# A Review of Health Assessment Techniques for Distribution Transformers in Smart Distribution Grids
## 4 Advanced Technologies for Real-Time Monitoring Transformer Condition in a Smart Distribution Grid
The investment cost of a distribution transformer is much lower than that of a power transformer. In order to get quality evaluation results, high-quality sensors are required. This means that the monitoring system will be very expensive, not consistent with the actual ability of the utilities. Therefore, the corresponding monitoring system for distribution transformers should be designed to be acceptable. To save installation costs, the monitoring parameters are usually voltage, current or ambient temperature that can get easily from electrical measurement systems. These parameters also contribute significantly in identifying the potential risks of transformers in operation, thereby making initial assessments of transformer status before making more thorough inspection decisions. Real-time monitoring transformers using advanced metering infrastructure (AMI) is an effective low-cost solution for managing smart transformers as well as smart grids. In [80], an AMI system is proposed to supervise transformer operation under fire hazards. The main parts of the detection system are sensors, valve systems and the control box. The control box will receive measured data from sensors and send commands to control the system. This smart monitoring system not only provides fast fault detection but also improves the overall health of transformers as well as the distribution system. In [81], the AMI systems are used to collect data in real-time including transformer loading, transformer aging, estimated ambient temperature, computed the hottest temperature from oil-immersed distribution transformers. The gathering information can be used indirectly to determine the priority candidate transformer for replacement before failure. The meter measurements can be utilized to develop temporally high-resolution views of transformer loading indirectly to determine each operational transformer's aging acceleration. In [49; 82], the papers provided a methodology that was used to predict the outage and congestion for distribution transformers. This paper uses the hourly usage data collected from Ameren Illinois' AMI meters to determine distribution transformer outage, failure, and overload. The proposed methodology not only detects and visualizes outage and congested areas in near real-time but also detects transformers and distribution areas with a long history of outage and congestion. More recently, portable systems allow diagnosing the dielectric part of many electrical apparatus including oil-insulated transformers [82; 83; 84]. The portable system uses a sensor which is a special wideband antenna. It allows to detect both partial discharges pulses and the signal of AC supply voltage remotely from the tested component; in this way, the need for direct connection or component turn-off is avoided. Table 5 compares the considered monitoring parameters, the advantages and disadvantages of real-time monitoring techniques in the literature. It can be seen that the advanced techniques are good solutions and have the potential to provide a low-cost health assessment system based on existing sensors, energy monitoring meters and the existing communication networks. These solutions can support utilities to monitor distribution transformers continuously and set up suitable transformer management strategies. However, low-cost does not directly translate into a short lifetime or lack of robustness. A good design practice may achieve higher quality and lifetime from lower-cost components (e.g., design for manufacturability,  
The low-cost health assessment system is also relative to the cost of the considered transformer. The US market for distribution transformers is currently standing at about 650,000-750,000 units per year. The price of a distribution transformer ranges from $700 to $12,000, depending on order quantity and kVA ratings [1]. Therefore, health assessment solutions for distribution transformers must be lower cost but sophisticated and precise solutions may not be as important.