Sebastian A. Richter

Active damping of LCL resonance with minimum sensor effort by means of a digital infinite impulse response filter

This paper presents a concept for active damping of an LCL filter connecting a digitally controlled PWM converter to the grid. Active damping is performed by means of a digital Infinite Impulse Response (IIR) filter. Digital filters, being well known in digital signal analysis, can easily be implemented when using digital controllers and require very low computational effort. In contrast to active damping solutions found in literature, no extra quantities except the controlled current must be sensed. This functionality is proven via experimental results. After a short introduction of the problem and an overview of solutions found in literature, the implemented fundamental procedure for active damping is described. Subsequently, the main idea of using an IIR filter for active damping, being an approximation to a derivative function, is qualified and the design procedure is presented. In this paper it is stated that with this method the sensors are minimized and that due to the approximation of the derivative function the damping performance is lower compared to solutions found in literature. However, the damping effect by using an IIR filter is proven. The use of IIR filters and the reduction of sensor effort for active damping of EMI filters with multiple passives looks promising also for other applications with a known load.

Control of a medium-voltage test generator

This paper describes the design of a digital control for a 3.3 kV, 1 MVA three-level PWM converter operating as test generator for disturbances in medium-voltage grids according to standards like IEC 61000, EN 50160, and IEEE 1547. The proposed control combines state-feedback with additional dynamic output feedback. It is designed fully in the stationary reference frame to avoid processing of complex transformations into different rotating reference frames. Resonant controllers provide zero steady-state error for up to ten harmonics. A DSP platform is used for implementing the control. It replaces the control unit of a commercial converter. Experimental results of a downscaled breadboard construction verify the functionality of the control.

Unified Control Strategy Covering CCM and DCM for a Synchronous Buck Converter

In general, transfer functions of switch-mode converters are treated differently when operating in continuous-conduction mode (CCM) or discontinuous-conduction mode (DCM). This complicates the development of a control algorithm for a converter operating in both operating modes. To solve this problem, a control algorithm is developed, which is applicable for CCM and DCM. Since the developed control algorithm is based on linear equations instead of differential equations, it is easy to implement and its calculation effort is minor. It is based on equating the volt-seconds across the inductor for both operating modes and adjusting the duty cycle d. All necessary equations are derived and the functionality of the developed algorithm is shown by means of simulation and measurements.

Design of a contactless rotary energy transmission for an industrial application

Till now, the common practice of transferring energy from a stationary to a rotating plane is realized via slip rings. These systems exhibit a small mechanical size but limit the achievable rotational speed. Due to abrasion, the lifetime of slip rings is limited. Contactless rotary energy transmissions can overcome these problems. They are not affected by fouling and are theoretically maintenance-free. This paper deals with the development of a contactless energy transmission for a drilling tool. The developed energy transmission transfers up to 200 W. The transferred energy is used to drive a piezoelectric actuator, which in turn controls the position of the cutting edge of the drilling tool. First, the mechanical constraints of this application are given and the consequences for the energy transmission are derived. Afterwards, the topology most suitable for this application is selected. Subsequently, the different methods to solve the difficulties, which arise from the mechanical constraints, with their advantages and disadvantages are compared and their influence on the design of the components is shown. At the same time, it is determined which degree of mechanical dislocation is allowed before the system will trip.

Discrete time modeling, implementation and design of current controllers

This work addresses discrete time modeling, implementation and design options for the current control of three phase grid tied PWM converters. Based on an accurate discrete time model of the PWM converter, closed loop current control is reviewed from the perspective of the synchronous and stationary reference frame. Then, implementation options for the synchronous frame proportional integral (SFPI) regulator and the proportional resonant (PR) regulator are discussed leading to the formulation of a general controller framework based on space vector resonators: the Resonant Space Vector (RSV) regulator. It can embody multiple SFPI regulators on different frequencies and allows a consistent design of state feedback controllers as well as an efficient implementation. For this control framework an insightful design procedure based on the root locus method for complex system models is introduced. Finally, the performance of the presented control design and implementation concepts is demonstrated experimentally.

