(VSC) Voltage Source Converter

In PMSG-based WECS, a back-to-back VSC is usually used to link the direct drive synchronous generator and the utility grid. The response of such power electronic converter to a disturbance is characterized by very high frequency phenomena. In spite of this, due to the objective of different studies, a voltage source converter is currently modeled with different ways in power system simulations. The topology of the back-to-back VSC comprises a double conversion from AC to DC and then from DC to AC. For the sake of analysis, only generator-side VSC is represented in the following, and the grid-side VSC is developed in an analogous way. 3.1. Detailed Model

Thanks to the development of the self-commuted devices, new generation of FACTS with better performance characteristics becomes available. The static synchronous compensator (STATCOM) is the first compensator based on voltage source converter (VSC) in FACTS to be conceived. It is shunt-connected, usually with a transformer, to the network and dynamically controlled to output required inductive or capacitive current. The VSC is the fundament component of the STATCOM which is able to convert DC energy to AC and control the frequency, amplitude and phase. Different from thyristor devices which only has turn-on control (turn-off depends on current coming to zero), a VSC consists of devices that possess turn-on and turn-off capability such as Gate Turn-Off Thyristor (GTO), Integrated Gate Bipolar Transistor (IGBT), MOS Turn-off Thyristor (MTO), and Integrated Gate- commutated Thyristor (IGCT) and so forth. These devices have higher power losses but advantages on overall system cost and performance. A VSC is the self-commutating converter in which the DC voltage always has one polarity, and the reversal of power is by reversing the DC current polarity [49]. Among all kinds of topologies, the simplest three- phase VSC is a 6-arm, 2-level converter which is shown in Figure 3.10. It consists of 6 devices (IGBT/diode is taken as an example here) and is connected to a capacitive device at the DC terminal while to the AC terminal usually via a filter.

Abstract—Hybrid multilevel converters are more attractive than the traditional multilevel converters, such as modular converters, because they offer all the features needed in a modern voltage source converter based dc transmission system with reduced size and weight, at a competitive level of semiconductor losses. Therefore, this paper investigates the viability of a hybrid multilevel converter that uses dc side H-bridge chain links, for high-voltage dc and flexible ac transmission systems. Additionally, its operating principle, modulation and capacitor voltage balancing, and control are investigated. This paper focuses on response of this hybrid multilevel converter to ac and dc network faults, with special attention paid to device issues that may arise under extreme network faults. Therefore the hybrid multilevel converter with dc side chain links is simulated as one station of point-to-point dc transmission system that operates in an inversion mode, with all the necessary control systems incorporated. The major results and findings of subjecting this version of the hybrid converter to ac and dc networks faults are presented and discussed.

In this paper, the three-phase voltage source converter with AC side voltage-doubling and DC common mode voltage suppression features is proposed. Due to the introduced IBBC, the DC-link utilization is extended to 1pu on the phase voltage (twice of that in two-level voltage source converter). Consequently, the sizes of DC-link capacitor and interfacing transformer are greatly diminished. Due to the inherent ground on the negative terminal of DC-link, the DC component in CM voltage seen from the AC side neutral point can be avoided. It has been shown that reliance of the proposed converter on IBBC to synthesize negative half of the output voltage introduces second order harmonics in the AC side voltage and current when operated in open loop. The d-q SRF models of the proposed converter when operated in islanding or as a grid connected converter are presented, which are in further used to design the necessary control loops for fundamental power transfer and elimination of the second order harmonic distortions observed during open loop operation. This paper also described the converter operating principle and brief assessments of the voltage and current stresses on the semiconductor switches of the proposed converter. Experimental results have substantiated the claimed attributes of the proposed ACVD-VSC, including its four-quadrant operation.

