what will mv switchgear look like in the future

What will MV switchgear look like in the future? by Jean-Marc Biasse

Table of contents

Introduction .............................................................................................. 2

Brief history of the technologies used in medium voltage

switchgear and control gear ...................................................................... 4

Evolution of the single-line diagrams .......................................................... 8

Future switchgear for MV consumer sites

and switching substations .......................................................................... 11

Conclusion ............................................................................................... 14

MV switchgear white paper | 02

What will MV switchgear look like in the future?

Introduction

The electricity industry is conservative. Among the reasons for this is the fact

that the lifetime of medium voltage and high voltage switchgear is around

40 years. Transmission system operators (TSOs) and distribution network

operators (DNOs) need stability. Maintenance and repair of such long-life

devices needs to be ensured. And of course, work is easier for service crews if

there is no change in technology.

However, some drastic evolutions appear about every 20 years.



Brief history of the technologies used in medium voltage switchgear and controlgear

In a substation are found all three categories of components of protection

chains: sensors, protection relays and circuit breakers (CBs).

Traditionally, the design of these components has evolved independently, but

with some constraints at interfaces to ensure interoperability.

Protection relays are particularly sensitive to the type of signal coming from

current transformers. Some association are possible; others are not. For

example, you may connect old technology 5A CTs to most modern protection

relays, but the opposite — connecting an LPCT to an old electromechanical

relay — is impossible.

Electrical switchgear need an insulation medium for two different functions: current

breaking and isolation between conductors or between conductors and earth.

For current breaking, the available technologies are air, oil, SF6, and vacuum.

To isolate conductors, the same technologies may be used plus solid insulation.

All elements of a medium voltage installation are subject to evolution

Available technologies for electrical switchgear

Voltage level Switching media Insulation medium

Circuit- breaking Load- breaking

High voltage SF6, vacuum NA SF6, air

Medium voltage SF6, vacuum Air, oil,

SF6, vacuum Air, oil,

Air, SF6, solids, oil,

Table 1: Insulation media

05 | MV switchgear white paper


The first technology used for breaking in CBs was air. These CBs were big

because the principle of breaking was a large expansion of the arc and noisy

because of the breaking in the air. They needed much maintenance and, for

that reason, were withdrawable (Fig 1).

In an effort to reduce the footprint, oil CBs came next (Fig 2). However,

they also needed much maintenance, for example to change oil after some

operations. Additionally, oil breakers are not safe to operate because of the

fire risk. Oil CB failures can easily result in a fatal accident among operators

and the public.

In the late sixties came SF6 and vacuum circuit-breakers. Both technologies

brought many similar advantages.

They are compact thanks to vacuum or SF6 insulation. They are much safer,

drastically reducing fire risk. They became more and more reliable. Electrical

endurance has been increased, thus CBs were able to perform a much higher

number of fault and load breakings. As a consequence of the improved

reliability, maintenance is less and less required and we can consider that

state-of-the-art CBs are now almost maintenance-free.

Often, they remain withdrawable because of installation in traditional metal-

enclosed panels.

From 1930 to 1950, most of the MV switchboards were in fact an assembly

of fixed components in an electrical room connected to visible busbars. Only

simple wire fencing prevented to access the live parts.

Then, because of more safety awareness, switching components and busbars

were integrated in metal-enclosed cubicles. Doors and sheet plates and frames

were earthed to avoid any accident from direct or indirect contact with live

parts. Busbars and connections were air insulated.

There were several generations of metal-enclosed air-insulated switchgear (AIS)

cubicles. The first generation, from 1950 to 1970, integrated withdrawable air

or oil CBs. The second generation, from 1970 to 1990, integrated withdrawable

SF6 and vacuum CBs. Another step in safety was introduced in the current,

third generation of metal-enclosed cubicles, which began in 1990. This new

generation introduced internal arc withstand capability to protect people

standing in front of the switchboard in case of an extremely rare internal fault.

Generally, CBs are withdrawable and installed in cassette to allow wall mounting

and front access cables. But more recently, in the 1990s, fixed CBs were also

used. This change was possible with modern highly reliable CBs and new

testing facilities of the protection relays.

Evolution of circuit-breaker technologies

Evolution of primary distribution switchboard technologies

1. Merlin Gerin™ circuit breaker DST

2. Drawout oil circuit breaker with arc control

3. Withdrawable vacuum CB

4. Withdrawable SF6 puffer CB

5. Air-insulated masonery cubicles

6. Metal-enclosed AIS panel with CB cassette

7. Metal-enclosed AIS panel with fixed CB

1

6

2

7

4

3

5



O/C

E/F

Secondary distribution switchgear also followed a similar evolution, but with

some differences.