Inverter based testbench for photovoltaic inverters in compliance with medium-voltage grid codes

Recently, the market for renewable energy sources like wind and photovoltaic have been growing rapidly and the total installed power has the range of conventional power plants. This leads to a need for power flow control especially under certain conditions e.g. grid failures and voltage rise along the power line. What happens if the line voltage or the frequency increases due to an excess power supply? Should the power source disconnect from the grid in the event of a voltage dip? All this questions have to be answered. Therefore, new grid codes have been established to have a standard which has to be fulfilled by all new high-power wind and photovoltaic inverters. In compliance with these new standards PV inverters have to be checked with test benches. This paper presents a new switching-inverter based test bench which has high dynamics and is capable of testing devices up to 500kW. It was recently used for certification measurements and is much cheaper than commercial linear amplifier test benches [1]. This paper focuses on fault-ride-through and frequency shift tests.

Control of a high power PWM current source rectifier

This paper describes the design of a digital control for PWM current source rectifiers (CSRs) in the range of 1 – 10MVA. A cascaded control structure is proposed: the inner loop controls the grid currents and the outer loop controls the dc-link current. The proposed grid-current control combines state feedback for active damping of the LC-filter with a servo compensator to increase robustness and guarantee zero steady-state error. It is implemented in the stationary reference frame, which avoids cross coupling as introduced by rotating reference frame transformation. The servo compensator contains a digital resonator and an integrator that eliminates offset currents. It is efficiently implemented as digital filter. The time varying reference for the instantaneous grid current is generated by a disturbance observer for grid voltage. This closed-loop technique inherently compensates the voltage drop across the filter inductance. DC-link current is controlled by a PI-controller. The power factor of the CSR is dynamically adjustable. The proposed control design is verified using a CSR prototype with 11 kVA rated output power. Experimental results are given.

Unified Control of a Buck Converter for Wide-Load-Range Applications

A new discrete-time state feedback controller is presented, which allows high-bandwidth voltage control of a buck converter for any load condition, whether it operates in discontinuous conduction mode (DCM), continuous conduction mode (CCM), or at the boundary of these regions. This makes the buck converter applicable for a wide range of applications. For the control design process, two large-signal models, which represent the buck converters discrete time dynamics in CCM and DCM, are developed. A simple proportional-integral regulator is used for the voltage control of the converter. The operation mode is detected and the voltage controller is connected in cascade to a current controller in CCM or to a nonlinear state feedback decoupling structure in DCM. In this paper, the modeling and design of the proposed control topology are introduced and its performance is demonstrated in simulation and experiment.

Unified control of a buck converter for wide load range applications

A new discrete time state feedback controller is presented, which allows high bandwidth voltage control of a buck converter for any load condition whether it operates in discontinuous conduction mode (DCM), continuous conduction mode (CCM) or in the boundary of these regions. This makes the buck converter applicable for a wide range of applications. For the control design process two large signal models are developed, which represent the buck converters discrete time dynamics in CCM and DCM. A simple PI regulator is used for the voltage control of the converter. The operation mode is detected and the voltage controller is connected in cascade to a current controller in CCM or to a nonlinear state feedback decoupling structure in DCM. In this work, the modeling and design of the proposed control topology are introduced and its performance is demonstrated in simulation and experiment.

Variable-Frequency Pulse-Width Modulation for an Improved Grid-Connected Converter Efficiency

This paper presents a high-efficient three-phase DC- AC grid-connected converter rated at 150 W. Two fundamental aspects led to the high efficiency, e.g. 96.2% at nominal power which corresponds to only 5.9 W losses. First, over the whole operation range, the converter operates as a resonant pole inverter (RPI), thus zero voltage switching at turn on is achieved. Second, the variable-frequency pulse-width modulation is optimized to a minimum rms-current in the filter inductor, leading to minimized conduction and inductor losses. Different experiments qualify the losses of the circuit components. A comparison between the conventional RPI and the PWM converter is given.