Igor Kondratiev

Synergetic control for DC-DC buck converters with constant power load

An algorithm for design of synergetic control is derived, using as an example a system containing buck choppers with constant power load J.G. Ciezki et al. (1998). The closed loop system behavior is compared to that of the same system controlled by feedback linearization control. As a result, it is shown that synergetic control has better dynamics and a faster response. This system nullifies steady state error not only of output voltage but also in current sharing among parallel converters.

Power sequencing approach to fault isolation in dc systems: Influence of system parameters

We show that medium-voltage dc power buses can be protected against short circuit faults by coordinating the action of a converter that supplies power to the bus with the action of contactors that are used to reconfigure the bus connections. Following a fault, the bus is de-energized (so there is no large current to interrupt), one or more contactors are reconfigured, and the dc bus is then reenergized. For a typical industrial dc bus, we show that it is possible to execute this de-energize-reconfigure-re-energize process 10 times faster than an AC bus can be protected and reconfigured using traditional circuit breakers. We show how the de-energizing and reconfiguring times depend on the output capacitance of the main converter and on the distance to the fault, and we show how to size each hold-up capacitor so that loads on unfaulted circuits can ride through the process uninterrupted.

Robust nonlinear synergetic control for m-parallel-connected DC-DC boost converters

This paper presents synergetic control design for an m-paralleled boost converter system under active current sharing. The presented design overcomes such problems of the system as multi-connectivity, nonlinearity, and high dimensionality. The set of macro-variables introduced during control design defines a set of invariant manifolds that accommodate different current sharing regimes such as master-slave and democratic current sharing simply by changing coefficients in the macro-variables. Invariant manifold formation contracts the system state space and makes it possible to perform stability analysis for an arbitrary number of paralleled boost converters. The stability analysis and current sharing analysis presented in this paper provide criteria for the choice of control law coefficients. The simulation results for a two-converter system validate the theory and show that the closed-loop system is characterized by stable operation, fast response, good voltage regulation, ability to nullify steady state error not only of output voltage but also of current sharing, capability to control the current drawn from each converter, ability to change current sharing during operation, and, most importantly, capability to withstand variation of the system parameters by 400%.

Nonlinear Synergetic Control for m Parallel-Connected DC-DC Buck Converters: Droop Current Sharing

In this paper we consider a general case of an m-paralleled buck converter system feeding dependent current source, which can easily represent resistive, constant current, and constant power loads. For control design, we use synergetic control theory, which is based on ideas of self-organization. The theory allows designers to derive analytical control laws for nonlinear, high-dimensional, and multi-connected systems. In the paper we derive general nonlinear PI control algorithms for the m parallel-connected DC buck converter under droop current sharing nullifying steady state current sharing error and improving voltage regulation. We study the stability of the closed loop system under different loading conditions. We clarify the choice of control law coefficients, and we show the benefits of dynamic power sharing provided by synergetic control design. The simulation results presented in the paper show good agreement with the theory.

Invariant based ship DC power system design

This paper introduces an alternative to traditional design approach - a design which provides the system components with the rules of interaction (invariants) and uses computational resources already available in the controller to implement these rules. We obtain rules of in-system interaction by analyzing power sharing in m-paralleled connected power sources feeding a complex load. We illustrate the design using the example of a system with an arbitrary number of paralleled buck converters feeding a constant power load. We introduce the found invariants into the system using synergetic control theory, analyze the closed loop performance, and define the stability conditions as well as conditions for compensating impact of constant power load, Finally, we exercise the design on a two-converter system, where we illustrate power sharing capabilities.

Controlled power sequencing for fault protection in DC nanogrids

For dc power distribution systems, we show how coordinated operation of electronic power converters and mechanical contactors (segmentizing switches) can permit rapid response to short circuit faults. It is possible to de-power the dc bus, open a contactor to isolate the faulted branch, then restore power to the remaining branches in less than 8 ms, so that the remaining loads can ride through the entire operation. This is faster than can typically be achieved in ac power networks using conventional technologies. Clamping diodes and point of load capacitors improve system robustness. Our results provide a detailed analysis, design guidelines, and explain how the protection scheme responds to different configuration of the system.

Ground fault protection for DC bus using controlled power sequencing

We present a new approach to protect a multi-branch medium-voltage DC bus from ground faults. Behavior analysis of a DC distribution scheme shows that it is possible to quickly de-energize the DC bus by controlling the main converter, open a contactor to remove the faulted line, then re-energize the system. This strategy reduces the out-of-service time and avoids the use of mechanical circuit breakers and their arc-eliminating equipments. The fast response time allows unfaulted loads to ride through the process with minimal local energy storage. Our research shows that it is possible to de-energize, reconfigure, and re-energize a typical industrial DC bus 10–20 times faster than methods traditionally applied in AC circuits, and that only 470 µFarad of bus capacitance is needed for unfaulted circuits to ride-through the process uninterrupted.

Branch circuit protection for DC systems

We present a new approach for protecting DC power distribution circuits against faults and negative incremental impedance instabilities. Like a circuit breaker, the device is passive until a fault occurs. Unlike a circuit breaker, the device operates in current limiting mode or in impedance transformation mode, according to the system requirements, and it can serve as a power buffer during transient upstream disruptions. The approach permits coordination between hierarchical levels of protection, it enables system reconfiguration, and it increases system stability. All three types of protection are achieved automatically by the controller based solely on local current and voltage measurements. The efficacy of this solution has been demonstrated through simulation. System stability with and without the proposed protection system has been analyzed according to the Brayton-Moser mixed potential criterion. The approach is proven to increase the stability of the systems in all configurations.

Synergetic control for induction motor based wheel-drive system

The paper presents the first anti-lock braking system controller for an induction motor based wheel-drive system that was designed using Synergetic Control Theory. The presented control algorithm not only ensures asymptotic stability of the closed-loop system in the entire range of admissible operating conditions but also insensitivity of the system to the change of coupling properties between tire and the surface of the road. Moreover, the operation of the control does not require identification of the traction coefficient between the tire and the road surface. Simulation results show that the proposed control strategy operates the wheel at the specified slip when the traction coefficient varies by 400%.

Protection of medium voltage DC power systems against ground faults and negative incremental impedances

Widespread concern about how to protect DC power distribution systems against high fault currents, and how to compensate for instabilities brought on by constant power load characteristics, has prompted us to develop a new approach for protecting these systems. Our approach employs a compact multiple switching topology converter, which fulfills three functions: it limits the line current to a predetermined value (which could be dynamically set); it works as buffer during short-duration faults on the power bus; and it compensates load instabilities that could arise due to the constant power characteristic of a load. The structure of the protection circuit, its positioning in the distribution network, its possible configurations, control strategies, and parameters selection will all be shown. Analysis of the performances and feasibility of the approach will be presented.