GE Grid Solutions
Power transformers are one of the critical assets of any electric power system required to maintain the reliable and efficient power supply. Yet more than half the transformers in the developed world are 40 years or older. With 52% of transformer failures caused by insulation degradation and electrical abnormalities due to aging, extending the life of these devices has become a top priority for power system engineers. Historically, transformer insulation monitoring is performed by manual oil samples, and these samples were sent to a lab for analysis and a condition report issued. As technology evolved, dissolved gas analysis (DGA) devices were introduced, capable of drawing oil samples on their own on a regular basis. One of the major root causes of transformer winding failures is due to electrical or thermal. However, electrical and thermal are measured for protection of the transformer only not for condition monitoring purpose.
This paper presents recent innovation of integrating electrical, chemical, and thermal monitoring and Diagnosis (M&D) of the transformers by interfacing next-generation protection relays with dissolved gas analysis (DGA) devices. All measurements electrical, chemical and thermal are synchronized to develop transformer health models using next-generation protection relays. For example, monitoring and diagnosing the impact of transformer events, such as energization, inter/external fault on the transformer insulation health is made possible. Integrated asset performance management using fault report and health report will be presented in detail.
Distribution networks are important in an electrical power system for transferring power from the transmission network or generators to end users e.g. industrial plants, commercial facilities, residential etc. Table 1 summarizes challenges and objectives in today’s distribution networks: Table 1 Today’s challenges and objectives in distribution netwworks:
This paper focuses on solutions for today’s distribution system challenges related to asset monitoring, personnel safety and reliability using next-generation protection relays.
Traditionally, electromechanical or single function protection devices have provided protection to electrical assets, such as transformers or feeders (breakers) from electrical short-circuit faults. As the technology has evolved, protective relays have become more intelligent, integrating multiple Protection & Control (P&C) functions and offering advanced communications.
Advanced protective relays now detect and notify operators of internal or external faults, collect and record electrical data, and perform pre- and post-fault analysis using high-accuracy transient records and fault reports. Transducer inputs can be added to measure temperature, pressure, cooling system characteristics, and mechanical operations (tap changer positions), etc. Next-generation protection relays are not only intelligent, but also possess enhanced capability to integrate asset monitoring, safety features e.g. arc flash, and reliability features e.g. environment monitoring, redundancy etc. proposed below in this paper.
INTEGRATION OF ASSET MONITORING
Power transformers are one of the most critical assets of a power substation required to maintain reliable and efficient power supply. Yet more than half the transformers in the developed world are 40 years or older. With 52% of transformer failures caused by insulation degradation and electrical abnormalities due to aging, extending the life of these devices has become a top priority for power system engineers.
Integrated M&D Approach
A transformer protection relay is connected with current and voltage transformers to obtain current and voltages to detect electrical faults in a transformer, and interfaced with breakers for control actions. On the other hand, dissolved gas analysis (DGA) devices on a transformer are capable of drawing oil samples on their own on a regular basis and performing chemical analysis continuously.
These two distinct systems (electrical protection and chemical monitoring) traditionally have not been interconnected. Yet since both systems provide much-needed asset and performance management data, there is an opportunity to bring them together to provide a more comprehensive view into the transformer’s overall health. This integrated approach of two systems (shown in Figure 1), (one protection relay with electrical functions; and second DGA with chemical analysis) allows trending incipient fault events with the same time stamps, correlation between the electrical and chemical models, and integrated asset health analysis and reporting.
Figure 2 shows electrical models for a transformer, such as power, current demand, load, harmonics, etc. In addition to continuously measuring voltages and current, protective relays measure numerous critical electrical parameters, including over-excitation and flux conditions. This allows operators to see loading imbalances and averages, including seasonal averages, average loading from a particular condition to another, or average loading for parallel or unparalleled conditions. For each of these different operational scenarios, the relay captures the electrical characteristics of the transformer and stores this data for a short- or long-term view. Data can be stored for up to one year of data within the relay, creating a repository of transformer behavior, allowing modeling transformer behavior over a period of time.
Several standard gas analyzer models exist that conform to IEC and IEEE standards, including the Duval triangle, gas ratios, and key gas. Figure 3 shows example of a Duval’s triangle model.
