ade7518 Analog Devices, Inc., ade7518 Datasheet - Page 54

• Manufacturer Part Number: ade7518
• Description: Single-phase Energy Measurement Ic With 8052 Mcu, Rtc, And Lcd Driver
• Manufacturer: Analog Devices, Inc.

ADE7518
Reactive Power Gain Calibration
Figure 58 shows the signal processing chain for the ADE7518
reactive power calculation. As explained in the Reactive Power
Calculation section, the reactive power is calculated by applying a
low-pass filter to the instantaneous reactive power signal. Note
that, when reading the waveform samples from the output of
LPF2, the gain of the reactive energy can be adjusted by using
the multiplier and by writing a twos complement, 12-bit word
to the VAR gain register (VARGAIN[11:0]). Equation 23 shows
how the gain adjustment is related to the contents of the watt
gain register.
The resolution of the VARGAIN register is the same as the
WGAIN register (see the Active Power Gain Calibration
section). VARGAIN can be used to calibrate the reactive
power (or energy) calculation in the ADE7518.
Reactive Power Offset Calibration
The ADE7518 also incorporates a reactive power offset register
(VAROS[15:0]). This is a signed, twos complement, 16-bit register
that can be used to remove offsets in the reactive power calculation
(see Figure 58). An offset may exist in the reactive power calcula-
tion due to crosstalk between channels on the PCB or in the IC
itself. The offset calibration allows the contents of the reactive
power register to be maintained at 0 when no power is being
consumed.
The 256 LSBs (VAROS = 0x100) written to the reactive power
offset register are equivalent to 1 LSB in the WAVMODE register.
Sign of Reactive Power Calculation
Note that the average reactive power is a signed calculation.
The phase-shift filter has −90° phase shift when the integrator
is enabled, and +90° phase shift when the integrator is disabled.
Table 43 summarizes how the relationship of the phase difference
between the voltage and the current affects the sign of the resulting
VAR calculation.
Table 43. Sign of Reactive Power Calculation
Angle
Between 0° to +90°
Between –90° to 0°
Between 0° to +90°
Between –90° to 0°
Reactive Power Sign Detection
The ADE7518 detects a change of sign in the reactive power.
The VARSIGN flag in the Interrupt Status 1 SFR (MIRQSTL,
0xDC) records when a change of sign has occurred according
to the VARSIGN bit in the ACCMODE register (0x0F). If the
VARSIGN bit is set in the Interrupt Enable 1 SFR (MIRQENL,
0xD9), the 8052 core has a pending ADE interrupt. The ADE
Output VARGAIN =
⎛
⎜
⎝
Reactive
Power
×
⎧ +
⎨
⎩
1
VARGAIN
Integrator
Off
Off
On
On
2
12
⎫
⎬
⎭
⎞
⎟
⎠
Sign
Positive
Negative
Positive
Negative
Rev. 0 | Page 54 of 128
(23)
interrupt stays active until the VARSIGN status bit is cleared
(see the Energy Measurement Interrupts section).
When VARSIGN in the ACCMODE register (0x0F) is cleared
(default), the VARSIGN flag in the Interrupt Status 1 SFR
(MIRQSTL, 0xDC) is set when a transition from positive to
negative reactive power occurs.
If VARSIGN in the ACCMODE register (0x0F) is set, the
VARSIGN flag in the Interrupt Status 1 SFR (MIRQSTL,
0xDC) is set when a transition from negative to positive
reactive power occurs.
Reactive Power No Load Detection
The ADE7518 includes a no load threshold feature on the reactive
energy that eliminates any creep effects in the meter. The ADE7518
accomplishes this by not accumulating reactive energy when
the multiplier output is below the no load threshold. When the
reactive power is below the no load threshold, the RNOLOAD
flag in the Interrupt Status 1 SFR (MIRQSTL, 0xDC) is set. If the
RNOLOAD bit is set in the Interrupt Enable 1 SFR (MIRQENL,
0xD9), the 8052 core has a pending ADE interrupt. The ADE
interrupt stays active until the RNOLOAD status bit is cleared
(see the Energy Measurement Interrupts section).
The no load threshold level is selectable by setting the
VARNOLOAD bits in the NLMODE register (0x0E). Setting
these bits to 0b00 disables the no load detection, and setting
them to 0b01, 0b10, or 0b11 sets the no load detection threshold
to 0.015%, 0.0075%, and 0.0037% of the full-scale output
frequency of the multiplier, respectively.
REACTIVE ENERGY CALCULATION
As for reactive energy, the ADE7518 achieves the integration of
the reactive power signal by continuously accumulating the
reactive power signal in an internal, nonreadable, 49-bit energy
register. The reactive energy register (VARHR[23:0]) represents
the upper 24 bits of this internal register.
The discrete time sample period (T) for the accumulation register
in the ADE7518 is 1.22 μs (5/MCLK). As well as calculating the
energy, this integration removes any sinusoidal components
that may be in the active power signal. Figure 58 shows this
discrete time integration or accumulation. The reactive power
signal in the waveform register is continuously added to the
internal reactive energy register.
The reactive energy accumulation depends on the setting of the
SAVARM and ABSVARM bits in the ACCMODE register (0x0F).
When both bits are cleared, the addition is signed and, therefore,
negative energy is subtracted from the reactive energy contents.
When both bits are set, the ADE7518 is set to be in the more
restrictive mode, the absolute accumulation mode.