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The Heart of Gas Insulated Switchgear (GIS): The Bay Local Control Cabinet (LCC)

A Local Control Cabinet (LCC) is typically installed at each bay location to accommodate the wiring of the GIS bay circuits and establish a connection with the substation control room. The LCC includes a mimic diagram, switches, indicators, and annunciator interlocks. Although not traditionally viewed as a component of GIS, it does exert control over its operation.


The Bay Local Control Cabinet (LCC)
The Brain of a Gas-Insulated Switchgear (GIS): The Bay Local Control Cabinet (LCC)

Regulatory provisions for substation equipment can be classified as local, situated within the substation itself, or remote, located in a separate facility such as an operations center. Considering that most switches are motor-operated, Gas-Insulated Switchgear (GIS) offers a broader spectrum of control options and functionalities than other systems.


In Gas-Insulated Switchgear, the functions are integrated into a Local Control Cabinet (LCC), which needs to be seamlessly integrated into the customer's existing automation system.


The compact and unique design of GIS equipment restricts the selection of components like switches, terminal blocks, and indicator lights for use within the system. The local control cabinet (LCC) or marshalling box (MB) is the sole exception, acting as the established connection point between the utility and the GIS equipment.


Users may opt to install control or monitoring devices within the cabinet, or they may simply use the enclosure as a point for wiring marshalling.


For example, many users prefer to organize their switchyard facilities so that the equipment control panels are located close to the protective relay panels. In such cases, it may not be necessary to include disconnect and ground switch controls in the Local Control Cabinet (LCC). The cabinet would mainly function as a hub for wiring terminations and might include a push-button control for circuit breaker maintenance.


Furthermore, the Local Control Center (LCC) can be equipped with indicator lights on a mimic board to assist operational personnel. Should the LCC (Local Control Cabinet) be located at a considerable distance from the control house or protective relay installation, operational switches can be installed to minimize the distance operators have to travel.

This article will discuss the following topics in brief.


Table of Contents:


1. LCC Access and Layout Requirements (Integrated or Free Standing LCCs?)

Typically, each bay's switchgear in GIS transmission substations is controlled locally by a dedicated bay local control cabinet (LCC), which is ideally situated near the switchgear and integrated within the GIS switchgear itself, as illustrated in Figure 1.


Local Control Cabinets (LCCs) are essential for managing local plant operations throughout the commissioning, testing, and maintenance stages. This requirement is applicable to both Remote Terminal Unit (RTU) and Station Control System (SCS) applications.


Figure 1 depicts the integration of Local Control Cabinets (LCCs) with GIS switchgear.


Figure 1 – LCCs integrated in GIS switchgear

Figure 1 – LCCs integrated in GIS switchgear

When freestanding Local Control Cubicles (LCCs) are positioned opposite to the GIS switchgear, they become the focal point of operation. The configuration will be such that the LCCs on the left side will have odd bay numbers, and those on the right will have even bay numbers. It is important to ensure that the LCCs paired with the GIS are located directly opposite the switchgear, as illustrated in Figure 2.


Figure 2 – An example of free sanding LCCs in front of GIS switchgear

Figure 2 – An example of free sanding LCCs in front of GIS switchgear

2. LCC Physical Requirements

Regarding ground level, the LCCs should be positioned at a height not exceeding 1800 millimeters. The cabinets must be placed at the rear, on gland plates, to facilitate the entry of control wires from the switchgear. The cabinet may feature a swing frame, with the door designed to open to the left and swing outward.


The front of the cabinet is usually where all control points, such as AC, DC, and VT MCBs, are mounted. This arrangement allows for easy operation and inspection without having to access the interior of the cabinet; however, alternative configurations are also available.


It is advised to label removable gland plates with permanent markers indicating the Safe Working Load they can support. Additionally, customers must ensure that the electrical control points on the LCC are positioned no less than 1800 millimeters from the floor level.


In addition to the design, supply, and installation of permanent platforms attached to the GIS, all control points must be situated within 1800 millimeters from the platform's top. Alternatively, supplying and installing freestanding cabinets positioned separately from the switchgear is an option. A person standing at the station's center, facing the LCCs, should see bays with odd numbers to the left of the mid sectionalizer and bays with even numbers to the right.

The LCCs must be mounted independently and positioned opposite the switchgear. It is essential that the operational control is designed to reflect this arrangement.


