What's a 2 1/2 Element Configuration?

PUBLISHED ON Mar 30, 2010

Q: I have 2 PTs and 3 CTs available to connect to your wattmeter.  Is that called  a 2½ element configuration?

A: In the context of measuring three-phase power, it all depends on whether your PTs are connected line-line or line-neutral.

Two-element wattmeters are generally used in delta connected systems.  They require two line-line voltages and only two phase currents.  Three-element wattmeters are used in wye connected systems.  They require three line-neutral voltages and three phase currents.  When applied that way, either type of meter can measure three-phase power accurately regardless of whether the load is balanced or unbalanced.

Each of the elements in a three-element wattmeter can be used to indicate the power on the corresponding phase.  The same is not true of two-element meters, however.  Although the total power is the sum of the power indicated on the two individual elements, the power measured by each element has no practical significance by itself.  So you can’t really measure the per-phase power of a delta connected system.

It is actually fairly common in substations for all three CTs to be available even though only two are required for measuring the power.  So this is one case where you might have two PTs and three CTs, but that would be an application for a two element wattmeter, not 2½ element.  For the purposes of measuring power on a delta system, you can ignore the third CT without consequence.

A few other types of connections are also used in the power industry and we see these from time to time:

Sometimes a wye connected system will have the PTs connected line-line.  That can either be done using two line-line PTs or three line-neutral PTs where the neutral conductor is not routed to the control house.  In either case, it results in only three voltage conductors being available to connect to the wattmeter.  In this case, a two element wattmeter must be used to indicate the total power, but then it will only be accurate as long as the load is balanced.

Occasionally we see wye connected systems having only two PTs that are connected line-neutral.  This is the application that is called 2½ element.  A two-element meter will not work for this kind of connection because the phase relationship between the voltage and current vectors is different from the delta application.  A three-element meter will not work because there are not enough PTs to calculate the power on the third phase.

Using only two PTs in a delta system makes sense intuitively because the relationships VAB + VBC + VCA = 0 and IA + IB + IC = 0 must always be true regardless of the state of balance.  In a wye system, one missing line potential may be calculated from  VAN + VBN + VCN = 0 by knowing any two of the vectors, but only when the voltages are balanced.  So there are always two restrictions inherent in the 2½ element application:
1.  Wye connected system having only two PTs connected line-neutral.
2.  System must be in balance.

In HV and EHV systems, conventional wound PTs tend to be very expensive so there may be a financial incentive to save the cost of one third of the PTs on a line.  And it is pretty safe to assume the voltage will always be in balance because the source impedance is so low when operating that high in the transmission grid.

So how does it work?  In an electromechanical 2½ element wattmeter there is usually a device called a “z-coil”.  The A-N and C-N potentials are routed internally to windings around a common iron core in such a way that the voltage produced on a secondary winding is equal to their vector-sum.  If the voltage is well balanced, the z-coil produces a voltage that should be nearly equal to VBN.  Once the meter has three line-neutral voltages (two connected to the terminals, one produced internally) and three phase currents connected normally, it can then measure the total power as if it were a conventional three-element meter.  Notice that the load current need not be in balance, only the voltage.  In a microprocessor based instrument, the output of a z-coil may be calculated but the voltage-balance requirement of the electromechanical wattmeter remains.

Email Bryan Gehringer with your questions about power measurement.