Siemens Reyrolle 7SR5, Руководство пользователя

Страница: 317 / 504

Parameter:
Gn 87T-BD > S1L
•
Default setting: 4 x In (4 ⋅ I
rated
)
Bias slope 2 (see S2 below) is used for bias current levels measured above this setting.
When a through fault occurs, saturation of 1 or more CTs may cause a transient differential current to be
measured by the relay. This bias slope 1 limit is chosen to ensure the biased differential function is stable
during this time. Lower setting values increase protection stability.
The 3 phase through fault current can be estimated from the transformer impedance. For a typical grid
transformer having a 15 % impedance, the maximum through fault will be 1/0.15 = 6.66x rating. Typi-
cally a setting value is chosen to introduce the increased bias at half of the transformer through fault
current level, so 6.66/2 = 3.33 and a setting of 3 would be selected as the nearest lower setting available.
Parameter:
Gn 87T-BD > S2 Type
•
Default setting:
Line
This defines the shape of the S2 bias characteristic.
Parameter:
Gn 87T-BD > S2
•
Default setting:
1.5x
(1.5 ⋅)
When a through fault occurs, saturation of 1 or more CTs may cause a transient differential current to be
measured by the relay. Bias slope 2 is chosen to ensure the biased differential function is stable during
this time. Higher setting values increase protection stability.
Parameter:
Gn 87T-BD > Delay
•
Default setting:
0 s
A 5 ms setting is recommended where the circuit is cabled to ensure stability during resonant conditions.
Parameter:
Gn 87T-BD > External Fault
•
Default setting: 2.4 x In (2.4 ⋅ I
rated
)
When any measured bias current is above this level the
Clearance Time
timer is started, see below.
Parameter:
Gn 87T-BD > Clearance Time
•
Default setting:
0.3 s
Outputs from the
External Fault
detector are raised after elapse of this time delay. These outputs
can be used to indicate transformer overload conditions.
Parameter:
Gn 87T-HS > Setting
•
Default setting: 20 x In (20 ⋅ I
rated
)
The 87T-HS function element provides unrestrained differential protection, it is used to provide fast trip-
ping for high fault currents within the protected zone.
The 87HS element is set as low as possible but to ensure stability must not be less than the maximum 3
phase through fault current and not less than half the peak magnetizing inrush current.
A setting of 7 ⋅ I
rated
or 8 ⋅ I
rated
has shown to be sufficiently sensitive for internal faults as well as
providing stability during external faults and transient system conditions. A setting of 7 ⋅ I
rated
will be
stable for a peak magnetizing inrush levels of 14x rated current.
Smaller transformers typically have lower impedance and therefore higher through fault levels and
magnetizing inrush currents. A setting of 8 ⋅ I
rated
can be used as CT saturation is reduced as system X/R is
usually relatively low and the peak level of magnetizing current does not usually exceed 16x rating.
Protection and Automation Functions
5.22 87T Transformer Differential Protection
Reyrolle 7SR5, Transformer Protection Device, Device Manual 317
C53000-G7040-C015-1, Edition 11.2019