A controlled power electronic converter can cause local instabilities when interacting with other dynamic subsystems in a power system. Oscillations at a certain frequency cannot, however, build up if the converter differential input admittance has a positive conductance (real part) at that frequency, since power is then dissipated. In this paper, input-admittance expressions for a voltage-source converter are derived. It is seen how the admittance can be shaped in order to get a positive real part in the desired frequency regions by adjusting the controller parameters.

Input-Admittance Calculation and Shaping for Controlled Voltage-Source Converters

The paper proposes several concepts of integrators for sinusoidal signals. Parallel and series associations of the basic PI units using the stationary frame generalized integrators are used for current control of active power filters. Zero steady state error for the concerned current harmonics are realized, with reduced computation, under unbalanced utility or load conditions. Designing of the PI constants, digital realization of the generalized integrators, as well as compensation of the computation delay etc. are studied. Extensive test results from a 10 kW active power filter prototype are demonstrated.

Stationary-frame generalized integrators for current control of active power filters with zero steady-state error for current harmonics of concern under unbalanced and distorted operating conditions

In this paper, a novel control method of grid-connected voltage-source converters (VSCs) is proposed. The method can be generally applied for all grid-connected VSCs but may be of most importance in high-voltage dc (HVDC) applications. Different from the previous control methods, the proposed method utilizes the internal synchronization mechanism in ac systems, in principle, similar to the operation of a synchronous machine. By using this type of power-synchronization control, the VSC avoids the instability caused by a standard phase-locked loop in a weak AC-system connection. Moreover, a VSC terminal can give the weak ac system strong voltage support, just like a normal synchronous machine does. The control method is verified by both analytical models and time simulations.

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Power-Synchronization Control of Grid-Connected Voltage-Source Converters

Control and operation of a dc microgrid, which can be operated at grid connected or island modes, are investigated in this paper. The dc microgrid consists of a wind turbine, a battery energy storage system, dc loads, and a grid-connected converter system. When the system is grid connected, active power is balanced through the grid supply during normal operation to ensure a constant dc voltage. Automatic power balancing during a grid ac fault is achieved by coordinating the battery energy storage system and the grid converter. To ensure that the system can operate under island conditions, a coordinated strategy for the battery system, wind turbine, and load management, including load shedding, are proposed. PSCAD/EMTDC simulations are presented to demonstrate the robust operation performance and to validate the proposed control system during various operating conditions, such as variations of wind power generation and load, grid ac faults, and islanding.

Control and Operation of a DC Microgrid With Variable Generation and Energy Storage

This thesis deals with the analysis, modeling, and control of the doubly-fed induction machine used as a wind turbine generator. The energy efficiency of wind turbine systems equipped with doubly-fed induction generators are compared to other wind turbine generator systems. Moreover, the current control of the doubly-fed induction generator is analyzed and finally the sensitivity of different current controllers with respect to grid disturbances are investigated.

The energy efficiency of a variable-speed wind turbine system using a doubly-fed induction generator is approximately as for a fixed-speed wind turbine equipped with an induction generator. In comparison to a direct-driven permanent-magnet synchronous generator there might be a small gain in the energy efficiency, depending on the average wind-speed at the site. For a variable-speed wind turbine with an induction generator equipped with a full-power inverter, the energy efficiency can be a few percentage units smaller than for a system with a doubly-fed induction generator.

The flux dynamics of the doubly-fed induction machine consist of two poorly damped poles which influence the current controller. These will cause oscillations, with a frequency close to the line frequency, in the flux and in the rotor currents. It has been found that by utilizing a suggested method combining feed-forward compensation and "active resistance," the low-frequency disturbances as well as the oscillations are suppressed better than the other methods evaluated.

The maximum value of the rotor voltage will increase with the size of a voltage dip. This means that it is necessary to design the inverter so it can handle a desired value of a voltage dip. For the investigated systems the maximum rotor voltage and current, due to a voltage dip, can be reduced if the doubly-fed induction machine is magnetized from the stator circuit instead of the rotor circuit. Further, it has been found that the choice of current control method is of greater importance if the bandwidth of the current control loop is low.

