Islanding detection in grid-connected power converters using harmonics due to the non-ideal behavior of the inverter

This paper analyzes the use of the voltage distortions in PWM voltage-source-inverters (VSIs) caused by the non-ideal behavior of the inverter for islanding detection purposes. The non-ideal characteristic of the inverters, mainly due to the dead-time needed to have safe commutations, produces fundamental frequency dependent harmonics (-5th, 7th...) in the output voltage. Although these harmonics are in principle an unwanted effect, since they reduce the power quality, they can potentially be used for islanding detection purposes. The physical principles of the method would be the same as for high frequency signal injection methods that have already been proposed but without the need of injecting a high frequency signa

Sensorless control of doubly-fed induction generators based on stator high frequency signal injection

High frequency signal injection based methods have been widely investigated for sensorless position/speed control of induction machines (IMs), permanent magnet synchronous machines (PMSMs) and more recently for doubly fed induction generators (DFIGs). When used with IMs and PMSMs, the high frequency signal is injected in the stator windings, an asymmetric (salient) rotor being required for this case. Contrary to this, both stator and rotor terminals are accessible and sensored in DFIGs, being therefore possible to inject the high frequency signal either in the stator or the rotor terminals. As consequence of this, the method can be used even if the machine is non-salient. In the implementation of the method with DFIGs, the high frequency voltage signal is typically injected in the rotor, the high frequency components (voltages of currents) induced in the stator being used for rotor position estimation. A drawback of this alternative is that the method is sensitive to the grid impedance in the stator side, which will be affected by the grid configuration, and is normally unknown. This paper proposes the sensorless control a DFIG injecting the high frequency voltage in the stator side, and using a high frequency current cancellation strategy in the rotor side. The main advantage of the proposed strategy is that the estimated position is independent of the grid characteristic

Coordinated operation of parallel-connected inverters for active islanding detection using high frequency signal injection

The high frequency impedance measured at the terminals of inverters connected in a microgrid by means of the injection of a small magnitude, high frequency voltage, has been shown to be a reliable metric to detect islanding. While the implementation of this method is simple when only an inverter injects the high frequency signal, this case is of limited applicability. On the other hand, several concerns arise when multiple inverters work in parallel, primarily due to risk interference among inverters. Islanding detection using high frequency signal injection in microgrids with multiple parallel-connected inverters is studied in this paper. A strategy for the coordinated operation of the inverters, without the need of communications or pre-established roles is proposed. Simulation and experimental results will be provided to demonstrate the viability of the concep

Islanding detection in three-phase and single-phase systems using pulsating high frequency signal injection

This paper analyzes the use of pulsating high frequency signal injection for islanding detection purposes. Active islanding detection using high frequency signal injection is an appealing option due to its reduced non-detection zone, reduced cost and ease of implementation. The use of a rotating high frequency signal has been reported and analyzed. However, this method can only be applied to three-phase systems. In this paper, the use of a pulsating high frequency signal injection is proposed. While it uses the same principles as rotating signal injection, it can be applied to both threephase and single-phase system

Operation and control of MMCs using cells with power transfer capability

Cells in conventional Modular Multilevel Converters (MMC) designs use a capacitor for energy storage. This means that the net power balance for each cell (neglecting losses) needs to be equal to zero, the MCC realizing therefore a power transfer between its DC and AC sides. This paper analyzes the design, operation and control of MMCs in which the cells have the capability to transfer (inject or drain) power. The use of such cells opens several new functionalities and uses for the MMC. On one hand, it would allow integrating elements like distributed energy storage (e.g. batteries), low-voltage/low power sources (e.g. PV) and loads at the cell level. Cells with power transfer capability can also be used connect the medium/high voltage DC and AC ports intrinsic to the MMC, with low voltage DC/AC ports at the cell level. This would result in multiport power converters, potential applications of this topology including solid state transformers (SST

Control strategies for MMC using cells with power transfer capability

IEEE Energy Conversion Congress and Exposition, ECCE 2015 (2015. Montréal)Conventional MMCs use cells which typically consist of a half-bridge and a capacitor. Due to their limited energy storage capability, the net power balance of the cells is zero (neglecting losses), the MMC therefore realizing a bidirectional power transfer between its DC and AC ports. It is possible however to provide the MMC with the capability to transfer power at the cell level. The use of such cells opens new functionalities and uses for the MMC, including integration at the cell level of distributed energy storage (e.g. batteries), low-voltage/low power sources/loads, and its operation as a multiport power converter, combining high and low voltage AC and DC ports. Existing control strategies for MMCs assume that all the cells have an identical design and operate identically. However, use of cells with power transfer capability can result in imbalances in their operation, provided that not all the cells transfer power, or that they do not transfer the same amount of power. This paper addresses the design and control of MMCs using cells with power transfer capability, with special focus on the design of suitable control strategies and on the definition of their limits of operatio

Design and implementation of the control of an MMC-based solid state transformer

Implementation of the control of a Solid State Transformer (SST) is described in this paper. The SST topology considered is derived from a Modular Multilevel Converter (MMC), in which the cells have the capability to transfer (inject or drain) power. The MMC is combined with an isolation stage formed by Dual Active Bridges (DABs) and a DC/AC power converter. The resulting modular multiport power converter can connect both high voltage and low voltage AC and DC ports, providing isolation between the high voltage and the low voltage terminals, and with full control of the power flow. Implementation of the control of this power converter is not trivial, due to the large amount of power devices and sensors involved, and to the complexity of the control algorithms. Furthermore, the need to provide isolation among the different stages adds further concerns mainly related with cost. This paper discusses the configuration, selection of the required hardware, as well as implementation aspects for the control of the proposed SST topolog

Operation of modular multilevel converters under voltage constraints

MMCs are normally designed to operate in the linear region of the PWM. This limits the peak-to-peak phase voltage in the AC port to be lower than the DC port voltage. It is possible to increase the AC voltage beyond this limit by the use of overmodulation strategies. However, this is at the price of an increase in the harmonic content (THD) of the voltages and currents, and consequently, of a decrease of the power quality. While this type of operation is not desired in normal conditions, there are exceptional circumstances in which the MMC could be forced to operate in this mode. These would include transient anomalies, e.g. a temporary decrease of the DC port voltage or a temporary increase of the AC port voltage, or quasi-permanent conditions, e.g. the failure (and subsequent disconnection) of one or more cells in one or more arms of the MMC. Under this circumstances, the voltage margin between the DC and the AC port voltages required for the normal operation of the MMC might be lost. Consequently, the MMC should operate in the overmodulation region, or turned-off otherwise. This paper addresses the use of overmodulation techniques in MMC under voltage constraints. Under these circumstances, the MMC control should guarantee stable operation, (i.e. a controlled power transfer between the DC and AC ports with the cell voltages maintained at their target values) and minimize the distortion of the currents, and consequently the adverse effect on the power qualit

Dynamic behavior of current controllers for selective harmonic compensation in three-phase active power filters

Current regulators are a critical part of active power filters (APFs). The design of current regulators capable of compensating high-frequency harmonics created by nonlinear loads is a challenging task. Selective harmonic current compensation using harmonic regulators is a viable method to achieve this goal. However, their design and tuning is not an easy task. The performance-and even the stability-of harmonic current regulators strongly depends on implementation issues, with the tuning of the controller gains being critical. Furthermore, the presence of multiple current regulators working in parallel can create unwanted couplings with the fundamental current regulator, which can result in a deterioration of APF current control, i.e., oscillations and settling times larger than expected. This paper addresses the design and tuning of selective harmonic compensators, with a focus on their stability analysis and transient behavio