CIRED 2021 Conference
20 – 23 September 2021
Paper 0260
OVERLOADABLE DISTRIBUTION TRANSFORMERS
FOR FLEXIBLE LOADING IN FUTURE
DISTRIBUTION NETWORKS
Radoslaw Szewczyk1*, Gokhan Kalkan2
1
Nomex® Electrical Infrastructure, DuPont, Lodz, Poland
2
KYTE Powertech, Cavan, Ireland
*radoslaw.szewczyk@dupont.com
Keywords: DISTRIBUTION TRANSFORMER, TRANSFORMER
CAPABILITY, FLEXIBLE LOADING, ARAMID INSULATION
REPLACEMENT,
OVERLOAD
Abstract
The power capacity demand in distribution networks grows continuously for various reasons. The increasing loading typically
requires larger and larger transformers (in terms of power rating and physical size), although there is not always space provided
for installing them (“brown field” installations).
While the increasing power capacity is needed, it must be also remembered that the typical distribution transformers are normally
not heavily loaded. Normally, they have quite high capacity margin. This study analyses design optimization efforts toward
optimal selection of the transformer power rating for its actual expected demand and expected loading pattern. Transformers
could be potentially rated lower comparing to typical conservative rules. The units could be optimized for cost and size, while
still allowing safe and reliable loading. If needed, they could be safely overloaded for extended periods of time.
The solutions evaluated are based on advanced insulation systems for transformers (based on aramid paper or thermally
upgraded cellulose paper enhanced with aramid). The designs could also use ester liquids for better fire safety or reduced
environmental impacts. A few example applications are discussed, like: pole type 200 kVA unit for rural installations,
overloadable 630 kVA unit for city substations, 1.7 to 4 MVA wind turbine replacement transformers.
1
Introduction
The power capacity demand in distribution networks grows
continuously. This may be related to:
•
•
•
•
increasing number of consumers,
connecting larger loads,
developing charging infrastructure for electric vehicles,
installing energy consuming heat pumps as a
replacement for gas heating,
• connecting and transmitting power from generated
distribution.
The increasing loading requires larger and larger
transformers (in terms of power rating and physical size),
although there is not always space provided for installing
them (“brown field” installations). The existing substations
may have limited dimensions and new transformers may not
fit. This problem may also happen if the new replacement
transformers must meet new efficiency requirements (e.g.
European Tier 2 efficiency requirements coming into force
in 2021). Lower losses will naturally drive larger
transformer dimensions.
While the increasing power capacity is needed, it must be
also remembered that the typical distribution transformers
are normally not heavily loaded. Normally, they have quite
high capacity margin. This study analyses design
optimization efforts toward optimal selection of the
transformer power rating for its actual expected demand and
expected loading pattern. Transformers could be potentially
rated lower comparing to typical conservative rules. The
units could be optimized for cost and size, while still
allowing safe and reliable loading. If needed, they could be
safely overloaded for extended periods of time. The
solutions evaluated are based on advanced insulation
systems for transformers (based on aramid paper or
cellulose paper enhanced with aramid). The designs
could also use ester liquids or biodegradable mineral oils
for better fire safety or reduced environmental impacts.
Certain specific transformer applications and design
optimisations efforts have been described in other
conference publications [1, 2, 3]. This included step-up
transformers for solar photovoltaic inverter applications. In
this paper a few cases for power distribution networks will
be discussed:
• pole type 200 kVA and 300 kVA units for rural
installations,
• overloadable 630 kVA unit for city substation,
• 1.7 to 4 MVA wind turbine replacement transformers.
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CIRED 2021 Conference
20 – 23 September 2021
Paper 0260
2 Case A: 200 kVA and 300 kVA pole type 2.2 Solution proposed
These challenges cannot be met by standard designs. One
transformers for rural installation
way to reduce the weight is to use low density and viscosity,
biodegradable hydrocarbon oil and thermally upgraded
cellulose paper enhanced with aramid (Nomex® 910).
Proposed oil can reduce winding temperatures due to its low
viscosity (more effective oil circulation in the cooling
system of the transformer). The thermally upgraded
cellulose paper enhanced with aramid allows to operate
transformer windings at higher temperatures (see Fig. 1;
normal operating temperature 120°C in mineral oil as
compared to 98°C for conventional Kraft cellulose paper or
compared to 110°C for TUK – the thermally upgraded Kraft
cellulose paper [4, 5]). As a result, the target weight limits
can be achieved for both scenarios. Additionally, due to
biodegradable oil, the proposed transformers are
environmentally friendly.
