System and method for ripple-free AC power determination

A power metering circuit includes a current input path for receiving an analog current input at a first analog to digital converter; a voltage input path for receiving an analog voltage input at a second analog to digital converter; a multiplier configured to multiply an output of the current input path and the voltage input path; a notch filter configured to receive an output of the multiplier, the notch filter having a stop band based on a line frequency; and a control circuit for setting a sampling frequency of the first analog to digital converter and the second analog to digital converter to a multiple of the line frequency.

TECHNICAL FIELD

The present disclosure relates to power metering and, in particular, to systems and methods for determining active and reactive power.

BACKGROUND

In a simple alternating current (AC) circuit, if the circuit is purely resistive, the current and voltage are in-phase, and true or active power is given by P=VI, in watts. If the loads are purely reactive, i.e., inductive or capacitive, then the voltage and current are ninety (90) degrees out of phase, and reactive power is given in Volt-Amperes-Reactive. In this case, there is no net transfer of energy to the load.

Practical loads have resistance, inductance, and capacitance, so both true and reactive power will flow to real loads. Even though the current associated with reactive power does no work at the load, it heats the wires, wasting energy.

In power metering applications, the most common way to measure active and reactive power is to convert current and voltage to digital words or streams with an analog to digital converter (ADC) and then apply a variety of processing to derive active and reactive power. For AC power, the current and voltage both have a fundamental frequency component at the line frequency ω. When doing the instantaneous power calculation, a multiplication is necessary between voltage and current channels. The result of the multiplication contains energy in DC (which is either active or reactive power depending on what quantities have been multiplied) and energy in the 2ω) component, where ω is the line frequency. Usually, this result is low pass filtered (LPF) to extract only the DC information but unless a large filter is applied, the remainder of the 2ω) is still large enough to create large jitters if a PWM (pulse width modulator) output is connected after the LPF.

Shown inFIG. 1is an example conventional power meter calculation block for AC active or reactive power. The block100includes a pair of analog-to-digital converters102a,102b,for digitizing current I and voltage V, respectively. The outputs of the ADCs102a,102bare filtered by highpass filters (HPF)104a,104b,respectively, to remove offset components. Then the current I is multiplied at multiplier110with the voltage (switch108being closed) to obtain the instantaneous active power or (switch108being open), the voltage is applied to phase shifter106and then multiplied with the current to obtain the instantaneous reactive power.

The instantaneous powers include a DC component and a 2ω) component due to the multiplication. The instantaneous powers are then lowpass filtered using one or more LPFs112,114to extract the desired quantity. Typically, the filtering is implemented as a simple IIR (infinite impulse response) first order LPF. As noted above, however, if a PWM116is applied at the output of the filtering, then jitter can still result from the 2ω) component.

Accordingly, there is a need for an improved and more stable output for power metering.

SUMMARY

According to various embodiments, the 2ω) component may be canceled using the systems and methods disclosed herein. A power metering circuit in accordance with embodiments includes a current input path for receiving an analog current input at a first analog to digital converter; a voltage input path for receiving an analog voltage input at a second analog to digital converter; a multiplier configured to multiply an output of the current input path and the voltage input path; a notch filter configured to receive an output of the multiplier, the notch filter having a stop band based on a line frequency; and a control circuit for setting a sampling frequency of the first analog to digital converter and the second analog to digital converter to a multiple of the line frequency.

In some embodiments, the control circuit includes a phase locked loop configured to receive one of the analog current input and the analog voltage input. In some embodiments, the control circuit comprises a frequency detector and a programmable oscillator configured to receive an output of the frequency detector. In some embodiments, the current input path and the voltage input path each including a highpass filter. In some embodiments, the voltage input path includes a selectable phase shifter. In some embodiments, the circuit further includes an output pulse width modulator.

A power metering circuit in accordance with embodiments includes a first input path for receiving one of an analog current input or an analog voltage input at a first analog to digital converter; a second input path for receiving another of the analog voltage input or the analog current input at a second analog to digital converter; a multiplier configured to multiply an output of the first input path and the second input path; and a notch filter configured to receive an output of the multiplier, the notch filter having a stop band based on a line frequency.

In some embodiments, the circuit includes a control circuit for setting a sampling frequency of the first analog to digital converter and the second analog to digital converter to a multiple of the line frequency. In some embodiments, the control circuit includes a phase locked loop configured to receive one of the analog current input and the analog voltage input. In some embodiments, the control circuit comprises a frequency detector and a programmable oscillator configured to receive an output of the frequency detector. In some embodiments, the first input path and the second input path each including a highpass filter. In some embodiments, the first input path includes a selectable phase shifter. In some embodiments, the circuit includes an output pulse width modulator.

A method in accordance with embodiments includes receiving an analog current input at a first analog to digital converter; receiving an analog voltage input at a second analog to digital converter; multiplying an output of the current input path and the voltage input path; notch filtering an output of the multiplier with a stop band based on a line frequency; and setting a sampling frequency of the first analog to digital converter and the second analog to digital converter to a multiple of the line frequency.

