In the mechanically insulated vehicle section, all the electric energy generated by the diesel generator set is supplied to the electrical equipment in the insulated vehicle, and these devices are all operated under the power frequency condition, which requires the three-phase alternating current generated by the generator set to have a relatively high frequency stability. Sex. In fact, the frequency of the electric energy emitted by the diesel generator is not fixed in the AC parameters. The normal fluctuation range is 49.5Hz~50.5Hz. When the load generates a wide range of fluctuations, the frequency will follow. Produce a wide range of fluctuations. In the detection process, for such a wide range of fluctuations, the staff sometimes cannot correctly judge whether it is caused by harmonics, thereby bringing extra labor to the staff. In order to monitor in real time whether the power generated by the generator set meets the normal standard, the CPU must be aware of the changes in the currently collected AC analog frequency, and can effectively filter out the harmonics, and process the measured frequencies in real time to ensure the sampling frequency and The actual signal frequency is the same.
1 basic principle of phase locking
1.1 hardware phase locking principle
The voltage signal and current signal of the three-phase circuit are converted into an AC input signal with an amplitude of -5V~+5V through the voltage transformer and the current transformer. In order to make the sampling frequency "follow" the change of the input signal frequency, generally adopt FIG. The traditional feedback system shown.
As can be seen from Figure 1: 
It is indicated that in order to make the sampling frequency track the system frequency, it is necessary to make n ≥ 2 in the open-loop transfer function F(S). Considering the stability of the system, take n=2, and take the feedback transfer function H(S) two 1/N (N is the frequency division coefficient, when the signal is stable, you can take 1), then the sampling frequency f0 can track the system frequency f The change.
The control link shown in Figure 1 can be conveniently implemented with a phase-locked loop hardware circuit, the principle of which is shown in Figure 2. In the figure, PD is a phase difference comparator whose transfer function is ud=kp·△θ; VCO is a voltage controlled oscillator, which is transmitted
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1.2 software phase locking mathematical model
The software phase locking principle is to implement the above phase locking process by computer software. Taking the division factor N of Fig. 2 as 1, setting the input u = Mcos θ, and output u0 = M0cos θ0, where θ and θ0 are quantities that change with time, then: 
The second term in equation (4) is a frequency doubling AC component. If θ-θ0 is a constant, the first term is a DC component. Further, if θ - θ0 is close to -π/2, then cos(θ - θ0) = sin(θ - θ0 + π/2) ≈ θ - θ0 + π/2. Therefore, for the DC component of ud, the analog multiplier is equivalent to a phase subtractor. In a voltage controlled oscillator, since the frequency can be a variable, they are not simply multiplied by time, but by time. In addition to the integral action of the voltage controlled oscillator, an integral term is required in the filter section, so that the DC component of ud can be stabilized to 0, thus θ-θ0. The difference can be stabilized to π/2. The value of uc is maintained by the integral term in the filter. In addition, if there is only an integral term in the filter, a constant amplitude oscillation will occur, so a proportional term needs to be added. Its mathematical model is shown in Figure 3.
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In the mathematical model, in addition to the input sampling of u to use the A/D conversion interface hardware, the multiplier, filter and voltage controlled oscillator can all be simulated by the algorithm in the program. If you need to output u0, you also need a D/A interface circuit. In the diesel generator set monitoring system, it is not necessary to output u0, and each variable in the phase-locked loop can be directly referred to in the program to participate in the next data processing.
1.3 Advantages of software lock-in
Compared with traditional hardware locks, software phase lock can achieve the requirements that hardware lock is difficult to achieve:
(1) Optimized filtering or adaptive filtering can be realized by using computer's flexible processing capability.
Filtering can form an infinite DC gain in the form of digital integration to achieve complete error-free regulation. If you want to filter out the multiplier component generated by the multiplier, you can design a digital filtering algorithm to make it zero at the multiplier of w, and modify the filtering algorithm according to the value of the measured w when the signal changes. Let its zero point also change to meet the test requirements. This is very difficult to achieve in hardware phase locking.
(2) The integral value can be forcibly changed to achieve fast locking.
The integral values ​​in the hardware circuit, such as the voltage of the capacitor, the current of the inductor, etc., cannot be mutated. However, the data in the program can be forced according to actual needs, which can achieve one-step locking.
Implementation of software phase lock in 2 diesel generator set monitoring system
In a diesel generator set, the frequency is essentially constant (50 Hz) so that w can be initialized to a value close to this frequency in the program. Then use the following algorithm; calculate θ and θ0. The actual difference can then be forced to modify θ0 to change the difference to π/2 to achieve one-step locking. Introducing a signal sin θ0 which is delayed by π/2 from u0, we can calculate usin θ0, set its DC component to X, and then set ucos θ0. The DC component is r, which is the same as in equation (4). The method is:
Ctg(θ0-θ)=Y/X
If X>0 then 0<θ0-θ<π;
If X<0 then -π<θ0-θ<0;
When X=0, if Y>0 then θ0-θ=0; otherwise θ0-θ=π.
Thus, as long as Y and X are obtained, the actual difference between θ and θ0 is determined.
When calculating X and Y, it is necessary to obtain an average value of the above-mentioned product for a period of time. If the signal is a single frequency, then the averaging only needs to eliminate the multiplier component in the product, so that only half of the cycle needs to be averaged, so the fastest one is to use one-step locking, which requires half a cycle time. If one considers that there may be harmonics in the input signal u, the average time can be lengthened to the entire period or an integer number of cycles. If there is a lower frequency component in u, the situation is more complicated, but the larger the average time, the smaller the error caused by the influence of the component. In short, it is not difficult to make the phase close to the exact lock.
In this system, since the input frequency rarely changes, in order to stabilize the phase-locked loop and avoid loss of lock due to interference, the time constant of the filter link is usually as large as possible, that is, as slow as possible.
As can be seen from the above, each intermediate variable in the phase-locked loop can be solved by a simple application, and finally the frequency of the acquired signal is obtained.
3 test results
On September 25, 2003, the Science and Technology Commission of the Shanghai Railway Branch organized relevant professional and technical personnel to test the diesel generator set monitoring system of the Shanghai mechanical insulation vehicle segment. The test report results are shown in Table 1.
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It can be seen from the table that the test software without software phase-locking technology produces large fluctuations due to harmonic interference, which affects the measurement results and causes workers to mistake the diesel generator set for non-compliance. . The software measurement results compiled by the software phase-locking technology are consistent with the measurement results of the high-precision meter, and the test is much better. When the performance of the diesel generator set is tested, it can completely replace the meter and meet the test standards.
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