2.1 Specification requirements; problem to be solved
Introduction of ECO losses, increasing popularity of electric
vehicles (EVs) and heat pumps as an alternative source of
heating at homes increase dimensions and power ratings of
pole mounted transformers. The challenge for transformer
design engineers is to find a solution that will address above
mentioned developments and limit the weight of the
transformer so that transformer weight will not exceed the
current weight carrying capacity of the pole.
Some European utilities predict that with increasing
demand for EVs and heat pumps, the 200 kVA pole
mounted transformers will be replaced by 300 kVA pole
mounted transformers that satisfy Tier-2 requirements of
European Commission Regulation 548/2014. At the same
time, the utilities would like to keep the existing pole
designs that limit the weight of the transformer (e.g. to
1100 kg). Example technical specification is shown in
Table 1.
Another request, from a different power utility, is to keep
the 200 kVA transformer weight limit under 1000 kg with
new ECO Tier-2 losses. The Table 2 compares the current
and Tier-2 compliant losses and weights.
Table 1. Example specification for 300 kVA pole mounted
transformer
Power
(kVA)
High
voltage
(kV)
Low
voltage
(V)
NLL
(W)
LL
(W)
Weight
limit
(kg)
300
21.5/10.75
423
358
2965
1100
Fig. 1. Normal operating temperatures for different
insulation materials at the same expected lifetime of 20
years (Kraft cellulose paper, TUK – thermally upgraded
Kraft paper, Nomex® 910 – thermally upgraded cellulose
paper with aramid)
Table 2. Comparison of current 200 kVA design and the
design with Tier-2 losses
High
voltage
(kV)
Low
voltage
(V)
NLL
(W)
LL
(W)
Weight
(kg)
Current
design
11
433
250
2750
860
Tier-2
design
11
433
225
2016
980
The solution proposed utilises advanced insulation system
in the transformer, as described above. The thermal design
allows for additional 10°C in winding temperatures due to
new type of paper and 5°C due to new type of oil vs. regular
thermally upgraded cellulose paper and mineral oil. The
proposed solution gives winding design optimisation
possibilities that have been verified in the described case.
The use of paper enhanced with aramid and the new
hydrocarbon oil allowed for compacting the design of
200 kVA and 300 kVA units to meet the stringent
efficiency requirement and be well within the weight
limit of 1000 kg and 1100 kg, respectively.
3 Case B: 630 kVA overloadable unit for city
substation
In both cases above, it should be considered that it is easier
to change the transformer design rather than the pole design.
Additionally, changing the pole design or replacing an
existing pole with one that has more weight carrying
capability is costlier. As a result, if transformers can be
designed to satisfy the weight limit criteria, this will be the
most cost-effective solution.
3.1 Specification requirements; problem to be solved
Similar concerns as described above are valid for ground
mounted transformers. For ground mounted transformers,
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CIRED 2021 Conference
20 – 23 September 2021
Paper 0260
dimensional limits are more important than weight
restrictions. In most cases, it is very difficult to increase the
footprint of existing transformers due to limited space
provided. Especially, if the existing transformers are in an
enclosure, increasing the power rating or upgrading the
transformer with Tier-2 rated ECO losses is a challenge.
Table 3. Comparison of existing and new designs of
630 kVA transformer
Many utilities are planning their future network based on
the use of EVs, heat pumps and rooftop solar installations.
In a particular case of a city substation, the utility has
calculated that current 630 kVA transformers will need to
be upgraded to 1000 kVA in future. Additionally, due to
spatial limitations, new transformers should fit in the same
10 K enclosures.
High voltage
(kV)
Low
voltage
(V)
NLL
(W)
LL
(W)
Weight
(kg)
Existing
unit
21.5/10.75
423
760
6025
~2120
New
design
21.5/10.75
423
540
4600
2936
In conclusion, introduction of Tier-2 ECO losses, future EV
charging requirements, increase in heat pump usage and
rooftop solar installations will increase the weight and
dimensions of existing transformer installations. Depending
on the requirements, biodegradable transformer oils and
aramid enhanced cellulose paper offer solutions where
the existing size limitations can be mitigated.
