1. Main load characteristics of urban rail transit system:

The harmonics in the electricity environment of the subway system mainly come from the rectifier and inverter devices in the power supply process of vehicle traction equipment, by DC power supply complete sets, lighting, elevators, display screens, air conditioners, drainage, etc. The harmonics that appear in the system Mainly the 5th, 7th, 11th, and 13th harmonics, and also contain a certain amount of 3rd harmonics.

In addition to increasing line losses and accelerating equipment aging, the harmonics present in the system also cause great damage to reactive power compensation devices such as capacitors in the system. It will cause electromagnetic interference to the signals of communication equipment, and harmonics of specific frequencies will also be amplified by capacitors and other devices in the system to produce resonance, which will cause huge damage to the entire power supply system.

2. The necessity of harmonic and reactive power control in urban rail transit systems:

Urban rail transit has strict requirements on power supply systems, especially subway power supply systems. It has very high requirements on power supply reliability. If the reliability of power supply is insufficient, it will easily have a major impact. Therefore, we need to control the power quality of urban rail transit such as subways. Here, harmonics are the main cause that threatens the reliability of subway power supply. At the same time, the problem caused by too low power factor is also a factor that cannot be ignored in reality. The power factor of urban rail subway power supply system is mainly composed of traction load and power lighting load. determined by its nature. For power lighting loads, inductive reactive power is mainly generated, and most power factors are relatively low, generally around 0.78. The power factor of the subway 0.4KV system is low, which will increase the loss of the subway's internal power supply lines and reduce the active output capacity of the power transformation and transmission equipment, making it vulnerable to fines from the power department.

The harm caused by harmonic pollution includes:

• Harmonics can lead to increased power loss in the power grid, shortened equipment life, malfunction of grounding protection, malfunction of remote control, overheating of lines and equipment, and other electrical energy safety issues;
• Being in a high harmonic environment for a long time will also cause the power grid to resonate, causing large-scale power supply interruptions and causing major accidents;
• Harmonics will cause additional losses in the equipment and components in the system, causing heating of transformers, cables, motors and other equipment, and accelerating insulation aging of equipment lines;
• Cause relay protection of electronic components or malfunction of automatic devices;
• Affect the normal operation of electronic instruments and communication systems and reduce communication quality.

The hazards caused by reactive power include:

An excessively low power factor will lead to an increase in the operation and maintenance costs of the urban rail transit system, and will also reduce the effective capacity of the power distribution system and reduce the service life of the equipment.

3. Solution:

a. Harmonic problem:

Subway harmonics are an important factor in the subway power quality environment. Subway harmonics mainly include frequency conversion equipment, UPS/ESE, escalators, comprehensive monitoring, lighting systems, access control systems and other weak current equipment.

In this regard, the solution we proposed is based on the actual situation, configuring corresponding active power filter APF equipment for active filtering, reducing harmonic content and reducing harmonic distortion rate.

Our APF product is shown in the figure below. It has multiple compensation functions such as harmonic compensation, power factor compensation and three-phase unbalance compensation. It can simultaneously filter out 2 to 50 times in a three-phase four-wire power distribution system. Harmonics, while improving the power factor of the power grid, is the perfect solution to manage power quality problems.

b. Reactive power compensation:

A large number of high-power equipment need to be installed in subway tunnels. These electrical equipment have relatively large capacity and strong working continuity, which can easily cause a large impact load on the power grid, generate a large amount of reactive power, and reduce the effective capacity of the distribution system.

One is to compensate through LC series products (capacitor reactor compensation cabinet) to improve the power supply quality of the power grid and improve the reliability of electricity consumption. It is easy to install, takes up a small area, and saves space.