Research on Energy - Saving Effect of Inverter Speed Regulation Technology in Electrical Automation

Edwiin

08/18/2025

As the core of industrial production, electrical automation systems directly influence overall production costs and environmental impact. Traditional constant-speed operation often leads to energy waste when responding to varying load demands and makes precise process control difficult to achieve. Variable frequency speed regulation technology, as an advanced motor control method, offers a promising solution to these issues. This study takes the electrical automation system of a power plant as an example to explore a retrofit scheme based on inverter speed control technology and its energy-saving effects, aiming to provide a reference for energy efficiency improvements in similar industrial scenarios.

1 Current Status and Retrofit Requirements of Inverter Applications in Electrical Automation

1.1 Existing Equipment

The electrical automation system of the power plant mainly consists of three parts: the power distribution system, motor drive units, and the control system. The power distribution system includes 10 kV high-voltage switchgear, transformers, and 400 V low-voltage switchgear, arranged in a tree structure for power distribution. Motor drives are primarily asynchronous motors controlled by direct-on-line or star-delta reduced-voltage starting methods. Pump loads account for the largest proportion of on-site equipment, including circulating water pumps, cooling water pumps, and feedwater pumps. These devices operate at constant speed, with flow regulated via valves, resulting in high energy consumption. The existing system architecture is relatively decentralized, with partial centralized management. The upper-level monitoring system communicates with field control systems via industrial Ethernet to enable centralized data display and remote operation. However, the current control system lacks advanced control algorithms for variable frequency speed regulation, leading to deficiencies in energy management and process optimization.

1.2 Retrofit Requirements

Based on the current equipment status, retrofit requirements for the electrical automation system primarily focus on improving energy efficiency and optimizing control. It is necessary to introduce inverter-based speed control technology to enable efficient operation of pumps and fans by adjusting motor speed to match load demands.

Meanwhile, leveraging existing pump stations and production facilities, there is an urgent need to build an intelligent monitoring platform compliant with Level 2 cybersecurity protection requirements. Centered on cloud computing and integrated with IoT technology, this platform will enable seamless integration between enterprise management and field control. The system architecture adopts a three-tier structure of "central platform + distributed subsystems + mobile terminals," ensuring real-time data acquisition, efficient processing, and secure storage.

The central platform, built on a high-performance server cluster, deploys advanced data analysis algorithms to provide accurate decision support. Distributed subsystems include modules for equipment condition monitoring, video surveillance, and environmental parameter collection, comprehensively covering all aspects of production operations. Mobile terminals, through customized applications, enable remote monitoring and instant notifications.

2 Theoretical Basis of Energy-Saving Effects

The analysis of energy-saving effects of inverter speed control technology in this study is primarily based on the affinity laws for fans and pumps and the energy conversion principles of variable frequency speed regulation. According to the operational status of the plant’s equipment, a large number of pumps and fans operate at constant speed with flow regulated by valves, resulting in significant energy losses. In contrast, variable frequency speed control adjusts motor speed to match load requirements, thereby achieving energy savings. The affinity laws for fans and pumps are established based on the relationships between flow rate, head, and power, with the relevant calculation formulas as follows:

where $Q$ is the flow rate (m³/h); $n$ is the rotational speed (r/min); $H$ is the head (m); $P$ is the power (kW), with $P_{1}$ representing the rated power and $P_{2}$ the power at reduced speed. The energy conversion formula for variable frequency speed regulation is:

Based on the above theoretical relationships, when system flow demand decreases, the motor automatically reduces speed via frequency control, significantly lowering power consumption and achieving energy savings. This provides a theoretical foundation for the subsequent retrofit design and energy-saving evaluation.

3 Retrofit Scheme of Inverter Speed Control Technology

3.1 Power Distribution System Upgrade

To effectively implement inverter speed control technology, this study upgraded the existing power distribution system. For the high-voltage system, the 10 kV switchgear was enhanced by installing intelligent vacuum circuit breakers with a rated current of no less than 1,250 A and a rated short-circuit breaking capacity of 31.5 kA. Microprocessor-based protection relays were integrated, providing multi-function protection including overcurrent, short-circuit, and ground fault, with a response time under 20 ms. An electric power quality monitoring system was also introduced, using Class A-grade high-precision sensors to monitor parameters such as harmonic content, voltage fluctuations, and three-phase unbalance in real time, ensuring system stability.