Fig. 3(a) displays three-phase currents of the M2L-VSC (continuous lines) superimposed on that of the C2L-VSC (doted lines), all measured at the phase interfacing reactors. The plots in Fig. 3(a) indicate that the C2L-VSC draws larger ac currents than the M2L-VSC. The plots for the dc link currents which are measured at the dc terminals of the active power regulators in Fig. 3(b) show that the C2L-VSC contributes larger transient current to dc fault than the M2L-VSC, and this is because of discharge of its dc link capacitor. Nonetheless, the C2L-VSC retains small residual dc voltage across the dc link capacitors compared to practically zero in M2L-VSC case, and as a results, the C2L-VSC exhibits slightly lower steady-state dc fault current than the M2L-VSC. Fig. 3(c) displays the current in the switch S a1 of the upper cell of the M2L-VSC. Notice that the

The power control system (PCS) of VSMES adopts double closed-loop serial control, including a voltage outer loop and a current inner loop. Since the direct cur- rent control strategy of current inner loop is introduced, the dynamic response of VSMES is improved to a large extent and also the anti-disturbance ability. Through the d-q transform of three-phase voltage and based on the target tracing power, the current value in the d-q coordi- nate is obtained, moreover, the target current value needed to be regulated can be derived from the decoup- ling transform of the current above, which achieves the purpose of direct current control [4].

In the last decade, a number of multilevel voltage source converters have been proposed for HVDC applications. Half- bridge (HB) and full-bridge (FB) modular multilevel converters (MMC) have emerged as the preferred topologies for commercial application to multi-terminal HVDC grids. In the meantime, several hybrid converters have been proposed as alternatives that offer important trade-offs between footprint, semiconductor loss, resiliency to AC and DC network faults and control range. Some of these hybrid converters are becoming increasingly attractive as they offer bespoke features, ranging from that of the HB-MMC to the FB-MMC. Although the construction of large-scale DC grids using different converter topologies is important from both practical and market point of view, particularly for prevention of monopoly in supply chains, ensuring safe and reliable operation of DC grids with such diversity, present significant technical challenges; especially, during fault conditions. Meaningful assessments of the interoperability of such complex DC grids, with equipment supplied by multiple vendors, require converter models with steady-state and transient behaviours that accurately resemble the typical behaviour of physical systems. All converter terminals (regardless of their topologies) must be designed to ensure that

Several ultra-high voltage dc (UHVDC) transmission systems based on the current source line commutating converter (LCC) with dc operating voltages up to ±800kV (800kV per pole) and 7200MW rated power have been installed to supply mega cities[1-9]. The choice of LCC is mainly driven by the established track record of LCC in bulk power evacuation over long distances for over 50 years. Proper operation of an LCC-UHVDC link with such large power rating requires the inverter side to be connected to a strong ac network in order to prevent converter commutation failure during ac network disturbances [10, 11]. LCC-HVDC links consumes large reactive power that can reach to 50% or 60% of the transmitted dc power, and it varies with the magnitude of dc power being exchanged between two ac networks [9, 12, 13]. Filter capacitors plus dedicated switched shunt capacitors are widely used to compensate the reactive power of the LCC in a discrete fashion, but this has proven to cause significant instantaneous reactive power mismatch at the filter bus that can create large over-voltages in weak ac networks. This drawback has been avoided in recent LCC-HVDC transmission links installations by replacing the switched capacitors with a line commutating dynamic reactive power compensator that autonomously and seamlessly adjusts its reactive power output in an attempt to maintain constant voltage at the filter bus[6-8, 12, 14-21].

implementation. Among the different droop control methods that can be adopted in DC MGs, two options have been considered in this paper; I-V and V-I droop. I-V droop controls the DC current depending on the DC voltage whilst V-I droop regulates the DC voltage based on the output current. The paper proposes a comparative study of V-I/I-V droop control approaches in DC MGs focusing on steady-state power sharing performance and stability. The paper presents the control scheme for current-mode (I-V droop) and voltage-mode (V-I droop) systems, derives the corresponding output impedance of the source subsystem including converters dynamics and analyzes the stability of the power system when supplying constant power loads. The paper investigates first the impact on stability of the key parameters including droop gains, local control loop dynamics and number of sources and then performs a comparison between current-mode and voltage-mode systems in terms of stability. In addition, a generalized analytical impedance model of a multi-source, multi-load power system is presented to investigate stability in a more realistic scenario. For this purpose, the paper proposes the concept of “global droop gain” as an important factor to determine the stability behaviour of a parallel sources based DC system. The theoretical analysis has been validated with experimental results from a laboratory-scale DC MG.

