| Abstract |
In this paper the prediction of harmonic instabilitities or limit cycles which may exist in HVDC systems, using describing function technique, is presented. These instabilities are caused due to the presence of low order uncharacteristic harmonics at the input of the converter firing system through
the DC current control loop. These low order harmonic may exist due to distorted or unbalanced AC
voltage or firing system error and or interharmonics. A computer model is developed to compute the system describing functions at different magnitude, phase and harmonic frequency of the control modulating input signal. The envelope of the inverses of the describing functions for each harmonic component can be determined. Therefore, domain of the limit cycle boundary for each harmonic can be obtained in the log magnitude-phase angle plane. The limit cycle instability is examined with two
quoted types of current control linear part transfer functions. The corresponding domains of the critical
control gains and time constants is computed for different degrees of AC systems imbalance and distortion. |
