1. The Hazards of Three-Phase Unbalance

In the low-voltage distribution network, due to the complex power load, wide geographical area, and mostly single-phase power load, as well as the randomness of each user's power consumption habits and power load, the problem of three-phase unbalance of the low-voltage distribution network load always exists. Three-phase load imbalance in the distribution network brings economic losses and risks to the power system and users.

Figure 1: Hazards of Three-Phase Unbalance

2. Methods for Correcting Three-Phase Imbalance

In the past, the power sector usually used manual commutation for management. However, due to the complex characteristics of the distribution network load structure, it is difficult to accurately grasp its changing patterns, cannot meet real-time requirements, and requires power outage operations. Practice has proved that manual commutation has the disadvantages of time-consuming, laborious, poor timeliness, and poor pertinence, and cannot well solve the problem of three-phase load imbalance.

The ETCR5500 Phase-Shifting Switch-Type Three-Phase Imbalance Mitigation Device is a real-time, intelligent automatic load control system that performs real-time, intelligent, and automatic phase-shifting control for single-phase load connections. It fundamentally resolves three-phase imbalance issues in low-voltage distribution networks, preventing problems such as single-phase overload tripping, high line losses, and low voltage at the end of the line caused by three-phase imbalance. Phase-shifting switch technology and devices for correcting three-phase imbalance have been widely deployed by State Grid and China Southern Power Grid, yielding significant results and substantially reducing the manual phase-adjustment workload for frontline personnel.

The ETCR5500 phase-shifting switch-type three-phase unbalanced power correction device consists of a main controller and phase-shifting switches, as shown in Figure 2. The main controller collects real-time load data from the distribution transformer area; analyzes the load voltage and current of each phase-shifting switch; and generates and transmits commands to the phase-shifting switches. The ETCR5500-PEX phase-shifting switch receives and executes commands from the main controller. Communication between the main controller and the phase-shifting switches is achieved via LoRa wireless communication, PLC carrier-wave communication, or RS-485 communication.

Figure 2: ETCR5500 Commutation-Type Three-Phase Unbalance Correction Device

Product Features: 

(1) Equal voltage 0 millisecond seamless commutation technology, no voltage sag during the commutation process, and no interruption of power supply.

(2) AI intelligent balance search algorithm accurately locates unbalanced points and ensures that each branch is balanced section by section.

(3) Reliable software and hardware interlocking technology ensures stable and reliable operation of the device.

(4) Significantly reduce the neutral line current and effectively increase the terminal voltage.

(5) Super long life, no limit on the number of daily phase changes, greatly reducing line loss and deformation loss, and significant energy saving effect.

(6) A variety of communication module combinations ensure full coverage of the stage area in any environment.

(7) Support multiple communication protocols to realize data upload and remote maintenance and management.

3. Situation in Taiwan area

The rated capacity of this transformer is 200kVA. The load fluctuates greatly in the station area. The annual average load rate is 30.51%, and the annual average three-phase unbalance is 48.54%. During the peak period, the load rate is 70.33%, the three-phase unbalance is 52.05%, the neutral line current is between 60 and 120A, and the current imbalance of each phase is large. The station area is located in a rural town. The station transformer outputs a total of 2 branches. The overhead lines of the 2 branches usually use single-phase wiring along the street after reaching the residential buildings. Load type in Taiwan area: composed of residents’ electricity consumption at night, small processing plants, charging piles, etc. Load situation in the Taiwan area: During the day, residents go out to work and work less and use less electricity. At night, residents mainly use electricity. Some of the load is used by charging piles to charge new energy vehicles in the early morning. Therefore, the overall load in the Taiwan area is higher from night to early morning. However, nighttime is also the time when the three-phase imbalance is most serious. During peak periods, there is still low voltage at the end of the heavy-loaded phase. The power consumption situation in the station area is complex, and manual periodic phase modulation is difficult to solve.

4. Governance plan

Through on-site investigation and theoretical verification analysis of the Taiwan area, it was decided to use a commutation switch-type three-phase unbalance control device to control the area. The governance plan is formulated as follows:

There are currently two low-voltage branches in this station area, which are metered by a set of CT. Therefore, this station area is equipped with 1 main controller and 8 phase commutation switches. The main controller is fixedly installed on a pole and detects the current of each phase through the low-voltage side CT. According to the distribution of users in the station area, 8 phase commutation switches are distributed at the beginning, middle and end of each branch, and one phase commutation switch is installed in front of the meter box with larger load. Finally, 3 phase commutation switches are installed along the street of branch 1, and 5 phase commutation switches are installed along the street of branch 2. Figure 3 shows the GIS map of the phase commutation switch device, Figure 4 is the installation diagram of the main controller, and Figure 5 is the installation diagram of the phase commutation switch. Eight phase commutation switches form a small local intelligent load automatic dispatching system under the control of one main controller. The load in the entire station area achieves balanced distribution of three-phase loads under orderly dispatch.

