Completing voltage regulation occupies lots of central processing unit (CPU) resources, which brings challenges in satisfying high real-time requirements of the system. Thus, a modular design approach was adopted in the design of software algorithms, which makes the program a clear structure, easy to debug, maintain and upgrade modules with easy expansion, high independence, and so on.
5.1. RMS Algorithm
Voltage regulation requires accurate measurement of generator output voltage value. Selecting the most suitable program for RMS calculation is vital to ensure the accuracy and real-time execution.
Several methods already exist that could be adopted for RMS calculation. Single point algorithm is a real-time method and requires only instantaneous values of three-phase voltage at synchronized instants, and no frequency information is required. Half period integral methods can complete calculation in half cycle only provided that the signal period is known, however, this integral method will bring measurement delay. Two-point algorithm requires sampling values of two points which need to be 90 degrees apart, so it could hardly be used in practice. Fast three-point algorithm in theory only needs three consecutive points to get the effective value, however, the sampling period needs to be a quarter of cycle in order to ensure the precision.
In summary, the single-point method is applicable to the calculation of three-phase symmetrical AC signal RMS value, which is in real time and independent of frequency, and is very suitable for voltage regulation system. Using a single point algorithm to calculate the effective value of 115 V, 400 Hz AC signal,
Figure 8 shows that the calculation accuracy is stable within ±1.5% in compliance with the technical requirements.
This paper presents the design of the generator controller with variable frequency system as the background. The low pass filter circuit exists in the voltage conditioning circuit, through which low-frequency signals changing from 0 to
ωH can pass with the gain of A0 under ideal conditions. The amplitude frequency response is shown in
Figure 9. However, in practice, the gain of the low pass filter circuit will decrease in the pass band, so a frequency change of input voltage brings about conditioning circuit capacitance decreasing while the frequency is increasing, leading to a decrease in conditioning circuit output voltage. Eventually, the calculated RMS value will be reduced in the program; therefore, the voltage regulation accuracy is affected. Thus, it is necessary to compensate for the RMS value calculated varying with frequency in DSP program, to ensure that it can achieve an accurate POR (point of regulation) voltage maintained at 115 V.
5.2. Voltage Regulation Algorithm
The generator has a plurality of working conditions, such as voltage buildup, overvoltage and normal operation, etc. It is difficult to balance all working states if only one single control strategy is used. Therefore, different control strategies are adopted for different stages in this system.
Voltage buildup stage: voltage starts to build when the generator reaches a certain speed. In the initial stage, the output voltage is very small, so it is conducive to a rapid rise in voltage by making the switch tube in a state of full conduction. However, due to the inertia of the generator, the output voltage change lags behind the change of the excitation current. Therefore, when the field current reaches a certain value, limitation operations are adopted until the build-up process is completed.
Normal operation stage: according to the frequency of the moment, the corresponding PID controller parameters for duty ratio calculation are adopted. The control strategy of Bang-bang is adopted to accelerate the dynamic response of the voltage regulation system. Set the error threshold emax and emin. Duty ratio is 0 when e(k) > emax, and 1 when e(k) < emim. When emim < e(k) < emax, use variable parameter PID control to calculate the duty cycle.
Overvoltage stage: when the output voltage of the generator is greater than 125 V, e(k) is greater than emax at this moment as described above, thus, both low-side and high-side MOSFET are closed in de-excitation circuit, and excitation current declines rapidly.
According to the above analysis, we can get the basic process of the control program as shown in
Figure 10. D1 and D2 are the high-side and low-side MOSFET control signal duty cycle, respectively. The upper and lower limits of the error
e(
k) need to be determined during practical tests.
In practice, since the motor parameters are not accurate, and the low-order transfer function of the three-stage mathematical model is not precise enough with a lot of dynamic factors ignored, it results in the controller parameters designed not being directly applicable for practical use, which can only provide a theoretical basis. This paper proposes a method for modifying control parameters with the operating characteristics of GCU.
The operating characteristic of the generator controller is a function relation between the output voltage of the main generator
and the excitation current
. For the same
shown as
Figure 11,
increases with downward in frequency due to the same Δ
D. Thus, in case of the same control parameters, it causes voltage fluctuation to meet the requirements at low speed, but becomes large at high frequencies. Therefore, parameter modification is required for the purpose of improving the voltage regulation accuracy. If the regulator uses proportional control, the following Equation (18) can be obtained, where,
is the proportional coefficient of the controller,
is the difference between the output voltage and the reference value of the generator:
Therefore, when the frequency increases, due to the increase in excitation voltage from to , should be reduced proportional to , where represents proportional coefficient at low frequency.