Open Phase Detection and Alarming Framework
Issue
The ION8650 meter incorporates line-to-line filter capacitors in its power supply circuitry to obtain conducted emissions class B compliance. Due to the line-to-line impedance, a partially powered or partially connected 3-phase system will result in phantom voltage measurements.
Product
ION8650A
ION8650B
ION9650C
Enviorment
ION Meters
Causes
Internally, 1nF capacitors couple each voltage phase together as well as each of those with neutral. At 60 and 50 Hz, these capacitors equate to an impedance of approximately 3.2 M? and 2.7 M? respectively. Under normal conditions, the voltage coupled on the phases by the line-to-line capacitors is unobservable due to the low source impedance of the system that the meter is measuring. However, if not every external fuse or potential transformer coil is removed from the circuit, these impedances, along with the measurement burden resistors, create a very high impedance voltage divider circuit, resulting in an internal voltage on the open phase.
When one phase in a three-phase system is in this open circuit state, the measured voltage comes from the other two phases. As such, the phase angle is reported as the mean of the other two, which, incidentally, is 180° from what it should be. The effect is that the power flow on this phase appears to be flowing opposite to when connected.
Under normal metering conditions, including sags and power quality events, this situation will not occur. Only a disconnected voltage input, or wiring fault, such as a damaged potential transformer or blown fuse, can cause this state.
A custom framework has been developed to allow customers to detect an open-phase fault. The fault will be reported in the meter's event log. If there is a phase-loss event occurring simultaneously, the open-phase event will take priority, cancelling the phase-loss event.
Resolution
The framework is outlined here:
The Voltage Symmetrical Components module is used to detect an open phase in the system. The magnitude of Zero Sequence Magnitude (ZSM) is compared to Positive Sequence Magnitude (PSM).
In a balanced system, the ZSM/PSM should be around zero; in a system with a lost phase (one phase at Vn) the ZSM/PSM will be around 45% (0.45); and in a system with an open phase, the ZSM/PSM will be around 70%. So a setpoint module reports an open phase if ZSM/PSM exceeds 60%. During an open phase event, phase loss events are cancelled and/or not reported.