Application Notes P54x/EN AP/La4
MiCOM P543, P544, P545 & P546
(AP) 6-19
AP
In the case of high resistance external faults on a short line, particularly under heavy power
exporting conditions, zone 1 will remain stable due to dynamic downwards tilting of the top
line as explained earlier but the detection of high resistance internal faults especially towards
the end of the line needs consideration. In such applications a user has a choice to either
detect high resistance faults using highly sensitive Aided DEF or Delta Directional schemes
or to clear the fault with distance ground protection. If distance is to operate, it is necessary
to eliminate over-tilting for internal faults by reducing the initial -3° tilting angle to zero so that
the overall top line tilt will equal to
(Iph-I2) angle only.
As per Figure 4, the internal resistive fault will then fall in the zone 1 operating chara
cteristic.
However, it should be noted that for short lines the load angle is relatively low when
compared to long transmission lines for the same transfer capacity and therefore the top line
dynamic tilting may be moderate. Therefore it may be necessary to reduce the zone one
reach to guarantee zone 1 stability. This is particularly recommended if distance is operating
in an aided scheme. To summarize, for very short lines with large R/X setting ratios, it is
recommended to set the initial tilt angle to zero and zone 1 reach to 70-75% of the line
impedance.
Note: The above discussion assumes homogenous networks where the
angle of the negative sequence current derived at relaying point is
very close to the total fault current angle. If the network is non-
homogenous, there will be a difference in angle that will cause
inaccurate dynamic tilting, hence in such networks either quad with
fixed tilt angle or even Mho characteristic should be considered in
order to avoid zone 1 over-reach.
Fixed tilt angle:
As an alternative to Dynamic tilting, a user can set a fixed tilt angle. This is applicable to
applications where the power flow direction is unidirectional.
Exporting end:
To secure stability, the tilt angle of zone 1 at exporting end has to be set negative and above
the maximum angle difference between sources feeding the resistive faults. This data
should be known from load flow study, but if unavailable, the minimum recommended setting
would be the angle difference between voltage and current measured at local end during the
heaviest load condition coupled with reduced zone 1 reach of 70-75% of the line impedance.
Note: Figure 4 shows that at sharp fixed tilt angl
e, the effective resistive
coverage would be significantly reduced, and therefore for the short
lines the dynamic tilting (with variable tilt angle depending on fault
resistance and location) is preferred. For all other over-reaching
zones set tilting angle to zero.
Importing end:
Set zone 1 tilt angle to zero and for all other zones the typical setting should be positive and
between +(5-10)°.
Note: The setting accuracy for over-reaching zones is not crucial because it
will not pose a risk for relay’s maloperation, the purpose is only to
boost zone 2 and zone 4 reach and improve distance aided schemes.
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