Improved dynamic security assessment for AC/DC power systems using transient energy functions

Due to rapid changes in electricity demand and disturbances caused by faults and equipment switching, power systems are often in a state of change. A method to predict transient stability that can accurately process a range of conditions frequently and faster than time simulation is highly advantageous. The Transient Energy Function (TEF) method is such a tool for this purpose.;Most TEFs that have been derived in the past are for AC-only systems. TEFs for HVDC systems are rare. Among them, the common approximation made is the omission of HVDC dynamics from the expression for transient energy. Since the HVDC system behaviour is dominated by its controls, the level of transient energy depends upon the behaviour of HVDC controllers. Thus, the above omission leads to an error in stability prediction.;In this thesis, a comprehensive TEF that can be applied for HVDC systems with different AC/DC converter control modes, and captures the dynamics of the HVDC network, converters, converter controllers and voltage-dependent current order limiter is developed by explicitly including their differential and algebraic equations in the TEF. The difficulty in the derivation of the new TEF is two-fold; (a) the equation representing interactions between rotor angles and DC power is complex, and (b) the number of equations required to model the dynamics in the HVDC system and controllers is large, and all of them must be included in the energy equation. The difficulty in (a) is overcome by modeling the DC power as a change of power injection into the AC system from the generators. The difficulty in (b) is overcome by taking two steps. First, the integral variable in the energy equation is changed from rotor angle to DC power, and then to time. Second, the term in the energy equation that represents energy due to HVDC dynamics is converted into a series of derivatives of DC power and rotor angles using integration-by-parts. When this series is evaluated using all HVDC equations, the higher order terms of the series diminish during fault-on-trajectory due to the acceleration of rotors, which makes the computation of the energy margin possible within a tolerance required for practical purposes.;The new method consistently uses all HVDC differential and algebraic equations to; (a) compute transient energy, (b) find the post-fault stable equilibrium point (SEP), and (c) perform the numerical integration. Due to (a), the new method provides improved stability predictions. Thus, the operating limits derived by the new method can now be used for operation of power systems in a more cost-effective manner with a greater degree of security. Due to (a), (b) and (c), the post-fault SEP must be found only once. Therefore, the new method takes less time to make the stability prediction. The operating limits derived in a shorter time enable faster operator intervention. These two advantages make the new TEF highly valuable to the industry in AC/DC power system operations.