Reactive Current Injection Research Articles

계통 연계형 3레벨 TNPC 인버터의 LVRT & HVRT

With the increase in renewable power generation facilities, strict regulations on grid connection standards are being presented. The standards of Low Voltage Ride Through(LVRT) and High Voltage Ride Through(HVRT), which are representative grid connection standards to improve grid stability by maintaining grid connection in low and high voltage situations when a grid failure occurs, are gradually being subdivided. In this paper, the LVRT and HVRT control algorithms suggested in the transmission and distribution system connection regulations of KEPCO(Korea Electric Power Corporation) are applied to the grid-connected 3-level TNPC(T-Type Neutral Point Clamped) inverter. And in the VRT situation, the active and reactive current injection strategies were applied to the grid connected inverter algorithm. In addition, various simulation cases were analyzed, such as LVRT & HVRT conditions of transmission and distribution system. In the distribution system LVRT simulation, the grid voltage operated at 150ms and 1500ms, respectively, within 0.5p.u. and 0.75p.u. In the transmission system simulation, the LVRT operation and the HVRT operation were also simulated. By analyzing various simulation cases, the characteristics of the VRT cases was analyzed in detail and VRT control performance of the designed TNPC inverter was verified.

Coordination of mode switching control and SMES unit for low‐voltage ride‐through fulfillment of power electronic transformer

AbstractPower electronic transformer (PET) should have a satisfying low‐voltage ride‐through (LVRT) ability to cope with different kinds and severities of voltage sags. To solve the LVRT issue, this paper proposes to coordinate mode switching control (MSC) and superconducting magnetic energy storage (SMES), and it is to realize the targets of promoting voltage recovery, keeping DC‐link stabilization, and alleviating current inrush. First, the theoretical modeling of a PET incorporating with a SMES unit is implemented, and the PET’s fault transient characterizes as well as the SMES’s potential supportability is analyzed. Then, the coordination philosophy of the MSC and the SMES is presented. For that the voltage sag reaches a certain threshold, the MSC is enabled to remove the PET’s normal control, and the SMES collaboratively operates to lower the DC‐link overvoltage, while avoiding the PET damage. In the process of the coordination control handling the voltage transients, a proper reactive current injection is carried out at the PET’s output stage to assist the voltage recovery, and the PET can possess a better LVRT margin. Using MATLAB, a 3 MW PET with an 8 H/1.2 MJ SMES unit is modeled, and different voltage sag scenarios and utilization ways of the MSC and the SMES are simulated. The comparative analysis results show that the proposed approach realizes a satisfying LVRT for the PET.

Evaluating Reactive Power-Based Voltage Support for the Low Voltage Ride Through Capability of the Distributed Energy Resource

Most of the grid codes generally require the Distributed Energy Resources (DERs) in order to remain connected throughout short-circuit grid faults and to provide the grid voltage support through reactive power-based regulation via additional Reactive Current Injection (aRCI) during the Low Voltage Ride Through (LVRT). It has been found out that the X/R ratio of the DER connection affects the effectiveness of the voltage support in improving the voltage sags. In this paper, the theoretical analysis of the system's X/R ratio impact on the effectiveness of the investigated voltage support is given. Furthermore, with the help of a MATLAB Simulink-based simulation study, the theoretical analysis is verified. The research is meant to substantiate a clear understanding of the effect of low X/R ratio to the voltage support in improving voltage sag. Finally, the rising number of the DERs on the distribution grid has been linked to the weakening of the grid strength, which often causes the system instability following the post-LVRT state. From the simulation, it has been found out that on a weak grid, the instability during the post-LVRT period could be prevented through correctly adjusted reactive power-based voltage regulation.

계통 연계형 인버터의 LVRT 동작 성능 분석

The scale of renewable power generation facilities such as wind power and solar power generation is being enlarged. Due to the increased scale of renewable power generation facilities, the system stability has been significantly affected by the increased distributed power sources, and various countries such as Germany and the United States are enacting standards to prevent deterioration of system stability. In this paper, we analyze the LVRT (Low Voltage Ride Through) conditions and reactive current injection standards established by Germany and Korea Electric Power Corporation (KEPCO). And, the LVRT control algorithm is applied to the designed inverter, and the operation of the grid-connected inverter was confirmed through simulation based on the LVRT boundary. In addition, the injection characteristic algorithm1 of active and reactive current in LVRT situation was modeled, and the modeled algorithm was verified by checking the simulation performance.