Figure 3 – An example of a Local Control Cabinet for RTU application

Figure 3 – An example of a Local Control Cabinet for RTU application

3. Application Of RTU & Substation Control System LCC

IFor RTU stations, it is mandatory to have a mimic diagram representing each bay, including all necessary operational controls and position indicators for the HV plant on the LCC, along with VT symbols. In stations integrated into the Substation Control System, each bay-specific LCC is equipped with a single Bay Control Unit (BCU). The LCD HMI mimic facilitates control over the BCU.


The construction of the LCC mimic must feature clearly positioned and labeled control switches and semaphores, utilizing only the standard ESBN plant designations in strict compliance with the project-specific single-line diagram (SLD) and signals list. The use of IEC designations on LCCs is expressly forbidden.


As shown in Figure 3, the "A" busbar should always be positioned above the "B" busbar in stations where two busbars are operating at the same time.


Figure 4 – LCC for a station control system (SCS) application

Figure 4 – LCC for a station control system (SCS) application

3.1 LCC Control Switches & Control On/Off Switches

To guarantee operational control over all circuit breakers, disconnectors, and earth switches, including both maintenance and high-speed types, the LCC mimic should be outfitted with illuminated controls and discrepancy-type switches. This requirement is specific to RTU applications. The operational control switches must be located near the HV plant symbol that they correspond to on the LCC mimic, as indicated by the red dotted box in Figure 3.


LCD Human Machine Interfaces (HMIs) are utilized to facilitate operational control over high-voltage facilities within Station Control Systems (SCS) applications.


Control On/Off switches for all HV Plant circuit breakers, disconnectors, and earth switches, both maintenance and high-speed types, must be installed on the LCC mimic diagram, as indicated by the blue dotted box in Figures 3 and 4. This requirement applies to implementations using both RTU and Station Control Systems (SCS).

When clients are provided with separate motors for disconnectors and maintenance earth switches, they must install individual control switches that include both on and off functions. The position indicators for these Control On/Off switches should be connected in series for each bay to provide the National Control Center with clear indications of the remote control status.


The National Control Centre (NCC) must receive Double Point Status Indications whenever one or more switches are in the "Off" position. The specifics of these indications will be elaborated in the project-specific signal list.


Figure 5 – Outline Arrangement of LCC

Figure 5 – Outline Arrangement of LCC

3.2 Local/Remote Control Key Switch

Furthermore, the LCC Local/Remote Control Key Switch is suitable for GIS transmission substations managed by either an RTU or a Station Control System (SCS). To facilitate the choice between local or remote control, each LCC should have a key switch dedicated to its respective bay. This key switch must also have the ability to secure a hold-off notice using a cable tie or a comparable fastening method.


Personnel working on high-voltage equipment are equipped with a switch as a last line of defense, ensuring that all remote commands have ceased and that the equipment is secure for commissioning, testing, or maintenance purposes.


The design of the switch should ensure that operational control is exclusively local at the LCC when it is set to the Local position. This design significantly restricts the capacity for remote operational control from the Station Control Cabinet (MIMIC/SCS HMI) or the NCC.

The switch should be operated in a manner that allows control only through the Station Control Cabinet when in the Remote position (Normal Operation), thereby limiting operational control from the Local Control Cabinet (LCC).



Figure 6 – Local/Remote switch in a Local Control Cabinet (LCC)

Figure 6 – Local/Remote switch in a Local Control Cabinet (LCC)

3.3 Earth Unlock Key Switch

RTU stations are equipped with specialized key switches on the LCCs (Local Control Centers) for Feeder Earth Switches and Customer Transformer Earth Switches, in accordance with the project's interlocking standards. Similarly, dedicated earth unlock key switches are also installed on the Station Control Cabinets/Mimics.


This key switch is designed as an additional safety measure, prompting operators to pause and reflect before engaging an earth switch. It should be noted that Local Control Cabinets (LCCs) in Station Control Systems do not necessitate a distinct earth unlock key switch.


The inherent command logic of the system provides adequate protection for Station Control System (SCS) applications. Requiring the operator to complete a two or three-step selection process to operate the earth switch is deemed to provide a comparable level of safety.


4. Control Wiring And Marshalling

The installation requires EMC-shielded control cabling, specifically in black, to connect the switchgear and LCCs. The control cables should come in pre-cut lengths, with either pre-termination on the switchgear or equipped with plug and socket systems for straightforward plug-in connections. Note that this excludes the circuit wiring for CT and VT.


All secondary connections for current and voltage transformers, along with controls, alarms, indicators, and both AC and DC supplies, should be wired to their respective designated terminals.


