Distance Relay
Distance Relay
Distance Relay
DISTANCE RELAY
Overcurrent protection scheme is essentially a simple protection scheme. Consequently, its accuracy is not very high. It is comparatively cheap as non-directional protection does not require VT. However, it is not suitable for protection of meshed transmission systems where selectivity and sensitivity requirements are more stringent. Overcurrent protection is also not a feasible option, if fault current and load currents are comparable. We now discuss about distance protection scheme which provides both 'higher' sensitivity and selectivity. Distance protection provides the following features: More accurate as more information is used for taking decision. Directional, i.e. it responds to the phase angle of current with respect to voltage phasor. Fast and accurate. Back-up protection. Primarily used in transmission line protection. Also it can be applied to generator backup, loss of field and transformer backup protection. Three Phase Fault Protection Consider a balanced (transposed) transmission line (fig 21.1)
Let T =
Then,
Where Z0 = Zs + 2Zm; [zero sequence impedance] Z1 = Zs - Zm; [positive sequence impedance] Z2 = Zs - Zm; [negative sequence impedance] Thus,
Now let a
bolted fault occur at percentage (%) distance, x of the line (fig 21.2)
For a solid
fault, Vn = 0. Thus,
For 3-phase fault, Ib = a 2 Ia Ic = aIa From equation (4), we get I0 = 0; I1 = Ia ; I2 = 0; So only positive sequence network is excited. Hence, (5)
It then follows that, a relay which monitor line current and phase voltages can locate fault by using equation (6). In the absence of fault currents I a, Ib and
Ic are smaller in magnitude. Consequently, apparent impedance seen by the relay is much higher. Hence, a simple logic to locate (6). fault is provided by equation
[Hint: Substitute Ib = a2 Ia, Vb = a2 Va, Ic = aIa,(7) Vc = a Va] Notice that, if equation (7) is used for locating fault, then the relay input voltage is the line voltage and not the phase voltage. Similarly, current input is the difference of line currents and not actual line currents. Thus, equation (7) provides an alternate way of locating fault. Note that per unit distance to fault is given by
ratio of apparent impedance seen by the relay to the positive sequence impedance of the line.
Line to Line Fault Protection Consider a bolted L-L fault on the phase b-c of the system (fig 21.3).
Again, system is considered unloaded for simplicity. Then the governing equations in 3-phase coordinates given by
(8)
Where
Further
Simplifying equation (8), we get (9) (10) (11) Now, subtract equation (11) from equation (10)
(12) From equation (7) and (12) we conclude that a relay input configured as per equation (7) can measure both 3-phase fault and L-L fault. Similarly, for a-c L-L fault
Therefore, traditionally the distance relays are configured as per equation (7) to detect and
locate both L-L and 3-phase faults. Therefore, distance to fault is given by where, l is length of line and Zapp is the impedance seen by the relay.
Earth Fault Protection Single Line to Ground Fault We now derive the governing equation for S-L-G fault case. Consider a single line to ground fault in phase 'a' on a unloaded transmission line at a per unit distance . (fig 21.4)
(13)
Thus, ratio
equals
and not
the relay input voltages and currents have to be configured in such a way that for any type of bolted fault (Zf = 0), the apparent impedance seen by relay is given by that we should modify equation (13) suitably. . Therefore, it follows
(14)
(15)
Now,
(16)
(17)
Since
, let
where m is called compensation factor for zero sequence current. Similarly, it can be shown for b-g and c-g faults. (19) And (20) It is clear that traditionally the ground fault relays require a different input configuration from phase fault relays (3-phase and L-L)
We had so far assumed bolted and unloaded faults. Therefore, there would be errors introduced when the fault has some impedance . Hence, the
apparent impedance seen by the relay will not exactly lie on transmission line impedance AB. Rather it would lie in a region shown by trapezoid in fig 21.5. Also, note that arcing faults are primarily resistive in nature.
Usually, distance relay characteristics are visualized by drawing the relay characteristics in R-X plane. If the apparent impedance seen by the relay falls inside the trip region (enclosed region), then relay declares a fault and issues a trip decision. This decision making can be done in about 1/2 - 1 cycle time, if no intentional time delays are introduced, e.g, for backup protection. While trapezoid or quadrilateral characteristics are quite popular with the numerical relays, previous generation of electromechanical and solid state relays used other characteristics like 'mho'
characteristics (see fig 21.6), which were easier to derive. Mho relay circles usually enclosed a larger area than the quadrilateral characteristics for identical line impedance and arcing impedance parameters. Thus, they are more susceptible to nuisance tripping. Hence, these characteristics have been superceded by the trapezoidal characteristics.