Current transformer operation

A current transformer (CT) scales a large conductor current down to a small current that an instrument can safely measure. For example, 1,000 A on the primary side can be measured as 5 A in the secondary circuit. When a CT is placed around a live wire, the current in that wire induces a current in the CT's secondary winding. This secondary current is proportional to the primary current, and the meter calculates its measurements from it. The turns ratio is the scale factor that links the measured secondary current to the actual conductor current. Some CTs have taps so the ratio can be matched to the operating current. The meter or controller must be set to the ratio of the CT that is actually installed. On one export power manager, a 1000:5 A CT is entered as 200:1. Gateways in PV and battery systems use CT readings to report energy production and consumption. If the CT ratio setting is wrong, the readings are wrong, and protective relays can raise a bad-CT-ratio alarm. How well a CT performs depends on the burden on its secondary, which is set by the wiring, the connected impedances and the relay or meter. In transformer differential protection, CTs are often connected in delta on the transformer's wye side and in wye on its delta side, so the CT currents match the transformer's phase shift.

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• sentence 2: support check: partial - Induction and proportionality are stated, but 'the meter' calculating is only the gateway using it for measurements.
• sentence 4: support check: partial - The 1000:5 A entered as 200:1 example is stated, but 'must be set to the actually installed CT' is only implied, and the device isn't named an export power manager.
• sentence 5: support check: partial - Gateway use of CT readings and the BAD.CT.RATIO alarm are stated, but the wrong-ratio-gives-wrong-readings link and the PV/battery scope are not stated.
• c1 dropped at review: The passage only says CT connections must match the transformer's delta-wye phase shift; it never makes CT operation a kind of winding configuration, and the claim's own note says it is not a subclass. -->