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
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.
MV switchgear white paper | 02
What will MV switchgear look like in the future?
Brief history of the technologies
used in medium voltage switchgear
and controlgear
All elements of a medium voltage
installation are subject to evolution
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.
Available technologies for
electrical switchgear
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.
Voltage level
Switching media
Insulation
medium
Circuit- breaking
Load- breaking
High voltage
SF6, vacuum
NA
SF6, air
Medium voltage
SF6, vacuum
SF6, vacuum
Air, SF6,
solids, oil,
Air, oil,
Air, oil,
Table 1: Insulation media
MV switchgear white paper | 04
What will MV switchgear look like in the future?
Evolution of circuit-breaker technologies
The first technology used for breaking in CBs was air. These CBs were big
1
2
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
3
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,
4
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.
1. Merlin Gerin™ circuit breaker DST
2. Drawout oil circuit breaker with arc control
Often, they remain withdrawable because of installation in traditional metal-
3. Withdrawable vacuum CB
enclosed panels.
4. Withdrawable SF6 puffer CB
Evolution of primary distribution switchboard technologies
From 1930 to 1950, most of the MV switchboards were in fact an assembly
5
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)
6
7
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
5. Air-insulated masonery cubicles
and front access cables. But more recently, in the 1990s, fixed CBs were also
6. Metal-enclosed AIS panel with CB cassette
used. This change was possible with modern highly reliable CBs and new
7. Metal-enclosed AIS panel with fixed CB
testing facilities of the protection relays.
05 | MV switchgear white paper
What will MV switchgear look like in the future?
There are some recent variants in metal-enclosed cubicles with fixed CBs,
8
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.
8. Metal-enclosed GIS switchboard with fixed CBs
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
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
9
10
11
12
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
O/C
more powerful MV/LV transformers and to bring more precise features in the
E/F
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.
9. Typical RMU arrangement with switch fuse
10. Oil switch unit
11. T
ypical RMU arrangement with CB
transformer protection
12. SF6 RMU with CB
MV switchgear white paper | 06
What will MV switchgear look like in the future?
Evolution of the technology of sensors and protection relays
Technologies of sensors and protection relays evolved in parallel because
13
14
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
15
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
13. Typical line distance electromechanical relay
conservative and many user specifications were still asking for 5A CTs even if
14. Typical overcurrent electronic relay Statimax type
no longer needed.
15. Digital relays VIP 400 (left) and Sepam 20 (right)
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.
07 | MV switchgear white paper
What will MV switchgear look like in the future?
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.
Diagram with withdrawable technology
The diagram with withdrawable CBs is the oldest one. It is still in use and not
16
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
16. Single-line diagram and typical panel for
withdrawable technology
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.
Typical diagram for GIS technology
To drastically eliminate the sensitivity to environment, gas-insulated switchgear
17
(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.
17. Single-line diagram and typical panel for
primary GIS technology
MV switchgear white paper | 08
What will MV switchgear look like in the future?
Simplified diagram with upstream two-position selector
In an attempt to simplify the five-position single-line diagram, it is possible
18
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.
18. Single-line diagram with upstream
two-position selector
Reverse diagram for GIS technology
Trying to improve ergonomy, moving to a direct cable earthing, some equipment 19
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.
19. Reverse single-line diagram with
GIS technology
All-in-one arrangement diagram for GIS RMU
For secondary applications, simplicity, insensitivity, and cost effectiveness often
20
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
21
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.
20. Typical three-position GIS RMU diagram
21. Examples of GIS RMU
09 | MV switchgear white paper
What will MV switchgear look like in the future?
New three-position diagram
The three-position arrangement for GIS RMU has experienced great success
22
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
22. New three-position diagram including vacuum
breaking and typical unit.
load-break switch or CB and a downstream earthing switch providing a doublegap 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
• 3rd position: cable earthing in one single operation
23
in a 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
23. Mimic diagram of three-position scheme using
vacuum interrupter
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.
24
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.
24. Dedicated cable testing device.
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
25
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 threeposition 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
25. Example of switchboard made with modular
2SIS units
breaking, has many advantages, it is necessary to analyse whether it is
completely adapted to the smart-grid deployment of today.
MV switchgear white paper | 10
What will MV switchgear look like in the future?
Future switchgear for MV consumer
sites and switching substations
The challenge of smart grids
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.
Smart grids will use more CBs than in the past
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
11 | MV switchgear white paper
What will MV switchgear look like in the future?
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,
• greater harsh climate withstand;
• reduced maintenance and spare parts.
low magnitude phase-faults and earth faults;
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.
Remote control will be mandatory for smart grids
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.
MV switchgear white paper | 12
What will MV switchgear look like in the future?
LPCTs and LPVTs will be essential for the huge
development of power management and metering
Control and monitoring will increase to properly manage the real-time
26
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
27
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
28
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.
13 | MV switchgear white paper
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. S
ize comparison between LPCT (left)
and CTs (right)
What will MV switchgear look like in the future?
Modularity is a must to meet the infinite number of
different applications
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.
Conclusion
The 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.
MV switchgear white paper | 14
©2013 Schneider Electric. All Rights Reserved. All trademarks are owned by Schneider Electric Industries SAS or its affiliated companies.
998-1173816_GMA-GB
09-2013