Grid impedance and automatic trip-out ability

Industrial installations

Power electrical installation's automatic trip-out ability is the largest part of evaluation during maintenance process. It protects the power transformer in case of overloading, overheating or shorting. The measurement includes Z line and Z loop impedances that can reveal bad contacts on busbars, condition of power switches, contactors or fuse holders. These issues happen already at very low resistances. The measurement solution must work with high resolution in milli-Ohm range. High accuracy is achieved by using a 4-wire connection to the tested unit, which automatically removes the resistance of the measurement wires, and high measurement current that directly affects the resolution of the result.

A switchyard with 100 A fuses must be tested with at least 10 A measuring current, the higher the better. MI 3144 impedance testing with 300 A can offer robust repeatable results with high noise immunity and reliability all up to 800 V. Results of L to N or L to L need to be compared between phases at the source and at the load. The instrument calculated the impedance of the circuit based on the difference. To avoid additional error, the measurement must be set up to only include the length of the conductor. Performing measurements close to the power transformer’s inductive impedance has a significant influence. Resistance, inductance and voltage drops need to be evaluated accordingly.

Prospective short-circuit current is calculated according to the standard IEC 60909-0 Short-circuit currents in three-phase a.c. systems - Part 0: Calculation of currents. The results of the calculation are the prospective short-circuit current Ipsc, prospective fault current Ipfc, and the HOT factors. The latter represent the predicted reserves in the system in case of an error or an overheated transformer.

Four-wire type of connection with Kelvin crocodiles for high precision and stable results eliminates problems of bad connectivity and test leads compensation.

Automatic trip out ability evaluation with high accuracy and stability of result even on powerful systems with fuses above 1000 A where classic methods became useless.

Impedance with Resolution of 1mOhm detailed evaluation of transformer’s Isc min, Isc max and Z HOT based on overheating reserve.

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