(1) In the design of distribution automation system, the internal energy assessment of users is increasingly important under the requirements of energy saving and emission reduction. The sum of the sub-measurements in the internal measurement assessment of the user feedback is always less than the total meter of the power supply bureau; the reason for the inaccurate measurement is unknown.
(2) User feedback The voltage transformer of the PT cabinet is often burnt out (especially the protection and accident audio circuits are operated by AC AC220V)
2. Analysis2.1 The reason for the inaccurate measurement is a. The ratio, capacity and accuracy of the current transformer. b. Capacity and accuracy of the voltage transformer. c. Voltage loss of the secondary circuit d. Accuracy of the meter itself and the like.
In the medium voltage switchgear, the accuracy of the current measuring instrument current transformer and the voltage transformer is 0.2. The transformer of the measuring cabinet is dedicated to the power supply metering, and only used for metering, so the metering accuracy can be guaranteed to the maximum extent. The accuracy of the current transformer and voltage transformer used in the common input and outlet cabinet measurement is 0.5. The accuracy of the metering of the power supply bureau has already been higher than that of the inlet and outlet cabinets. Therefore, the metering value of the meter must be different, but it should be within the cumulative error range of the transformer and meter. However, the actual difference value is far more than the accuracy error. Through the analysis of the actual situation of some 10KV high-voltage power distribution systems, we explore other reasons that affect the measurement difference. The following figure shows the secondary schematic diagram and the primary system diagram of the 2CT, 2PT three-phase three-wire metering circuit of the inlet and outlet cabinets (the meter is installed on the high-voltage cabinet as an example)
For example, a power distribution system of a thermal power plant in Beijing (the meter is installed on a high-voltage cabinet as an example) has about 17 inlet and outlet cabinets, and its AC voltage signals are taken from the 100V voltage of the PT. In the system, equipped with Changsha Weisheng Electronics Co., Ltd. multi-function meter DSSD3313X100V 1.5 (6) A (0.5 active) AC voltage loop power consumption = = 4VA a total of 17, only the total power consumption of the energy meter 17X4VA = 68VA; installed ABB The power consumption of the microcomputer REX521 (AC voltage 100V loop) is “=0.5VA total 17 pcs, the total power consumption of the microcomputer AC voltage loop is 17X0.5=8.5VA; for the whole system, the total power consumption of AC 100V voltage is 68+ 8.6=76.5VA (the secondary conductor power consumption is not considered). Therefore, if the PT is selected according to the capacity of 76.5VA, the 0.5/100VA thermal limit output of 500VA should be selected to meet the measurement accuracy of the system with 0.5 levels, and the energy measurement is accurate. If the PT is wrongly selected 0.5/30VA, the thermal limit output is 150VA. Firstly, the load capacity of the meter has far exceeded 30VA, and the measurement accuracy must have a large deviation. Secondly, the PT long-term overload is like a small horse-drawn cart, which is easy to burn. The current loop CT of each cabinet, such as LZZBJ9-12/150B/2S, has a capacity of 0.5 VA. The accuracy and capacity of the current loop installed in this cabinet can be guaranteed. The distance of the cabinet) Also consider the selection of the capacity of the CT.
For example, Xiamen Yuyu Terminal Power Distribution System has 20 inlet and outlet cabinets, equipped with 20 Shanghai Jinling mechanical watches DS862-2 3X100V 3 (6) A power consumption "= 8VA, total power consumption of energy meter 20X8 = 160VA Another 20 Schneider's microcomputer Sepam20, because there is no AC voltage 100V input, so the power consumption of the microcomputer AC 100V voltage is no longer considered; the total power consumption of the AC 100V voltage is 160VA for the whole system (the secondary conductor work is not considered) Consumption). If the 3PT connection selects the PT of JDZX10-10 0.5/30VA thermal limit 150VA, it will overheat and burn out due to exceeding the limit capacity. Therefore, only the voltage transformer of JDZX9-10 1.0/180VA thermal limit 400VA can be selected; this can ensure a certain measurement accuracy and protect the PT from damage.
Therefore, in the substation of the 10KV system, there are more outlets that may increase the load of the transformer, or the power monitoring device required for the distribution network automation, or the various operating meters, or even the AC operating power source from the PT, etc. . When the load of the current loop or voltage loop in the system exceeds the rated capacity within the accuracy range indicated by the transformer, the measurement accuracy of the transformer will decrease significantly; the larger the load exceeds the rated output, the more the secondary output attenuation will be, resulting in a large measurement value. Decrease, and form a significant gap with the metering meter of the power supply bureau; if the load exceeds or approaches the thermal limit output, long-term operation will inevitably lead to overheating of the voltage transformer, resulting in a series of accidents such as the voltage transformer drying up.
Safe and reliable power supply metering must evaluate the energy consumption of each part of the entire power distribution system, and select the transformer of the appropriate specification, which must meet the measurement accuracy and the capacity of the voltage transformer. 2.2 There are three reasons for the voltage transformer burning of the PT cabinet (3PT-Y connection method): (1) resonance of the voltage transformer; (2) overload of the voltage transformer; (3) PT = secondary short circuit. (Note: Voltage transformer referred to as PT)
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