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Analysis, Modeling and Control of Doubly-Fed Induction Generators for Wind Turbines

This paper presents a voltage source converter connected to a grid with software specially designed for limited control voltage. The voltage source converter uses a deadbeat vector current controller. The paper deals with limiting reference voltage, integrator windup and delay time compensation. Simulations and experimental verifications of the proposed controller are included.

Vector current controlled voltage source converter-deadbeat control and saturation strategies

This thesis focuses on design and analysis of the control system structure for back-to-back converter squirrel-cage induction machine drives. Particularly, sensorless control of induction machines, meaning vector control without a mechanical shaft sensor, and vector control of pulsewidth-modulated (PWM) rectifiers are considered. The back electromotive force is used as the basis for sensorless control in this thesis. A variant of the classical "voltage model" is adopted for sensorless flux estimation. It is shown that the estimator must be redesigned for the purpose of arbitrarily placement of the closed-loop poles. A thorough stability analysis of the redesigned estimator shows that asymptotical stability can be guaranteed at nominal speeds. The stability at very low frequencies is, however, largely affected by the knowledge of the stator resistance. The presence of a singularity for zero stator frequency is found, which makes it impossible to guarantee stable operation at very low frequencies, except for the case of zero external load torque. The underlying mechanisms behind the two widely acknowledged instability phenomena for sensorless control at low frequencies are revealed. The most critical form of instability is the infamous flux collapse: the flux collapses to approximately zero, giving nearly total loss of torque, and uncontrolled rotation in the direction of the external load torque. The less well-known instability phenomenon frequency lockup is not as critical: the field orientation deteriorates, such that the torque reduces but not vanish, and the stator frequency and rotor speed lock on to constant values close to zero. A control system structure is developed for the PWM rectifier. The previously proposed concept of virtual flux is adopted for grid-voltage synchronization, and three different synchronization algorithms are analyzed. The PWM rectifier is also considered for an active filtering application, for which a vector current control system designed for the deadbeat response is designed. An analysis shows that the resulting deadbeat control system is equivalent to previously proposed Smith predictor structures.

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On Control of Back-to-Back Converters and Sensorless Induction Machine Drives

In this paper, a unified theory for sensorless flux estimation and vector control of induction motors and nonsalient permanent-magnet synchronous motors (PMSMs) is developed. It is shown that an estimator and vector controller for one of the motor types can also be applied to the other, with only minor modifications necessary. Two candidate estimators are considered: a variant of the well-known "voltage model" (VM) and a phase-locked-loop-type speed and position estimator. These are applied to both motor types, and evaluated experimentally. For the nonsalient PMSM, an important result is that synchronization can be guaranteed from any initial rotor position.

Unified sensorless vector control of synchronous and induction motors

In this paper, a grid-connected voltage source power converter with a digital vector current controller is used as a shunt active filter at a moderate switching frequency. The system is investigated analytically as well as experimentally. Four different active filtering methods are compared: two direct methods based on the instantaneous active and reactive power theory; and two selective active filtering methods. Experimental results show that the active filtering methods operate successfully at a moderate switching-frequency. Individual harmonic cancellations up to the 13th order are successful for the selective filters, and also the direct method works satisfactorily when compensating for the time delay of the current controller.

Shunt active filtering of vector-current controlled VSC at a moderate switching-frequency

The voltage sag response of PWM rectifiers is analyzed for a candidate dc-link voltage control system. It is shown that the voltage sag response is mainly determined by the dynamics of the dc-link voltage and the control system. PWM rectifiers are found to be robust against voltage sags, but the remaining grid voltage limits the maximum active power that can be delivered to the utility grid.

Rolf Ottersten

Papers

Vector current controlled voltage source converter-deadbeat control and saturation strategies

On Control of Back-to-Back Converters and Sensorless Induction Machine Drives

Unified sensorless vector control of synchronous and induction motors

Shunt active filtering of vector-current controlled VSC at a moderate switching-frequency

Voltage Sag Response of PWM Rectifiers for Variable-Speed Wind Turbines