Voltage Measurements
The MicroLogic P trip unit is equipped with a three-phase voltage power supply that, with respect to the distribution system, may be considered a delta load. The three-phase power supply reinjects voltage on an open phase. The voltage-protection functions react as indicated below.
Minimum-Voltage Protection
This function is based on the measurement of the phase-to-phase voltages.
In diagrams 1, 3 and 4 on the next page, a fuse has blown. The trip unit reinjects voltage on the failed phase and measures a phase-to-phase voltage higher than the actual voltage. The phase-to-neutral voltage should be zero, but the value measured is not zero.
In diagram 2, the phase-to-neutral voltage is effectively zero and the measurement indicates zero as well.
By limiting the pickup threshold of the minimum-voltage protection to the 80% - 100% range of the rated distribution-system voltage, the differences between the real voltages and the measured values are not significant and the MicroLogic trip unit will operate under all circumstances in the expected manner.
Voltage-Unbalance Protection
This function is based on the measurement of the phase-to-phase voltages.
In diagrams 1, 3 and 4 on the next page, a fuse has blown. The trip unit reinjects voltage on the failed phase and measures a phase-to-phase voltage higher than the actual voltage. The phase-to-neutral voltage should be zero, but the value measured is not zero.
In diagram 2, the phase-to-neutral voltage is effectively zero and the measurement indicates zero as well.
By limiting the pickup threshold of the voltage-unbalance protection to the 0% - 20% range, the differences between the real voltages and the measured values are not significant and the MicroLogic trip unit will operate under all circumstances in the expected manner.
Phase Failure
Detection of phase failure is not possible on the basis of the minimum-voltage and voltage-unbalance protection functions. The MicroLogic power supply requires at least two phases (between 100 and 690 V).
In diagrams 1, 3 and 4, if two phases have failed, the MicroLogic P trip unit measures for the three phases the value of the single voltage present (e.g. U12 = U23 = U31= 410 V).
Diagram 1
Diagram 2
Diagram 3
Diagram 4
Selection of the Voltage Measurement Inputs
The voltage measurement inputs are standard equipment on the downstream connectors of the circuit breaker.
It is possible to measure distribution-system voltage externally using the PTE external voltage measurement input option.
With this option, the internal voltage measurement inputs are disconnected. The PTE option is required for voltages greater than 690 V (in which case a voltage transformer is required).
When the PTE option is implemented, the supply circuit of the voltage measurement input must be protected against short-circuits. Installed as close as possible to the busbars, this protection function is ensured by a P25M circuit breaker (1 A rating) with an auxiliary contact (cat. no. 21104 and 21117).
The supply circuit of the voltage measurement input is reserved exclusively for the trip unit and must never be used to supply other circuits.