In some embodiments, setting the sampling frequency includes using a phase locked loop configured to receive one of the analog current input and the analog voltage input. In some embodiments, setting the sampling frequency comprises using a frequency detector and a programmable oscillator configured to receive an output of the frequency detector. In some embodiments, the current input path and the voltage input path each including a highpass filter. In some embodiments, the voltage input path includes a selectable phase shifter.

DETAILED DESCRIPTION

The disclosure and various features and advantageous details thereof are explained more fully with reference to the exemplary, and therefore non-limiting, embodiments illustrated in the accompanying drawings and detailed in the following description. It should be understood, however, that the detailed description and the specific examples, while indicating the preferred embodiments, are given by way of illustration only and not by way of limitation. Descriptions of known programming techniques, computer software, hardware, operating platforms and protocols may be omitted so as not to unnecessarily obscure the disclosure in detail. Various substitutions, modifications, additions and/or rearrangements within the spirit and/or scope of the underlying inventive concept will become apparent to those skilled in the art from this disclosure.

As will be discussed in greater detail below, embodiments provide an improved ripple-free calculation block for determining active or reactive power, removing the 2ω component. In particular, embodiments use a notch filter to remove the 2ω component. If the notch filter has a notch at 2ω, the component is removed and after the filter, the output contains only the desired DC quantity.

A difficulty can arise, however, because the 2ω frequency must be known in order to place a notch at the 2ω frequency. Typically, however, the line frequency is not known and can move and change with time, temperature and network inaccuracies.

Accordingly, embodiments are provided that include a control circuit which can track the line frequency and provide the notch at the correct 2ω frequency. One such embodiment employs a phase-locked loop (PLL) and is shown, for example, inFIG. 2.

The block200ofFIG. 2includes a pair of analog-to-digital converters202a,202b,for digitizing current I and voltage V, respectively. The outputs of the ADCs202a,202bare filtered by highpass filters204a,204b,respectively, to remove offset components. Then the current I is multiplied at multiplier210with the voltage (switch208being closed) to obtain the instantaneous active power or (switch208being open), the voltage is applied to phase shifter206and then multiplied with the current to obtain the instantaneous reactive power.

The instantaneous powers include a DC component and a 2ω) component due to the multiplication. The 2ω) component is filtered using a notch filter212in accordance with embodiments. Active or reactive power signal is then filtered at214and provided to PWM216from which a pulse output is provided.

In addition, a phase locked loop218is provided to receive and lock the line frequency. As shown, the PLL218outputs Nfline, which is used by the ADC as the sampling frequency. Thus, the data rate of the ADC is a multiple of the line frequency. If the converter is an oversampling converter, then oversampling rate is NflineOSR, OSR being the oversampling ratio. In this case, all calculations are performed at the data rate frequency Nfline. The notch filter212may be implemented as a first or second order sinc filter. In such a case, if the data rate of the ADC is Nfline, an oversampling rate or averaging factor of N/2 can be used to get a notch at precisely the 2fline frequency.

The N factor depends on the desired bandwidth of the converter. For example, if the application requires convert32harmonics of the line frequency, N >64. Since the PLL has to multiply OSR×N, where OSR is typically between 32 and 512, and N is usually between 8 and 64, the PLL might be a relatively complex analog component.

Consequently, other embodiments may use a programmable master clock oscillator and digital processing blocks as the control circuit. Such an embodiment is illustrated inFIG. 3. The block300ofFIG. 3includes a pair of analog-to-digital converters302a,302b,for digitizing current I and voltage V, respectively. The outputs of the ADCs302a,302bare filtered by highpass filters304a,304b,respectively, to remove offset components. Then the current I is multiplied at multiplier310with the voltage (switch308being closed) to obtain the instantaneous active power or (switch308being open), the voltage is applied to phase shifter306and then multiplied with the current to obtain the instantaneous reactive power.

The instantaneous powers include a DC component and a 2ω component due to the multiplication. The 2ω component is filtered using a notch filter312in accordance with embodiments. Active or reactive power signal is then filtered at314and provided to PWM316from which a pulse output is provided.

In addition, a control circuit including lowpass filter318, frequency counter320, and oscillator322is provided. In this embodiment, the line frequency is measured by LPF318(e.g., 1st or second order IIR) to avoid the noise and harmonics to disturb the measurement, followed by a zero-crossing detector and frequency counter320. The line period is then extracted with a number of clock cycles of the frequency detector320. The frequency of the oscillator322is then programmed and adjusted as a function of this line frequency detection to be as close as possible to an integer multiple of the line frequency calculated through the zero crossing path.

In one embodiment, employing a relatively simple notch filter (sinc with an N/2 averaging rate being a power of 2), the number of clock periods detected should be a power of 2 when the oscillator322is tuned so that the data rate can be a power of 2 multiple of the fline. In such a case the digital processing needed to implement the notch filter is relatively small (a first order sine filter can be implemented with a simple adder) while the frequency evaluation block size remains in most cases substantially smaller than the PLL of the first case.

With both cases of the PLL and frequency evaluation and tuning of the oscillator, the output result after the notch filter is a DC-only component. The other advantage the notch filter induces is that it cancels also all the 2ω components from all the harmonics of the power line. Therefore the output signal is clean of any harmonics, as long as the frequency tracker (PLL, or programmable oscillator with digital processing) tracks correctly the flinefrequency.