Bogdan Marinescu

Synchronverter-Based Emulation and Control of HVDC Transmission

This paper presents a new control strategy for high voltage direct current (HVDC) transmission based on the synchronverter concept: the sending-end rectifier controls emulate a synchronous motor (SM), and the receiving end inverter emulates a synchronous generator (SG). The two converters connected with a DC line provide what is called a synchronverter HVDC (SHVDC). The structure of the SHVDC is firstly analyzed. It is shown that the droop and voltage regulations included in the SHVDC structure are necessary and sufficient to well define the behavior of SHVDC. The standard parameters of the SG cannot be directly used for this structure. A specific tuning method of these parameters is proposed in order to satisfy the usual HVDC control requirements. The new tuning method is compared with the standard vector control in terms of local performances and fault critical clearing time (CCT) in the neighboring zone of the link. The test network is a 4-machine power system with parallel HVDC/AC transmission. The results indicate the contribution of the proposed controller to enhance the stability margin of the neighbor AC zone of the link.

An intrinsic algebraic setting for poles and zeros of linear time-varying systems

In this paper, poles and zeros are defined for linear time-varying systems using suitable ground field extensions. The definitions of the system poles, transmission poles, invariant zeros, hidden modes, etc, are given in an intrinsic module-based framework and are consistent in the sense that the poles are connected to the stability of the system and the zeros to the zeroing of the output for non zero inputs. In particular, it is proved that the necessary and sufficient condition for a continuous-time system to be exponentially stable is similar to the well-known condition in the time-invariant case.

Brief paper: Output feedback pole placement for linear time-varying systems with application to the control of nonlinear systems

The output feedback pole placement problem is solved in an input-output algebraic formalism for linear time-varying (LTV) systems. The recent extensions of the notions of transfer matrices and poles of the system to the case of LTV systems are exploited here to provide constructive solutions based, as in the linear time-invariant (LTI) case, on the solutions of diophantine equations. Also, differences with the results known in the LTI case are pointed out, especially concerning the possibilities to assign specific dynamics to the closed-loop system and the conditions for tracking and disturbance rejection. This approach is applied to the control of nonlinear systems by linearization around a given trajectory. Several examples are treated in detail to show the computation and implementation issues.

Model-matching and decoupling for continuous- and discrete-time linear time-varying systems

Solutions to the exact model-matching and block-decoupling problems for both continuous- and discrete-time linear time-varying systems are presented. The parametrisation of the whole class of proper solutions is given. For the decoupling, the minimal delay problem is also considered in a time-varying setting. The approach is algebraic and based on the Smith–MacMillan form at infinity of a transfer matrix of a time-varying system which has been recently introduced in systems theory. This avoids the difficulties related to the inversion of the transfer matrices with entries in non-commutative fields over which the determinants (of Dieudonné or Ore type) are much more complicated. The solutions presented here involve only standard matrix computations excluding direct matrix inversions and are thus easy to implement in practice. Examples are treated in detail to illustrate the theoretical results and the way in which the computations are done and a physical example is also shown.

Improvement of transient stability in an AC/DC system with synchronverter based HVDC

The HVDC emulation by the synchronverter concept is investigated in a realistic power system. A specific tuning method for the parameters of the regulators based on the sensitivity of the poles of the neighbor zone of the HVDC with the respect to the latter parameters is used. As consequence, not only the local performances of the HVDC link, but also overall transient stability of the AC zone in which the HVDC is inserted are improved. Extensive tests are provided using Matlab/Simulink implementation of the IEEE 9 bus/3 machines test system.

Improvement of fault critical time by HVDC transmission

This paper investigates the impact of High Voltage Direct Current (HVDC) transmission on the transient stability of a two-machine power system, considering three transmission line configurations: parallel HVAC-HVAC, parallel HVDC-HVDC, and a hybrid HVAC-HVDC operation. The faults are balanced three-phase short-circuits in AC lines, and single phase faults on DC lines, applied in the mid-point of the interconnection. For each configuration, transient stability of the AC systems is assessed in terms of the fault critical clearing time (CCT), and for different DC power levels. The results indicate the contribution of HVDC transmission in increasing the critical clearing time; and therefore enhancing the systems stability margin and operational security.

