Proof of Connections When Instrument Transformers Are Used

Author: Hans Lohninger

When instrument transformers are used in measuring three-phase power by the two-wattmeter method, verification of the connection of the low-reading element may be made by removing the potential-transformer primary connection at the Common line (it is assumed that the upper meter is the one of lower reading), as at a in Fig. 16-10, and touching it at b. If Wi reverses, its reading is to be taken as negative. If it does not reverse, then its reading is to be taken as positive. The potential-transformer connection must be restored to its original position at a.

The connections of a polyphase wattmeter may be checked in the same manner as are those of two separate wattmeters. The wattmeter should first be connected so that each element alone produces a positive reading. With the potential coil of the higher-reading element disconnected, the algebraic sign of the lower-reading element is then checked in the usual way. If this test shows a negative reading, then that element must be reconnected so that its torque on the needle is opposed to the torque of the higher-reading element. The same procedure should be used in proving the correctness ot connections of a polyphase wattmeter when potential transformers are used. Fig. 16-11 shows a circuit diagram in which both current transformers and potential transformers are used with a polyphase wattmeter to measure power in a three-phase circuit.

Fig. 16-11. Polyphase Wattmeter Connections Using Current Transformers and Potential Transformers to Measure Polyphase Power