A Residual Voltage Transformer is used to measure the residual voltage of three phase system during single phase fault. During normal operating condition, the summation of three phase voltage is zero but in case of single phase fault, the scenario changes and there exists a residual voltage.
Let us first discuss residual voltage in case of single line to ground fault. Let us consider a solidly grounded system as shown in figure below.
Let us assume that a ground fault takes place in A phase (In many industries and numerical relays, normally the phases are said as A, B and C instead of R, Y and B, though they represent the same thing i.e. A phase means R phase, B means Y phase and C means B phase). Ea, Eb and Ec are the Generator terminal voltage per phase. Bold letters here represent vector form.
Because of ground fault in A phase, the voltage at the point of fault will become zero but the voltage of other two healthy phases will remain normal as the neutral is solidly grounded therefore the neutral potential will be maintained to earth potential.
Va = 0
Vb = V ∠-120°
Vc = V ∠120°
Here V is per phase voltage under normal condition.
Thus the residual voltage of system = Va+Vb+Vc
= 0 + V ∠-120° + V ∠120°
= V ∠-60°
Thus we observe that, there exists a residual voltage in case of single line to ground fault. This residual voltage is measured by Residual Voltage Transformer.
The primary of Residual Voltage Transformer is connected to three phase system and its secondary is connected in Broken Delta as shown in figure below.
The output of the secondary windings connected in broken delta is zero when balanced sinusoidal voltages are applied (as Va+Vb +Vc = 0), but under conditions of unbalance a residual voltage equal to three times the zero sequence voltage (V0) of the system will be developed.
To measure this component i.e. 3V0, it is necessary for a zero sequence flux to be set up in the Residual Voltage Transformer (RVT), and for this to be possible there must be a return path for the resultant summated flux. Therefore, RVT core must have one or more unwound limbs linking the yokes in addition to the limbs carrying phase windings. Usually the core is made symmetrically, with five limbs, the two outermost ones being unwound. This two outermost unwound limbs provide return path for zero sequence flux.
In case where three single phase transformer units are used to measure residual voltage, no extra limbs are requires as each single phase transformer has a core with closed magnetic path.
It is very important to earth the primary winding neutral of Residual Voltage Transformer to provide return path for zero sequence current else zero sequence current cannot flow and hence the flux will contain 3rd harmonic component that is reflected in primary and secondary voltages of Residual Voltage Transformer. This voltage appearing at the secondary terminals of RVT is not the residual voltage of the system in any way.