Rated currents at the distribution level are lower and the number of substations

is higher. Then, looking for money saving, only simple switches with fuse

protection were used. A typical switchboard includes three functions, two

switches and a switch fuse to protect the MV/LV transformer.

The same evolution as for primary distribution appeared from masonery

cubicles to modular metal-enclosed AIS cubicles.

But, due to the typical three-function repetitive arrangement, a special ring main

unit (RMU) configuration appeared in the 1950s.

For more compactness, the three functions have been fitted in one metallic

tank. The first RMUs of this type were oil RMUs with the same inconvenient fire

risk. Modern RMUs now use SF6 as it provides compactness and insensitivity

to ambient environmental conditions. Moreover, both with the need to protect

more powerful MV/LV transformers and to bring more precise features in the

protection scheme, modern RMUs are now equipped with CBs for MV/LV

transformer protection.

The same evolution in safety concerns resulted in new designs having internal

withstand capabilities. Sometimes the advantage of a compact and repetitive

RMU solution becomes inconvenient when extension is needed or if more than

four- or five- function switchboard are needed.

There are some recent variants in metal-enclosed cubicles with fixed CBs,

where the insulation of busbars and all components, including CBs and

connections, are made with epoxy or some other resin. These panels are

generally called a solid insulation system (SIS).

However, always looking for better electricity availability, utilities started to

require more and more insensitivity to ambient environmental conditions. And,

all AIS and SIS panels are still sensitive to environmental conditions if not

properly installed in protected rooms.

That was the reason for the arrival of metal-enclosed gas-insulated switchgear

(GIS) in the 1990s. All components, busbars, and connections are fitted in one

or several hermetically sealed tanks filled with SF6. Thanks to SF6 insulation,

this type of equipment is very compact.

Both AIS and GIS panels coexist today. The final choice may differ for

each application, depending on the importance given to many criteria such

as compactness, insensitivity to the environment, the availability of high

performance, criticality of the application, power restoration mode in case of

failure, ergonomy of operation, and/or ergonomy of cable testing.

Evolution of secondary distribution switchboard technologies

8. Metal-enclosed GIS switchboard with fixed CBs

9. Typical RMU arrangement with switch fuse

10. Oil RMU with switch fuse

11. Typical RMU arrangement with CB

transformer protection

12. SF6 RMU with CB

11

9

12

10

8



Technologies of sensors and protection relays evolved in parallel because

both types of components are closely linked. Sensors, like current

transformers, shall permanently give an image of the current and this image is

transmitted to the protection relay. We can consider the relay to be the brain,

as it is able to receive the signal and analyse it to decide whether the signal

is normal or represents a fault. In case of a fault, the protection relay sends a

tripping message to the circuit-breaker mechanism.

Up until the 1970s, protection relays were made using electromechanical

technology. Coils and disks were parts of these relays that needed high

auxiliary power to operate. Consequently, the current transformers had to

supply high burden. 5A on secondary output was necessary to operate these

protection relays.

In the 1980s, electronic protection relays occurred with less need of auxiliary

power from the CTs. They could be operated by current transformers

having 1A rating on secondary winding. But the high voltage sector is very

conservative and many user specifications were still asking for 5A CTs even if

no longer needed.

Later in the 1990s came the first digital relays. With this technology, the need

of signal power from the CTs becomes very low. A new category of CTs

was developed: the low power current transformers (LPCT). They deliver a

voltage signal representing the primary current. In spite of the advantages in

space and flexibility, their deployment has been very slow because of users’

conservatism, still asking for 1A or even 5A CTs to feed digital relays. This

overpower in input needs adapter transformers in the protection relay to lower

the input power.

Now, the situation is finally going to change. Digital relays are very common

and advantages of LPCT are recognized. Moreover, clear IEC standards

have been published, making interchangeability of LPCTs or protection

relays easier.

Evolution of the technology of sensors and protection relays

13. Typical line distance electromechanical relay

14. Typical overcurrent electronic relay Statimax type

15. Digital relays VIP 400 (left) and Sepam 20 (right)

13

15

14



16. Single-line diagram and typical panel for

withdrawable technology

17. Single-line diagram and typical panel for

primary GIS technology

16

17

Evolution of the single-line diagrams Even if sometimes conservative, customers tend to look after reduced

dimensions, lower cost, better reliability, and better ability to withstand

harsh environments. To meet these needs, there is a progressive move from

withdrawable to fixed equipment.

