Stability of DC voltage droop controllers in VSC HVDC systems

Abstract

Abstract-Future multi-terminal HVDC systems are expected to utilize dc voltage droop controllers and several control implementations have been proposed in literature. This paper first classifies possible dc droop implementations in a simple framework. Then, the small-signal stability of a VSC-based converter station is analyzed for all the identified droop control schemes. The stability range for the system is determined as a function of the droop gain and is used to compare the flexibility and robustness of the implementations. The comparisons reveal that the droop implementations based on the ac current and dc voltage achieve a wider stability region than the other schemes and a limited sensitivity to the droop gain.

Interoperability of Modular Multilevel Converters and 2-level Voltage Source Converters in a Laboratory-Scale Multi-Terminal DC Grid

“…In both cases, the outer loop is assumed to be based on a simple PI controller. It should be noted that an active power droop can easily be included in the dc voltage controller, or a dc voltage droop could be included in the power controller [16], [23]. For simplicity, operation with a single converter terminal controlling the dc voltage will be assumed in the following, while all other terminals will be operated with ac-side power control.…”

Interaction of droop control structures and its inherent effect on the power transfer limits in multi-terminal VSC-HVDC

“…The d-axis current reference is provided by one of the various dc voltage droop controllers, identified and categorized in [19] and shown in Fig. 4.…”

“…Additionally, the applicability of droop control is proven by its use in the Nanao MT-HVDC grid [4]. In technical literature several alternative droop control schemes have been proposed [6], [9]- [18], which have been categorized in a simple framework in [19]. The general operation principle for all the dc voltage droop implementations is identical, i.e.…”

“…The contribution of this work encloses a methodology for a stability analysis of different droop control implementations, regarding the active power transfer capability as a function 2 of the droop gains. This methodology is applied to analyse a MT-HVDC grid, which extends the work in [19], to emphasize the impact of different control structures. In the study case the flexibility against active power flow variations and robustness against disturbances will be compared and analyzed for all the various implementations.…”

Methodology for the analysis of dc‐network resonance‐related instabilities in voltage‐source converter‐based multi‐terminal HVDC systems

“…Typically, two methods have been applied to study stability in MTDC systems: eigenvalue [4][5][6][7][8] and frequency-domain analysis [9][10][11]. The impact on the system stability of different dc-bus voltage droop control strategies has been studied in [4][5][6]. In the same fashion, Zadeh et al [7] investigate stability in the Cigré dcgrid benchmark system, and oscillatory instabilities related to the dc-bus voltage controllers are found.…”