Basics of Direction Sensing

In this examples we have mentioned that the fault is in forward direction with respect to relay R3. Here we have decided this by observing the position of the relay with respect to fault and source. However in actual practice relay has to discriminate between forward and reverse direction fault. How this can be achieved is discussed in next sections.

Direction of AC current flow.

The name “alternating current” suggests that there is no specified physical direction of current flow. To elaborate this let us compare measurement of alternating current and direct current by clip on meter as shown in following figures.

 
DC clip on meter has either arrow marking or polarity marking on jaw of the meter. If the direction of current flow is reverse with respect to this marking it is indicated by –Ve sign by clip on meter display. Thus current flowing from A to B displayed as 17.32 A and it is  -17.32 A if flowing from B to A.

  However for AC tong tester there is neither such marking nor any –Ve sign for displayed current. Whether current flowing from A to B or B to A display will be +Ve only.

Though the change in direction of current flow in case of A.C. system does not changes sign for clip on meter display still it is wide practice to show currents in A.C system by direction. Obviously then someone may ask the question that; what is the meaning of showing the direction of current flow in AC system?; and answer to this is; it is actually direction of active power flow. Obviously to determine power flow in circuit we require current as well as voltage. How this is achieved is explained in next section. 

Direction of power flow

As mentioned in previous section to determine physical direction of current flow (power flow) we require additional quantity; this additional quantity is voltages causing current (respective phase voltage). The meaning of direction of AC current flow (power flow) can be best understood with the help of power system as shown in figure.

As shown here consider a SLD for three bus power system feeding the lagging power factor load. Let direction sensing devices D_A and D_C are connected to Line-AB and Line-BC at Bus-B. Bus-B CT and PT connections are shown in the figure as per common conventions listed below.

1)    Connect PT secondary terminal “a” to device and “n” to earth

2)    CT shall be installed such that its primary P1 terminal shall be towards Bus

3)    Connect CT secondary terminal S1 to relay and S2 to earth (with other phase CT S2 terminal)

Let us consider an instance where Bus-A is positive. Thus as per CT polarity the current flowing through CT primary and secondary has the directions as shown in the figure.

Now as seen from the figure direction of instantaneous current and voltage for device D_C is same (conventionally both entering into the device). Thus current phasor shown lagging behind voltage phasor by the power factor angle of the load.

While for device D_A direction of instantaneous current and voltage is opposite (conventionally current is leaving the device while voltage is entering into the device). Thus here current phasor is reversed.

By observing the vector representation of voltage and current in the figure (as shown below respective device) we can easily conclude that; if standard connection convention followed then-

Whenever angle between voltage and current is less than 90⁰ (lagging or leading) physically direction of power (current) flow is away from bus.

AND

Whenever angle between voltage and current is more than 90⁰ (lagging or leading) physically direction of power (current) flow is towards bus.