection coordination is proposed in this paper. First, using the high controlled-ability of the converter, the control strategy for fault ride-through (FRT) operation is divided into two stages by switching control modes. Then, according to the electrical quantities under two stages, four uncorrelated equations can be established, and the equations still have four unknown parameters. The fault distance can be calculated by solving them. The simulation results show that the proposed method has high accuracy and is immune to fault resistance and fault location, it solves the problem of insufficient equations in principle. [ABSTRACT FROM AUTHOR]

This paper presents a new voltage support (VS) strategy for photovoltaic (PV) systems based on voltage sensitivity analysis to enhance the dynamic voltage stability (DVS) of islanded microgrids (MGs). Every network contains different characteristics. In particular, MGs have unique features compared to utility grids such as high resistance to reactance ratio (R/X). Consequently, injection of both active and reactive current is likely to improve the DVS of the islanded MG. Therefore, instead of engaging the entire capacity to reactive current injection which is performed in conventional voltage support strategy, both the active and reactive current is injected in the proposed strategy. On contrary, without considering the fact that each network contains distinctive characteristics, other strategies with both active and reactive current injection provide fixed active and reactive current which are uniform for all networks. To convey the network distinctive features on VS strategy, voltage sensitivity subject to active and reactive current injection is used in the proposed strategy. As a consequence, unlike other fixed active and reactive current strategies, the proposed strategy provides unique combination of active and reactive current for every network. The concept and derivation of the proposed strategy is clearly described. Network sensitivity analysis is carried out to determine the voltage sensitivity coefficients. The effectiveness of the proposed strategy is theoretically demonstrated and also compared with other strategies. Case studies are carried out on two different islanded MGs. The results show that the proposed voltage support strategy is capable of enhancing the DVS of islanded MGs by mitigating a possible instability, and by restoring the voltage faster than other strategies. [ABSTRACT FROM AUTHOR]

This paper presents a novel phase-tracking robust-synchronization-loop (RSL) to track the phase and frequency of the grid voltage. A detailed analytical model of the proposed RSL algorithm is presented. Further, a state-space small-signal model is developed, including the dynamics of a grid-connected converter and its controller. From the small-signal analysis, it is shown that the eigenvalues associated with the states of the RSL remain in the left half-plane even for a very weak grid condition with a grid impedance of 1.0 pu, thus, it provides better stability property with the grid-connected converters. In order to show the effectiveness of the proposed RSL, simulation results are provided for different scenarios of the grid conditions such as voltage and frequency disturbance, unbalance grid condition and grid faults. Further, the performance of the RSL algorithm is tested by experiment in lab. The results show that the proposed method effectively tracks the phase and frequency of the grid voltage and maintains the good synchronism of the converters during different disturbances. [ABSTRACT FROM AUTHOR]

To improve the reliability of power supply, reclosing schemes are required after transient faults, which commonly occur in overhead line based high voltage DC (HVDC) systems. However, in the event of permanent faults, the auto-reclosing scheme may cause a severe strike. To avoid the severe impacts caused by permanent faults, the fault type should be discriminated before activating the reclosing scheme. Therefore, an adaptive reclosing scheme based on phase characteristics is proposed in this paper. Firstly, the modulation of a periodic voltage by actively controlling the hybrid DC circuit breaker (DCCB) is introduced. Then, a cascaded π equivalent model and its decoupling algorithm are presented to analyze the frequency-domain characteristics of the measured impedance of the coupled overhead lines. From the frequency-domain characteristics, the frequency of the periodic detecting voltage is determined to analyze the phase features of the measured impedance at primary frequency. The permanent or transient faults can thus be accurately identified by using these different phase characteristics, with negligible influence on the healthy lines. In addition, the proposed scheme is robust to various fault resistances, leading to improved reliability. The effectiveness of the proposed scheme is verified in PSCAD/EMTDC. [ABSTRACT FROM AUTHOR]

With the increasing number of controllable devices in the grid, devices owned by different entities are supposed to cooperatively support the grid operation. To balance the various interests of the customers and the utility, this paper proposes a two-level scheduling algorithm to address voltage regulation by coordinating the customer-owned battery energy storage system (BESS) and utility-owned load tap changers (LTCs) in power distribution systems. The objective is to find the optimal operation scheduling for both BESSs and LTCs that simultaneously maximize the economic payback and satisfy system operation rules. For BESSs, a local optimization problem is formulated in a chance-constrained manner in major consideration of the economic performance. For the utility interest, a Markov decision process (MDP) and a computationally efficient reinforcement learning (RL) algorithm are utilized to solve the optimal tap setting problem. To coordinate BESSs and LTCs, a global voltage adjustment is applied. The effectiveness of the proposed algorithm is validated via simulations on the IEEE 13-node and 34-node distribution test feeders. [ABSTRACT FROM AUTHOR]

High penetration level of single-phase solar PVs in low voltage distribution systems has raised concern about the Voltage Unbalance (VU) level. Real-time phase reconfiguration using Dynamic Switching Devices (DSD) has been proposed as a cost-effective solution to mitigate the excessive level of Voltage Unbalance. DSDs are fast power electronics devices that could change the phase connection of a solar PV with minimum disturbances. Most of the proposed methodologies for controlling DSDs are based on a central controller and the assumption of availability of Advanced Metering Infrastructure (AMI) and a communication system. In this paper, we propose a distributed control methodology to control DSDs based on the measurement from the local node only and using the sensitivity of nodal voltage to power injection. Each DSD decides on phase swapping if it rectifies the unbalance index of the local node. We show that decreasing the Voltage Unbalance Factor (VUF) of individual nodes will improve the unbalance situation of the whole feeder. The proposed scheme does not require a communication system nor a central controller. The result of simulations confirms the efficacy of the proposed methodology. [ABSTRACT FROM AUTHOR]

The current-limiting inductors (CLIs) are commonly used to suppress DC fault current to safely trip DC circuit breakers in MMC-HVDC systems, but it significantly jeopardizes DC dynamics and causes instantaneous power mismatch between AC and DC sides of islanded rectifier station (RS), exposing MMC internal states to large disturbance. This paper firstly provides a detailed analysis of the influence of CLIs on AC fault, establishes RS mathematical representation including AC side dynamic, and reveals the mechanism of DC voltage oscillation, modulation saturation and PCC waveform distortion. The analysis shows that the AC and DC fault characteristics are decoupled under unsaturated AC modulation, but coupled under excessively low submodule capacitor voltage, where AC modulation is saturated and PCC waveforms are distorted. To solve this issue, a novel RS AC fault ride through method for MMC-HVDC system with CLIs is proposed to stabilize RS DC voltage, desaturate AC modulation and decouple fault characteristics by regulating the zero-sequence modulation of circulating current suppression. In addition, a current reference disturbance and an AC energy dissipation device coordinated control are designed, which adaptively recovers the submodule capacitor voltage. Finally, the feasibility of the proposed method is verified by simulation results. [ABSTRACT FROM AUTHOR]