ABB RELION REX640 Technical Manual page 554

Protection functions
intermittent) or the fault resistance value (low or high ohmic). MFADPSDE replaces
traditional sensitive directional earth-fault protection (such as Iocos) and transient
earth-fault protection (such as Wischer principle) combining the same functionality
into a single function block (see
Table 612: Comparison of the MFADPSDE functionality with traditional methods in
resonant earthed networks
Traditional Iocos
Traditional Wischer
MFADPSDE
As shown by numerous practical field tests, MFADPSDE provides better sensitivity
and selectivity compared with traditional methods with less complexity in
settings and configuration. MFADPSDE provides enhanced earth-fault protection
performance, especially in networks with cable feeders where distributed
compensation coils are applied and the earth-fault type is typically intermittent.
It is applicable in feeders with distributed compensation regardless of the size or
localization of distributed coils. Selective protection can be achieved also in case of
overcompensated feeders.
The neutral point in compensated networks is earthed via a controllable centralized
arc suppression coil located typically in the primary substation. Additionally, (fixed)
distributed compensation coils can be used, which are installed in relevant locations
along the protected feeders.
In unearthed networks, the neutral point in the primary substation is not connected
to earth. However, in unearthed networks (fixed) distributed compensation coils
can also be used to compensate a part of the total capacitive earth-fault
current. Typically, the resulting compensation degree is less than 50 percent. Thus,
unearthed networks with or without distributed compensation coils are generally
referred to as unearthed networks in this manual.
MFADPSDE operation is based on multifrequency neutral admittance measurement
using the cumulative phasor summing technique. This concept provides
extremely secure, dependable and selective earth-fault protection also when the
residual quantities are highly distorted and contain non-fundamental frequency
components.
The sensitivity that can be achieved is comparable with traditional fundamental
frequency based methods such as IoCos/IoSin (DEFxPTOC), Watt/Varmetric
(WPWDE) and neutral admittance (EFPADM).
MFADPSDE can detect faults with dominantly fundamental frequency content as
well as transient, intermittent and restriking earth faults. The function can be used
as an alternative solution to transient or intermittent function INTRPTEF.
MFADPSDE supports fault direction indication both in operate and in non-operate
direction, which can be used during fault location process. The in-built transient
detector can be used to identify restriking or intermittent earth faults, and
discriminate them from permanent or continuous earth faults.
The operation characteristic is defined by a tilted operation sector, which is
universally valid for unearthed and compensated networks.
554
Table 612
).
Earth-fault type
Transient Continuous Restriking/
x
x
x x
1MRS759142 F
Low-ohmic
Intermitten
t
x
x x
x x
Technical Manual
High-
ohmic
x
x
REX640