Capacitor placement and sizing to minimize losses in a radial distribution network considering uncertainty using modified affine arithmetic division

In a distribution system, load and distributed generation connected to a feeder are subject to uncertainty. Conventional capacitor placement and sizing methods do not consider uncertainty in power injections, due to which the results may be erroneous. In literature, affine arithmetic (AA) is one of the tools used for incorporating uncertainty in power system analysis. However, conventional division operation in AA gives rise to extra noise terms in the resulting affine form, which are not due to the actual uncertainty sources but are due to the nonaffine operations. The present work incorporates modified AA division which does not generate any additional noises, thereby improving the accuracy. Buses in the distribution network are ranked in decreasing order of rate of change of active power loss in line with respect to the change in the effective reactive power flow in that line. The top three ranked buses are chosen as candidate buses for capacitor connection. The required affine reactive current injection at the candidate buses is calculated, and the corresponding affine capacitive kVAR required is obtained through modified AA-based backward/forward sweep (BFS) power flow analysis. The intervals for the cost incurred in losses, voltage profiles with and without capacitor are calculated, and savings for capacitor connected system is calculated. Switchable capacitors are used for reactive power injection. The proposed method is tested on 15, 33 and 118 bus radial distribution systems. The results show that the proposed modified AA method is accurate than existing interval arithmetic-based method.

Current limiting strategy for grid-connected inverters under asymmetrical short circuit faults

Grid-connected inverter plays an essential role as an interface between energy resources and the power grid. The performance of the inverters is adversely affected by the grid disturbances such as imbalances and asymmetrical short circuit faults. Then, it is necessary to enhance the functionality of the inverter under such conditions. This paper proposes an unbalance current limiting strategy for grid-connected inverters under asymmetrical short circuit fault conditions. In the proposed current limiting strategy, two main features are included: (i) second-order harmonic elimination from instantaneous active power injected into the grid, and (ii) reactive current injection according to the grid code demand. In the proposed current limiting strategy, proper positive and negative sequence current components are calculated, by which the aforementioned goals are achieved. Furthermore, to be compatible with the well-known control structure of the power converters, the current references are obtained in the form that is applicable with the commonly-used double-synchronous-references-frame control structure. To verify the effectiveness of the proposed method, the behavior of the current limiting strategy is examined by simulation of an asymmetrical short circuit fault condition in a test system. In the simulation, both the transient response upon fault occurrence and steady-state of fault condition are considered and analyzed. Furthermore, the simulation results are supported by experimental test results, verifying the effectiveness of the proposed strategy.

Experimental evaluation of an energy storage system for medium voltage distribution grids enabling solid‐state substation functionality

“Smart grids” is a new concept of managing the power transfer in modern grids that will rely not only on the on-line processing of a variety of consumption and generation power data but will also need new hardware to facilitate a flexible and efficient conversion and storage of electrical power to maintain grid stability under fast changing operating conditions. This paper presents the results of the experimental evaluation of a 1.5MJ/25kW energy storage system connected directly to a medium voltage grid to provide fast and flexible grid control capabilities. The demonstrator consists of a supercapacitor stack connected to a low-voltage DC bus via interleaved converters whilst connection to the medium voltage grid is done either via a low voltage inverter and step-up 50Hz transformer or via a cascaded modular multilevel converter fed by dual active bridge converters isolated by medium frequency transformers. The experimental features demonstrated are fast power peak levelling, reactive current injection, grid fault ride through whilst maintaining a high round trip efficiency over a wide power range. The concept on which the demonstrator was designed facilitated the implementation of the solid-state substation with integrated energy storage concept that can further increase flexibility and reliability of future grids.

