**Statement of Problem**
(1) In the design of distribution automation systems, the internal energy assessment of users has become increasingly important under the requirements of energy saving and emission reduction. However, a common issue arises where the sum of sub-measurements in the internal user measurement system is consistently less than the total reading from the power supply bureau’s main meter. The root cause of this measurement inaccuracy remains unclear, creating challenges in ensuring accurate energy monitoring and billing.
(2) Another recurring problem involves the frequent burning out of voltage transformers (PTs) in the PT cabinet, especially in circuits that use AC 220V for protection and alarm functions. This not only leads to operational disruptions but also increases maintenance costs and safety risks.
**2. Analysis**
2.1 The primary causes of inaccurate measurements include:
a. The ratio, capacity, and accuracy class of the current transformer (CT).
b. The capacity and accuracy class of the voltage transformer (PT).
c. Voltage drop in the secondary circuit.
d. The inherent accuracy of the meter itself, among other factors.
In medium-voltage switchgear, the CTs and PTs used for measurement are typically rated at 0.2 accuracy class, as they are dedicated to power metering. This ensures high precision in energy measurement. However, in general input and output cabinets, the CTs and PTs are usually rated at 0.5 accuracy class. While the power supply bureau's metering system has higher accuracy, the difference between the two systems should still fall within the cumulative error range of the transformers and meters. In practice, however, the discrepancies observed often exceed these expected tolerances.
Through field analysis of several 10kV high-voltage distribution systems, it became clear that additional factors contribute to the measurement differences. For example, in a thermal power plant in Beijing, there are around 17 input and output cabinets. Each uses a 100V PT to provide voltage signals. The multi-function meters installed on the high-voltage cabinets, such as the DSSD3313X100V 1.5 (6) A model, consume about 4VA each. With 17 meters, this totals 68VA. Additionally, the microcomputer protection devices (e.g., ABB REX521) connected to the same PT loop consume 0.5VA per unit, adding another 8.5VA. The total load on the 100V PT loop is therefore 76.5VA. If the PT is selected with a capacity of only 30VA, it will be overloaded, leading to significant measurement errors and potential damage to the PT due to overheating.
Similarly, in the Xiamen Yuyu Terminal Power Distribution System, 20 DS862-2 meters are used, each consuming 8VA, totaling 160VA. Since the microcomputers do not require an AC 100V input, their contribution is negligible. In this case, a PT with a capacity of at least 180VA is required to avoid overloading and ensure reliable operation. Using a smaller PT, such as a 30VA model, would lead to overheating and failure, posing serious risks to the system.
Therefore, in 10kV substations, multiple outlets, monitoring devices, and operating meters can increase the load on the PTs. When the load exceeds the rated capacity of the PT within its accuracy range, the measurement accuracy drops significantly. The greater the overload, the more the secondary output is attenuated, leading to large deviations in measured values compared to the main meter. If the PT operates near or beyond its thermal limit, it may overheat, dry up, and eventually fail, causing accidents and downtime.
Accurate and reliable power metering requires a comprehensive evaluation of the entire power distribution system. Selecting the appropriate PT and CT specifications is essential to meet both measurement accuracy and capacity requirements.
2.2 There are three main reasons for the burning of voltage transformers in PT cabinets using the 3PT-Y connection method:
(1) Ferroresonance caused by the PT.
(2) Overloading of the PT.
(3) A short circuit in the secondary winding of the PT.
These issues highlight the importance of proper selection and regular maintenance of PTs to ensure safe and accurate operation of the electrical system.
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