Inside the Modern Digital Low Voltage Switchgear: Devices and Communications

LV Digital switchgear evolution

Low voltage switchgear is one of the significant pieces of equipment utilized for power distribution in electrical systems of all types of process and chemical plants. There is a migration from conventional low-voltage switchgear to the current digital version due to the advancements in digital technologies during the last 30 years.

By having a continually self-supervising system with maximal fault detection, using digital signals and the cutting-edge IEC 61850 communication network offers considerable benefits. It makes it simple to add new components in the future by keeping an eye on all component connections and looking for open circuits.

And so the digital low voltage switchgear becomes intelligent and versatile!

In the early 1990s, when current and voltage sensors were introduced into low-voltage switchgear applications, they replaced conventional instrument transformers because of their better performance in relay protection and metering applications. That caused a significant shift in accepting new low-voltage switchgear design technologies.

Non-conventional instrument transformers (NCITs), which employ a novel measuring technique, are able to transmit signals across longer distances without sacrificing high accuracy levels.

Without the significant heating losses of conventional measurement instruments, these signals can be produced and sent. NCITs have secondary outputs in the millivolt (mV) range.

Table of Contents:

1. IEC 61850 and Digital Low Voltage Switchgear
2. LV Switchgear Architecture In Substation Automation
3. Devices and Components of Digital LV Switchgear
   1. Power Section
   2. Control Section
   3. Ethernet Switches
   4. Ethernet/Communication Cabling
4. Case Study of LV Switchgear With Two Incomers and One Bus Coupler
5. Communication Protocols In Digital Low Voltage Switchgear
6. Advantages of LV Digital Switchgear

1. IEC 61850 and Digital Low Voltage Switchgear

IEC 61850 is a general standard to define the architecture, communication, control methods and functions of different devices used for substation automation. This standard defines process bus communication, which improves the overall system reliability and operation of digital switchgear when we use different types of original equipment/component manufacturers in power distribution automation.

IEC 61850 helps an Electrical design engineer define a simple architecture for substation automation systems using digital switchgear, which can be extended to smart grids and other complex systems.

With the addition of distributed energy sources to the grid and the introduction of battery energy storage and charging systems, the power networks are becoming more complex, smarter and intelligent and it demands the use of Digital switchgear.

So IEC 61850 standard is necessary for introducing a common language for power automation and easy upgrade.

Learn More – IEC 61850 for digital substation automation systems

IEC 61850 for digital substation automation systems

Due to the general nature of the IEC 61850 standard, the plant owner has the choice to define and select the switchgear and its components among various competitive vendors and select any vendor which follows this standard for their manufacturing and makes them part of the substation automation.

IEC 61850 comprises ten major sub-parts thoroughly explaining the configuration, monitoring, recording control and protection of electrical networks defined under the digital version of switchgear in substation automation. This standard provides software methods and develops data models, reducing switchgear upgrades and modification operational and maintenance costs.

The figure below shows the substation structure that offers site-wide information where a field bus connection for process control information and switching commands is utilized.

Figure 1 – Intelligent Substation Control System

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2. LV switchgear architecture in substation automation

Based on IEC 61850, the general system architecture with low-voltage switchgear can be divided into three different levels:

1. Station level
2. Bay level
3. Process level

The station level generally consists of an ESCADA (Electrical Supervisory Control and Data Acquisition), which is a local intelligence of the power automation system. It consists of an engineering workstation (EWS), operator workstation (OWS), Maintenance workstation (MWS), servers for the storage of power system data and events, and gateways to other outside networks with proper redundancy arrangements.

At the station level, redundancy of the whole system is achieved using the protocols like High-availability Seamless Redundancy (HSR) and Parallel Redundancy Protocol (PRP) for all the networking devices from different levels of automation.

This will help maintain a standby system when one of the systems fails and make the substation automation system redundant.

Figure 2 – HSR (High-availability Seamless Redundancy) IEC 62439-3 Clause 5

If the ring is broken, messages will still arrive over the intact path. A broken ring is easily detected since duplicate messages are no longer received. Normal condition Message from IED is sent via both links (“A” & “B”) to the SCADA via HSR ring.

Operation under failure condition: Failure recognized in HSR ring (“A” link). SCADA receives the message via the healthy part of the ring (“B” link).

Figure 3 – PRP (Parallel Redundancy Protocol) IEC 62439-3 Clause 4

The communication network is fully duplicated. If only one packet is received, the receiver knows the other path is broken. Normal condition Message is received in Substation management unit (COM600) via both parallel links (LAN “A” & LAN “B”)

The bay level consists of control circuits, control-circuit grouping devices, and intelligent electronic devices (IEDs), which are used for grouping control functions and individual control of each load (fed from digital low-voltage switchgear).

Control circuits for each load are singly fed or group controlled and connected to the central control system at the station level using an Ethernet bus.