DIRECTIONAL OVER CURRENT RELAY

Experiment No.: – 13

Title of the Experiment:

Directional over current relay.

OBJECTIVE:

  1. To perform the testing of operating characteristic of directional over current relay.
  2. Determine the pick-up current of an induction type over current relay corresponding to each plug setting.
  3. Observe the operating inverse time characteristic of an induction type over current relay.

THEORY:

Directional over-current relay:

A directional over-current relay develops maximum positive torque when the current and voltage supplied to the current coil and the pressure coil are in phase. If we define the maximum torque angle (MTA) as the angle between the voltage and current at which the relay develops maximum torque, then a directional over-current relay can be called a directional relay with MTA of zero degree.

Fig. 13.1 : Directional over-current relay construction and Phasor diagram

If the polarity of the current is appropriate, then directional unit picks up. If the current magnitude is above pickup, then the over-current unit also picks up and when both units pickup, the trip coil is energized and CB tripping is ensured.

Design of Directional Units for three phase system:

In a three phase fault system, choice of the reference phasor can be the phase voltage itself. For a purely reactive circuit, the fault current in the correct direction lags the reference phasor by 900 . With respect to reference phase ‘Va’, we can draw operating line (also called as zero torque line due to legacy of electromechanical relays) which separates the plane into two regions marked as ‘operate’ and ‘Do not operate’. If the fault is in the operating region, then Ia lags Va and the relay issues the trip decision. In case, fault is behind the relay, the fault current leads Va and hence lies in the “do not operate” region.

Over-current Relay Characteristic:

An over-current (OC) relay operates when the current exceeds the current setting value. The relay has two settings namely, plug setting (current setting) and time setting. The plug setting decides the current required for the relay to pick up while the time setting decides the operating time of the relay. There are three operating time characteristics such as instantaneous, definite time and inverse characteristics. The inverse characteristics has three types (i) Inverse definite minimum time (IDMT), (ii) Very inverse (VI) and (iii) Extremely inverse (EI). Generally, the operating time in the inverse characteristics is represented with respect to the plug-setting multiplier, PSM, is defined as follows:

\[\color\red{PSM=\ \frac{{I\ }_{relay}}{PS}….(1)}\]

where Irelay is the current through the relay operating coil and PS is the plug-setting of the relay. The value of PSM tells us about the severity of the current as seen by the relay.

The IDMT characteristics is widely used in the distribution network and equipment protection.

The mathematical relation between the current and the operating time of IDMT characteristic can be written as

\[\color\red{T_{RB}=\frac{0.14\ \ast T M S}{{PSM}^{0.02}\ -1}….(2)}\]

where PSM is the plug-setting multiplier and TMS is the time-multiplier setting of the relay. Thus, the operating time is directly proportional to the TMS and inversely proportional to the PSM. The characteristics of the IDMT relay are shown in Figure.1.

Figure 1. IDMT characteristics

CIRCUIT DIAGRAM:

Figure 2. Experimental setup for over-current relay testing.

  PRECAUTION:

  1. Main supply is in OFF condition.
  2. Make sure that varic is in initial condition.

PROCEDURE:

Directional test:

  1. Place the current direction selector switch in forward direction and check the relay operation with current higher than set value.
  2. Now place the current direction selector switch at reverse direction and check the relay operation.

Pick-up test:

  1. Make the connection as per the diagram.
  2. Set the desired the setting in the plug setting and TMS setting in TMS dial.
  3. Turn on the test switch.
  4. Vary the variac linearly from zero value.
  5. Note down the current value at which relay disc starts to move from standstill condition.
  6. Repeat the steps from 2 to 5 for different setting.

Inverse time characteristics test:

  1. Make the connection as per the diagram.
  2. Set the desired the setting in the plug setting and TMS setting in TMS dial.
  3. Turn on the test switch.
  4. Vary the variac linearly from zero value to current value to make required PSM.
  5. Turn off the supply and change the toggle switch to test position.
  6. Note down the operating time (shown in timer) of the relay corresponding to set PSM.
  7. Repeat the steps from 2 to 6 for different setting.
  8. Draw the observed inverse characteristics in the graph sheet.

TABULATION:

Directional test:

Direction of current flowExpected operationActual operation
ForwardO/C unit is enabled 
ReverseO/C unit is disabled 

Pick-up test:

Plug setting200%175%150%125%100%75%50%
Expected pick-up current in A108.757.56.2553.752.5
Actual pick-up current in A       

Inverse Time characteristics test:

PSMOperating time corresponding to PSM in sec
258
TAOTEOTAOTEOTAOTEO
Plug setting = 50%TMS= 1      
TMS = 0.5      
Plug setting = 100%TMS= 1      
TMS = 0.5      
Plug setting = 150%TMS= 1      
TMS = 0.5      
Plug setting = 200%TMS= 1      
TMS = 0.5      

Note: TAO – Actual operating time; TEO – Expected operating time;

CALCULATION:

CONCLUSION:

To be written by student,

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