This thesis is divided into two parts; The analysis of HVDC convertor transformer core saturation instability, and the design of a flexible data acquisition system for its assessment. A linearised direct frequency domain analysis of the mechanism behind the core saturation instability is presented. Using linearised convertor transfer functions and transformer saturation characteristics on ac and dc equivalents, the system equations are solved to describe the phenomenon. A term, referred to as Saturation Stability Factor, is derived to measure the system susceptibility to such an instability. The direct and quick solution of the method is used to reveal the characteristics of vulnerable systems. It is also used to determine effective and appropriate control measures. The predicted dynamics are validated with time domain simulations. Harmonic instability problems with long time constants, such as core saturation instability, are suited to real time computer analysis. Such real time analysis requires a compatible data acquisition system to process and present the simulated data. The second part of this thesis describes the design of a powerful and yet flexible data acquisition system for these purposes. A locally developed harmonic monitor, called CHART (Continuous Harmonic Analysis in Real Time) possesses several unique qualities, distinguishing it from other commercial systems. This thesis details the third generation of this system, known as CHART III, and its software architecture, referred to as the CHART III Virtual Operating System, which has transformed the dedicated harmonic monitor to a flexible multipurpose data acquisition system. The capability provided in the system is illustrated by several recent field measurements. The understanding resulting from the theoretical analysis of the instability, and the development of CHART III, have paved the way for the real time analysis of the core saturation instability.