Chatter is a time step to time step, symmetrical oscillation phenomenon inherent in the trapezoidal integration method used in the Dommel algorithm for transient simulation of electrical networks [1].
Chatter is usually initiated by the closing of a switch in a branch containing inductors. It does not matter if the switching occurs between time steps, or at a natural current zero [8]. Figure 4-5 illustrates the presence of voltage chatter, due to a natural turn-off of a series thyristor/inductor, series branch.
Figure 4-5 - Voltage Chatter Across and Inductor
Since chatter does not represent any electrical network behaviour, it must be suppressed. EMTDC includes a chatter detection algorithm to continuously detect such spurious oscillations and remove them if so required. Chatter is detected by continuously monitoring every node voltage and branch current and is assumed to be present if these quantities change direction successively for five consecutive time steps. For example: 1.0, -0.9, 0.8, -0.7 and 0.6. In addition, the chatter detection algorithm continually monitors for branch switching events. In this way, chatter introduced by any sudden changes in the electric network (even those not initiated by switching events) is detected.
NOTE: If chatter detection is disabled and chatter removal is enabled, only chatter due to branch switching will be removed. This is sufficient in most situations. The default Chatter Detection Level (CDL) is set to 0.001 pu in the PSCAD Project properties.
Either when chatter is detected or when a switching event takes place, a chatter removal algorithm in invoked. Chatter is removed using a half time step interpolation. The user has the option to enable or disable the chatter algorithms in PSCAD, however it is a good practice to keep them enabled for all circuits.
For more detailed information on Chatter and its effects, please see [6] and [8].