The full form of BCU in a substation is Bay Control Unit. Actually, you may say this is the digital version of the total control and relay panel of a bay. Sometimes, a BCU is also used as an auxiliary BCU, which performs the auxiliary control, indication, and annunciation functions of the substation.
Basic Architecture of a BCU
The basic concept is very simple. It has a central processing unit. It is a single modern unit with a central processing unit, and it has a number of input terminals and output terminals associated with it.
Electrical Inputs Received by the BCU
Actually, a control panel receives different signals from a bay of the substation. It receives current signals from the secondary of the current transformers associated with the bay, and it receives secondary voltage signals from the potential transformers associated with that bay or from the common voltage transformer of the system. So, the Bay Control Unit has some input terminals which directly receive the current input and voltage input from the current transformer and voltage transformer respectively.
Analog Inputs to the BCU
Next, the BCU also receives analog inputs through transducers for different analog quantities like temperature, tap position, etc.
Binary Inputs and Potential-Free Contacts
Further, for judging the status of different auxiliary contacts of the circuit breaker and isolator, it has binary input terminals that use potential-free contacts of the equipment. In that case, the BCU sends its own DC voltage, for example 5 volts or 24 volts, and that voltage returns to the Bay Control Unit (BCU) through the contacts. If the contact is closed, the voltage returns and the status becomes one. If the potential-free contact is open, no voltage returns to the sending end, and the status becomes zero.
So, we are having three types of inputs: direct electrical inputs for CT and PT, analog inputs such as 4–20 milliampere inputs or 0–10 volt inputs from transducers for processed electrical signals like temperature and similar parameters, and binary inputs through the potential-free contacts of different equipment.
SCADA Communication Through BCU
Data Exchange Between HMI, BCU, and Field Equipment
Not only that, the BCU can also receive inputs from SCADA through its communication ports. So, if any command is initiated from the Human Interface Machine (HMI) of the substation, it comes to the Bay Control Unit (BCU) through the station bus and through the Ethernet switch. The Central Processing Unit of the BCU processes that command and sends instructions to the Intelligent Electronic Device (IED) of the associated equipment like the circuit breaker or transformer.
The status of that circuit breaker or transformer is then received by the BCU through its binary inputs. Through the Central Processing Unit of the BCU, it is converted into a communication-compatible signal and sent back to the SCADA monitor through the same station bus network. In this way, the position of the circuit breaker or isolator can be seen on the HMI screen of the SCADA system.
Interlocking Logic and Digital Safety in BCU
Additional Protection Through Programmable Interlocking
Although physically all the equipment associated with a bay have their physical, mechanical, and electrical interlockings, still a BCU can have the facility of programmed operation with interlocking logic. So, whenever a command is received either from SCADA or directly from the Bay Control Unit (BCU) itself through its input module, the Central Processing Unit verifies the interlocking logic. If everything is okay, then only it sends the instruction signal to the IED, that is, the Intelligent Electronic Device, of the associated circuit breaker or isolator to be operated.
So, it provides double safety, which means additional interlocking safety to the system digitally, in addition to its physical interlocking.