Siemens siprotec SJ62 User Manual page 218

Resonant-ground-
ed System
Grounded System
Electrical Machines
Angular Error Com-
pensation (CTs)
Note Regarding
Settings List for
Sensitive Ground
Fault Detection
SIPROTEC 4, 7SJ62/63/64 Handbuch
C53000-G1140-C147-A, Edition 07.2015
In a resonant-grounded system, directional determination on the occurrence of a
ground fault results more difficult since the small residual wattmetric current for mea-
surement is usually dwarfed by a larger reactive current (be it capacitive or inductive)
which is much larger. Therefore, depending on the system configuration and the posi-
tion of the arc-compensating coil, the total ground current supplied to the device may
vary considerably in its values with regard to magnitude and phase angle. The relay,
however, must evaluate only the active component of the ground fault current, that is,
cos ϕ. This demands extremely high accuracy, particularly with regard to phase
I
Ns
angle measurement of all instrument transformers. Furthermore, the device must not
be set to operate too sensitive. When applying this function in resonant-grounded sys-
tems, a reliable direction determination can only be achieved by connecting cable core
balance current transformers. Here the following rule of thumb applies: Set pickup
values to about half of the expected measured current, thereby considering only the
residual wattmetric current. Residual wattmetric current is mainly due to losses of the
Petersen coil. Here, the COS ϕ measuring type is used since the resistive residual
wattmetric current is relevant.
In grounded systems, a value is set below the minimum anticipated ground fault cur-
rent. It is important to note that INs dir (current value RELEASE DIRECT.) only detects
the current component that is perpendicular to the directional limit line defined at ad-
dresses 3124 and 3125. COS ϕ is the type of measurement used, and the correction
angle is set to –45°, since the ground fault current is typically resistive-inductive (right
section of Figure "Directional characteristic for cos-ϕ-measurement in the functional
description of the sensitive ground fault detection).
One may set the value COS ϕ for the measurement type and use a correction angle
of +45° for electrical motors supplied from a busbar in an ungrounded system, since
the ground current is often composed of an overlap of the capacitive ground current
from the system and the resistive current of the load resistance (Figure "Directional
characteristic for cos-ϕ-measurement" in the functional description of the sensitive
ground fault detection, left part).
The high reactive component in a resonant grounded system and the inevitable air gap
of the cable core balance current transformer often require the angle error of the cable
core balance current transformer to be compensated. In addresses 3102 to 3105 the
maximum angle error CT Err. F1 and the associated secondary current CT Err.
I1 as well as another operating point CT Err. F2/CT Err. I2 are set for the ac-
tually connected burden. The device thus approximates the transformation character-
istic of the transformer with considerable accuracy. In ungrounded or grounded
systems angle compensation is not required.
In devices with sensitive ground fault input, which is independent of the nominal
current rating of the device, settings may in general also be entered as primary values
under consideration of the current transformer ratio. However, problems related to the
resolution of the pickup currents can occur when very small settings and small nominal
primary currents are given. The user is therefore encouraged to enter settings for the
sensitive ground fault detection in secondary values.
2.12 Ground Fault Protection 64, 67N(s), 50N(s), 51N(s)
216