Impact of the switching frequency on the DC-side admittance in three-phase converter systems

Abstract

Three-phase converter systems can be prone to negative impedance instability of the DC-bus. This paper studies the dynamic interactions between the DC-and AC-sides of a current-controlled three-phase system in order to reveal possible risks of instability. The system is modelled by using a small-signal method from a viewpoint of the DC-side. Time delays due to pre-calculation and discretization are considered. The stability analyses are performed for the small-signal model. The results of the stability analyses are compared with time-domain simulations. Comparisons demonstrate good agreement. In addition, a stabilizing controller is developed for the example system. According to simulations, the proposed controller can stabilize the system.

Modeling and Analysis of VSC-Based HVDC Systems for DC Network Stability Studies

“…Option 1 has been typically adopted in works such as [11], [12]. However, it can be shown that G 1 , as well as G 2 , are unstable if VSC 2 acts as a dc load, while G 3 is unstable for P 20 = 0.…”

“…However, it indicates that there is a risk of instability. To determine if the system is unstable, Nyquist stability criterion can be applied in the SISO system as in [11], [12]. From (27) and Fig.…”

“…Using (7), (11), and considering R 10 and R 20 as part of the dc network, the following state space model is defineḋ Finally, using (36) and (35) the closed loop stated space model, in the form indicated in [19], iṡ…”

“…In high power dc networks, such as VSC-HVDC systems, instabilities related to dcnetwork resonances are studied through eigenvalue analysis in [8]- [10], and it is found that the operating point and the gains of the Direct-Voltage Controller (DVC) have an impact on the dc-network stability. Similarly to low power dc networks, impedance analysis has been also applied in [11], [12] to study instabilities in high power dc networks. In [11], a dcside admittance is derived for a converter supplying power to a load and, again, Nyquist stability criterion is applied.…”

“…Similarly to low power dc networks, impedance analysis has been also applied in [11], [12] to study instabilities in high power dc networks. In [11], a dcside admittance is derived for a converter supplying power to a load and, again, Nyquist stability criterion is applied. However, a VSC that controls the direct-voltage is not included in the analysis.…”