System design

Bay unit (500BU04)

The bay unit is the interface between the protection and the primary system process comprised of the main CTs,

isolators and circuit-breaker and performs the associated data acquisition, pre-processing, control functions and bay

level protection functions. It also provides the electrical insulation between the primary system and the internal

electronics of the protection.

The bay unit contains four input CTs for measuring phase and neutral currents with terminals for 1 A and 5 A.

Additional interposing CTs are not required, because any differences between the CT ratios are compensated by

appropriately configuring the software parameter of the respective bay units.

The bay unit optionally contains five input voltage transformers for the measurement of the three-phase voltages and two busbar voltages to allow the recording of

voltage disturbances. (see Chapter 10).

After acquisition, the analog current and voltage signals are converted to numerical signals which are preprocessed and filtered.

Zero-sequence voltage and zerocurrent signals are also calculated internally. 

The process data are transferred at regular intervals from the bay units to the central processing unit via the REB500 process bus.

Every bay unit has a minimum of 21 binary inputs and 19 binary outputs. The binary I/O module can be used to

detect and process the positions of isolators and couplers,blocking signals, starting signals, external resetting signals, etc.

The binary input channels operate according to a patented pulse modulation principle in a nominal range of 24 to 250 V DC. 

The PC-based HMI program provides settingsfor the threshold voltage of the binary inputs.

Out of the 19 binary outputs, 6 are equipped with power output relays and 13 are signal output relays (see contact data in Chapter 10).