Control of DFIG for wind energy in a network context: A new formulation and interpretation of the control specifications

This paper investigates the interaction of a Doubly Fed Induction Generator (DFIG) based wind system with the other components of the power system, like other rotating machines and branches of the grid. The power delivered by the DFIG is controlled via PI regulators, which are usually designed neglecting the interaction between the grid and the wind turbine, although the grid voltage and frequency cannot be considered as perfectly constant. This study emphasizes the impact of considering a network context on DFIG control performances and the need of a new formulation and interpretation of the DFIG control specifications. The DFIG based Wind Turbine, both the converters and their detailed controls are simulated with a grid benchmark model under faulty grid conditions.

Virtual synchronous generators dynamic performances

In this paper, dynamic performances of virtual synchronous generator are investigated. The virtual synchronous generator is based on the synchronverter (VSG) concept which is an inverter that mimics a synchronous generator (SG). The real and reactive power delivered by VSGs connected in parallel and operated as generators can be automatically shared using the well-known frequency and voltage drooping mechanisms. The VSG method is tested in comparison with the standard SG of the same parameters and capacity on a two bus test network. A comparative assessment is presented in terms of local performances and the fault critical clearing time (CCT). The results indicate good local performances and the contribution of the synchronverter to enhance the stability margin of the neighbouring AC zone.

Robustness and coordination in voltage control of large-scale power systems

In this paper it is shown how the robustness and the coordination of the voltage regulation actions for the transmission grid can be improved. Simpler approaches which ensure higher robustness and performances can be used if the control objectives are pursued at two hierarchical levels of different nature. Also, this is a way to coordinate means of control of different nature with a sufficient time and methodological separation in order to avoid negative mutual influence. At the first level, called the static level, optimal reachable set-points are computed for the second control level, called the dynamic level. The static level can be combined with the shunt reactive power compensation. The system non-linearities are taken into account at the static level while the dynamic level is a linear robust predictive control which takes into account the presence of asynchronous transmission delays. The predictive control strategy is based on the separation property; the output delays are handled using an original steady-state Kalman predictor of order equal to the length of the state of the system without delays. The robustness is improved at the dynamic level against uncertain delays, parametric uncertainties (like, e.g., moderate topological errors and load variations not taken into account in the control model) and unmodelled dynamics. The two-level organisation of the control allows, on one hand, to take into account the important evolutions of the system (like, e.g., large and known topological and load changes) and, on the other hand, a coherent hybrid reactive power control: the switched control of the grid shunt compensation for the reactive power is done at the static level while the reactive power injection provided by the generators is continuously handled at the dynamic level. This is a theoretical analysis of how concepts of automatic control and voltage regulation of power systems can be combined. To be applied as a control scheme, the results presented here should be adapted to a specific context (particularities of the power system and of the organisation of the power industry). They can be used, eventually in conjunction with other improvements, to existing horizontally-organised interconnections (in which all generators of a controlled region can be easily managed since owned by the same utility) or to face specific requirements of moving to the open access in the electric power industry like, e.g., tolerating simplified models in order to cover larger regions, taking into account the interaction between regions, recalibrating set-points, assisting human operator when necessary or facilitating implementation of mechanisms for the management of the reactive power based on price signals.

Quasi-poles of linear time-varying systems in an intrinsic algebraic approach

Abstract In a previous piece of work it has been shown that the exponential stability of a linear time-varying (LTV) system can be evaluated using new definitions of the poles of such a system. The latter are given by a fundamental set of roots of the skew polynomial P ( ∂ ) which defines the autonomous part of the system. Such a set may not exist over the initial field K of definition of the coefficients of the system, but can exist over a suitable field extension K ⊃ K . It is shown here that conditions for stability can also be obtained using linear factors of the polynomial P ( ∂ ) over another field extension K which may be smaller: K ⊃ K ⊃ K . The roots of these factors are called the quasi-poles of the system. The necessary condition for system stability, expressed in function of these quasi-poles, is more restrictive than the one involving a fundamental set of roots.