  Impedance measurements have been widely used with the Nyquist plot to
estimate the stability of interconnected power systems. Being a black-box
method for equivalent and aggregated impedance estimation, its use for the
identification of sub-components bandwidth is not a straightforward task. This
paper proposes a simple method that will enable to identify the specific part
of the equivalent impedance (e.g. controller's bandwidth) that has major impact
on the stability of the system. For doing that, the paper analyses the
stability of an interconnected system of wind farms and high voltage dc (HVDC)
transmission system. The impedance frequency responses of the wind farms and
HVDC system from the ac collection point are measured and it is shown by the
method proposed in this paper, which controller has major impact in the
observed oscillation. A mitigation technique is proposed based on re-tuning of
the critical controller bandwidth of the interconnected converters. The method
suggested can reveal the internal controllers' dynamics of the wind farm from
the measured impedance combined with an analytical expression of the impedance
and a transfer function identity when no information about the controllers is
provided by the vendors due to confidentiality and industry secrecy.
