ADE7754 datasheet(25/44 Pages) AD

25 / 44 page

REV. PrG 01/03

PRELIMINARY TECHNICAL DATA

ADE7754

– 25 –

Thus the IRQ line can also be used to signal the end of a

calibration. From Equations 8 and 12.

VI dt

ft dt

()

cos

=−

+ 

























⋅

()

∫∫

(14)

where n is a integer and T is the line cycle period.

Since the sinusoidal component is integrated over an integer

number of line cycles, its value is always zero.

Therefore:

E(t)

VI dt

∫

(15)

E(t)

VInT

(16)

The total active power calculated by the ADE7754 in the Line

accumulation mode depends on the configuration of the

WATMOD bits in the WATMode register. Each term of the

formula can be disabled or enabled by the LWATSEL bits of

the WATMode register. The different configurations are

described in Table III.

WATMOD

LWATSEL0

LWATSEL1

LWATSEL2

0VA x IA

+ VB x IB

+ VC x IC

1VA x (IA

*-I

*)+ 0

+ VC x (IC

*-I

2VA x (IA

*-I

*)+ 0

+ VC x IC

Table III -Total Line Active Energy calculation

Note: IA

*, I

* and I

* represent the current channels samples

after APGAIN correction and High-Pass Filtering.

Important: The Line Active Energy accumulation uses the

same signal path as the Active Energy accumulation. How-

ever, the LSB size of these two registers is different. If the

Line Active energy register and Active energy register are

accumulated during the same amount of time, the Line Active

energy register will be 4 times bigger than the Active Energy

The LAENERGY register is also used to accumulate the

reactive energy by setting to logic one bit5 of the WAVMode

this bit is set to one, the accumulation of the Active Energy

over half line cycles in the LAENERGY register is disabled

and is done instead in the LVAENERGY register. As the

LVAENERGY register is an unsigned value, the accumula-

tion of the active energy in the LVAENERGY register is

unsigned in this mode. The reactive energy is then accumu-

lated in the LAENERGY register - see Figure 31. In this

mode (reactive energy), the selection of the phases accumu-

lated in the LAENERGY and LVAENERGY registers is

done by the LWATSEL selection bits of the WATTMode

In normal mode, bit5 of WAVMODE register equals 0 the

type of active power summation in the LAENERGY register

(sum of absolute active power or arithmetic sum) is selected

by bit2 of the GAIN register.

In the mode where the Active powers are accumulated in the

LVAENERGY register, bit5 of WAVMODE register equals

1, it should be noticed that the sum of several active power is

always done ignoring the sign of the active powers. This is

due to the unsigned nature of the LVAENERGY register that

does not allow signed addition.

REACTIVE POWER CALCULATION

Reactive power is defined as the product of the voltage and

current waveforms when one of this signal is phase shifted by

90º at each frequency. It is defined mathematically in the

IEEE Standard Dictionary 100 as:

Reactive Power

=⋅ ⋅

()

∞

∑

sin

where Vn and In are respectively the voltage and current rms

values of the n

th harmonics of the line frequency, and

n is the

phase difference between the voltage and current n

th harmon-

ics. The resulting waveform is called the instantaneous

reactive power signal (VAR).

Equation 19 gives an expression for the instantaneous reac-

tive power signal in an ac system without harmonics when the

phase of the current channel is shifted by -90º.

( )

sin(

)

=−

ωϕ

(17)

i t

( )

sin(

)

’( )

sin(

)

−

ωω

(18)

VAR t

v t

i t

()

’()

=×

VAR t

V I

( )

sin(

)

sin(

)

1 1

ϕω

(19)

The average power over an integral number of line cycles (n)

is given by the expression in Equation 19.

VAR

VAR t dt

V I

∫

( )

sin(

)

(20)

where T is the line cycle period.

VAR is referred to as the Reactive Power. Note that the

reactive power is equal to the DC component of the instan-

taneous reactive power signal VAR(t) in Equation 19. This is

the relationship used to calculate reactive power in the

ADE7754 for each phase. The instantaneous reactive power

signal VAR(t) is generated by multiplying the current and

voltage signals in each phase. In this case, the phase of the

current channel is shifted by -89º. The DC component of the

instantaneous reactive power signal in each phase (A, B and

C) is then extracted by a low pass filter to obtain the reactive

power information on each phase. In a polyphase system, the

total reactive power is simply the sum of the reactive power

in all active phases. The different solutions available to

process the total reactive power from the individual calcula-

tion are discussed in the following paragraph.

Figure 30 shows the signal processing in each phase for the

Reactive Power calculation in the ADE7754.

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