The power measurement in a balanced three-phase circuit can be done by

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any of the above

Explanation: In a balanced three-phase circuit, the total active power can be measured using one, two, or three wattmeters depending on the circuit configuration and availability of the neutral point. While the two-wattmeter method is the most common industrial practice for three-wire systems, all three methods are theoretically valid for balanced loads.

Working Principle: The one-wattmeter method is used only for balanced loads by connecting the current coil in one phase and the potential coil between that phase and neutral (or an artificial neutral). The three-wattmeter method measures power in each phase individually and sums them. The two-wattmeter method measures total power by calculating the algebraic sum of the readings of two wattmeters connected in two lines, assuming a balanced load where the phase angles are identical.

Key Formulas: $P = \sqrt{3} V_L I_L \cos(\phi)$ — Total Power in balanced 3-phase circuit; $P = 3 	imes V_{ph} I_{ph} \cos(\phi)$ — Equivalent power expression in terms of phase values

Key Points: One-wattmeter method requires a balanced load and access to the neutral point.; Two-wattmeter method is applicable for 3-phase 3-wire systems, even if unbalanced (though the total power remains accurate for balanced systems).; Three-wattmeter method is universal and can be used for both balanced and unbalanced systems with or without a neutral wire.

Advantages: Flexibility in instrumentation based on available equipment; Two-wattmeter method reduces the number of sensors needed for 3-wire systems

Disadvantages: One-wattmeter method is extremely sensitive to load unbalance; Three-wattmeter method is more complex to set up

Applications: Industrial power consumption monitoring; Electrical laboratory testing of motors and transformers

Additional Info: In a balanced system, the power factor can be determined using the ratio of the readings of two wattmeters.. If the load is unbalanced, the two-wattmeter method still provides the total power, whereas the one-wattmeter method will provide an incorrect result.

Answer: D is correct — All three methods are valid for measuring power in a balanced three-phase circuit given the appropriate connections.

Exam Tip: Always remember that the two-wattmeter method is valid for any 3-phase 3-wire system, regardless of whether the load is balanced or unbalanced, but for purely balanced circuits, all these methods converge on the same result.

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