1 Introduction

With the widespread application of nonlinear loads such as LED lighting, frequency converters, computers, and office communications in modern buildings and industries, three-phase four-wire power supply and distribution systems are facing increasingly prominent power quality problems. Excessive N-line current caused by factors such as the convergence of the 3Nth zero-sequence harmonic and three-phase imbalance can easily lead to neutral wire overheating, insulation aging, and even electrical fires, posing a serious threat to the safe and stable operation of the power supply and distribution system.

 

To address the core issue of overcurrent caused by the superposition of zero-sequence harmonics on the neutral line, current research primarily employs techniques such as flux compensation, zero-sequence filtering, and zigzag-connected reactors, achieving some success in suppressing harmonics and eliminating neutral line overcurrent. Building upon this foundation, this paper proposes a comprehensive electrical mitigation scheme for terminals with excessive N-line current, aiming to suppress zero-sequence harmonics at their source and balance three-phase currents, providing safe, efficient, and reliable technical support for low-voltage power distribution systems.

 

 

2. Mechanism of N-line current generation

2.1 Causes of N-line current generation

In actual project sites, the energization of the neutral (N) line is particularly common due to the 3nth harmonic and three-phase imbalance. This is especially true in commercial plazas or sports center plazas, which often contain numerous LED fluorescent lights/floodlights and LED screens, leading to a energized N line. The load type is a switching power supply, and it exhibits the following characteristics:

1. The load contains a high level of harmonics. When the internal switching devices operate in a high-frequency switching state, the input and output current and voltage waveforms will be distorted. The harmonic current is mainly the third harmonic current, and the current distortion rate THDi is generally between 70% and 120%, while also containing higher harmonics.

 

2) Switching power supplies often employ power factor correction (PFC) technology, therefore their reactive power characteristics are generally predominantly capacitive, with a power factor above 0.9. Actively connecting capacitors in areas where they are present can actually increase the system's reactive power, leading to a rapid decrease in the power factor.

 

 

2.2 Reasons for N-line current generation

The abnormal situation of the neutral line being energized frequently in 0.4kV low-voltage power distribution systems is generally due to the following factors: 1) Unbalanced currents in the three-phase loads (A/B/C); 2) 3n-th harmonic currents generated by nonlinear load equipment; 3) Open circuit in the neutral line, resulting in infinite impedance and preventing current from forming a closed loop, causing the neutral point potential on the load equipment side to shift from zero; 4) Mixed connection of the neutral line and the protective earth (PE) line; 5) Damage to the insulation layer between the A/B/C phase lines and the neutral line, causing leakage between the phase lines and the neutral line; 6) Grounding fault. In TN-S systems, excessive neutral point grounding resistance or poor grounding can cause the neutral point potential to rise when a single-phase grounding fault occurs, energizing the neutral line.

 

 

3. Terminal Electrical Comprehensive Management Solution

The terminal electrical power quality comprehensive management system solution consists of four parts: terminal electrical power quality comprehensive management equipment, physical gateway, server, and service terminals. The terminal electrical power quality comprehensive management equipment serves as the underlying hardware foundation, implementing specific service actions such as data collection and power quality compensation at the end of the power supply and distribution network. The physical gateway enables data transmission between the terminal management equipment and the server, as well as policy function allocation for the equipment. Equipment operation data is visualized for users via the server and service terminals. The terminal system topology is shown in Figure 3.

Figure 3 Terminal System Topology Diagram

 

The terminal electrical comprehensive management system solution integrates "interconnection, monitoring, analysis, and management," achieving the goal of addressing harmonics, reactive power, and three-phase imbalance by monitoring the power distribution system from the equipment level to predicting power quality and analyzing abnormal data. It also includes features such as abnormal temperature detection, neutral current management, and overcurrent feedback protection for the neutral (N) line. Compared to active power filters (APF) and passive neutral current interruptors, its advantages are:

1) It integrates "interconnection-monitoring-analysis-governance" into one system, involving hardware governance equipment, physical gateways, servers, and software system service platforms;

2) Added N-line current monitoring and mitigation functions, including N-line temperature monitoring and early warning, N-line current mitigation, and overcurrent feedback protection;

3) Added functions for stabilizing terminal voltage and managing three-phase imbalance.

 

 

4 ANSNP Neutral Line Security Protector

The ANSNP series neutral line protection device, connected in parallel in low-voltage power distribution systems with harmonic loads, can quickly and in real-time track and compensate for dynamically changing harmonic currents. The basic principle of the neutral line protection device is as follows: Overcurrent information on the neutral line is collected through a current detection circuit; the controller quickly calculates and extracts the content of each harmonic current, generates a harmonic current command, and through a power actuator, generates a compensation current with the same amplitude but opposite direction to the overcurrent, which is injected into the neutral line, thereby eliminating excessive current in the neutral line.

