Quartz watches are generally more accurate than mechanical watches, and the Swiss standard has a monthly error of 15 seconds. Some precision movements can reach the annual error within a few seconds, but even the most expensive Swiss watches, there will be errors in time, what is the reason?
The movement in the watch is very important, just like the human heart. In the quartz clock, the 32.768 kHz crystal oscillator is used to generate the oscillating signal. After 15 divisions, the 1 Hz signal is obtained, and each cycle is driven for a second. The accuracy and stability of the crystal frequency determines the accuracy of the watch's travel time. One of the most important parameters of a crystal is PPM, a PPM equal to one in a million. Ordinary quartz watches use a crystal precision of 20 PPM, and the most expensive Swiss watches use crystals with an accuracy of up to 5 PPM.
In the power measurement, a clock is also required. The PA6000 power analyzer uses a constant temperature crystal frequency of 100 MHz and an accuracy of less than 1 PPM. Compared with the 32.768 kHz and 20 PPM used in the watch, it is several orders of magnitude higher. What is the use of this egg?
First, why does the power measurement require a clock?
The power conversion efficiency of various types of power sources such as PV inverters, inverters, and UPSs has reached 98%. In the future, it will enter a long-term and slowly increasing phase. The 0.2% efficiency gap is enough to affect customers' choices. Power measurement is the basis for calculating power efficiency. The following is an analysis of the role of high-precision synchronous clocks in power measurement.
According to the use of AC power, four common modes can be selected for its effective value: true rms value, rectified average value, calibration average value, fundamental RMS value, and accuracy of voltage and current measurement. However, voltage and current are only It is the basis of power measurement, and the synchronization error control between different channels is the key point of power measurement.
According to the calculation formula of active power: P=U*I*cosφ, when the power supply is working at high efficiency, φ is very close to 0. When measuring 50Hz, if there is a synchronization error of 1ms in voltage and current channels, calculate cosφ ≈ 0.95. The active power deviation caused by the 1ms delay between the voltage and current channels actually reaches 5%!
It can be seen that ensuring the synchronization of voltage and current channels is the core of accurately measuring electric power, and the main factors causing channel synchronization error are:
1) The difference between the characteristic impedance of the voltage and current channels, the current channel adds a shunt to the voltage channel, and the characteristic impedance of the shunt affects the transmission delay of the high-frequency signal and limits the bandwidth of the instrument;
2) Three-phase AC power is input through different power boards, and there is synchronization error between the boards;
3) There is temperature drift in the synchronous sampling clock, which affects the accuracy of current and power integral data;
In order to ensure accurate measurement, the PA6000 power analyzer adopts a new hardware architecture, which ensures that the synchronization error between voltage and current channels reaches the industry's minimum 10 ns.
Current channel shunt: common sampling resistor has large stray inductance, PA6000 uses Kelvin non-inductive shunt, stray inductance is less than 5nH, guarantees optimal high frequency performance and minimum transmission delay, ensuring voltage and current channel Consistency with signal transmission delays;
100MHz synchronous clock between channels: The traditional power analyzer uses a common crystal oscillator of 1-10MHz as the synchronous clock, and the synchronization error reaches 1us, and the frequency stability is easily affected by the crystal temperature drift. The PA Series uses a 100M synchronous clock with high stability temperature compensation to avoid measurement errors introduced by clock drift caused by temperature changes. At the same time, the sampling phase of any channel ADC is synchronized, and the error is within 10 ns.
Second, what is the synchronization source?
For the measurement of DC power, the values ​​of voltage and current are relatively stable, and the power is obtained by multiplying the numbers read by the voltmeter and the ammeter. For AC power, the instantaneous power is also fluctuated due to the positive and negative alternating voltage and current. For the calculation of electric power, it is generally necessary to intercept the waveform of the whole cycle to intercept the average power of the interval to the standard power value.
The synchronization source is the signal referenced when the voltage and current waveforms are intercepted, and the voltage and current waveforms are intercepted according to the zero-crossing point of the synchronization source. Therefore, the synchronization source is required to have a significant zero-crossing point, and the frequency is the same as the signal to be measured.
For the inverter and UPS source, the voltage is generally selected as the synchronization source, and the inverter test generally selects the current as the synchronization source.
With a Kelvin non-inductive shunt, 100MHz synchronous clock, and 10ns synchronization error, the PA6000 provides you with a high-precision power test solution.
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