End-to-end monitoring systems feeds situational awareness

Brian D. Sparling, Senior Member of IEEE, and Regional Manager with Dynamic Ratings Inc.

Historically, substation assets have been loaded beyond nameplate ratings and the need to accommodate these emergencies or contingency, conditions are best served using End-to-End electrical apparatus monitoring systems.

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-five years, the typical technology adoption period, from concept to commercial reality in the electric utility industry.

What are the benefits of implementing End-to-End monitoring of power transformers and circuit breakers?

• Provides Situational Awareness of electrical assets operating closer to their capacity without compromising safety or reliability.
• Fully optimize real-time substation loading/overloading based on actual site conditions, including asset condition or operating modes.
• Assist in making intelligent decisions about load management based on actual circumstances.
• Forecast (predict) operating conditions used to facilitate condition-based maintenance (CBM) programs or agency reporting (such as environmental reporting of SF6 release).
• Collect 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 other systems and SCADA. Until recently, the operation of electrical apparatus would fit 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 ambient condition. Of course, this loading condition rarely happens over the life of a transformer, where both load and ambient temperature vary over time.

Cyclical Load

This loading implies a cyclical load at a normal constant ambient (30°C) where the hottest-spot conductor temperature varies as the load cycles above and below the nameplate MVA of the apparatus. From the thermal ageing standpoint, this cycle is equivalent to the case of rated constant load at normal ambient temperature (30°C).

Overloading Apparatus

The consequences of loading apparatus beyond its nameplate rating are as follows:

• 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. 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.
• Bushings, On-Load Tap-Changers, cable-end connections and current transformers will also be 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, can provide the triggers for CBM (Condition Based Maintenance).

The size or criticality of the unit is no longer

• the deciding factor for on-line monitoring as
• the cost of deployment is minimal and the
• tangible benefits related to monitoring far outweigh the cost.

Long-time Emergency Loading

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.

• 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 solid and liquid insulation. It will reduce the effective life of the asset.
• Cooling system is operating for extended periods, increasing maintenance and 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 because of elevated temperatures.

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 the loss of supply.
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.

• The main risk for short-term failures is the reduction in the dielectric strength due to the possible presence of gas (water vapour) 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.

End-to-end Performance

The benefits of on-line monitoring have primarily accrued to reduce maintenance costs 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 follows the adoption of load management technology for power equipment such as oil-filled transformers and oil or gas-filled circuit breakers.

The use of on-line monitoring systems as controlling devices, such as the cooling system on transformers providing communications with external networks, enables the system to be integrated in the transformer control system resulting in minimal incremental cost to implement. The additional information gathered on the asset greatly offsets this cost. The size or criticality of the unit is no longer the deciding factor for on-line monitoring as the cost of deployment is minimal and the tangible benefits related to monitoring far outweigh the cost. Clearly, sustaining electrical apparatus operation and uptime, reliable operations and increased asset life are the benefits of continuous monitoring systems.

Technology solutions are essential for communications to connect devices installed on-field assets to communicate to networks via substation control rooms. One suitable technology is the powerline communications system (iBridge), which provides high- speed, encrypted communications over existing cables. The iBridge system is substation hardened, designed to perform in high EMC environments, available as point-to-point or point-to-multipoint, protocol-independent with the rapid installation time and suitable for existing or new substation installations.
The cost of deploying such systems is non-recurrent, totally flexible and extremely competitive with fibre or wireless systems. The encryption level ensures full security of data.
On-line monitoring of power transformers and circuit breakers in a substation, with communications to SCADA and engineering networks, is mature technology suited for new installations as well as retrofitting to existing assets. The future is here and available for all asset classes, with the objectives of understanding Situational Awareness of the equipment and its ‘reliability, whilst minimizing operational expenditure.

This article was first published on Electricity Today.