Design - ABB REC 561 User Manual

1MRK 580 141-XEN
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3 Design

Disturbance recorder
The calculation of the RMS values takes some time. The minimum time
between the start of two different recordings must be about 300 ms to
obtain correct calculation. If this time is less, the RMS values for the sec-
ond disturbance are indicated as 0.
The disturbance recording function is an optional function in the REx 5xx
terminals. The processing of analogue signals is handled by a dedicated
DSP (digital signal processor). Other functions are implemented in the
main CPU. The memory is shared with other functions.
The numerical signals coming from the A/D conversion module in serial
form are converted to parallel form in a dedicated DSP. The analogue trig
conditions are also checked in the DSP.
A check of the start conditions is performed by searching for a maximum
value. This is a positive peak. The function also seeks a minimum value,
which is the negative peak.
When this is found, the absolute average value is calculated. If this value
is above the set threshold level for the overfunction on the channel in
question, an overfunction start on that channel is indicated. The overfunc-
tion is indicated with a greater than (>) sign.
Similarly, if the average value is below the set threshold level for under-
function on the channel in question, an underfunction start on that channel
is indicated. The underfunction is indicated with a less than (<) sign.
The procedure is separately performed for each channel. This method of
checking the analogue start conditions gives a function that is insensitive
to DC offset in the signal. The operating time for this start is typically in
the range of one cycle, 20 ms in a 50 Hz network.
The numerical data, along with the result of the trigger condition evalua-
tion, are transmitted to the main CPU. The main CPU handles these func-
tions:
• Evaluation of the manual start condition
• Evaluation of the binary start condition, both for true binary input
signals and for internally created logical signals
• Storage of the numerical values for the analogue channels
The numerical data for the analogue channels are stored in a cyclic pre-
fault buffer in a RAM. When a trigger is activated, the data storage is
moved to another area in the RAM, where the data for the fault and the
subsequent post-fault period are stored. Thus, a complete disturbance
recording comprises the stored data for the pre-fault, fault, and post-fault
period.
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