Although all Valhalla wattmeters are compatible with general purpose current transformers, there are distinct accuracy and bandwidth advantages to using a current shunt when making a power measurement. Current shunts generally provide greater accuracy over a broader frequency range than current transformers (CT’s). Rather than measure actual in-series current flow, the indirect, inductive magnetic-field principle employed by current transformers is subject to a variety of potential error sources.
The inductive, magnetic field sensing technique employed for a CT measurement is often subject to inherent phase errors. Handheld clamp-on CT’s specifically have centering (placement) problems, stray magnetic field (proximity-based) interactions, bandwidth limitations, and CT turn ration errors. Phase offset errors are of no significance when measuring simply the current level, however, if used for a true power watts (EI cos θ) measurement, then CT phase errors significantly magnify the true power watts uncertainty.
Measuring current and power using a current shunt basically employs a voltage measurement made across a known resistive shunt. The voltage potential across the shunt is directly proportional to the current flow. Valhalla current shunts, in particular, are designed to maintain a stable resistance with minimal heat rise, as well as have a flat frequency response. A magnetic field cancellation effect is achieved specifically by the shunt for greater frequency response.
Valhalla wattmeter shunts use a proprietary, low-temperature coefficient resistive alloy, lined with a high-thermal conductive, electrically-insulating layer (Kevlar) in a high-efficiency heat dissipation package. Using Valhalla’s temperature-stabilized shunt design allows a 100-ampere shunt to withstand 150 amperes without shifting the calibrated value of the shunt (no overheating).