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Selection calculation of the capacity of the voltage transformer

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Selection calculation of the capacity of the voltage transformer

Date of release:2018-06-08 Author:Compson Technology Click:

statement of problem

(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. Analysis

         2.1 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)

(1) Ferromagnetic resonance of PT

In a neutral point ungrounded system, the high voltage cable and the switching device busbar have a capacitance to ground, which is connected in parallel with the high voltage winding of the PT grounded to the neutral point. - Once the high voltage windings of the electromagnetic equipment such as transformers, voltage transformers, and arc suppression coils are disturbed for some reason, the saturation of the inductor is different, and a resonant circuit is formed with the capacitance to ground, which may cause continuous ferromagnetic resonance. The ferromagnetic resonance can be fundamental resonance, high frequency resonance, and frequency division resonance. The fundamental resonance usually shows that the two-phase voltage rises, the voltage of one phase decreases, the fundamental frequency resonance and the power frequency voltage are the same frequency, and it is easier to obtain energy from the power source. Therefore, the fundamental frequency resonance can generate a large overcurrent, sometimes manifested. Very strong, it is easier to cause PT dryness. The main hazard of high-frequency resonance is that the overvoltage multiple is high, which often causes the insulation of the main equipment to be broken down or the PT to explode. The consequences are very serious.

The frequency-divided resonance always shows that the two-phase voltage rises at the same time. Its main hazard is also the overcurrent. The over-current surge caused by the frequency-divided resonance has no large fundamental resonant overcurrent, but it also often exceeds the thermal stability allowable current of PT. The value may also burn the PT under the action of long-term frequency division resonance.

Although the amplitude of the overvoltage generated by the resonance is not high, the overvoltage frequency is often far below the rated frequency, and the core is in a state of high saturation. The expression may be that the relative ground voltage is increased, the excitation current is too large, or the low frequency is oscillated. , causing insulation flashover, evasive bursting, high-value zero-sequence voltage component generation, illusory grounding and incorrect grounding indication. In severe cases, it may also induce protection malfunction or overcurrent in the voltage transformer causing PT burnout of the PT cabinet. The specific performance is that the body bursts and the internal insulation material is sprayed out, which makes the system unable to operate normally.

In addition to the PT with better excitation characteristics, the common elimination method of ferromagnetic resonance is as follows:

(a) The neutral point of the PT is added with a nonlinear damping resistor (see Figure 4). The neutral point resistance is used to absorb the energy of the resonance. Therefore, as long as the resistance is large enough, the resonance condition can be destroyed to eliminate the harmonics. The voltage of the point is around 1kV, which does not cause insulation problems for the PT, and has a good effect on the frequency division resonance. This method has been widely used, and there are many manufacturers producing stereotyped products, and have achieved satisfactory results in practical applications. The disadvantage is that when the resistance is too large, the open triangle voltage is too low, and the grounding protection may be invalid;

(b) Connect the fixed (or variable) damping resistor in parallel with the PT opening 3 corner (see Figure 5). Connect the damping resistor to the PT opening triangle and use the current to cancel the zero-sequence flux of the core. The higher the current, the better the effect. . The choice of resistors must follow the following principles: Ensure the sensitivity of the grounding indication; Can effectively eliminate the resonance, the PT will not be burnt out when the system is single-phase grounded. This method has a good effect on high frequency and fundamental frequency resonance.

(c) Adopt 4PT connection (see Figure 6)

The closed resistor of the 4PT connection, the large resistance of the zero-sequence PT and the large inductance can eliminate the harmonics of different properties. The large resistance of the zero-sequence PT can effectively prevent the fuse of the high-voltage fuse, and solve the neutral point insulation problem and solve the opening. The value of the triangular parallel resistance is difficult to determine, but the cost is high and the external dimensions are large.

(d) Microcomputer harmonic elimination

The microcomputer device monitors the PT three-phase voltage and the open triangle voltage. When the system fails, the device starts data acquisition and digital signal processing on the signal, and then analyzes and calculates the data to obtain the fault type. If it is ferromagnetic resonance, the system produces a harmonic elimination circuit, which makes the ferromagnetic resonance disappear rapidly under the action of damping. If it is an overvoltage or single-phase ground fault, after the microcomputer system detects, the display and alarm are given respectively. This method is relatively simple, with openings. The advantages and disadvantages of the triangle scheme are similar, so you don't have to worry about the resistance.

(2) PT overload

The secondary load of the voltage transformer is biased, and the primary and secondary currents are large, so that the sum of the secondary side load current exceeds the rated value, resulting in an increase in the internal winding heat of the PT, especially when the voltage is higher than the PT rated voltage (10 kV). The internal heating of the PT is more serious. Moreover, the system is a neutral point non-effective grounding system, which is prone to ferromagnetic resonance, resonance causes PT overload, and PT is more prone to thermal expansion and burst.



a. The following is the secondary operation power supply AC220V circuit of PT cabinet 3PT

The above high-voltage system (1 incoming line ten metering ten PT cabinet 10 1 outlet) uses vacuum circuit breaker for ABB's VD4-12/630-25KA AC220V closing trip unit to consume power 250VA; opening and closing, 1 unit consumes power 250VA; The microcomputer protection adopts ABB's SPAJ140C AC220V microcomputer power supply and output relay module under the operating condition of 6VA, the power consumption under static conditions is 4VA; when the system is short-circuited; the microcomputer operates; the command jumps VD4; The alarm sounds; the total power consumption of the system is 250+6+4+40=300VA; (in the AC 220V operating system, the power supply XMa and XMC are often installed by the company and also wired to the PT's KMa and KMC AC220V circuits. At this time, the power consumption of the accident circuit whistle DDJ1AC220V is 40VA. The power consumption of other relays and lamps is not calculated. If the PT cabinet voltage transformer selects 0.5/30VA thermal limit output 150VA; PT capacity is not enough overload; thermal expansion burst occurs ( 300VA" 150VA). Therefore, only the JDZX9-10 1.0 class / 180VA thermal limit 400VA voltage transformer can be selected, which can meet the requirements and can be installed in the handcart; or choose the AH3 system as shown on the right. Case; use fuse holders and a variety of large-capacity non-standard voltage transformers fixed in the cabinet; such as JDZX11-10C 0.5/120VA heat and limit 800VA. This system solution can meet the measurement accuracy and capacity. Claim.



(3) Secondary short circuit of PT

PT second time does not allow short circuit, the voltage transformer secondary side coil turns ratio PT is bound to be less in the secondary side coil, but the wire diameter is larger, once the secondary side is shorted, the secondary side causes a large short circuit current, The short circuit causes the PT to resonate in a single phase, causing the transformer to burn out.

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Key word:Voltagetransformer

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