Accurate Thermal Information Drives Dynamic Rating
(Loading) Calculations for Power Transformers
Mark Tostrud, P.E.
| Dynamic Ratings, Inc.
Brian Sparling, SMIEEE | Dynamic Ratings, Inc.
Nov. 2012
Historically, substation assets have been loaded beyond nameplate ratings and the need to
accommodate these emergency or contingency conditions are best served using
on-line electrical apparatus monitoring systems.
Why monitor and dynamically load
power transformers?
For many years, the limit for normal apparatus
loading was based on the maximum nameplate
rating or an arbitrarily set value, this limit is often
called “the red line.”
On-line monitoring of power transformers and
circuit breakers, for condition assessment, has
gained popularity over the past twenty years.
This period is the typical technology adoption
period, from concept to commercial reality, in the
electric utility industry.
What are the benefits of implementing
On-line monitoring?

Utilization of electrical assets closer to their real
operational capacity without compromising safety
or reliability.

Assisting in making intelligent decisions to fully
optimise real-time substation loading /
overloading based on actual site conditions,
including asset condition and/or operating mode.

Forecasting (predict) operating conditions used to
facilitate condition based maintenance (CBM)
programs or agency reporting (such as
environmental reporting of SF6 gas released).

Collecting operational and accumulated loss of life
data to enable estimation of the residual.
Loading Management
Today’s sophisticated monitoring solutions
continuously calculate the maximum safe load
capability of the assets and display (locally or via
embedded web servers) and communicate with
SCADA or other systems.
Until recently, the operation of electrical
apparatus fits into one of the following loading
categories: Continuous Load or Cyclical Load.
Continuous Load
This is the constant loading at rated nameplate
output in (MVA) when the apparatus is operated
under a constant 20°C / 30°C (the IEC / I.E.E.E
loading
guidelines
respectively)
ambient
condition.
Of course, this loading condition rarely happens
over the life of a transformer, where both load
and ambient temperature vary over time.
GRAPH 1
IEC AND I.E.E.E. AGING RATE
Cyclical Load
Long-Term Emergency Loading
This loading implies a cyclical load at a normal
constant ambient (20°C) where the hottest-spot
conductor temperature varies as the load cycles
above and below the nameplate MVA of the
apparatus.
Long-term emergency loading of transformers
occurs and may persist for extended periods.
This can lead to significantly increased ageing of
the solid insulation system, not limited to:

From the thermal ageing standpoint, this cycle is
equivalent to the case of rated constant load at
normal ambient temperature (20°C).
Deterioration of the mechanical properties of the
conductor insulation will accelerate at higher
temperatures.
This ageing acceleration is also impacted by the
moisture content of the conductor insulation.
Taken together, it is an exponential function in
terms of ageing rate of the transformer insulation.
It will reduce the effective life of the asset.
Overloading Apparatus
The consequences of loading apparatus beyond
its nameplate rating are as follows:

Cooling system operation, for extended periods
will increase maintenance and life reduction.

Contact resistance of the breaker and / or OLTC
will increase at elevated currents and
temperatures. In extreme cases thermal runaway
could occur.
Gasket materials may become more brittle as a
result of elevated temperatures.

The temperatures of the windings, cleats, leads,
insulation and oil will increase, accelerating
insulation consumption.

The core leakage flux increases, causing additional
eddy-current heating in metallic parts.

As the temperature changes, the moisture and
gas content in the apparatus changes.
Short-Time Emergency Loading

Short-time increased loading can result in an
increased risk of a system failure. However,
acceptance of this risk for a short time may be
preferable to loss of supply.
If SF6 gas or oil is leaking to the environment,
moisture is entering. This gas loss can significantly
alter a circuit breakers performance and gas leaks
often must be recorded and reported. Moisture in
electrical apparatus accelerates condition
deterioration.

NOTE: The permissible duration (typically) of
short-time emergency loading is shorter than the
thermal time constant of the whole transformer,
and depends on the operating temperatures
before the increase in loading.
Bushings, On-Load Tap-Changers, cable-end
connections and current transformers will be
exposed to higher stresses, encroaching upon
their design and application margins.
2

The main risk for short-term failures is the
reduction in the dielectric strength due to the
possible presence of gas (water vapor) bubbles in
the regions of high electrical stress, (leads &
windings). Bubbles are likely to occur when
winding hot-spot temperature exceeds 140°C for
a transformer with winding insulation moisture
content of 2.5% or more. This critical temperature
will decrease as the moisture concentration in the
winding insulation increases.