Additionally, these concepts are with bio-degradable oils
and offer environmentally friendly solutions.
3.2 Solution proposed
The 630 kVA rated transformer cannot be directly replaced
with 1000 kVA Tier-2 transformer in the already defined
enclosure. To enable the transformer upgrade, a study was
made to verify if 630 kVA Tier-2 rated transformer with
natural ester liquid and thermally upgraded cellulose paper
enhanced with aramid (Nomex® 910) could be overloaded
during the number of hours where the load was expected to
reach maximum value of 1000 kVA. In this approach, the
recommendations from IEC 60076-14 standard were
followed [6]. Keeping in mind the 10 K enclosure, the top
oil rise was set to 65 K whereas winding temperature rise
was set to 80 K.
The first unit of this concept was installed in the grid in the
beginning of 2021. The unit will be closely monitored
during its operation for collecting more data to prove the
solution at the user.
4 Case C: Wind turbine replacement
The prototype unit was built and equipped with embedded transformers and new installations
fibre optic sensors. The heat run test at 1000 kVA was
performed in the enclosure. Based on the calculated and
tested temperatures the insulation life was calculated and
the lifetime expectancy of the transformer was confirmed.
4.1 Specification requirements; problem to be solved
Initial development of wind turbine liquid filled
transformers for installations inside the wind turbines used
high temperature insulation systems with Nomex® and with
silicon or synthetic ester liquid. To be able to fit the
transformer design into given dimensional restrictions, high
thermal performance characteristics of aramid insulation
material were utilised. Today, windfarms often equipped
with turbines in the range of 1.5 to 4 MW are to be
repowered. Solutions for transformers are required with
further capacity and efficiency enhancement. After
introduction of ECO losses, any replacement step-up
transformer in onshore installations should be ECO
directive compliant or it should be demonstrated that
dimensional restrictions cannot be met by the given loss
levels. This requirement is also applicable to old turbines
equipped with cast resin transformers, generally known to
have higher no load losses than liquid filled transformers.
For offshore installations, the European ECO directive does
not apply, but the users often apply the same guidelines for
efficiency or develop their own loss capitalization rules.
Additionally, the special testing included short circuit
withstand test and lightning impulse dielectric tests. The
unit passed all these tests and verified the design concept.
In the described case, the developed new specification of
630 kVA transformer for city substations required safe
long-term overloading capability of 1000 kVA.
For meeting the size restrictions of city substation and
fulfilling the overload requirement, a solution with aramid
enhanced cellulose paper and natural ester liquid was
developed. The below Table 3 compares the losses and
weight of the existing and new designs of 630 kVA
transformer.
Although ECO directive allows an increase of 15% and 10%
on no-load and load losses respectively for dual voltage
transformers (as per Table I.3 of the Regulation 548/2014),
these increases were not considered here. That was to
reduce the dissipated losses over 24 hours period during
normal or lower loading conditions or to allow for increased
seasonal loading conditions like for heat pumps operating.
4.2 Solution proposed
Introduction of ECO losses relaxed the cooling
requirements and thermal gradients of the windings due to
losses reduction. As a result, the highest thermal
performance solid insulation materials are not always
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CIRED 2021 Conference
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Paper 0260
• with synthetic ester liquid and with insulation system of
lower thermal class (aramid enhanced cellulose paper
Nomex® 910) for cost optimization.
required. While the transformer liquid inside the wind
turbines should still be rated for K class fire point (i.e. above
300°C), the thermally upgraded paper or aramid enhanced
cellulose paper can be used. This solution is also costeffective, and the lower cost of insulation system almost
compensates the increase in transformer price related to
lower loss design.
Both designs have been offered for evaluation at the user.
The type tested designs verified the suitability of the
concept for using the given innovative insulation systems
in the replacement of wind turbine transformers in
selected projects (onshore and offshore). Additionally,
transformers will normally operate at lower temperatures
where liquid stray gassing risk is reduced. Lower
temperature cycle will also reduce the mechanical fatigue
on transformer tanks.
