Where:
• t
op
is the operating time of the relay
• T
01
is fixed time delay (setting)
• K is a constant (setting)
• I
S
is measured subsynchronous current in primary amperes
The existing relay was applied on a large 50Hz turbo generator which had shaft
mechanical resonance frequency at 18.5Hz. The relay settings were T
01
= 0.64
seconds, K= 35566 Amperes and minimal subsynchronous current trip level was
set at I
S0
=300 Amperes primary.
Solution:
First the IED configuration shall be arranged as shown in
Figure 294. Then the
settings for SMAI HPAC filter and multipurpose function shall be derived from
existing relay settings in the following way:
The subsynchronous current frequency is calculated as follows:
50 18.5 31.5
s
f Hz Hz Hz= - =
EQUATION13000030 V1 EN-US (Equation 524)
In order to properly extract the weak subsynchronous signal in presence of the
dominating 50Hz signal the SMAI HPAC filter shall be set as given in the
following table:
Table 45: Proposed settings for SMAIHPAC
I_HPAC_31_5Hz: SMAIHPAC:1
ConnectionType Ph — N
SetFrequency 31.5
FreqBandWidth 0.0
FilterLength 1.0 s
OverLap 75
Operation On
Now the settings for the multi-purpose overcurrent stage one shall be derived in
order to emulate the existing relay operating characteristic. To achieve exactly the
same inverse time characteristic the programmable IDMT characteristic is used
which for multi-purpose overcurrent stage one, which has the following equation
(for more information see Section “Inverse time characteristics” in the TRM).
1MRK 506 369-UEN B Section 13
System protection and control
Line distance protection REL670 2.2 IEC 583
Application manual
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