SUBJECT Energy Systems and Power Electronics Please answer q

SUBJECT: Energy Systems and Power Electronics

Please answer question and show all work.

4. Would only one wattmeter be needed to measure the total three-phase active power on a balanced three-phase four-wire system Explain. Yes No

Solution

NO

Three-phase balanced networks are used in the power industry for reasons of economy and performance. Three-phase generators and motors run smoothly, with no torque pulsations, unlike single phase machines. In addition balanced three phase systems may be operated as three wire or four wire systems, with much less copper needed for the power delivered as compared with three single phase systems.

At a power generating plant, the windings of a three phase machine are arranged to provide three voltages, each 120° apart in time and, in the common balanced system, usually all of the same magnitude. These three voltage sources may be connected in a wye (Y) or a delta () configuration. Three phase loads may also be connected in wye or delta connections. The wye connection has a central node to which a neutral wire may be joined, but the delta connection is a three wire system without a node for a neutral or ground) connection.

To measure power in a 3-phase system, it would seem necessary to use three wattmeters, each connected to neutral for a common terminal, and each responding to a line-to-neutral voltage and a line current. One would then add up the powers indicated on each wattmeter. Analysis of such a circuit shows that one wattmeter is redundant, hence the two-wattmeter method of measuring 3-phase power was developed for three wire systems. This method is satisfactory even if the loads are unbalanced. It is necessary to connect the wattmeters taking into account the polarity of their coils. When the current enters the marked terminal of the current coil and the voltage positive is connected to the marked terminal of the voltage coil, the reading represents power absorbed. In that case the algebraic sum of the wattmeters determines the total load power. In reactive circuits it may be necessary to reverse the current coil of one wattmeter in order to get an upscale deflection. This reading is taken as negative when the total power is determined algebraically.

If a 3-phase system has four wires, it is necessary to use three wattmeters, unless it is known that the system is balanced and therefore no current is flowing in the neutral wire. For any balanced N wire system it is necessary to use N 1 wattmeters to measure the total power.