A 2600 kVA prototype replacement wind turbine
transformer with natural ester and thermally upgraded paper
was subjected to short and long-term partial discharge test,
lightning impulse full wave test with 200 kV and switching
impulse test with 140 kV (Fig. 2). It was successfully tested
and verified. Additionally, the unit was equipped with
embedded fibre optic sensors inside the windings and
subjected to temperature rise test. All tests were successful
and this 2600 kVA transformer with 33 kV to 690 V voltage
ratings has been in operation for more than two years now.
5
Conclusions
Both utility and private networks are in transition to meet a
global goal that is to reduce carbon footprint and use greener
products to protect environment. ECO directives set general
rules in terms of losses to contribute to the greener
environment. On the other hand, there is a global interest
in renewable energy sources, EVs and heat pumps. All
these developments will require transformer power
ratings to be upgraded. Especially in city centres, where
the space is limited, and transformer dimensions need to
be the same, new concepts are required to meet these
demands.
New solid and liquid materials offer several advantages and
are perfect candidates that offer attractive solutions to
address above mentioned challenges. The advantages of
such solutions are listed below.
1. As most of these solutions involve bio-degradable oil,
greener transformers will replace the old installations.
Depending on local regulations, bunding might not be
required.
Fig. 2. Wind turbine transformer 2600 kVA, 33 kV / 690 V
under 140 kV switching impulse test (unit equipped with
fibre optic sensors for direct temperature measurements)
2. Transformers can be designed for known loading
conditions especially for solar and battery storage
applications.
In another example, replacement wind turbine transformer
of 1700 kVA power rating and 22 kV (HV) to 12 kV (LV)
voltage levels was designed and manufactured with aramid
enhanced cellulose paper (Nomex® 910). Top oil
temperature rise was limited to 70 K and hot-spot
temperature was limited to 150ºC. The challenge was to
come up with a design with same footprints and this was
achieved with the described insulation materials
combination.
3. Lower loss transformers can be replaced within the same
footprint by operating with the hot-spot temperatures
higher than transformers with conventional insulation.
4. Application is important in determining the solid and
liquid insulation composition. For wind turbines or long
duration overloading conditions, ester oils plus
thermally upgraded cellulose paper enhanced with
aramid offers the best solution whereas for solar, battery
storage or short time overloading, renewable
hydrocarbon oil plus thermally upgraded cellulose paper
enhanced with aramid is the best combination. For wind
turbine replacements, ester oil is the required oil type.
The next example is the Tier-2 compliant 4 MVA unit for
offshore wind turbine application (33 kV to 690 V). The unit
is with KDAF forced cooling system where the liquid is
directed into the windings and the forced air flow is utilised.
This replacement unit has been designed in two options:
• with synthetic ester liquid and aramid Nomex® based
insulation system for the highest thermal performance,
5. Pole mounted transformers can be designed with
renewable hydrocarbon oils plus the thermally upgraded
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CIRED 2021 Conference
20 – 23 September 2021
Paper 0260
cellulose paper enhanced with aramid to fulfil the
weight restrictions.
[3] Kalkan, G., Szewczyk, R.: 'Optimization of
transformers for solar or battery storage installations based
on a cyclic loading pattern', CIRED, Geneva, Switzerland,
2021, paper #261
[4] Marek, R., Wicks, R., Szewczyk, R., et al.: 'New
cellulose paper enhanced with aramid - practical example
of material thermal evaluation acc. to IEEE Std C57.1002011', CIGRE SC D1 Colloquium, Rio de Janeiro, Brazil,
2015, paper ID27
[5] Szewczyk, R., Marek, R., Ballard, R., et al.: 'Innovative
insulation materials helping in cost reduction of modern
transformers', CIRED, Madrid, Spain, 2019, paper #2108
[6] IEC 60076-14 'Power transformers - Part 14: Liquidimmersed power transformers using high-temperature
insulation materials', 2013
Lastly, it is recommended that end users develop certain
design rules and guidelines depending on the application to
prevent under-designing of the transformers for these
specific applications.
6
References
[1] Szewczyk, R., Duart, J.-C., Trifigny, P.: 'Mitigation of
lock-in effect for compact substations with transformers
meeting future EU efficiency regulations', CIRED,
Madrid, Spain, 2019, paper #2089
[2] Szewczyk, R., Duart, J.-C.: 'Improved reliability and
performance of transformers in solar installations by use of
advanced insulation materials', Matpost, Lyon, France,
2019, paper #51
Nomex® is a trademark owned by affiliates of DuPont de
Nemours, Inc.
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