Provisions should be made at the file terminals for the connection and grounding of multi-core screened cables of 6 mm2 (for CT and VT circuits) and control cables of 1.5 mm2. Furthermore, spare auxiliary switch contacts should be connected to the assigned terminals. Adequate space should be reserved for terminating and connecting additional wires required for future external interlocking. An extra 10% of terminals will be provided for future use.


The terminal type specified for the file should be as follows:

  • Terminals for CT secondary circuits (for example Phoenix UGSK/S and URTK/SP)

  • Terminals for VT secondary circuits (for example Phoenix URTK/S)


Figure 7 – PT bay front view diagram

Figure 7 – PT bay front view diagram

Customers must submit drawings that clearly depict the physical layout of the proposed cabinets, detailing the location of all terminals, devices, and the size of the trunking. Within the LCC, PVC trunking with PVC covers is mandatory.


Trunking must be sized to accommodate all the wiring, ensuring there is sufficient extra capacity for any future additions.


The cabinet layout drawings are set for an extensive design review. It is expected to be possible to link the secondary circuits of the current transformer at each corresponding file terminal block, thereby establishing a short circuit. Connections between LCCs for shared supplies, position indicators, voltages, and so on, should be made with isolatable terminals. The design of terminals and connections in the LCC should be such that they minimize disruptions during future expansions of the station to accommodate additional bays.


Careful consideration is required in the design of busbar and interlocking schemes to ensure they support the future expansion of GIS installations. Terminals and connections should be crafted to permit the disconnection, bypassing, and extraction of a bay and its corresponding LCC, without disrupting the secondary control systems or the functioning of neighboring LCCs.

The wiring for the Local Control Cabinet (LCC) extension should incorporate disconnector-type connections on critical circuits to minimize disruptions. Consequently, the interlocking and busbar protection associated with the initial Gas-Insulated Switchgear (GIS) development will remain operational.


Figure 8 – PT bay front view diagram

PT bay front view diagram
Figure 8 – PT bay front view diagram

5. Local Metering

In RTU applications, every bay requires a Voltmeter and Ammeter at the Local Control Cabinet (LCC) to display the line voltage and current for each feeder and transformer outlet. The URTK/SP type, particularly the Test disconnect terminal block, is the suggested terminal for ammeter connections.


In Station Control System (SCS) applications, it's crucial to ensure local metering is accessible through the BCU (Bay Control Unit) at the LCC (Local Control Center).


Figure 9 – URTK/SP – Test disconnect terminal block with slide, nominal voltage of 500 V, nominal current of 41 A, screw connection method, rated cross section of 6 mm², and a cross section range of 0.5 mm² to 10 mm².


Figure 9 – URTK/SP – Test disconnect terminal block with slide, nominal voltage of 500 V, nominal current of 41 A, screw connection method, rated cross section of 6 mm², and a cross section range of 0.5 mm² to 10 mm².

6. Fault Signaling

In addition to any alarms and displays for switchgear faults present on-site, each fault-indicating device should be fitted with a voltage-free, normally closed (NC) contact.


This contact must be wired to terminals that are integrated into the substation's signaling system.


7. Additional Requirements

Each cabinet should be fitted with permanent lamps that have door switches, cubicle heating, and 230 V AC anti-condensation heaters. The preferred method for wiring, other than CT & VT circuit wiring, is to employ a pre-wired plug and socket configuration.


Figure 10 – Wiring diagram of a LCC heating, illumination and heating of HV equipment

Figure 10 – Wiring diagram of a LCC heating, illumination and heating of HV equipment

8. Swichgear Interlocking

Interlocking conditions are meticulously engineered to prevent the operation of disconnectors under load and the engagement of earthing switches on a locally energized circuit. A fail-safe interlocking mechanism is essential to guarantee that a component's failure does not lead to accidental operation. Primary connections from the high voltage switchgear are required to signal the position for the interlocking system.


Incorporating auxiliary relays into an interlocking scheme necessitates their operation in a fail-safe mode to ensure system integrity.


Accidental commands to open or close switchgear, through hold-on circuits or other methods, will be prevented. The station's interlocking design should be developed with future upgrades in mind, taking into account potential changes in interlocking during subsequent phases.

The early phases of the interlocking design must be made future-proof to reduce harm to existing interlocking circuits during system expansion for future bays. It is essential to devise future bay busbar disconnect interlocking scenarios that minimize changes to the current interlocking circuits in operational stations.


Figure 11 – An example of a feeder interlocking conditions

Figure 11 – An example of a feeder interlocking conditions

Power Engineering Guide
Power Engineering Guide by Siemens

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Power Engineering Guide by Siemens

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