1. Introduction Driven by the rapid development of computer networks and related network communication technologies, UPS (Uninterruptible Power Supply) is increasingly being widely used in various sectors of the national economy to ensure the high authenticity, reliability, continuity, and high fidelity of data, files, and graphics obtained during information resource sharing. Simultaneously, with the development of information technology, new technologies such as intelligent information processing and network-based remote monitoring are gradually being applied to UPS systems, forming fully intelligent UPS systems, which facilitate users and improve reliability. This paper focuses on UPS control technology, analyzes the characteristics of UPS PID control technology, and delves into the composite control strategy based on PID control and repetitive control. 2. Overview of Control Strategies Digital control technology for UPS inverters has become a hot topic in the current inverter research field, resulting in various digital control methods for inverters, including digital PID control, state feedback control, deadbeat control, repetitive control, and fuzzy control, which have powerfully promoted the development of UPS technology. Each control scheme has its own advantages and disadvantages. While some control methods offer good dynamic response speeds, their steady-state output voltage harmonic distortion falls short of requirements; some methods, while possessing high dynamic and steady-state accuracy, are highly sensitive to parameter changes and lack robustness; some methods exhibit excellent steady-state accuracy but poor dynamic response; and some methods are currently limited by hardware capabilities and cannot be widely applied. Therefore, an inevitable trend is the mutual integration and complementary use of various control schemes to form composite control solutions. 3. Digital PID Control In UPS inverter control, the most common and simplest method is PID control. Specific implementations include instantaneous voltage feedback control and voltage-current dual closed-loop feedback control. Figure 1 shows instantaneous voltage feedback control. The advantage of instantaneous voltage feedback control is that it uses only one voltage sensor; however, its disadvantages include poor system dynamic response characteristics, poor tracking characteristics, and suboptimal waveform quality. Figure 2 shows the output voltage waveform of a 10KVA inverter with a capacitive load using this control method. As can be seen from the figure, the waveform distortion is significant, making it difficult to meet the requirements of a high-quality power supply. One method to improve the dynamic characteristics of a voltage source inverter is to add a current closed loop. In this control strategy, the current of the filter capacitor (i.e., the derivative of the output voltage) is introduced into the control system as a feedback variable to improve the output waveform quality. It requires a Hall sensor to detect the filter capacitor current, increasing the system's complexity and cost. 4. Composite Control Based on PID and Repetitive Control The inverter controller is a regulating system with a sinusoidally varying reference setpoint, not a constant setpoint regulating system. Simultaneously, the system disturbance, i.e., the load current, is not a constant disturbance. When there is a linear load, the load current varies sinusoidally; however, when there is a nonlinear load, the current varies non-sinusoidally. To address the problem of zero steady-state error tracking of sinusoidal commands, a sinusoidal signal model with the same frequency as the reference setpoint can be embedded in the controller. Here, ω is the angular frequency of the sinusoidal command. It can be verified that when both the command and the disturbance vary sinusoidally at the angular frequency ω, a stable regulating system containing the internal model shown in (1-1) is zero steady-state error. However, this conclusion is only obtained under the assumption of a linear load. Actual loads are much more complex and are mostly rectifier loads. Such load currents are non-sinusoidal, containing the fundamental frequency and multiple harmonics that are integer multiples of the fundamental frequency. Therefore, the actual disturbance frequency components are very rich. If zero steady-state error is to be achieved for all these frequency disturbances, the method of embedding a sinusoidal signal internal model is unsuitable. Disturbance signals share a common characteristic: they repeat with the same waveform in each fundamental frequency cycle. Therefore, repetitive control based on the internal model principle can adopt a "repetitive signal generator" internal model as follows. Its transfer function is: where L is the fundamental frequency cycle of the inverter output. Discretizing this yields a positive feedback loop delayed by one cycle. This delay loop is the fatal flaw of repetitive control; its adjustment of tracking error lags behind by one power frequency cycle. Therefore, combining PID control with repetitive control is considered, forming a new UPS inverter waveform control method based on PID and repetitive control. Repetitive control improves the steady-state output waveform quality of the system, while digital PID control enhances the dynamic characteristics of the system, enabling the system to possess both good steady-state and dynamic characteristics. The purpose of feedforward control is to improve the control effect of the digital PID controller, further reduce the fluctuation and waveform distortion of the output voltage during dynamic processes, and improve the stability of the digital PID control system. The discrete repetitive controller is used to eliminate the periodic tracking error of the system and reduce the output voltage waveform distortion when the UPS inverter is driven by a nonlinear rectifier load. The digital PID controller adjusts the output voltage tracking error in real time, reducing the output voltage fluctuation and distortion when the system is disturbed. The control block diagram is shown in Figure 3. The main components in the diagram are described as follows: 1) z-N: Periodic delay element, which makes the error information of the current period affect the correction amount from the next period. 2) Q(z): Set to overcome the inaccuracy of the object model and enhance the system stability. It can be a constant less than 1. 3) S(z): Compensation element, used to modify the object characteristics. 4) zk: Phase compensation, to meet the system frequency response requirements. 5) a: Scale factor. Used to maintain the stability of the system. 5. Experimental Results Based on the above model, the controller parameters can be preliminarily determined using Simulink simulation. Initially, conservative parameters were used in experiments on a 10KVA inverter module. The parameters were adjusted to achieve better static and dynamic characteristics. The system parameters were: input DC voltage 200V, output frequency 50Hz, switching frequency 19.6Hz, filter inductor 120uH, and filter capacitor 15uF. Figure 4 shows the voltage and current waveforms with a resistive load (left) and a current of 36A (right); and with a rectifier load (right) and a current of 20A. From these two figures, it can be seen that the composite control strategy based on PID control and repetitive control has good waveform control performance, especially for nonlinear rectifier loads, exhibiting excellent harmonic suppression. The system also has good dynamic response characteristics. Therefore, the composite control strategy based on PID control and repetitive control introduced in this paper has high application value. References [1] Hou Zhenyi, Wang Yiming, “UPS Circuit Analysis and Repair”, Science Press, 2001 [2] Li Chengzhang, “Modern UPS Power Supply and Circuit Diagram”, Electronic Industry Press, 2001 [3] S. Karve, “Three of a kind [UPS topologies, IEC standard],” IEE Review, vol.46, no.2, pp.27-31, March 2000 [4] Ma Xudong, UPS Intelligent Control Technology Based on DSP, Electrical Automation, 2003, No.1, pp.26-27 [5] H. Pinheiro, P. Jain, “Comparison of UPS topologies based on high frequency transformers for powering the emerging hybrid fiber-coaxial networks,” IEEE-INTELEC '99, pp.9-12, 1999 [6] Lin Xinchun, Duan Shanxu, DSP-based fully digital control system for UPS, Power Electronics Technology, 2001, No. 2, pp. 51-53 (end)