Date Code 20010731 Control Logic 5-1
SEL-352-1, -2 Instruction Manual
SECTION 5: CONTROL LOGIC
This section describes relay elements and logic with figures and accompanying text. Details on
setting ranges and default settings are listed in Section 7: Setting the Relay.
SIGNAL PROCESSING
The relay filters the current and voltage channels and samples the signals 64 times per power
system cycle. The relay stores the analog low-pass filtered samples for event reporting. The
analog low-pass filter has a 3 dB cutoff at 1200 Hz. Refer to Figure 1.3 in Section 1:
Introduction and Specifications for a graph of the analog low-pass filter. The microprocessor
digitally filters each signal using a cosine digital filter, except for the 50FT elements where a
half-cosine digital filter is used.
Frequency Tracking Specification
All SEL-352 Relay calculations and decisions are performed every 1/8 or 1/4 of a power system
cycle depending on the logic. Refer to Table 5.5 for relay logic processing order and intervals.
The SEL-352 Relay NFREQ setting determines the nominal frequency of the system (50, 60 Hz).
As the power system frequency fluctuates, the relay tracks to that frequency to determine
processing intervals. With NFREQ = 50, the relay tracks over the range of 45–55 Hz. With
NFREQ = 60, the relay tracks over the range of 55–63 Hz. If the system frequency goes beyond
the tracking range, the relay stops tracking at that boundary until the system frequency returns.
The tracking frequency follows the A-phase voltage frequency of the voltage source that is
greater than the 59L setting. If both X and Y voltage sources satisfy this criterion, the relay
calculates an average frequency using both X and Y voltage input zero crossings.
The event reports display the tracking frequency at the time of trigger.
RELAY LOGIC OVERVIEW
The SEL-352 Relay is fully programmable for complete custom logic control but has selectable
fixed logic for secure traditional breaker failure schemes. Minimal fixed logic settings make the
relay easy to use. The optional custom logic provides flexibility when it is needed.
As an overview to the relay, consider relay function in four separate parts. Link the four parts
together to make the relay complete. These four parts for overview purposes are the Input
Assignments, Scheme Logic, Trip and Close Logic, and Output Assignments. Refer to Figure
5.1 for a block diagram of this overview. The Analog Measurements are part of the Scheme
Logic.
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