Comparison of Active and Reactive Power Oscillation Damping With PV Plants

This article proposes a flexible control scheme (FCS) of a photovoltaic (PV) power plant to support the voltage stability and provide fast recovery after the grid faults in a power system with high penetration renewable energy sources (RES). To achieve these goals, the proposed FCS includes two control loops: a reactive current (I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</sub> ) injection loop and a frequency-damping control (FDC) loop. The I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</sub> injection loop injects I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</sub> in proportion to the terminal voltage of the PV during a fault. After the fault clearance, the FDC loop provides active and/or reactive power modulation to stabilize system frequency; then, the FDC loop puts the system recovery forward by damping out the remaining synchronous machines' frequency oscillations. The FDC loop comprises active power damping function without PV power curtailment and reactive power damping function. We verified the proposed FDC's performance in a two-area system for various penetration levels of inverter-based resources using a PSCAD simulator. We investigated the control schemes' effectiveness by comparing the efficiency and the response in mitigating the post-fault oscillations and restoring the voltage regulation.

Validation of EMT Digital Twin Models for Dynamic Voltage Performance Assessment of 66 kV Offshore Transmission Network

The increase in Power Electronic (PE) converters due to the increase in offshore wind energy deployment have given rise to technical challenges (e.g., due to unprecedented fast dynamic phenomena) related to voltage and frequency stability in the power system. In the Offshore Wind Farms (OWFs), the currently available current injection-based voltage control for PE converters are not suitable for voltage control in PE dominated systems due to the absence of continuous voltage control and ineffectiveness during islanding. Moreover, in such power systems, the conventional controllers are not suitable for frequency control due to the absence of dynamic frequency control. The paper presents the Direct Voltage Control (DVC) strategy in a real-time environment to mitigate challenges related to voltage and frequency stability during islanding of OWFs. The control strategy is implemented in the average Electro-magnetic Transient (EMT) model of Type-4 Wind Generator (WG) in RSCAD® Version 5.011.1. It is compared with the benchmark model of the control strategy in DIgSILENT PowerFactoryTM 2019 SP2 (×64) in EMT platform. The comparison based on short-term voltage stability and reactive current injection reveals that both the models provide similar results, confirming the validation of the RSCAD model. Moreover, the detailed representation of the converters in the RSCAD model provides a better depiction of the real-world operation.

Reactive power control of photovoltaic power generation systems by a wide‐area control system for improving transient stability in power systems

AbstractAs the installed capacity of renewable energy sources is increasing, the role of inverters is becoming more important in the maintenance of power system stability. Therefore, in this study, we propose the use of a wide area control system (WACS) to control the inverters of photovoltaic (PV) systems in a coordinated manner to improve transient stability. In this method, we select PV systems whose reactive power injections are deemed to suppress the acceleration of the critical generator, based on a time‐domain simulation for each possible fault before the fault occurs. When a fault occurs, we trigger the reactive current injections with the selected PV systems that is, the WACS operates as an event‐based emergency control system. Numerical simulation results demonstrate that this method can further improve transient stability compared with an autonomous reactive current support function that is applied to existing PV inverters. The proposed WACS could be a promising approach to address the transient instability phenomena under unscheduled power flow conditions by periodically updating the control table based on real‐time information.

Low‐voltage ride‐through control for photovoltaic generation in the low‐voltage distribution network

The increasing penetration of photovoltaic (PV) energy in power grids will impose system instability issues, especially in the occurrence of faults. However, very limited research has been conducted on the low-voltage ride-through (LVRT) control of PV systems in the low-voltage distribution networks (LVDNs) with predominantly resistive line impedances. To fulfil this remaining gap, the effects of active current injection (ACI) on the grid voltage support in LVDN is mathematically analysed in this study. Subsequently, a novel LVRT control scheme for the PV grid-forming inverter is proposed, where the control distinguishes itself from other existing methods due to its optimisation of ACI and PV energy harvesting with the premise of system safety and specified reactive current injection as per grid codes. Multi-mode control modes are involved in the proposed method for dealing with different environmental conditions and voltage dips. Meanwhile, the DC-link voltage is adaptively operated in a self-adjustable control structure for improving grid resilience. The effectiveness of the proposed control method is verified by simulations in MATALB Simulink and hardware experiments on a PV microinverter. Compared with the traditional LVRT control, the post-fault power recovery and voltage support capability can be significantly improved.

