To a great extent future electric power systems will use HVDC technologies, based on either current sourced or voltage sourced converters (CSC, resp. VSC). Both technologies have their merits and range of applications being connected to technical capabilities, costs and operational properties and performance. Voltage stability is an important issue in classical CSC-based HVDC systems, while voltage stability and angle stability (both transmission angle and rotor angle) are of concern in VSC-HVDC systems. The systems are vulnerable against static instability when the AC connection point at either transmission end becomes weak in a very slow and unobserved manner. These systems may also be subject to transient instability at sudden major changes of grid structure and power flow. The present work intends to provide pertinent information for the development of diagnostic tools for stability assessment and of stabilizing controls relating to HVDC systems.
The proposed approach is to implement online stability assessment (OSA) based on
1. Synchrophasors obtained from WAMS for measurement based assessment,
2. Off-line predictive static and dynamic stability computations from D-2 to near real-time considering uncertainties and the inclusion of detailed HVDC dynamics
3. Online static and dynamic stability computations utilizing continuous power flow (CPF) and fast dynamic simulations using detailed HVDC models
This article proposes developments in necessary concepts and methodologies to support different tasks in point 3 above, focusing on angle and voltage stability where VSC-HVDCs are considered. Such methods can enhance operator tools such as those currently being developed in different European projects (, ). This paper deals primarily with the computation of the voltage sensitivity factor (VSF) of VSC-HVDC systems and the impact of converter controls and controls limitation on VSF curves and stability margins. Sensitivity curves holding for normal undisturbed operation and for credible (n-1)-contingencies are calculated and from these curves thresholds are determined. The study shows the effect of VSC current limitation on the stability margin. The limitation can be imposed either on the reactive current or on the real current. In both cases the limitation reduces the stability margin.
Transient simulations on the PSCAD/EMTDC simulator are performed with static voltage sources feeding the HVDC converter converters. Comparison of the power dependent voltage and angle curves obtained from these simulations with the static curves obtained from load-flow computations show differences due to control lags. This, however, does not impair the validity of the static curves for normal power ramps and their applicability for undisturbed operation.
By replacing the static voltage sources with rotating synchronous generators it can be shown that suddenly occurring negative steady state margins must not necessarily lead to instability. Immediate DC power reduction can prevent instability. Required are a sufficiently fast response and an adaptive reduction value. It needs to be further investigated whether and how pre-calculated VSF curves from contingency computations can be used for this task.
CIGRE (International Council on Large Electric Systems), 2014.
HVDC System stability Wide Area Power Systems PMU WAMS WACS Weak AC Grids Stability Analysis Voltage Sensitivity Factor Angle Sensitivity Factor Stability Margin Steady State Stability Transient Stability Stabilizing Control