Normally, for a forward fault, phase difference between voltage and current of relay (theta) lies between -90 and +90; and for a reverse fault, theta is outside of this range. Some works, however, use negative components of voltage and current of relay to identify fault direction. I want to know that what is the criterion for a forward/reverse fault in this condition? In my simulation studies, phase difference between negative components of voltage and current lies between -90 and +90 for a reverse fault.
H Iman,
Depends entirely on the type of relay logic you wish to realize.
You may wish to consult the following book, unless you have already done so:
S. H. Horowitz, and A. G. Phadke, "Power System Relaying", John Wiley, 2013.
-Sanjay
Hi Sanjoy Roy
I found an answer to my question in following paper:
M. Amin Zamani, Tarlochan S. Sidhu, and Amirnaser Yazdani, A Protection Strategy and Microprocessor-Based Relay for Low-Voltage Microgrids, IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 26, NO. 3, JULY 2011.
"The negative-sequence directional element is based on the same algorithm as the one described for a regular directional element, except that the algorithm output is logically inverted, compared to the positive-sequence directional element."
Hi Iman,
Good to see that you have found an answer to the problem. I would however advise some caution !
The statement that you have gathered from Zamani et al is strictly true only if the prefault network is perfectly balanced ! I imply not only balanced lines and apparatus, but also balanced loads !
As you know, in a distribution system it is quite common to have single phase loads, which lead to some negative sequence content in a three phase circuit even though the operation is perfectly healthy.
In such cases, the negative sequence directional logic may not be a straightforward inversion of the positive sequence directional logic under all circumstances! Thresholds and limits may have to be set more carefully.
With best wishes.
-Sanjay
The negative- and zero-sequence networks are passive, no "source" behind the relay. The fault (unbalance, in general) acts as a source that excites the V2 and I2 quantities at the relay location. So for a forward fault (the Thevenin source in front of the relay), V2 = -I2*Z2SYS_LOC, and for a reverse fault (the Thevenin source behind the relay) V2 = +I2*(Z2LINE+Z2SYS_REM). Similarly for the zero-sequence and incremental-quantity directional elements. More details in this primer (paper and presentation): Sequence Component Applications in Protective Relays - Advantages, Limitations, and Solutions
negative sequence current will be flowing opposite direction of positive sequence current.. thus the angle for negative sequence current has 180 degree difference from positive sequence current.. when a fault occurs at a line, positive sequence current will be flowing toward the fault point while negative sequence current will be flowing toward the terminal..
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