Oil expansion could cause an overflow in the
transformers conservator.
Planned Loading beyond Nameplate
– Normal Operation
This loading [2] results in the conductor
hottest-spot or top oil temperature exceeding the
limits suggested for normal life expectancy
loading. The user accepts this loading as a
normal, planned-for operating condition. There is
no
associated
equipment
outage
or
emergencies with this type of loading. Cyclic
loads resulting in hottest-spot conductor
temperatures in the range of 120 – 130°C would
be associated with this loading requirement.
This type of loading would occur frequently, and
in some cases daily, during a short part of the
transformer’s load cycle.
Insulated
Conductor
Hottest-Spot
Temperature
Other Metallic
(Supports,
core, etc.)
Top Oil
Temperature
Load Factor
Per Unit
Current
120°C
Planned
Loading Beyond
Nameplate
130°C
Long-Time
Emergency
1-3 Months
140°C
Short-Time
Emergency
0.5-2 Hours
180°C
IEC
120°C
(N/A)
140°C
160°C
IEEE
140°C
150°C
160°C
200°C
IEC
140°C
(N/A)
160°C
180°C
IEEE
IEC
IEEE
105°C
105°C
(N/A)
110°C
(N/A)
(N/A)
110°C
115°C
(N/A)
110°C
115°C
1.5 p.u.
IEC
1.3 p.u.
(N/A)
1.3 p.u.
1.5 p.u.
Standard
Normal Life
Expectancy
IEEE
TABLE 1
SUGGESTED MAXIMUM TEMPERATURES
Table 1 summarizes the suggested maximum
temperatures documented in ANSI/IEEE
C57.91-1995 [2] and IEC Standards 60354 and
60076-7 [3], for the four types of transformer
loading. In addition to these criteria, it is always
advisable to calculate the loss of insulation life
and make sure it is acceptable for the loads
beyond nameplate. Acceptable limits of loss of
insulation life for various loadings are very
important in developing a loading policy and
thermal model limits to facilitate real-time
Dynamic Loading.
Risk and Consequences of
Overloading Transformers
NOTE: All of these loading conditions make one
very important assumption: that the solid
insulation is DRY. The definition of DRY is: a
solid insulation system (most importantly the
winding conductor insulation) with moisture
content of less than 0.5% (weight or
water/weight of solid insulation).
The consequences of loading a transformer
beyond its nameplate rating are as follows [3]:
3

The temperature of the windings, cleats, leads,
insulation and oil will increase and can reach
unacceptable levels.

The leakage flux density outside the core
increases causing additional eddy-current heating
in metallic parts linked by the leakage flux.

As the temperature changes, the moisture and
gas content in the insulation and in the oil will
change.

Bushings, On-Load Tap Changers (OLTC’s),
cable-end connections and current transformers
will also be exposed to higher stresses, which
encroach upon their design and application
margins.
Things to consider before using
Dynamic Loading Calculations
Limiting Factors for the Dynamic
Loading Calculations
Prior to stressing any utility asset beyond their
nameplate value, a thorough review of the
asset’s health is recommended. Among the
items that should be considered in a health
review are:
 Maintenance history
o Past and present DGA results
(including moisture)
o Past and present Power Factor (PF)
test reports
o Oil quality
o Oil level
o OLTC problems
Prior to using the dynamic loading calculations
in your DRMCC, the limiting factors of
temperature and loading limits for the calculation
must be programmed. The calculation is based
on both the short time and long time limits
programmed in the monitor.

