Identification and small-signal analysis of interaction modes in VSC MTDC systems

IEEE Trans. Power Delivery

Suul

et al. 2018

Self Cite

Abstract-The DC-side dynamics of Modular Multilevel Converters (MMCs) can be prone to poorly damped oscillations or stability problems when the second harmonic components of the arm currents are mitigated by a Circulating Current Suppression Controller (CCSC). This paper demonstrates that the source of these oscillations is the uncontrolled interaction of the DC-side current and the internally stored energy of the MMC, as resulting from the CCSC. Stable operation and improved performance of the MMC control system can be ensured by introducing closed loop control of the energy and the DC-side current. The presented analysis relies on a detailed state-space model of the MMC which is formulated to obtain constant variables in steady state. The resulting state-space equations can be linearized to achieve a Linear Time Invariant (LTI) model, allowing for eigenvalue analysis of the small-signal dynamics of the MMC. Participation factor analysis is utilized to identify the source of the poorly damped DC-side oscillations, and indicates the suitability of introducing control of the internal capacitor voltage or the corresponding stored energy. An MMC connected to a DC power source with an equivalent capacitance, and operated with DC voltage droop in the active power flow control, is used as an example for the presented analysis. The developed small-signal models and the improvement in small-signal dynamics achieved by introducing control of the internally stored energy are verified by time-domain simulations in comparison to an EMT simulation model of an MMC with 400 sub-modules per arm.

Section: Introductionmentioning

confidence: 99%

Improving Small-Signal Stability of an MMC With CCSC by Control of the Internally Stored Energy

Freytes

IEEE Trans. Power Delivery

Suul

et al. 2018

Self Cite

“…Note that for the non-conventional control structure, the first two equations in (10) are replaced by (12), as the active current takes on the task of energy regulation.…”

Section: A) Conventional Controlmentioning

confidence: 99%

“…This interaction mode identification method is proposed by [12]. With this method, the hybrid ac/dc system is divided into subsystems.…”

Section: B Participation Factor Based Interaction Analysismentioning

confidence: 99%

“…A parameter ηαi is defined in [12] as a measure of the cumulative participation of subsystem α in mode i. Mathematically, ηαi is defined as the ratio of L1-norm of the vector of participation factors of states in subsystem α in mode i (pα,i) to the L1-norm of the vector of participation factors of all states in mode i (pi).…”

Section: B Participation Factor Based Interaction Analysismentioning

confidence: 99%

See 1 more Smart Citation

Avoiding AC/DC grid interaction in MMC based MTDC systems

Endegnanew¹,

2017 Twelfth International Conference on Ecological Vehicles and Renewable Energies (EVER)

Uhlen

2017

Abstract-In this paper, the interaction between ac and dc grids is studied for two types of MMC control structures; conventional and non-conventional control structures. Linear analysis methods that are based mode shapes and participation factors are used to identify dynamic interaction in a hybrid ac-dc power system. The analysis uses as a test system a three terminal Modular Multilevel Converter (MMC) based Multi-terminal High Voltage DC (MTDC) grid connecting three multi-machine asynchronous ac grids. The results show that there is some dynamic interaction between ac and dc grids under conventional MMC converter control structure, while the non-conventional MMC converter control structure decouples the ac and dc side of the MMC converter, thus avoiding interaction between the ac and dc grids.

“…[3]), including the effect of droop controllers (e.g. [4]), and from the more practical transient stability perspective (e.g.…”

Section: Introductionmentioning

confidence: 99%

HVDC meshed multi-terminal networks for offshore wind farms: Dynamic model, load flow and equilibrium

Garcés

Sánchez

2017 IEEE 18th Workshop on Control and Modeling for Power Electronics (COMPEL)

et al. 2017

This paper presents sufficient conditions for the existence of an equilibrium point of multi-terminal HVDC (MT-HVDC) network for offshore applications. Newton's method is used both as a computational tool and as the basis for proving the existence and uniqueness of the equilibrium. A simple but generalized dynamic model of the MT-HVDC grid for offshore wind farm applications is presented which is non-linear due to the constant power loads with droop regulation. The classic Kantorovitch theorem is used to define the requirements for the existence of the equilibrium and the super-convergence of Newton's method starting from voltages close to 1pu. Finding this equilibrium is equivalent to the power flow in power systems applications. Computational results corroborate the requirements for equilibrium as well as the convergence of the algorithm in a realistic MT-HVDC grid. Index TermsOffshore wind energy, Multiterminal HVDC systems, HVDC, transient stability, equilibrium point, dynamic systems.