Together with the evolution of the technology of medium voltage switchgear,

single-line diagrams of incomers and feeders were regularly challenged.

It is possible to make some comparisons between the most typical single-line

diagrams, just highlighting some points of importance.

The diagram with withdrawable CBs is the oldest one. It is still in use and not

obsolete in some primary distribution applications. Disconnection is made

by racking out the CB truck, providing visible disconnection and usually an

earthing switch is directly acting on cable ends.

Maintenance of the CB is very easy and this was necessary for old CBs. In

addition, access to terminals for cable testing is quite easy.

However, some points have to be carefully considered. Remote control of

the disconnector is not really practical because of the truck to be racked

out. Earthing the busbar needs a dedicated earthing truck, which is heavy to

handle. Testing the cables needs a direct access to cables, opening the cable

compartment. And finally, the equipment should be installed in clean air rooms

as it is sensitive to environmental conditions because of the AIS technology.

To drastically eliminate the sensitivity to environment, gas-insulated switchgear

(GIS) were developed. First derived from HV GIS technology, these equipment

are fitted with fixed CBs and separate disconnectors.

This technology was made possible thanks to the design improvements of

CBs that now need very little maintenance. Gas insulation and plug-type cable

connectors ensure the highest degree of insensitivity to harsh environments.

Among the points to be aware of is that operation is not so intuitive because of

a five-position scheme. Particularly, cable earthing is made through CB closing

that must remain closed to ensure end-user safety when working.

Diagram with withdrawable technology

Typical diagram for GIS technology



18. Single-line diagram with upstream

two-position selector

19. Reverse single-line diagram with

GIS technology

18

19

20

In an attempt to simplify the five-position single-line diagram, it is possible

to design an upstream two-position selector. This arrangement reduces the

number of positions thanks to the two-position selector. As the cost is also

reduced, it has been possible to use this arrangement in secondary distribution.

However, there are still four positions that make the operation not so intuitive,

especially for secondary distribution. And, earthing the cable remains made

through CB closing. When the cable is earthed, the CB must stay closed to

ensure safety. The positive earthing indication depends on the status of the

combination of two devices.

Trying to improve ergonomy, moving to a direct cable earthing, some equipment

uses a reverse diagram with GIS technology. Now earthing the cables is made

directly via an earthing switch having making capacity. This also gives the

possibility to design a dedicated device for cable testing via a removable link.

But there are still four positions and a need of keys for safety interlocks. Cost is

increasing because of separate earthing switch having making capacity.

For secondary applications, simplicity, insensitivity, and cost effectiveness often

are a must. These criteria were the drivers to move to an all-in-one arrangement

for GIS RMU.

The main device is an SF6 disconnecting load-break switch or circuit breaker

allowing for a very simple three-position diagram. Breaking and disconnection

are performed in a single operation, leading to the three-position scheme (line,

open and disconnected, earthed).

Local or remote operations are very simple. The mimic diagram is very easy to

interpret. Earthing of the cables is made directly. Interlocking safety is inherent

between the different positions. It is very easy to implement a cable testing

device, allowing access to cable without opening the cable box nor interfering

with the cable terminations.

Simplified diagram with upstream two-position selector

Reverse diagram for GIS technology

All-in-one arrangement diagram for GIS RMU

20. Typical three-position GIS RMU diagram

21. Examples of GIS RMU

21



22. New three-position diagram including vacuum

breaking and typical unit.

23. Mimic diagram of three-position scheme using

vacuum interrupter

24. Dedicated cable testing device.

25. Example of switchboard made with modular

2SIS units

22

23

24

25

The three-position arrangement for GIS RMU has experienced great success

for around 30 years now and is still well appreciated. Nowadays, even if the

technology is not much changing, there is a trend to use vacuum breakers

in secondary applications. The question was whether it was still possible

to keep the same simplicity of the three-position diagram using another

technology. Recent developments brought an original solution, keeping the

same advantages of the three-position arrangement of GIS RMUs, but using

vacuum breaking.

The new proposed arrangement includes an upstream vacuum disconnector

load-break switch or CB and a downstream earthing switch providing a double-

gap isolation between cables and busbars. All previous advantages are kept

with this real three-position scheme (line, open and disconnected, earthed).

• 1st position: CB or load-break switch closed

• 2nd position: CB or load-break switch opened and disconnected

in a single operation

• 3rd position: cable earthing in one single operation

Breaking and disconnection are made in one single operation of a vacuum

interrupter. Earthing the cables is done directly, using an earthing switch having

making capability. This diagram facilitates the implementation of clear mimic

indications, making operations very intuitive and thus safer. Safety interlocks

are built-in, short, key free, and positively driven. This diagram also allows the

use of a dedicated cable testing device, increasing the safety of people and

switchgear. As it is well known that MV cables are generally much older than

switchgear, they will need more and more testing and conditional replacement.