In this paper, some of the results presented in [16] on the stability study of diagonal dominant systems are applied to the power system context, to assess the stability performance of the inerconnection be- tween a power converter unit and the grid. The approach proposed by the impedance-based stability criterion is taken, and the typically verified diagonal dominance property of the converter-grid system in the sequence-frame is used. A stability margin is presented, which is based on perturbation theory [17]. Compared to the gain and phase margin figures, which are used as a result of treating the system as two decoupled SISO sub-systems, i.e. ignoring the cross-coupling terms of the converter impedance, the proposed stability margin takes such coupling into account. A safer and more conservative evaluation of the system stability robustness can be obtained in this way. Moreover, negative values of the proposed stability margin, which are associated to scenarios where the diagonal dominance property of the system is not verified, are likely to indicate a less performing system, characterised by poorly damped dynamics. The paper is organised as follows. In section 2, a description of the methodology used to model a grid- converter system in the sequence-frame is presented, discussing how its diagonal dominance property is defined and how this is used to define the presented stability margin. Thereafter, in section 3, the study of the absolute and relative stability of a grid-connected Voltage Source Converter (VSC) prototype con- nected to a weak grid is conducted, comparing the SISO stability margins calculated according to the impedance-based stability criterion [8] with the proposed stability margin. Experimental results are also included to support the analytical study. Final conclusions are provided in section 4.

The function of PV array is to convert the solar irradiation which is comes from solar energy into dc power. The MPPT algorithm is also connected to the PV array which allow PV array P-V array to produce maximum power. The unidirectional power is obtained and then changed into ac power with the help of three level inverter and then this ac power is filter through LC filter and fed to utility grid .A boost converter is also to provide link between MPPT and inverter for boost purpose. In order to match inverter output current with the grid voltage and reduce the total harmonic distortion. The voltage source converter is used in this paper.

The function of PV array is to convert the solar irradiation which is comes from solar energy into dc power. The MPPT algorithm is also connected to the PV array which allow PV array P-V array to produce maximum power. The unidirectional power is obtained and then changed into ac power with the help of three level inverter and then this ac power is filter through LC filter and fed to utility grid .A boost converter is also to provide link between MPPT and inverter for boost purpose. In order to match inverter output current with the grid voltage and reduce the total harmonic distortion. The voltage source converter is used in this paper.

With the increased levels of offshore wind power penetration into power systems, the impact of offshore wind power on stability of power systems require more investigation. In this paper, the effects of a large scale doubly fed induction generator (DFIG) based offshore wind farm (OWF) on power system stability are examined. The OWF is connected to the main onshore grid through a voltage source converter (VSC) based high voltage direct current (HVDC) link. A large scale DFIG based OWF is connected to the New England 10- machine 39-bus test system through a VSC-HVDC. One of the synchronous generators in the test system is replaced by an OWF with an equivalent generated power. As the voltage source converter can control the active and reactive power independently, the use of the onshore side converter to control its terminal voltage is investigated. The behaviour of the test system is evaluated under both small and large grid disturbances in both cases with and without the offshore wind farm.

In comparison with the requirements of protection schemes in VSC-HVDC networks, fault location is less time-critical, and is more focused on accuracy. From this point of view, the simulation results, in addition to their poor performance under resistive ground fault conditions indicate that the active impedance method is not as reliable in locating the faults in VSC-HVDC systems. In contrast, the TW based methods using CWT are capable of accurately estimating the fault position for various fault scenarios occurring at different locations. They are also applicable to hybrid systems where a two-ended method is applied. In addition to accuracy, there are also other aspects which need to be considered when committing to choice of fault locating techniques implementation. The advantages and disadvantages regarding different perspectives of the two approaches can be summarised as follows:

Plans have been proposed for the interconnection of various offshore wind farms into a HVDC based European Supergrid with the clear aim of satisfying a large portion of the power demand throughout Europe with renewable generation [1–3]. Due to the numerous control and network support capabilities offered, Voltage Source Converter (VSC) based HVDC (VSC-HVDC) transmission has become a widely acknowledged technology for carrying out such a task. Thanks to the system control capabilities of VSC, there is the potential to interconnect numerous VSC-HVDC lines into a single multi-terminal DC (MTDC) network that can allow the efficient pooling and dispatch of energy over a vast geographical area [4–6].

