Recent events related to power system failure have shown that voltage collapse can be a cause of widespread outages.The thrust of this paper is to discuss and establish means of mitigating system voltage instability by using a combination of both reactive current droop compensation and line drop compensation.It is shown that the point that the voltage regulator controls can be defined by a new method which is based on a widely accepted voltage stability analysis tool.This tool can be used to determine which generators will have an impact on the maximum permissible loading of a bus.Dynamic analysis was carried out on the CIGRE Nordic test system to study the impact of control point location on time to collapse and it is shown that the new scheme can improve the voltage stability.
It presents a method which can equivalently arrange the capacitance distribution along with the winding of the generator on the terminal and neutral respectively in a reasonable partition,particularly for a type of high-voltage graded insulated cable wound generator,Powerformer.The winding of the Powerformer adopts graded insulation,which leads to the various cable thicknesses in different portion of the winding,and thus,the uneven capacitance distribution.The large capacitive currents and large transient outrush currents resulting from the cable consisting of the stator winding of Powerformer may cause problems to the generator differential protection.Most of literatures available in the field of compensated differential protection focus on the charging current compensation for long transmission line,instead of for generator.The authors give a method which can be used to compensate the capacitive current wholly to improve the reliability of the differential protection of Powerformer.It is proved that the distributed capacitance can be equivalent as the lump circuit with a capacitance partition coefficient ρ,and ρ is proved as a constant no matter whether the generator experiences the normal operation,external phase(s) fault or internal phase(s) fault.The formula of the partition coefficient is provided and the corresponding equivalent circuit of the Powerformer cable to calculate capacitance currents is given.An analysis programming resolving the minimum value of the coefficient ρ is written in MATLAB 7.0/m according to this formula,using the function fmincon() which can work in any type of constraint condition.The program always gets the same global minimum points under the different initial values condition which proves our point by mathematical test.With this new approach to winding capacitance dividing method,the protection scheme used for Powerformer can be validated and improved accordingly.
