White Paper
End-to-End Monitoring Systems
Historically, substation assets are loaded beyond nameplate ratings. The need to accommodate
emergency, or contingency, conditions are best served using End-to-End apparatus monitoring systems.
INTRODUCTION
For many years the limit for normal apparatus loading was based on the maximum
nameplate rating or an arbitrarily set value, called ‘the red line.’ On-line monitoring
of power transformers and circuit breakers, for condition assessment, has gained
popularity over the past twenty years.
Long-Time Emergency Loading
Until recently, the operation of electrical apparatus would fit into one of the following
loading categories: Continuous Load or Cyclical Load.
Long-term emergency loading of transformers occurs and may persist for extended
periods. This can lead to significantly increased aging of the solid insulation system including:
• Deterioration of the mechanical properties of conductor insulation will accelerate
at higher temperatures. This aging acceleration is also impacted by the moisture
content of the conductor insulation. Taken together, it is an exponential function in
terms of the aging rate of transformer insulation, reducing the effective life of the asset.
• Cooling systems operating for extended periods, increasing maintenance &
reducing life expectancy.
• Contact resistance of the breaker and/or OLTC will increase at elevated currents
and temperatures and, in extreme cases thermal runaway could occur.
• Gasket materials may become more brittle as a result of elevated temperatures.
Continuous Load
END-TO-END PERFORMANCE
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 other systems and SCADA.
This is the constant loading at rated nameplate output in (MVA) when the apparatus is
operated under a constant 20°C ambient condition. Of course, this loading condition
rarely happens over the life of a transformer, where both load and ambient temperature
vary over time.
This loading implies a cyclical load at a
normal constant ambient (20°C / 68°F) where
the hottest-spot conductor temperature
varies as the load cycles above and below
the nameplate MVA of the apparatus. From
the thermal aging standpoint, this cycle is
equivalent to the case of rated constant load at
normal ambient temperature (20°C / 68°F).
100000
IEC and IEEE Age Rate
10000
Age Rate (pu)
Cyclical Load
1000
100
1
0.1
0.01
0.001
Overloading Apparatus
The size or criticality of the apparatus is no longer the deciding factor for monitoring,
as the cost of deployment is minimal and the tangible benefits related to monitoring far
out-weigh the cost.
Proven, utility-grade communication solutions are installed on field assets providing
real-world data to networks via substation control rooms. One suitable technology is
the powerline communications system (PCS), which provides high-speed, encrypted
communications over existing cables.
10
0.0001
0
The benefits of on-line monitoring have primarily reduced maintenance costs and
improved T&D system reliability. The prospect of using on-line monitoring to make
intelligent decisions on asset load optimization allows the adoption of load management
techniques in substations.
20
40 60
80 100 120 140 160 180 200
WHS
IEEE
IEC
The consequences of loading apparatus beyond its nameplate rating are as follows:
• Temperatures of windings, cleats, leads, insulation and oil will increase, accelerating
insulation consumption.
• Core leakage flux increases, causing additional eddy-current heating in metallic parts.
• If SF6 gas or oil is leaking to the environment, moisture is entering. 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.
• Bushings, On-Load Tap-Changers, cable-end connections and current transformers
are exposed to higher stresses, encroaching upon their design and application margins.
• Detection of restrikes on breakers plus, knowledge of trip timing, clearing time, and
arc duration, may trigger CBM (Condition Based Maintenance).
Short-Time Emergency Loading
Short-time increased loading can result in an increased risk of a system failure.
Acceptance of this risk for a short time may be preferable to loss of supply.
• The main risk for short-term failures is the reduction in dielectric strength due to
the possible presence of gas (water vapor) bubbles in the regions of high electrical
stress, (leads and windings). These 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 conservator.
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.
©2013 Dynamic Ratings Specifications subject to change without notice. All rights reserved. The PCS system is substation hardened, designed to perform in high EMC environments,
available as point-to-point or multipoint, protocol independent with rapid installation time
and suitable for existing or new substation installations. The cost of PCS is non recurrent,
totally flexible and deployed for 60% less cost than fibre systems.
end-to-end monitoring benefits
The benefits of implementing End-to-End monitoring of power transformers and circuit
breakers include:
a. Utilize electrical assets closer to their real operational capacity without
compromising safety or reliability.
b. Fully optimise real-time substation loading/overloading based on actual site
conditions, including asset condition or operating modes.
c. Assist in making intelligent decisions about load management based on actual
circumstances.
d. Forecast (predict) operating conditions used to facilitate condition based
maintenance (CBM) programs or agency reporting (such as environmental
reporting of SF6 release).
e. Collect operational and accumulated loss of life data to enable estimation of:
• How fast is insulation consumed
• View of an asset’s remaining life
• Risk level(s) for an asset’s end-of-life
CONCLUSION
On-line monitoring of transformers and circuit breakers in a substation, with
communications to SCADA and engineering networks, is mature technology suited
for new installations as well as retro-fitting to existing assets. The future is here and
available for all asset classes, with the objectives of sustaining electrical apparatus
operation and uptime, reliable operations and increased asset life are the benefits of
continuous monitoring systems, while minimizing operational expenditure.
WP 130131 End-to-End Monitoring Systems