This paper proposes a frequency control strategy for voltage source converter based multi-terminal HVDC systems (VSC-MTDC) to facilitate the exchange of primary frequency reserves among asynchronous AC systems, thus providing frequency support from each AC system to the others. The proposed frequency control utilizes a reference signal calculated from global measurements which reflects the overall frequency dynamics of all connected asynchronous AC systems. The proposed control outperforms the traditional frequency droop scheme by reducing impact on the DC voltage profile and improving frequency nadir. The performance of the designed frequency control with different DC voltage droop controls is evaluated. The adaptation of the proposed frequency control for converter outages is analyzed. The robustness of the proposed control to communication latency and the sensitivity of frequency support to power limits are also investigated. The VSCMTDC model and the proposed control are implemented in the commercial grade software PSS/E and thus is suitable to study large-scale realistic systems and can be incorporated into the power system planning process at utilities and ISOs. The effectiveness of the proposed control is illustrated on a developed AC-MTDC test system and also on a large-scale realistic model combining the North American Western Interconnection (WI) and Eastern Interconnection (EI) with continental HVDC interconnections.

High Voltage Direct Current Transmission (HVDC) systems are called to play an important role in modern electric power systems. Among the many aspects of HVDC currently under active research, this paper presents a proposal to design supplementary controllers in multi-terminal HVDC systems based on Voltage Source Converters (VSC-MTDC, for short) to damp electromechanical oscillations (Power Oscillation Damping or POD). In the proposed POD controllers (PODC), each VSC station compares the average of the frequencies measured at the AC connection points of the VSC stations of the MTDC system (frequency set point) with its own AC output frequency. This difference is then used to manipulate the active-(P) and reactive-power (Q) injections of each one of the VSC stations. The proposed PODCs are designed systematically by using a coordinated-design method using the concept of eigenvalue sensitivity, in order to achieve the required damping ratio of a set of electromechanical modes of interest in the power system where the VSC-MTDC is connected. The proposed designed methodology has been tested in the Cigre Nordic32A system with an embedded VSC-MTDC system. Results show that the proposed PODCs are effective damping successfully the electromechanical modes of interest and they are robust against communication latencies.

Grid-forming voltage source converter (GF-VSC) has been identified as the key technology for the operation of future converter-dominated power systems. Among many other issues, transient stability of these types of power systems remains an open topic of research because it is still a key limiting factor for stressed power systems. Previous studies have proposed control strategies for GF-VSCs to improve the transient stability of systems of this type by suitable current-limitation algorithms and/or control of active-power injections. As an alternative, this paper proposes two fast voltage boosters to improve the transient stability of power systems with 100 % of GF-VSC-based generation with virtual synchronous machine (VSM). One control strategy uses local measurements, whereas the other one uses global measurements of the frequency of the centre of inertia (COI). Both strategies improve transient stability of these types of systems significantly. The advantage of using fast voltage boosters for this purpose is that the set points linked to frequency/active-power injection (i.e., set points linked to the primary energy source of the VSCs) will not be modified. Furthermore, strategies such as current-limitation, active-power control and fast voltage controllers for transient stability improvement are compatible and complementary.

Power transmission is the main purpose of high voltage direct current systems based on voltage source converters (VSC-HVDC). Nevertheless, this type of system can also help to improve transient stability by implementing suitable supplementary controllers. Previous work proposed active- (P) and reactive-power (Q) control strategies in VSC-HVDC multi-terminal systems (VSC-MTDC, for short) to improve transient stability, producing significant improvements. In those strategies, each VSC station of the MTDC system compares its frequency measurement with the average of the frequencies measured by all converter stations of the MTDC system (weighted-average frequency, WAF) in order to modulate its own P and Q injections. Hence, a communication system is required. This paper presents a detailed analysis of the impact of communication latency on the performance of those control strategies. The communication delays have been modelled using a Padé’s approximation and their impact on the performance of the control strategies have been assessed by means of time-domain simulation in PSS/E. The effect of the control strategies on transient stability has been quantified with the critical clearing time (CCT) of a set of faults. Results show that the control strategies analysed present good results for realistic values of communication delays.

Inter-area oscillations have been extensively studied in conventional power systems dominated by synchronous machines, as well as methods to mitigate them. Several publications have addressed Power Oscillation Damping (POD) controllers in Grid-Following voltage source converters (GFOL). However, the performance of POD controllers for Grid-Forming voltage source converters (GFOR) in modern power systems with increased penetration of power electronics requires further investigation. This paper investigates the performance of GFORs and supplementary POD controllers in the damping of electromechanical oscillations in modern power systems. This paper proposes POD controllers in GFORs by supplementary modulation of active- and reactive-power injections of the converter and both simultaneously (POD-P, POD-Q and POD-PQ, respectively). The proposed POD controllers use the frequency imposed by the GFOR as the input signal, which has a simple implementation and eliminates the need for additional measurements. Eigenvalue-sensitivity methods using a synthetic test system are applied to the design of POD controllers in GFORs, which is useful when limited information of the power system is available. This paper demonstrates the effectiveness of POD controllers in GFOR converters to damp electromechanical oscillations, by small-signal stability analysis and non-linear time-domain simulations in a small test system and in a large-scale power system.