Directional Negative Sequence Overcurrent Logic; Phase Rotation; Figure 34: Negative Sequence Overcurrent Logic - Directional Operation - GE P24DM Technical Manual

Chapter 6 - Current Protection Functions
6.3.1

DIRECTIONAL NEGATIVE SEQUENCE OVERCURRENT LOGIC

I2 I2
I2>1 Current Set I2>1 Current Set
CTS Block CTS Block
I2> Inhibit I2> Inhibit
I2H Any Start I2H Any Start
I2> Blocking I2> Blocking
2H Blocks I2>1 2H Blocks I2>1
I2>1 Direction I2>1 Direction
V2
I2> V2pol Set I2> V2pol Set
VTS Slow Block VTS Slow Block
I2> Blocking I2> Blocking
VTS Blocks I2>1 VTS Blocks I2>1
I2> Char Angle I2> Char Angle
I2>1 Tmr Blk I2>1 Tmr Blk

Figure 34: Negative Sequence Overcurrent logic - directional operation

Directionality is achieved by comparing the angle between the negative phase sequence voltage and the negative
phase sequence current. The element may be selected to operate in either the forward or reverse direction. A
suitable characteristic angle setting (I2> Char Angle) is chosen to provide optimum performance. This setting
should be set equal to the phase angle of the negative sequence current with respect to the inverted negative
sequence voltage (–V2), in order to be at the centre of the directional characteristic.
For the negative phase sequence directional elements to operate, the device must detect a polarising voltage
above a minimum threshold, I2> V2pol Set. This must be set in excess of any steady state negative phase
sequence voltage. This may be determined during the commissioning stage by viewing the negative phase
sequence measurements in the device.
When the element is selected as directional (directional devices only), a VTS Block option is available. When the
relevant bit is set to 1, operation of the Voltage Transformer Supervision (VTS) will block the stage. When set to 0,
the stage will revert to non-directional.
6.4

PHASE ROTATION

A facility is provided in the P24xM to maintain correct operation of all the protection functions even when the
motor is running in a reverse phase sequence. This is achieved through user configurable settings available for the
four setting groups.
The default phase sequence for P24xM is the clockwise rotation ABC. However, some applications may require an
intermediate anti-clockwise phase rotation of ACB.
In process industry there is often a common practice to reverse two phases to facilitate the process, using phase
reversal switches. The following sections describe some common scenarios and their effects.
For such applications the correct phase rotation settings can be applied for a specific operating mode and phase
configuration in different setting groups. The phase configuration can then be set by selecting the appropriate
setting group. This method of selecting the phase configuration removes the need for external switching of CT
circuits or the duplication of relays with connections to different CT phases. The phase rotation settings should
only be changed when the motor is off-line so that transient differences in the phase rotation between the relay
and power system due to the switching of phases don't cause operation of any of the protection functions. To
ensure that setting groups are only changed when the machine is off-line the changing of the setting groups could
be interlocked with the IA/IB/IC undercurrent start signals and an undervoltage start signal in the PSL.
104
&
&
Directional
check
&
&
&
IDMT/DT
Timer Settings
Note: This diagram does not show all stages. Other stages follow Note: This diagram does not show all stages. Other stages follow
similar principles. similar principles.
P24xM
I2>1 Start I2>1 Start
I2>1 Trip I2>1 Trip
V00608
P24xM-TM-EN-2.1