Figure 3: Diagram of Commutation Switch Positions

Figure 4: Main Controller Installation Diagram

Figure 5: Phase-Switching Switch Installation Diagram

 5. Analysis of Governance Outcomes

On June 11, 2024, the correction device was installed. After verifying that the wiring was correct, it was put into operation at 8:00 p.m. on June 11. The voltage and current trend graphs at the secondary output of the substation transformer are shown in Figure 6. In the figure, the yellow line represents the current in Phase A, the green line represents the current in Phase B, the red line represents the current in Phase C, and the blue line represents the neutral line current.

Prior to the device’s commissioning, it was observed that the C-phase load in the service area was heavily loaded, the B-phase was moderately loaded, and the A-phase was lightly loaded, with these conditions exhibiting periodic variations; the three-phase current imbalance in the service area was severe. After the device was commissioned, it was observed that the three-phase load currents in the service area were essentially balanced, the neutral line current decreased from approximately 90 A to approximately 15 A, and the three-phase imbalance remained below 20%, with the imbalance level remaining essentially stable.

Figure 6: Current Trends in the Front and Back Areas Before and After Treatment

The load trend chart of the backstage area before and after treatment is shown in Figure 7. The green line in the figure is the average load rate and the purple line is the average three-phase unbalance. After the device has been running stably for a period of time, the daily load data of the production command system in June 2024 was retrieved. The three-phase load tends to be balanced. It can be found that the daily average three-phase imbalance in the station area dropped from 48.66% to 19.56%, a decrease of 58%.

Figure 7: Load Trend Chart for the Service Area in June 2024

 On June 4, 2024, the maximum three-phase imbalance in this service area was 87.57% (at 17:45 that day). As shown in Figure 8, at 18:00 and 18:30, the three-phase imbalances in this service area were 70.1% and 71.97%, respectively. The three-phase imbalance exceeded 50% for a cumulative duration of more than two hours. It was not until 19:45 that day, after the load had dropped, that the three-phase imbalance in this feeder’s load began to decrease. 17:45 also marked the peak load of the day. During the summer peak period, a three-phase imbalance greater than 50% that persists for an extended period poses a risk of single-phase overload tripping.

Figure 8: Metering Automation System—Load Curve for the Service Area on June 4, 2024 (Before Governance)

On June 19, 2024, the maximum three-phase imbalance in the station area was 58.45% (at 07:15 on that day). As shown in Figure 9, the three-phase imbalance in the station area dropped to 18.20% at 08:15. After installing a three-phase imbalance control device, the station area can dynamically adjust the three-phase load in real time according to the current load situation. The duration of the three-phase imbalance exceeding 50% does not exceed 15 minutes. The maximum load point is at 19:30 on that day. At this time, the unbalance degree is only 5.3%, which can effectively avoid tripping caused by single-phase overload.

Figure 9: Metering Automation System—Load Curve for the Service Area on June 19, 2024 (After Governance)

 6. Summary

(1) After the device was put into operation, it significantly reduced three-phase imbalance. Under equivalent operating conditions, the three-phase imbalance of this transformer was reduced from 48.7% to approximately 19.5%, a decrease of 60%. When sudden changes in load occur, the device can immediately and dynamically adjust the three-phase loads to achieve a more balanced distribution, thereby greatly reducing the risk of single-phase overload tripping caused by three-phase imbalance.

(2) After the device was put into operation, the neutral line current dropped from an average of 90 A to an average of 15 A, thereby significantly reducing line losses and transformer losses and yielding notable energy savings. At the same time, the end-point voltage has improved markedly.

(3) The application of this device has greatly improved the power supply reliability and power quality of the grid. To date, there have been no complaints regarding low voltage issues, which is of far-reaching significance for enhancing the grid company’s brand image and service reputation among users.

In summary, the three-phase unbalance control device shows good control capabilities in field applications. The device has small size, low cost, strong adaptability, high reliability and good effect. It has great reference significance for the treatment of three-phase unbalance problem in the entire network, and has the possibility of being widely promoted and applied.

Guangdong Yidian Measurement and Control Technology Co., Ltd. specializes in the research, development, and production of power testing instruments and power quality management solutions. With nearly two decades of deep involvement in the power industry, the company remains committed to technological innovation and independent R&D, meticulously crafting every product and providing attentive service to every customer. The company actively supports OEM and ODM customization services, tailoring solutions to meet specific customer needs and creating greater value for them. Yidian Measurement & Control is the manufacturer of the ETCR5500 zero-millisecond phase-switching unit, three-phase imbalance correction devices, and single-phase load automatic regulation devices.

ETCRMETER is a high-end brand of ETCR. All products are manufactured by ETCR, which also provides technical support and after-sales service.

For more information, please visit the Yidian Measurement & Control official website: www.etcrmeter.com