Prior to the cable test, opening the switch or CB disconnector and closing the

earthing switch provides a double gap between cable and busbar. Then a safe

and fully interlocked earth link switch may be opened to give direct access to

the cable conductor. During testing, the cable box remains closed, the cable

connections remain intact, and the main contacts of the earthing switch remain

in the same position. This recommended test procedure ensures the highest

safety for people doing the tests and also avoids any damaging of the main

circuit or cable connections.

To meet the same advantages of GIS RMUs, the new arrangement shall be

insensitive to harsh environment. This is ensured by a complete Shielded and

Solid Insulation System (2SIS) solution. Busbars and a vacuum interrupter

encapsulation and earthing switch enclosure are made of solid insulation that

is covered by a conductive layer connected to the earth. The equipment can

support any kind of harsh environment as well as GIS RMUs.

Compared to GIS RMUs, this 2SIS technology associated with this new three-

position diagram arrangement offers much better modularity as the general

architecture is based on single units. Thus, it is easy to build switchboards for

many kind of applications requiring a large number of units While it is obvious

that this modular architecture, based on 2SIS technology using vacuum

breaking, has many advantages, it is necessary to analyse whether it is

completely adapted to the smart-grid deployment of today.

New three-position diagram



Future switchgear for MV consumer sites and switching substations

Smart grids have two main objectives. One is to optimise the relation between

the demand and the supplying of energy. The second is to provide the

necessary conditions to integrate more distributed and renewable energies.

Comparing the two-way flow that is needed for these objectives with the simple

one-way flow still valid with centralised energy production, the challenge is

big. As for each other link in the chain, one question arises: are MV switchgear

ready for this challenge, or is an evolution necessary? Looking at existing grids

and at some experimentations, it is possible to highlight some switchgear

values that will help to meet this challenge.

For some years, experimentations have proven that adding CBs in distribution

network loops is an efficient way to decrease the number of customers affected

by an outage and to reduce power restoration time. The distribution network is

generally operated in an open loop, allowing a backup solution in case of fault.

It is historically equipped with manual switches, with only one protection device

per feeder, located in the HV/MV substation. The increasing demand for quality

of supply led to the deployment of remote controlled substations, bringing lower

shortage duration. Nevertheless, in case of fault, all the customers supplied by

the faulty feeder are disconnected.

But in fact, the customers upstream of the fault could have been unaffected.

The use of CBs instead of switches in the loop allows disconnecting only the

customers connected to the faulty part, a significant benefit regarding the

number of affected customers compared to the traditional solutions.

On an ideal point of view, solutions including low cost CBs, low cost sensors,

no communication, without specific network architecture and easy possible

upgrade could reduce the outage at a cost-effective level. Today, adequate and

economically viable answers to the needs of MV/LV substations do exist in both

following areas:

• optimised integrated CBs for network applications, including LPCTs,

• adaptation of existing protection systems by the reduction of

time discrimination interval or the use of logic discrimination in

substations between incoming and outgoing feeders.

In a similar way, it is more efficient and precise to use CBs to protect

MV/LV transformers.

Traditionally, MV/LV transformers have been protected by switch-fuses because

of the significant cost differential compared to withdrawable CBs and relays.

The main advantage of using an RMU with a fixed integral low cost CB is that

The challenge of smart grids

Smart grids will use more CBs than in the past



it allows to have improved transformer protection at an equivalent lifetime cost,

thus making transformer CB protection affordable.

An MV/LV transformer generally has a very low failure rate. All faults are starting

interturn faults or earth-phase faults and are located inside the primary or

secondary windings or on the LV zone. Only CBs can quickly and surely detect

the faults at early stage when they are of low or very low magnitude. At the

same time, fuses are sometimes not able to break or have to wait until the fault

has degenerated into a two-phase or three-phase fault of high magnitude to

operate properly.

The main advantages of the CB solution are:

• better discrimination with other MV and LV protection devices;

• improved protection performance for inrush current, overloads,

low magnitude phase-faults and earth faults;

• greater harsh climate withstand;

• reduced maintenance and spare parts.

Migration of withdrawable CBs towards fixed CBs and the use of vacuum

breaking make them cost-effective. Dissemination of modern highly reliable CBs

was a key factor for the acceptance of fixed CBs. In this respect, the modular

architecture of 2SIS, based on highly reliable vacuum interrupters, is very

flexible and allows for an infinite number of combinations.