 

4.1 Functions and Purposes

(1) Detect and analyze the current of the end circuit, and control the harmonics and three-phase imbalance. This can solve the problem of excessive N-line current caused by the 3Nth harmonic and three-phase imbalance, and eliminate the electrical fire safety hazards caused by it.

(2) Intelligent voltage regulation function;

(3) It realizes the control of the N-line current of the system, and at the same time has the functions of harmonic current, reactive power, stray current and system current change;

(4) System energy-saving function; reduces system cable loss and the loss caused by the skin effect, thus achieving comprehensive energy saving. At the same time, it can improve the service life of system components and reduce maintenance costs.

(5) Wireless data transmission function and mobile APP interaction function.

 

4.2 Product Appearance

Figure 1. ANSNP module chassis exterior view

1) This equipment is a forced-air-cooled wall-mounted unit, designed for indoor wall mounting. The bottom panel is the air inlet for heat dissipation, and the front panel is equipped with a 7.0-inch LCD screen for near-end human-machine interaction. The top panel is the air outlet for heat dissipation and contains wiring ports for primary and secondary cables. 2) The top panel includes primary wiring terminals (A/B/C/N/N for three-phase four-wire system and A/B/C for three-phase three-wire system) and secondary wiring terminals for current transformers, accessory power supply, emergency stop, and communication. 3) The module requires grounding; a grounding terminal is located behind the equipment's output terminals.

Note: The A/B/C phase lines and the N line of a three-phase four-wire system must not be reversed. Reversing them will cause a phase-to-phase short circuit and serious damage to the equipment.

 

 

4.3 Current Transformer Selection and Wiring

Current transformer: open/closed type, accuracy class 0.5, compatible range 100:5~10000:5. The transformer capacity can be referenced from the circuit breaker capacity of the terminal distribution box being managed, such as 400/5 or 500/5.

The requirements for current transformer cables are: low smoke, halogen-free, flame retardant, and 2.5mm².

Figure (1) below shows the wiring method for a single module, and Figure 2 shows the wiring method for modules in parallel.

Figure 1 Wiring method for a single module

Figure 2. Parallel wiring method for two modules

 

4.4 Equipment Installation Requirements

When installing the wall-mounted module, air should enter from the bottom and exit from the top. Heat should be dissipated from bottom to top. At least 150mm of space should be reserved for the air inlet and outlet of the module, and the installation position should be more than 1.5 meters away from the ground. Do not obstruct the air outlet.

Wall-mounted modules are typically installed on walls or inside small cabinets, and come standard with a pair of mounting ears, as shown in the picture. Φ10 holes can be drilled in the wall, and M8 expansion screws can be used for fixing. Wall-mounted modules are equipped with a 7.0-inch color touchscreen.

 

 

 

5 Examples of N-line Current Control Projects

 

5.1 Project Overview

A commercial center in Shenzhen mainly involves loads such as elevators, LED light sets (numerous), LED screens, air conditioners, and lighting, which caused abnormal temperature of the switches and excessive current in the neutral (N) line, affecting the electrical safety of the loads. On-site testing was conducted at 22 end-point treatment locations, with the neutral line treatment test performed using a terminal floor distribution room as an example.

There are two main reasons why the N line generates current:

1) The imbalance of the three phases A/B/C leads to the presence of zero-sequence current on the N line;

2) The 3Nth harmonic current of the phase line will be superimposed on the N line (for example: if there is a 10A third harmonic current on phase A, a 20A third harmonic current on phase B, and a 5A third harmonic current on phase C, there will be a 10+20+5=35A harmonic current on the N line; similarly, the 9th harmonic current, the 15th harmonic current, etc. have the same characteristics).

 

5.2 Governance Effectiveness

Equipment configuration: 1 ANSNP100 wall-mounted module;

Treatment effect:

(1) The current distortion rate is significantly reduced and the current waveform tends to be sinusoidal.

(2) The N-line current is reduced. The third harmonic current is reduced from 125A before treatment to about 5A. The compensation rate of the ANSNP for the third harmonic in the N-line current is 96% or above, which meets the expected treatment requirements in the early stage;

(3) Reduce cable heating.

 

 

6. Conclusion

This paper proposes a comprehensive terminal electrical quality management solution based on the problem of excessive neutral line current. This solution integrates "interconnection, monitoring, analysis, and management," encompassing hardware management equipment, physical gateways, servers, and software system platforms. Compared to traditional passive neutral line current interruptors and active power filters (APFs), it adds functions such as abnormal neutral line temperature detection, neutral line current management and overcurrent feedback protection, stabilizing terminal voltage, and managing three-phase imbalance. Finally, a project engineering case study verifies the effectiveness and reliability of the comprehensive terminal power quality management solution, ensuring the electrical safety of the terminal power supply and distribution system.