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ADE7752B 数据表(PDF) 16 Page - Analog Devices |
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ADE7752B 数据表(HTML) 16 Page - Analog Devices |
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16 / 27 page  ADE7752B Preliminary Technical Data Rev. PrA | Page 16 of 27 TYPICAL CONNECTION DIAGRAMS CURRENT CHANNEL CONNECTION Figure 19 shows a typical connection diagram for the current channel (IA). A current transformer (CT) is the current trans- ducer selected for this example. Notice the common-mode voltage for the current channel is AGND and is derived by center tapping the burden resistor to AGND. This provides the complementary analog input signals for IAP and IAN. The CT turns ratio and burden resistor Rb are selected to give a peak differential voltage of ±500 mV at maximum load. In theory it is better to center tap Rb; however, this requires very careful attention to the layout and matching of the resistors to ensure that the channels have the same resistance. A single resistor may be more practical and is a valid design choice. IAP ±500mV Rb Rf Rf CT NEUTRAL PHASE IP IAN Cf Cf Figure 19. Typical Connection for Current Channels VOLTAGE CHANNELS CONNECTION Figure 20 shows two typical connections for the voltage chan- nel. The first option uses a potential transformer (PT) to pro- vide complete isolation from the main voltage. In the second option, the ADE7752B is biased around the neutral wire, and a resistor divider is used to provide a voltage signal proportional to the line voltage. Adjusting the ratio of Ra, Rb, and VR is a convenient way of carrying out a gain calibration on the meter. VR can be implemented using either a potentiometer or a binary weighted series of resistors. Either configuration works, however, the potentiometer is subject to noise over time. Two fixed value resistors can be used in place of VR to minimize the noise. ±500mV Ra * Rb * VR * VAP AGND Rf Rf PT NEUTRAL PHASE VN Cf Cf VAP Rf NEUTRAL PHASE VN Cf Cf * Ra >> Rf + VR; * Rb + VR = Rf ±500mV Figure 20. Typical Connections for Voltage Channels METER CONNECTIONS In 3-phase service, two main power distribution services exist: 3-phase 4-wire or 3-phase 3-wire. The additional wire in the 3-phase 4-wire arrangement is the neutral wire. The voltage lines have a phase difference of ±120° (±2π/3 radians) between each other (see Equation 6). () ( ) () () ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ π + ω × × = ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ π + ω × × = ω × × = 3 4 cos 2 3 2 cos 2 cos 2 t V t V t V t V t V t V l C C l B B l A A (6) where VA, VB, and VC represent the voltage rms values of the different phases. The current inputs are represented by () ( ) () () ⎭ ⎬ ⎫ ⎩ ⎨ ⎧ φ + π + ω × = ⎭ ⎬ ⎫ ⎩ ⎨ ⎧ φ + π + ω × = φ + ω × = C l C C B l B B A l A A t I t I t I t I t I t I 3 4 cos 2 3 2 cos 2 cos 2 (7) where: IA, IB, and IC represent the rms value of the current of each phase. φ A , φB, and φC represent the phase difference of the current and voltage channel of each phase. The instantaneous powers can then be calculated as follows: PA(t) = VA(t) × IA(t) PB(t) = VB(t) × IB(t) PC(t) = VC(t) × IC(t) Then: ( ) ( ) ( ) () () () () ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ φ + π + ω × × − φ × × = ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ φ + π + ω × × − φ × × = φ + ω × × − φ × × = C l C C C C C C B l B B B B B B A l A A A A A A t I V I V t P t I V I V t P t I V I V t P 3 8 2 cos cos 3 4 2 cos cos 2 cos cos (8) As shown in Equation 8, in the ADE7752B, the active power calculation per phase is made when current and voltage inputs of one phase are connected to the same channel (A, B, or C). Then the summation of each individual active power calcula- tion gives the total active power information, P(t) = PA(t) + PB(t) + PC(t). |
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