To be more compact and efficient, switchgear with integrated control and

monitoring features provide better optimisation. Remote control of the

switchgear becomes essential and must be very easy. End users will no longer

accept long power outages. Feeder automation, self healing using remote

control is the only way to shorten the time of loss of power. Optimising the

loads in some parts of the distribution network will also be possible using

remote control to operate the switchgear and change the protection settings.

Of course, manual operation mode will also be very easy. For that, no matter

the technology,, the three-position operation mode (line, open/disconnected

and earthed) is the simplest one, also increasing safety. One big advantage of

this three-operation mode is that it is the same for remote control as for local

manual operation.

Remote control will be mandatory for smart grids



26. One phase analog ammeter with a 1.1VA

consumption and numerical power meter with

a 0.15VA consumption of the current input

27. LPCT sensor principle

28. Size comparison between LPCT (left)

and CTs (right)

28

27

26Control and monitoring will increase to properly manage the real-time

connections to the grid. For that purpose, more and more sensors will be

used. Thanks to modern control & monitoring devices and digital protection

relays, compact low power current transformers (LPCT) and low power voltage

transformers (LPVT) can replace heavy traditional CTs and VTs.

The introduction of digital technology for measurement and protection

(Figure 26) has modified the requirements of current transformer burden.

The manufacturers have developed protection devices based on low power

microprocessor technology with wide range of use, low consumption and

innovative current sensors that allow constituting a consistent protection chain.

Perfectly adapted to these small burdens, the LPCT consists of a current

transformer having a small core secondary winding connected to a integrated

shunt resistor (Figure 27). The shunt resistor converts the secondary current

output into a low-voltage signal. The iron core LPCT is based on the well

known CT technology. LPCT technology is an optimised technology with

several advantages:

• Simpler choice: engineering is simplified due to the wide operating

range. One type LPCT can cover applications from 5A to 1250A

where the traditional CTs require a range of five sizes. A single

sensor is performing both measurement and protection purposes;

• Easy and safe installation: LPCT output is plugged directly into the

protection relay with no risk of over voltage when disconnecting;

• Flexibility of use: easy adaptation to the power consumption

changes and/or protection setting during the MV system design or

operating life. High accuracy up to the short-time circuit current with

low saturation;

• Compactness: the reduced size and weight allows for an easy

integration and therefore MV switchgear dimension reduction. Figure

28 shows the size comparison between CTs 24 kV and 36 kV and

LPCT, meeting the same MV network protection and measuring

technical requirements.

Power management will increase as it will be very important to have a real-time

view of the available power. Metering equipment will need to be cost-effective,

compact and integrated. As a great advantage of 2SIS architecture, it is now

possible to have 2SIS LPCTs and LPVTs, making metering equipment insensitive

to harsh environments.

LPCTs and LPVTs will be essential for the huge development of power management and metering



The variety of electrical installations resulting in an infinite combination of

switchboard sizes and configurations will increase with the integration of

renewable energies and with the need of energy efficiency to save energy.

Modularity of switchgear is a key to answer the need of flexibility. MV switchgear

also will be more distributed in the network.

With this respect, the 2SIS system brings the highest flexibility. As each part of

the busbar and each part of cable connection are 2SIS technology, there is no

external influence, no matter the arrangement of the switchboard. As a result,

many possibilities of cable entries are provided and extension of a switchboard

is very easy.

Moreover, high insensitivity to harsh environmental conditions and less

maintenance will be very appreciable.

Modularity is a must to meet the infinite number of different applications

ConclusionThe development of smart grids will result in the inclusion of more intelligence

in MV equipment. This network evolution may be the opportunity to introduce

new criteria for the choice of products, such as flexibility, insensitivity to harsh

environments, compactness, optimisation of remote control, etc.

In conclusion, the physics are the same but some technological points are

changing as well as the way to optimise them.

For all these reasons, there is a great confidence that the 2SIS modular

architecture using the three-position scheme and vacuum interrupters is very

well adapted for the coming deployment of smart grids. This architecture

can address a large number of applications in secondary distribution but,

thanks to its modularity, can also challenge some low-end applications where,

traditionally, primary equipment is used. In this respect, this architecture is able

to bridge the gap between secondary and primary specialised equipment.

what will mv switchgear look like in the future

Documents

mv consumer sites

ambient environmental conditions

solid insulation system

medium voltage switchgear

enclosed ais panel

environmental conditions

distribution network

insulation medium