For the low-voltage system, the 400 V system was the focus of the upgrade. Dedicated inverter feeder circuits were added to the existing system using independent feeder cabinets equipped with intelligent molded-case circuit breakers. The rated current was selected between 400 A and 630 A based on load requirements, featuring electronic trip units for precise overload and short-circuit protection. Each inverter circuit is equipped with an isolating switch matching the circuit breaker’s rated current and includes a visible break feature to facilitate equipment maintenance.

For harmonic mitigation, active power filters (APF) are installed at the inverter input side, with specific specifications as listed in Table 1.

For the optimization of grounding systems, this study adopted the TN-S wiring method, separating the neutral line (N) from the protective earth line (PE) starting from the distribution cabinet. The main PE line uses copper conductors with a cross-sectional area of no less than 95 mm² to ensure a ground resistance of less than 1 Ω. Equal potential bonding bars were added at critical equipment locations such as inverters and motors, using copper conductors with a cross-sectional area greater than 16 mm². This effectively suppresses common-mode interference and enhances the system's EMC performance [21].

3.2 Selection and Parameter Optimization of Inverter Equipment

The selection of inverters is based on precise matching of load characteristics and process requirements. For pump loads, vector control inverters are chosen, with their rated power strictly corresponding to the motor's, and an overload capacity of 150%/1 min. This study selected the ABB ACS880 series inverter, which features DTC (Direct Torque Control) technology, with a torque response time of less than 5 ms and speed control accuracy of ±0.01%. Considering the on-site environment, a sealed inverter with an IP54 protection rating was used, equipped with a forced air cooling system, ensuring a cooling airflow of no less than 1 m³/(min·kW).

For parameter optimization, the focus is on adjusting the PID control parameters and utilizing the self-tuning algorithm built into the inverter. Through step response testing, the optimal proportional gain $K_{p}$ , integral gain $K_{i}$ , and derivative gain $K_{d}$ are automatically calculated. The calculation formula for the PID controller output $u (t)$ is:

The built-in auto-tuning algorithm of the inverter is used to automatically calculate the optimal proportional gain $K_{p}$ (range: 0.1–100), integral time $T_{i}$ (range: 0.1–3600 s), and derivative time $T_{d}$ (range: 0–10 s) through a step response test. Acceleration time is set to 10–30 s and deceleration time to 15–45 s to effectively prevent water hammer effects. Torque limiting is enabled with a setting of 120% of the motor's rated torque to prevent overload. For fan loads, the inverter's energy-saving mode is activated: under light-load conditions (load rate < 50%), the output voltage is automatically reduced, with a maximum reduction of up to 20%. Meanwhile, the V/F curve is optimized by increasing the voltage output in the low-speed range (0–10 Hz) to ensure sufficient starting torque.

A sleep-wake function is configured: when the operating frequency remains below 10 Hz for 60 s, the inverter enters sleep mode; it automatically wakes up when the system pressure drops by 5%, further improving system efficiency. In the basic inverter settings, the carrier frequency is set to 4 kHz. Based on the actual requirements of the power plant, the overvoltage and undervoltage protection thresholds are set to 418 V and 304 V, respectively. Additionally, the motor's rated parameters and multi-speed operation settings are configured as detailed in Table 2.

The calculation formulas for current limitation and minimum current optimization are respectively as follows:

where $I_{lim}$ is the maximum current limit; $I_{n}$ is the motor rated current; $I_{smin}$ is the minimum stator current; $I_{dopt}$ is the optimal excitation current; and $I_{q}$ is the torque current component. By incorporating current limiting and minimum current optimization strategies, fine-grained control of motor operation is achieved. Overvoltage and undervoltage protection settings ensure the motor operates within a safe range. Stall protection and current limiting measures effectively prevent overload. Additionally, this control method supports communication via the Modbus-RTU protocol, enabling remote monitoring and parameter adjustment, thereby significantly enhancing the system's intelligence level.

