  The stability of multi-vendor, multi-terminal HVDC systems can be analyzed in
frequency domain by black-box impedance models using the generalized Nyquist
stability criterion. Based on the impedance stability analysis, a multi-level
sensitivity analysis approach using frequency-domain sensitivity functions is
proposed to identify the root cause of potential instability. Case studies on a
four-terminal HVDC system are carried out for stability and sensitivity
analysis based on the impedance measurement in PSCAD. The analysis results are
finally validated by electromagnetic transient simulations.
