Appendix D—Trip Unit Voltage Supply Architecture
The trip unit has an integral internal three-phase voltage power supply which appears as a three-phase delta configured load to the system (Integral Internal Three-phase Power Supply). This power supply is a three-phase load by itself and will inject voltage on an open phase (Open Phase on Three-phase Power Supply). The impact of a three-phase delta configured load on the voltage-based protection functions is as follows:
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Hazard of equipment damage
Setting undervoltage protection (Vmin) below 80% or voltage
unbalance (Vunbal) above 20% can cause the trip unit to not perform
as expected.
Failure to follow these instructions can result in equipment damage.
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Minimum Voltage Protection
The minimum (under) voltage protection function is based on phase-to-phase voltage measurement.
For circuit configuration 1 (Circuit Configuration 1), 2 (Circuit Configuration 2) or 3 (Circuit Configuration 3), if a fuse opens the trip unit will inject voltage on the open phase. Therefore the trip unit will meter the voltage being injected on the open phase accurately. The phase-to-phase voltage (VLL) measurement will be higher than when the open phase is at zero volts. The trip unit will also accurately meter the phase-to-neutral voltage (VLN) injected on the open phase and display a value greater than zero.
For circuit configuration 4 (Circuit Configuration 4), the trip unit has a return path through the transformer and the injected voltage on the open phase will be zero. In this configuration the trip unit will accurately meter zero volts on VLN.
To ensure the MicroLogic trip system will perform as expected regardless of system configuration the user should limit the undervoltage pickup range to 80%–100% of the nominal phase-to-phase system voltage.
Integral Internal Three-phase Power Supply
Open Phase on Three-phase Power Supply
Circuit Configuration 1
Voltage Unbalance Protection
The voltage unbalance protection function is based on phase-to-phase voltage measurement.
For circuit configuration 1 (Circuit Configuration 1), 2 (Circuit Configuration 2) or 3 (Circuit Configuration 3), if a fuse opens the trip unit will inject voltage on the open phase. Therefore the trip unit will meter the voltage being injected on the open phase accurately.
The phase-to-phase voltage (VLL) measurement will be higher than when the open phase is at zero volts. The trip unit will also accurately meter the phase-to-neutral voltage (VLN) injected on the open phase and display a value greater than zero.
For circuit configuration 4 (Circuit Configuration 4), the trip unit has a return path through the transformer and the injected voltage on the open phase will be zero. In this configuration the trip unit will accurately meter zero volts on VLN.
To ensure the MicroLogic trip system will perform as expected regardless of system configuration the user should limit the voltage unbalance protection settings to 0–20%.
Circuit Configuration 2
Circuit Configuration 3
Circuit Configuration 4
Loss of Multiple Phases
Do not use either minimum voltage protection or voltage unbalance protection to determine the loss of multiple phases.
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The internal voltage power supply requires voltage of two phases to operate. (The voltage power supply has an operating range from 100 V to 690 V.)
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In circuit configurations 1 (Figure 103), 2 (Figure 104) and 3 (Figure 105), when multiple phases are lost the trip unit will measure the system voltage of the remaining phase on all three phases. For example, if two phases are lost on a 480 V three-phase delta system, the trip unit will meter 480 VLL on all three phases.