Cooling system health / condition

Bushing and OLTC ratings

Load profile of the transformer to ensure the
hottest phase is being monitored
The safe maximum load is the maximum load
the transformer can carry without exceeding any
of the limiting factors. IEC / I.E.E.E. load limits,
transformer manufacturers, regional system
operators and your utility loading guidelines
should be reviewed prior to programming the
limits in the monitor.
The limiting factors of temperature or current or
time may be configured via SCADA or
programmed from the interface unit (IU) as
shown here:
Dynamic Ratings Loading Limits Temperature
Dynamic Loading Calculation
Requirements
The dynamic loading calculations in Engineered
to Order transformer monitoring solutions are
heavily dependent on the accuracy of the
thermal information programmed in the monitor.
The following describes the functionality within
Dynamic Ratings Monitoring, Control and
Communication Systems. (DRMCC)
Dynamic Ratings Loading Limits Current
Dynamic Ratings Loading Limits Time
Prior to relying on dynamic loading calculations,
the accuracy of the thermal model should be
verified. Thermal model accuracy may be easily
verified by comparing the measured vs.
calculated top oil temperature values stored in
the data archive. The accuracy of the calculated
winding hot spot temperatures may also be
verified if fiber optic probes are installed in the
assets windings.
Factors which will affect the
Calculation
Ambient temperature can have a significant
impact on the cooling efficiency of the
transformer.
At
low
temperatures,
the
transformer cooling system is more efficient so
the monitor will report a higher safe maximum
loading limit.
At higher temperatures, the
cooling efficiency is reduced so a lower safe
maximum load limit will be reported.
Dynamic Ratings assists customers with
modification(s) of their systems thermal model
inputs to improve the accuracy of the model
based on the archive data. When verifying the
thermal model of a transformer, having the
transformer fully loaded is recommended.
4
Secure System Status
Tap position is used to calculate the system
losses. Tap positions resulting in higher system
losses will result in a lower safe maximum load
calculation.
The DRMCC features a built in WEB server to
display the “safe” loading capability of the unit.
Cooling system availability will also affect the
safe maximum load calculation. Hence if a
cooler failure alarm is active in the DRMCC, that
stage of cooling will be considered as not
available and the safe maximum load calculation
will be adjusted accordingly.
Dynamic Loading Calculations
DRMCC allowable loading is calculated in a
similar way to IEEE C57.91, Clause 7.3. A flow
chart of the iterative process used for the
calculations in provided in Flow Chart 1.
FIGURE 1
Safe Load Calculation
182% of
nameplate
FIGURE 2
Emergency ONAF (2 Hour) Transformer Rating using IEEE
Default Temperature Limits
FIGURE 3
Emergency ONAN (2 Hour) Transformer Rating using
I.E.E.E. Default Temperature Limits
The local display can display real-time safe
loading limits, as well as all information and
alarms from the monitor.
FLOW CHART 1
Top Oil and Winding Hot Spot Calculation Flow Chart
5
Effects and Hazards of Long-Time
Emergency Loading
Described below are a few selected short-term
loading hazards and their effects, not limited to:
This is not a normal operating condition and its
occurrence is expected to be rare, but it may
persist for weeks or even months at a time, and
can lead to significant aging of the solid
insulation system, not limited to:
 The main risk for short-term failures is the
reduction in the dielectric strength due to the
possible presence of gas (water vapor) bubbles in
the regions of high electrical stress, that being the
windings and the leads.
 Deterioration of the mechanical properties of the
conductor insulation will accelerate at higher
temperatures. This aging acceleration is also
dependent on the moisture content of the
conductor insulation. Taken together, it is an
exponential function in terms of aging rate of the
transformer. It will reduce the effective life of the
transformer, particularly if the unit is subjected to
system short-circuits or transportation events.
These bubbles are likely to occur when the
hot-spot temperature exceeds 140°C for a
transformer with winding insulation moisture
content of 2.5% or more. This critical temperature
will decrease as the moisture concentration (in the
winding insulation) increases.
 Gas bubbles can also develop (either in oil or in
solid insulation) at the surfaces of heavy metal
parts heated by leakage flux or be produced by
the super-saturation of water in the oil. Such
bubbles usually develop in regions of low electric
stress and have to circulate in regions where the
stress is higher before any significant reduction in
the dielectric strength occurs.
 The contact resistance of the OLTC could increase
at elevated currents and temperatures and, in
extreme cases thermal runaway could take place.
 The gasket materials in the transformer may
become more brittle as a result of elevated
temperatures.