3.3 Control System Upgrade and Integration

The control system upgrade employs the Siemens S7-1500 series PLC, specifically the CPU 1517-3 PN/DP model, which features a 2 ns bit operation speed and a 40 ns word operation speed. The PLC is equipped with 1.6 GB of working memory and 32 MB of load memory, supporting communication protocols including PROFINET, PROFIBUS, and OPC UA. The system adopts a distributed architecture with ET 200SP series remote I/O modules, achieving a 250 μs communication cycle via PROFINET.

The software architecture is based on the TIA Portal V16 integrated development environment. The PLC program includes function blocks (FBs) for inverter communication, PID control, Model Predictive Control (MPC), data acquisition preprocessing, and alarm management. The detailed system framework is illustrated in Figure 1.

4 Energy-Saving Effect Analysis

The energy-saving benefits of inverter speed control technology are primarily reflected in reduced power consumption and improved system efficiency. By comparing energy consumption data before and after the retrofit, the energy-saving performance can be quantitatively evaluated. The post-retrofit system data in this study were collected using the following methods:

Energy Metering System: Smart meters were installed on the power supply lines of major electrical equipment to collect electricity consumption data before and after the retrofit. The meter model is Schneider PM5560, with an accuracy class of 0.2S and a sampling interval of 15 minutes.
Inverter Built-in Functions: The built-in energy monitoring function of the ABB ACS880 inverter was used to record operating time, output power, and energy consumption. Data were transmitted to the central control room via the Modbus-RTU protocol.
SCADA System: A real-time data acquisition and storage system was built using the Siemens WinCC V7.5 platform. Key parameters such as motor speed, load rate, output voltage/current, and power factor were monitored with a 1-second sampling cycle.
On-site Testing: The Fluke 435 II power quality analyzer was used to conduct spot measurements under various load conditions, capturing instantaneous power, harmonics, and power factor data.

Based on the measured data, the annual average load rate was calculated. By comparing energy consumption before and after the retrofit, the power-saving rates under different load conditions were determined, as shown in Table 3.

The results show that the energy-saving effect gradually increases as the load rate decreases, which aligns with the cubic law principle of energy saving in variable frequency speed control. Under full-load operation, the power-saving rate is 10%, primarily attributed to the high efficiency and precise control capability of the inverter, indicating that even under high-load conditions, frequency control technology still offers significant energy-saving potential.

At a 75% load rate, the power-saving rate increases to 49.4%, highlighting the advantages of variable speed control under partial load. When operating at 50% load, the power-saving rate reaches 75%, demonstrating its excellent performance under medium load. At 25% load, the power-saving rate reaches as high as 93.7%, fully showcasing the substantial energy-saving potential of variable speed control under low-load conditions.

Overall, the annual average power-saving rate is 56.8%, indicating that the optimized system achieves good comprehensive energy-saving performance during actual operation cycles. Considering the time distribution across different load states, these results provide valuable guidance for energy efficiency optimization in industrial electrical automation systems.

For fan loads, the inverter’s built-in energy optimization function automatically adjusts the V/F curve under light-load conditions. Measurements using the Fluke 435 II power quality analyzer at various load rates reveal the relationship between voltage reduction and power consumption reduction.

Results show that in the load rate ranges of 20%–30%, 30%–40%, and 40%–50%, power consumption reductions are 36.7%, 25.3%, and 15.8%, respectively. This indicates that the lower the load rate, the greater the proportional reduction in power consumption achieved by the same degree of voltage adjustment. For ultra-light-load conditions below 20%, more aggressive control strategies or equipment shutdown should be considered to further improve overall system efficiency.

5 Conclusion

This study, through the retrofitting practice of an electrical automation system in a power plant, verifies the significant energy-saving effectiveness of variable frequency speed control technology. By optimizing control strategies and system integration, substantial reductions in power consumption and improvements in operational efficiency are achieved. The findings provide a technical reference for energy-saving retrofits in similar industrial scenarios. Future work will further consider the uncertainties in operation and travel times by developing a probabilistic operation resource scheduling model to better address various uncertainties in real-world operations.

Topics

Variable Frequency Drive Design and Simulation

Edwiin

Hello，I'm Wdwiin. A decade of hands-on experience in electrical engineering, specializing in high-voltage systems, smart grids, and renewable energy technologies. Passionate about technical exchange and knowledge sharing, committed to interpreting industry trends with professional insights to empower peers. Connection creates value—let’s explore the boundless possibilities of the electrical world together!