Modelling fault behaviour of power electronic converters

Renewable energy sources primarily employ power electronic converters to deliver their energy to the grid. These converter-based distributed generators (CBDGs) have a fault behaviour different from conventional generators, capable of compromising the reliability of traditional protection schemes. This paper investigates the fault behaviour of CBDGs by presenting both an analytical steady state model and a numerical transient simulation model developed in MATLAB/Simulink. Using these models, the impact of grid code requirements regarding reactive current injection during faults is analyzed. The results underline that different modelling techniques can support the design of grid codes.

An Active Distribution Network Equivalent Derived From Large-Disturbance Simulations With Uncertainty

A reduced-order, “grey-box” model of an active distribution network, intended for dynamic simulations of the transmission system, is derived. The network hosts inverter-based generators as well as static and motor loads, whose dynamic parameters are affected by uncertainty. This issue is addressed using Monte-Carlo simulations. The parameters of the equivalent are adjusted to match as closely as possible the average of the randomized responses, while their dispersion is accounted for through the weights of the weighted least-square minimization. A procedure is used to remove from the identification the parameters with negligible impact. To avoid over-fitting, the equivalent is tuned for multiple large-disturbance simulations. A recursive procedure is used to select the smallest possible subset of disturbances involved in the least-square minimization. Simulation results with a 75-bus MV test-system are reported. They show that the equivalent is able to reproduce with good accuracy the discontinuous controls of inverter-based generators, such as reactive current injection and tripping.

Avoiding overvoltage problems in three‐phase distributed‐generation systems during unbalanced voltage sags

During voltage sags, distributed generation systems must fulfil specific grid-code requirements for reactive current injection. This ancillary service can produce overvoltage problems in networks operating in unbalanced conditions when the amplitude of one of the phase voltages is higher than the others and a balanced reactive current is injected through large grid impedance. This study proposes a control scheme to avoid these overvoltage problems, thus reducing the risk of cascade disconnection that this incident may produce. The derivation of the control scheme starts from the flexible oscillating-power control, introduces the necessary modifications so that this control meets the grid-code requirements for current injection and combines it with a slope voltage control to achieve a good voltage regulation. A theoretical analysis is included to determine the expressions that quantify the voltage support characteristics of the proposal. Finally, selected experimental results are reported to validate the characteristics of the proposed control.

Voltage support strategy for PV inverter to enhance dynamic voltage stability of islanded microgrid

This paper presents a voltage support (VS) strategy for grid feeding photo-voltaic (PV) inverter with new coordination between the active and reactive current injection to improve the dynamic voltage stability (DVS) of islanded microgrid (MG). In order to limit the supplied current from the inverter within the maximum allowable current, a new variable, margin residue, is defined in the proposed VS strategy. Moreover, the coordination between active and reactive current is segmented by taking into account the voltage level as well as the margin residue. The DVS of two islanded MGs is investigated to present the effectiveness of the proposed strategy in enhancing it. From the illustrated results, it is shown that the new strategy can provide an improved DVS in comparison with other strategies under similar network conditions.

Power control strategies for grid connected converters applied to full-scale wind energy conversion systems during LVRT operation

Some grid codes establish that during voltage sags, the generation units employing static converters must favor the electrical mains support. Unbalanced voltage sags generate grid power oscillations with twice the fundamental frequency. This way, the converter control can be modified to mitigate the unbalanced sag effects to the grid converter. The present work analyzes different power control methods presented in literature, in order to study how they affect the active and reactive power oscillation and the generated current waveform. A case study with a simulation of a 2(MW) WECS is performed in order to analyze the strategies behavior in terms of power fluctuation and converter dc bus impacts. It is evaluated the system behavior over different operation points of reactive current injection curve, defined in the modern grid codes.