Effects and Hazards of Short-Time
Emergency Loading
Short-time increased loading will result in a
service condition having an increased risk of
failure.
Pressure build-up in the bushings may result in a
failure due to oil leakage. Gassing in
condenser-type bushings may also occur if the
temperature of the insulation (inside the bushing)
exceeds approximately 140°C.
 The expansion of the oil could cause an overflow
of oil in the conservator (if equipped).
 Breaking and making excessive high current in the
On-Load Tap Changer (OLTC) could be hazardous.
Short-time emergency overloading causes the
conductor hot-spot to reach a level likely to
result in a temporary reduction in the dielectric
strength. However, acceptance of this risk for a
short time may be preferable to loss of supply.
Adding Real-Time Communications
Drives Customer Benefits
As discussed, on-line monitoring systems
provide benefits which are optimized when
coupled with the addition or inclusion of
Real-Time communication systems.
This type of loading is expected to occur rarely,
and it should be rapidly reduced or the
transformer disconnected within a short time in
order to avoid its failure.
The DRMCC system can function as the
communication hub. Acting as the link to the
customer’s network, the link can use either
Serial or Ethernet communications via a
Copper wire, Optical Fibre, Wireless Radio or
Powerline Communication System (PCS).
NOTE: The permissible duration (typically) of
this load is shorter than the thermal time
constant of the whole transformer, and depends
on the operating temperatures before the
increase in loading.
6
The two most significant benefits of this
coupling are improved system reliability and
maintenance
savings.
This
strategic
combination allows electrical apparatus
owners to confidently migrate from failure or
time based maintenance strategies to
conditions
based
maintenance
(CBM)
programs. (Graph 2)
PCS is rapidly becoming
the industry’s most secure, reliable and
economical
solution
for
sub-station
communications. This includes communication
between devices in the sub-station, to customer
service or control buildings or among preferred
networks. PCS is as an alternative to installing a
new wire or pulling optical fibre.
PCS transfers critical loading information via the
native communication protocol(s) without
interference as the transceiver encodes the data
onto any existing conductor. In the control
building, an inductive coupler is used with a
second transceiver to retrieve the signal passing
the monitored information onto the substation
RTU or desired gateway.
Typically, PCS installs in less than 30 minutes
without tools or specialized labor, customer’s
report high data throughputs over various
physical configurations including; Peer-to-Peer
or Many to One.
GRAPH 2
Customers enjoy increased system availability
and Reliability Improvements when on-line
monitoring
is
coupled
with
real-time
communications by receiving condition based
alarms when problems first arise. This early
detection allows the appropriate actions to be
taken before conditions escalate.
Further, customers are achieving increased
Maintenance Savings as the DRMCC
advanced analytics filter through the condition
data to automatically identify the issues requiring
maintenance attention. This allows utilities to
focus their Operations and Maintenance crews
on problem resolution versus manual or off-line
condition assessment followed by problem
identification and verification prior to resolution.
SCHEMATIC 1
Regardless of topology or protocol, when a
DRMCC featuring on-line monitoring is
coupled with real-time communications it
drives economic benefit for users.
7
Conclusion - On-Line Monitoring for
Dynamic Loading
On-line monitoring of power transformers, for
condition assessment, has gained popularity
over the past twenty years, the typical
technology adoption period, from concept to
commercial reality, in the electric utility industry.
The benefits of on-line monitoring have primarily
accrued to reduction in maintenance cost and
overall improvement in T&D system reliability.
The prospect of using on-line monitoring to
make intelligent decisions on how to optimize
the load on such important substation assets as
transformers follows the adoption of load
management technology for power equipment
such as oil-filled transformers.
The size of the unit and/or its system criticality is
often the key deciding factor when selecting
on-line monitoring expenditures. However, the
tangible benefits related to Dynamic Loading are
far greater and more easily identifiable than
those attributed to reduced maintenance cost.
References
In the final analysis, if the unit is not fit for the
purpose or if its poor condition leads to sudden
failure, the value of monitoring dynamic load
capability becomes academic at the time.
[1] Life Management Techniques for Power Transformers,
Cigre Technical Brochure 227, 2004.
[2] IEEE Guide for Loading Mineral-Oil-Immersed
Transformers, IEEE C57.91-1995
Clearly, good condition assessment and
dynamic (on-line) load monitoring go together
hand-in-glove.
[3] IEC Loading Guide for Oil-Immersed Power
Transformers, IEC 354.
8