DOI: 10.4236/epe.2019.113007 103 Energy and Power Engineering In this paper, we considered a microgrid as a small single-area power system where the inertia constant ( H ) is highly time variant and fluctuates between 3 and 6 s due to the deployment of Wind-PV-Wave power generation. The idea of using ultra-capacitor (UC) at the DC-link to emulate a virtual inertia of the VSG is implemented while the droop control is emulated by the battery as a virtual speed governor. A hypothetical inertia coefficient is implemented in the control loop which defines a time constant of the capacitor to respond during the transient processes. Due to the inertia coefficient, the UC responds quickly to the transients and protects battery for a fast discharge. This combination of the ESS connected with the UC at the DC-link forms a DC-grid structure. The proposed scheme controls the three phase inverter connected with the ESS with a DC-DC controller in parallel and this structure operates as VSG as shown in Figure 1. A CSD-scheme control for the voltage and the frequency regulation at the point of common coupling (PCC) is presented. To regulate the PCC voltage and the frequency of the system, a dynamic control estimates the fundamental reference source active and reactive power components. The proposed scheme has a bi-directional capability of active and reactive powers flow which controls the frequency and the terminal voltage at the PCC. In Figure 1, the components are shown in detail: a WEC as a RES, a battery based ESS and a power converter followed by the AC-microgrid through a harmonic filter. In this study, a WEC enables the system to harvest energy directly from a buoy or a floater by ocean waves, as discussed in detail in [44] [45] [46] [47] [48]. The performance of the controller is demonstrated under varying loads and RES power conditions.

This paper is proposing and evaluating experimentally the performance of a hybrid converter solution, shown in Fig. 1 that consists of a slow switching medium to high voltage inverter whose switching harmonics are actively filtered by an auxiliary inverter having ultra-low installed power as a result of significantly lower voltage and current ratings compared to the main inverter. This can be a solution suitable also for retrofitting older implementations that cannot comply with newer power quality standards/limits. The auxiliary inverter is a Current Source Inverter (CSI) which is known to be capable of directly synthesising the AC current reference without the need of a current control loop, therefore requiring the lowest switching frequency and causing less switching losses. This aspect is very important as the reference current is the switching current ripple of the main inverter (hundreds Hz-few kHz). The use of a series capacitor to cancel most of the fundamental (50Hz) voltage enables the use of standard/fast switches with ratings up to 1.2kV in a medium voltage rated converter.

In a CPT system, a high frequency voltage is desired to drive the electric field coupler so that the alternating current can flow through it to provide the load with the required power. Therefore, a Class-E converter is designed as the high frequency converter at the transmitter to convert DC source to AC. The best converter is selected

Figure 7 shows that CFII is generally slightly improved, but not significantly, with an SVC connected at the PCC. The rating of the SVC doesn’t appear to have a significant effect on the CFII improvement. This seemingly poor performance may be due to the speed at which the SVC reacts to the fault on the system. As both LCC HVDC and SVC use thyristor technology they will have very similar response times. If the LCC system cannot respond fast enough to avoid a commutation failure by adjusting the rectifier and inverter firing angles then the SVC system is unlikely to be able to react much faster to support it significantly. Another potential cause for the apparent non-improvement in performance is that the switching of TSR or TSC banks could cause further unwanted disturbance to the AC voltage waveform due to the sudden large increase/decrease of reactive power. This could be mitigated by ensuring that no large banks of components are switched if a commutation failure is likely, but this effectively reduces the reactive compensation available to support the voltage.

This paper assesses the suitability of ACVD-VSC applied as a STATCOM, where it is able to synthesize higher ac voltage from a given dc-link voltage, surpassing the dc voltage limit and approaches the current limit for reactive power generation, in contrast to the conventional two-level VSC counterpart. Thus, the reactive power generation range of the STATCOM can be achieved using the ACVD-VSC. Also, in the proposed solution, the negative dc bus can be grounded without introducing any dc voltage stress to the transformer neutrals.