Power flow management scheme of hybrid renewable energy source to maximize power transfer capability using I2HOSOA approach

Due to the intermittent nature of renewable sources, the generation of power is varied which is the main problem in renewable energy system. Miss-matching between the power generation and load power causes a deviation from the desired voltage and frequency in power supply. Therefore, a new efficient smart grid system is required for an optimal power flow management. In this paper, a hybrid approach is presented for power flow management of HRES connected smart grid system. The novelty of the proposed approach is the combined execution of IHHO with SOA named as I2HOSOA technique. In the established work, the HHO is integrated by crossover and mutation function, it is known as IHHO. The main contribution of the proposed strategy is to control the power flow based on source and load side parameters variations. In the proposed approach, the control signals of the voltage source are developed by the IHHO based on the variety of power exchange between the source and load side. Similarly, the online control signals are located by the SOA procedure by utilizing the parallel execution against the active and reactive power varieties. The multi-objective function is shaped by the grid required active and reactive power varieties created based on accessible source power. Here, the control parameters of the power controller are enhanced by the proposed technique based control models in light of the power flow varieties. The comparison between established and existing methods is analyzed in terms of reactive current injection, grid code, current amplitude limitation control, active power control, zero active power oscillations, and injection of active and reactive power. Furthermore, the statistical evaluation of established, and existing methods of mean, median, and standard deviation, is evaluated. Finally, the proposed model is executed in MATLAB/Simulink working platform and the execution is compared with the existing techniques.

A novel low voltage ride through strategy for cascaded power electronic transformer

In view of the operating characteristics for voltage sags of AC side of the power electronic transformer(PET), a low-voltage ride through(LVRT) strategy adapted to bidirectional power exchange of PET is proposed for the purposes of maintaining the system stability, assisting the system voltage recovery and protecting PET safety. During the asymmetric voltage sag, the negative sequence current of PET is eliminated to ensure the symmetry of the injected current. According to the degree of positive sequence voltage sag, the reactive current injection is provided to assist in voltage recovery. According to the PET active power condition before the voltage sag, the level and direction of which are maintained as far as possible without exceeding the limit, for which the disturbance to the AC and DC grids is reduced. Finally, the effectiveness of the proposed LVRT strategy is verified by simulation model.

A Novel Reactive Current Injection (RCI) Control for Microgrid Protection with Inverter Based Distributed Generation (IBDG)

As the development of renewable distributed generations (DGs) is growing rapidly, the autonomous self-healing microgrid had emerged as an effective solution for integrating renewable DGs in the distribution networks. However, before the autonomous self-healing microgrid can be realized, one of the main issues that needs to be resolved is the ability to utilize the most cost-effective protection system—overcurrent relays—to achieve the goal. However, the overcurrent relay is insensitive to the limited fault current contributed by the inverter-based distributed generation (IBDG). Therefore, this paper will propose a novel inverter fault current control with a reactive current injection (RCI) that injects the correct fault current vector, albeit with a limited magnitude, for detection by the cost-effective directional overcurrent relay. This paper will also evaluate the performances of the different RCI controls in delivering an efficient self-healing microgrid protection based on a directional overcurrent relay. The proposed self-healing protection scheme is tested with both a simple distribution test network and also the IEEE 16 bus test system, considering random system parameters like variations in IBDG location, fault location, load capacity and load power factor. Moreover, the performance of the proposed inverter RCI control is also tested under changing weather conditions.

Control Techniques for the Back-to-Back Neutral-Point Clamped Converter in Asynchronous Operation

This paper investigates the operation of the back-to-back neutral point clamped (BTBNPC) converters when the ac voltages on both ends are asynchronous. The zero-sequence injection (ZSI) technique has been used to extend the linear modulation range of the pulsewidth modulation operation. This paper investigates the ZSI of the BTBNPC to maximize the dc-bus voltage utilization while the ac voltages of both ends have different magnitudes and phases. This study further explores the reactive current injection to reduce the risk of overmodulation when grid voltages surge. The combination of these techniques can extend the operation range of the BTBNPC over varying ac voltages of both ends with the available dc-bus voltage. In addition, the issue of the neutral-point balance is also taken into consideration. Laboratory test results are provided to verify the proposed technique.