Bay Controller (BCU): Parts, Functions, Testing

Short answer

The bay control unit (BCU) is the IED that operates the circuit breaker and disconnectors of a bay, applies the interlocking, checks synchronism before closing and delivers measurements, positions and alarms to the control system. It is the point where local and remote operation meet the primary equipment.

What a bay controller is

The bay controller, or BCU (bay control unit), is an intelligent electronic device (IED) dedicated to the control of one bay: a line, a transformer, a bus coupler or a feeder. It receives the positions and alarms of the primary equipment, executes the open and close commands, applies the interlocking and delivers measurements and events to the substation control system and to the control center.

In a conventional substation these tasks were done by a mimic panel with control switches, auxiliary relays and extensive wiring. The bay controller concentrates them in one configurable, networked device. It is installed in the panels of the control building and is a piece of the bay level of substation automation.

Main parts

Binary inputs for positions (as double-point), alarms and primary equipment status.
Binary outputs for open and close commands to the circuit breaker, disconnectors and earthing switches.
Analog inputs for current and voltage, or subscription to sampled values from merging units in digital substations.
Local interface with a bay mimic, local/remote selector and command keys.
Communication ports, with PRP or HSR redundancy when the architecture calls for it, and time synchronization.
Programmable logic for interlocking and automatic sequences.

How it works

The controller sits at the bay level of the IEC 61850 architecture, between the station level (local HMI, RTU or gateway) and the process level (primary equipment or merging units).

Its main functions, with their IEC 61850 logical nodes:

Control (CSWI) of the circuit breaker (XCBR) and the disconnectors (XSWI), normally with select-before-operate (SBO).
Interlocking (CILO): bay interlocking, with its own positions, and station interlocking, with positions from other bays arriving as GOOSE messages.
Synchrocheck 25 (RSYN) before closing the circuit breaker.
Measurements (MMXU): voltages, currents, power and frequency.
Event recording with time stamps and alarm management.
Automatic sequences, depending on the project: transfers, switching sequences, voltage regulation or capacitor bank control.

The configuration of an IEC 61850 substation is described with SCL files: the manufacturer’s ICD, the SCD of the whole substation and the CID loaded into each IED. The IEC 61850 configuration defines which data each device publishes and who subscribes.

Operation follows a command hierarchy: from the primary equipment itself, from the bay controller, from the station HMI or from the remote control center. The local/remote selector at each level defines who has control at any moment and prevents two operators from operating the same device at once. At high and extra-high voltage, the owner’s philosophy also usually requires critical interlocks to keep working even if the station network is lost, with hardwired backup or local logic.

Types

Dedicated controller at high and extra-high voltage: one device per bay, separate from the protection. Several manufacturers have specific lines for this function.
Combined protection and control IED at medium voltage, in industry and in solar plants: the feeder relay also operates the circuit breaker and shows the mimic.
Controllers in digital substations, which receive sampled values and positions over the process bus.
PLCs and distributed control systems in industry, when the substation is part of the plant control.

How each manufacturer implements it

Siemens SIPROTEC, SEL, ABB Relion, GE Multilin and Schneider MiCOM or Easergy offer bay controllers or IEDs with control functions. Each organizes the interlocking logic, the mimic and the commands differently, and is configured with its own software (DIGSI, AcSELerator, PCM600, EnerVista, Easergy Studio) as well as the IEC 61850 system configurator. This is why a change of interlocking in a substation with equipment from several brands requires knowing each tool and keeping the SCD file consistent.

Implementation details that matter

The manual of the SIPROTEC 5 6MD85/6MD86 high-voltage bay controller (edition 07.2017, published in Russian) shows design decisions that exist, under other names, in all devices:

One block per function. Each circuit breaker or disconnector is modeled with three blocks: the switching device, which stores the real unfiltered position, the control, and the interlocking. They correspond to XCBR or XSWI, CSWI and CILO. Other functions, such as synchrocheck or reclosing, are added to the breaker even though they are not control.
Position filter and chatter blocking. Position signals are filtered in the control block, not in the switching device block. The software filter time is set from 0 ms to one day in steps of 1 ms, and the time stamp is corrected by subtracting that time and the fixed hardware filter, so that it matches the real change. In addition, a signal that changes more times than allowed within a test time is blocked so as not to fill the event list; with zero changes allowed, the function is disabled. An auxiliary contact with bounce or vibration is a classic cause of saturated event lists.
Command authority. The local, engineering software, station and remote levels exclude one another, and the station level is defined in edition 2 of IEC 61850. The state of the key and of the authority can be read in the logic, so that rules can be automated, such as switching to local when the key is left without interlocks. See command levels and local/remote selector.
Acquisition blocking and manual update. If an auxiliary contact is faulty, the device allows blocking the acquisition of that switching device and setting its position by hand, only from the local panel or through IEC 61850, and it stays until someone removes it. While it is active no position changes are detected, so the interlocks do not respond to that device.
Double-command blocking. It is active from the factory and prevents two commands from being executed at once on a switching device. It can also be extended to other bays with a “not selected” signal published by GOOSE.

The detail of how commands are checked and what is skipped in interlock-free mode is in substation switching operations and in interlocking release.

Testing at commissioning and in maintenance

Point-to-point of inputs and outputs with the real primary equipment.
Local and remote commands of each device, with SBO and supervision times.
Complete interlocking matrix, including the conditions that arrive from other bays.
GOOSE: publication, subscription, quality, behavior on message loss.
Synchrocheck with secondary injection of two voltages.
Measurement accuracy with current and voltage injection.
Time synchronization and event time stamping.
Signals up to the control center, as part of the integrated system tests.

Common failures

Interlocks that block because of an intermediate or invalid position (00 or 11) of a disconnector with misadjusted auxiliary contacts.
GOOSE subscriptions broken after changing the configuration of an IED without updating the SCD.
Data with “test” or “invalid” quality that the logic treats as false.
Local/remote selector forgotten in local.
Commands that expire because execution arrives after the selection time.
Loss of time synchronization that puts events out of order.

Testing, configuration and commissioning with Tiza Group

At Tiza Group we configure and test bay controllers of the main brands, on site (in Colombia, or elsewhere on request) or through remote configuration, and we commission the control system with its signals to the control center. Contact us with the brand and model of your equipment.

References

IEC 61850-6, SCL configuration language.
IEC 61850-7-4, logical nodes (CSWI, XCBR, XSWI, CILO, RSYN, MMXU).
IEC 61850-8-1, mapping to MMS and GOOSE.
IEC 61850-9-2, sampled values.
IEC 62439-3, PRP and HSR redundancy.
IEEE C37.2, device function numbers.
Siemens, SIPROTEC 5 6MD85/6MD86 bay controller manual (switching device blocks, filter and chatter blocking, command authorities), edition 07.2017, in Russian.

Frequently asked questions

Why does the bay controller not let me close the circuit breaker?

The usual causes are an interlocking condition not met, an invalid or intermediate disconnector position, the local/remote selector in the wrong position, a synchrocheck condition not met, or a GOOSE signal from another bay with invalid quality or lost. The device event log usually shows which one.

What is the difference between a bay controller and a protective relay?

The relay detects faults and trips. The bay controller carries out operating switching and applies the interlocking. At medium voltage and in industry a single IED usually does both; at high and extra-high voltage it is common to separate them.

What is a select-before-operate (SBO) command?

It is a two-step command: first the device is selected and the controller checks that the operation is possible, and then the execute order is sent within a time limit. It prevents operations by mistake or from a corrupted message.