Bushings Health Monitor+ | Dynamic Ratings
instrument transformer bushing

In the utilities industry, bushings are essential components connecting transformers to the grid, and monitoring their health is essential for maximizing uptime and reducing damage to equipment and danger to workers.

You already know that bushing failures can occur from moisture contamination or partial discharge — and that they make up 25% of transformer failures. You also know that regular monitoring of bushings allows utilities to implement preventive maintenance strategies that identify potential issues early on, allowing maintenance teams to address problems before they escalate, reducing the likelihood of unexpected failures and minimizing downtime.

What you might not know is that with the launch of the Bushings Health Monitor+ (or BHM+) from Dynamic Ratings, utilities stakeholders now have access to an all-in-one asset monitoring solution uniquely positioned for voltage reference and sum of currents reporting in the same monitor.

Retrofitting voltage reference bushing monitoring is difficult and expensive because of the availability of voltage measurements required. This is why asset management stakeholders often have to choose between a sum of current or a volt reference solution.

Now, there’s a third option: a single monitoring solution — applied to both old and new sites — ensures the reliability, efficiency, and safety of power transformers.

Our Bushing Health Monitor + supports both voltage reference and sum of current techniques and allows end users to standardize on a single model of bushing monitor, which simplifies support and training for the field operation teams.

Users can deploy voltage reference on new sites, and sum of currents on existing sites where the cost of supplying voltage measurements are cost prohibitive.

transformer bushings

The Bushings Issue

Bushings contribute heavily to the overall reliability of power transformers, and a malfunctioning bushing can lead to catastrophic transformer failure, causing power outages and significant disruptions to the electrical grid. Faulty bushings pose a risk of oil leaks, flashovers, or even explosions.


Regular online condition based monitoring helps utilities maintain a high level of system reliability. After all, health monitoring helps utilities identify signs of degradation, preventing these risks and ensuring the safety of personnel and surrounding infrastructure. Timely detection allows for proper mitigation measures to be implemented and is crucial for maintaining a reliable power supply.

With BHM+ from Dynamic Ratings, you have instantaneous real-time access to the performance of bushings under different operating conditions. This data can be used to optimize the operation of transformers and ensure that they are working within their design parameters, which leads to increased efficiency and reduced energy losses.

Testing Parameters

Bushings can degrade over time due to factors such as electrical stress, environmental conditions, and aging. Health monitoring systems can detect early signs of degradation, allowing utilities to take corrective actions before the bushings fail. Early detection is key to preventing costly repairs and replacements.

Online bushing monitoring is a proven method of detecting bushings that are operating in a degraded condition. Since the test parameters used for online monitoring and offline bushing tests are different, the results can differ wildly as a result.

There are two predominant technologies for online bushing monitors. The sum of currents methodology monitors for imbalance in the bushing leakage current to detect deterioration of bushings. Under balanced system conditions, the vector sum of leakage currents is zero, assuming the power factor and capacitance of each bushing is identical.

Voltage reference, on the the hand, calculates power factor from the bushing leakage current and the voltage reference signal for detecting degradation of the bushings. Each bushing is measured independently when using the voltage reference technique, making it less affected by grid fluctuations.

C50 Polar Plot
walk in substation

The sum of current test is the most standard configuration for bushings monitoring and is easily retrofitted for legacy transformers. It requires a current input from each phase of the high and low side bushings to be monitored.

Voltage reference requires an additional voltage input from each bushing in order to be monitored. These voltage inputs can make retrofitting reference methodology cost prohibitive, especially if a voltage source is not already available because of additional hardware and cabling requirements.

The sum of current method will provide stable current under most three-phase system operating conditions. This imbalance current is small, but finite, as no power system is equally balanced and the bushings are never identical. The rate of change of this current imbalance is useful for detecting bushing health changes.

Power system induced voltage fluctuations from ungrounded power sources such as distributed energy resources can result in system imbalances which may impact the sensitivity of some current calculations. In these circumstances, the voltage reference method may be used since they are single phase calculations.

While the voltage reference method will improve the accuracy of the online power factor and capacitance measurements, one common misconception is that online and offline measurements should match. It’s important to understand the differences to maintain confidence in online monitoring.

C50 on transformer
employee repairing bushing

The sum of current method has proven to be easy to retrofit and is a reliable method — unless there are excessive system voltage fluctuations.

The sum of currents method has a field-proven track record in successfully detecting bushing failures, as it monitors C1 capacitance, and a change in the capacitance as well as changes in the dielectric losses of C1. Of course, the imbalance current is a vector, where the vector phase position indicates defective phase and type of defect.

