I have seen relay+meter products in projects.
End users and consultants have the impression that it must be cheaper with the combined product.
However, I want to know technically, what is the benefit of separating these two functions instead of one?
So I can tell them that they're probably saving money at cost of system reliability... bla bla bla...
Relay always connects to higher saturate point CT but less accurate while meter always connects to high accuracy CT at its rated value giving more accurate reading.
Typically, the relay CTs do not have the same level of accuracy that metering CTs have, as Yut mentioned. The specific metering need will drive the decision of discrete metering or protection + metering. If the customer only wants basic metering, and not concerned about accuracy and power quality, then influence the customer to use Schneider relays that have the metering functionality. If it is a revenue meter application, I would expect the customer to prefer separate metering, since the meter is the customer's cash register. Has the relay received third party accuracy certification? Likely not. A device that is specifically designed for metering is the best choice in a critical application, as opposed to using a secondary function (metering) of a protective device. Looking at it from the other direction, our meters could, in theory, be used as protective devices, but they are not used that way because it is not the primary function of the device. The same philosophy applies to using protective relays for metering. Use the right device for the right application.
The point is power quality. I've not heard of a protective relay having certified compliance with IEC 61000-4-30 Class A, which our meters have. Relays typically have slower sample rates, and therefore poorer waveform capture capability. Also keep in mind that our meters can compensate for transformer losses, so they can be placed on the transformer secondary, where the CTs and PTs will be much less expensive. Our meters are designed for PQ applications including sag/swell and transient detection and waveform capture, and harmonic analysis. Like the revenue metering situation, if power quality is a concern, it is best to use the device designed for that application. Fortunately, we have successfully combined revenue and PQ metering into a single device.
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