An integrated approach offered by the protection relay eliminates the need for offline DGA analysis. When the DGA device is connected to the protection relay, it imports the chemical data on an ongoing basis, compiling reports, and presenting the analysis back in a practical, actionable fashion. This reduces reliance on offline experts and laboratories; the data is readily available in the system. This type of continuous monitoring can show a transformer fault’s characteristics moving from one point to another, as well as the ratio of gases. This is a powerful tool in understanding the speed at which degradation or a fault condition is evolving. Integrated electrical and chemical analysis is further used to achieve the following:
Data correlation models:
These models provide the ability to correlate electrical, chemical and thermal data, allowing for the creation of individual trending models. Such models help identify correlations between different but interrelated sets of data, such as a record of transformer loads compared to gas concentration data. A correlation model will help spot, for example, that a gas concentration is occurring every time a transformer is loaded in a particular way. This model can then be used to track transformer load, temperature, and gas concentration changes in order to identify any relationship between one measure and another. This in turns allows operators to proactively safeguard transformer assets, extending the time before a shut-down for maintenance or decommissioning is required.
During energization, a transformer goes through tremendous stress, experiencing significant inrush current and application of voltage. Every time a transformer is energized, the protective relay can capture a snapshot view of what is happening inside the transformer during that event. That data is assembled into an energization record that includes such measures as:
- Current and voltage samples for initial cycles
- Inrush parameters:
- Peak inrush current
- Max voltage dip
- 2nd and 5th harmonics
This is a particularly valuable tool when working with aged transformers. In this type of transformer, incipient fault conditions can start developing when the asset is energized. A protective relay can provide an accurate representation of energization behavior of the various inrush parameters. This allows the benchmarking of a healthy energization condition that helps determine any deviations from one event to another. In addition to being an invaluable instrument in the commissioning of new transformers, this record is an excellent re-commissioning tool, capturing and analyzing data for a transformer after repair.
Transformer fault report:
When a protective relay detects an internal fault, it trips the transformer. External or through faults, however, can result in high current flow through the transformer prior to the device being tripped offline, causing significant stress on the transformer. A typical distribution utility experiences about 15 to 20 through faults each year, even in advanced distribution systems. Research demonstrates that many transformers that fail catastrophically start showing indications of problems immediately following a thorough fault event. Developing situations such as these are more easily detected with visibility into pre- and post-fault electrical, chemical, and thermal data. In the 845, this data is delivered via the user-friendly integrated transformer fault report. This graphical fault report can provide clear visual indication of the fault condition and any changes to the transformer’s health. This report can help in two ways. First, it provides greater visibility into the safety of a transformer after it has been tripped, allowing operators to make judgments about whether it is safe to put the device back into operation.
Second, it allows for proper diagnosis of any new conditions that have started evolving in that transformer, such as an increase in a gas concentration, shortly after a fault event. This integrated solution provides a heightened level of visibility, allowing the data to be applied in a meaningful way to maintenance or troubleshooting needs.
Transformer health report :
Transformer health report (shown in Figure 5) is a compilation of operational and fault data, allowing benchmarking and identification of deviations from the normal operating conditions. An asset management team can utilize this readily available report for post-event analysis and record archival.
It provides a snapshot of tripping alarm events on a transformer, including the following:
- Nameplate data
- Trend in energization behaviors
- One year learned data
- Historic maximums
- DGA models
- Alarm/trip history
Having all this data in a single device helps eliminate multi-layered monitoring and recording devices, providing one-stop operational, monitoring and fault analytics. The simple and intuitive analytics can be archived for reference and future comparison.
Environmental factors have a direct impact on the reliability of a protection relay. The products are designed and tested to industrial standards but infrastructure failures can cause the environment to exceed product specifications. The next generations of protection relays are equipped with measurement capabilities to monitor the surrounding environment in which the product is installed. Monitoring parameters such as temperature, humidity, and vibration and surge events can provide an early indication of a change in an environment before it becomes detrimental to the protection relay (shown in Figure 6.)
Comparative analysis can be performed on a fleet of protection relays and unexpected behavior can be identified for further investigation during a maintenance schedule. Histograms that trend the data give insight into the operating environment over the life of the product. The functionality of protection relays providing environmental data for customer consumption gives more control in managing the reliability of asset or process.
Today’s power engineers and asset managers need transformer monitoring and diagnostics solutions that provide clear, actionable intelligence through a combination of electrical, chemical and thermal data. In unifying these different types of data, the protective relay provides a holistic view into multiple aspects of transformer health, including overloading and gassing, over-voltage and bushing problems, hotspots, and DGA,and tap changer and cooling system monitoring.
Data correlation models are time aligned and trended among electrical, chemical and thermal characteristics through a range of transformer conditions. Reports from the relay– including the energization record, integrated transformer fault report and transformer health report – allow capturing the key operational dynamics that provide insights around both internal and through fault conditions. Critical transformer data is compiled into reports andmodels that are easy to interpret, supporting informed decision-making. This ultimately delivers actionable analytics for asset optimization and transformer life extension, enabling operators to reduce capital andoperational expenditures while maintaining the power system reliability demanded by today’s market.