But there are known limitations with the sum of currents method, and factors which may affect the sum of currents method include system voltage and balances, system power factor and phase angle fluctuations, and reverse power flow. Power factor and capacitance values are estimated and can be influenced by system anomalies, as well.

The voltage reference technique solves these problems and can help deal with some of the challenges being posed by the fluctuations present in the sum of currents method.

When bushings are energized and operating at higher temperatures, the stresses in the bushing are increased. This makes the online monitoring more sensitive and may result in cases where the online and off-line values do not match.

The New BHM+

The Dyanmic Ratings BHM+ allows asset managers to select the method that best suits their installation and application concerns.

With the BHM+ the monitor continuously takes measurements and provides a warning or alarms on key parameters, which allows asset managers to make smart decisions and plan ahead.

Dynamic Ratings is offering a new bushing health monitor that supports both voltage reference and sum of currents testing to improve the accuracy of the bushing power factor and capacitance values, using both methods simultaneously.

image looking up from inside a transmission tower


The Dynamic Ratings solution will allow the use of both the sum of currents and voltage reference simultaneously, and our Bushing Adapter Unit (or BAU) allows for the ability to choose between either or both.

Dynamic Ratings BAU+ Sensor

The Bushing Adapter Unit (BAU+) sensor serves multiple purposes which eliminates the need to purchase a unique sensor for each application. It provides

  • Bushing signals for bushing insulation characteristics
  • Partial discharge in bushings and transformers
  • Additional circuitry for noise filtering using Rogowski coils

C50 Success

The BHM is itself part of C50 Transformer Monitor, which makes condition monitoring easier and more affordable than ever before.

Harnessing the power of real-time data analytics enables utilities to make decisions about their high voltage transformers quicker and with increased accuracy.

Dynamic Ratings has streamlined the C50 configuration process, and backwards compatibility allows configurations to be copied from previously programmed C50 online transformer monitoring system which enables large-scale development with ease.

The BAU+ sensor has a robust, environmentally hardened design rated to IP-67 specifications. These sensors are ideally suited for use in power transformers, isophase bus ducts, and HVCTs.

Dynamic Ratings C50

Easy Removal and Installation for Offline Testing

BAU+ Sensors are installed in the test tap of the bushing. If a test tap isn’t available, the voltage tap may be used. BAU+ Sensors are designed with three sections: body, adaptor and cable connector. The body of the sensor can be easily detached from the adapter by the removal of three screws. Once the body is disconnected, the test tap of the bushing can be accessed directly or the adapter can be removed from the bushing to provide additional room when needed. The adapter head and contact change to accommodate the design of the bushing tap, the body and cable connector stay the same.

Better Protection Means Better Performance

Our Bushing Adapter Unit comes with three distinct levels of protection.

Dynamic Ratings BAU+ Sensor
  1. Open Circuit / Voltage Limiter – Six voltage limiters provide open circuit protection. A current balance circuit is used to distribute the stress equally between limiters. Dynamic Ratings offers the lowest open circuit protection in the industry.
  2. Surge Protection – Surge protection provides safety for equipment and personnel. Two surge protection circuits provide protection for switching and system transients.
  3. Fail Safe Circuit – A fail safe circuit automatically grounds the tap inside the body of the sensor should the open circuit and / or surge protection fail.

The DynamicMetrix® Difference 

Offline testing alone does not track estimated Tan Delta and capacitance, imbalance percentage, or temperature coefficient, and our DynamicMetrix™ software platform uses a secure file transfer protocol (digital communication) to pull the info out of the BHM and visualizes it providing analytics. DynamicMetrix™ runs in place of Athena, and now acts as the industry standard, and the best digital asset management software for your data reporting needs.

Our software is flexible enough to allow the use of voltage references requiring a phase shift. This will allow you to use a PT from your Delta winding as a reference for bushings on the Wye winding

Adding voltage reference to your BHM+ monitoring provides the ability to assess Bushing health data using only power factor and capacitance values, provides more accurate C1 Power factor values and more accurate C1 capacitance values, and allows for faster reporting of changes in power factor and capacitance.

BHM+ comes with easy configuration tools that are intuitive and easy to use, including automatic hardware detection, a simplified interface, auto-gen SCADA files, and easy alarm configuration.

Online C50 monitoring provides access to all critical operating data, such as

  • Power Factor and Capacitance
  • Cooling Systems
  • Partial Discharge
  • Internal and Ambient Temperatures
  • On Load Tap Changers
  • Multi-gas DGA
  • Bushing Health

Online monitoring rather than periodic offline testing gives utilities preventative solutions that lower the risk of bushing failure. The Bushing Health Monitor+ supports both voltage reference and sum of current techniques and allows end users to standardize on a single model, making online bushing health reporting more streamlined and effective.

Author: Tyler Willis, Dynamic Ratings