High-speed rail and subway

The Impact of Grid Power Quality on High-Speed Rail Operations

On August 13, 2016, a power outage occurred on the G79 high-speed train from Beijing West to Shenzhen North Station. Thousands of passengers were trapped in carriages at temperatures exceeding 40°C for nearly two hours, and many children and elderly people suffered from discomfort and dehydration. The safe and stable operation of high-speed rail is inextricably linked to the power quality of its grid, and ensuring the quality of this grid has become a hot topic of discussion.

By 2020, China will have built over 16,000 kilometers of new high-speed rail, forming a high-speed rail network anchored by the "four vertical and four horizontal" high-speed rail network. Currently, high-speed rail operates at speeds of 300 km/h, and calls for increasing speeds to 350 km/h are growing. The safe and stable operation of high-speed rail is inextric linked to the power quality ofably the high-speed rail grid. Power quality issues can lead to grid failures and train electrical equipment failures. To explore the impact of high-speed rail grid power quality issues on high-speed trains, we must understand the power supply and drive mechanisms of high-speed rail.

High-speed rail operations rely on the traction power supply system. Electrified railway traction power supply methods primarily include BT (current-absorbing transformer), AT (autotransformer), and TR (direct power supply). Due to the high power and traction current requirements of high-speed rail, the AT method, with its highest power transmission capacity, is generally used.

The traction power supply system primarily consists of a traction substation (substation), an autotransformer (AT), a catenary (T), a feeder line (F), rails (R), and high-speed trains. The basic principle is that the traction substation provides power for the entire traction system. Current flows from the traction substation, supplies energy to the high-speed train through the catenary, and then returns to the traction substation via the feeder line.

High-speed rail power supply is divided into "power supply sections," with each section averaging tens of kilometers. Each substation extends two power branches, each providing current of different phases. As a train passes through the "power supply section" between two substations, it passes through four power branches: A1, B1, A2, and B2. To ensure power supply safety, each power supply branch is electrically insulated (isolated) to prevent short circuits between them. The transition from one power supply branch to another is instantaneous.

High-Speed Rail Drive Principle

The traction power supply system provides power to high-speed rail, which relies on electricity for its propulsion. The basic principle is as follows: High-speed trains receive high-voltage AC power via a pantograph in contact with the overhead catenary. This power is then stepped down by a transformer and converted to DC by a four-quadrant rectifier. This power is then converted to three-phase AC with adjustable amplitude and frequency by an inverter. This power is then fed into a three-phase asynchronous/synchronous traction motor, which drives the wheels through the transmission system.

High-Speed Rail Power Quality Analysis

High-speed rail is a unique type of high-power single-phase load. For a three-phase symmetrical power system, this traction load exhibits characteristics such as fluctuation, nonlinearity, and asymmetry.

1. Load Fluctuation and Impact: Acceleration, coasting, braking, and other factors during train operation can cause load fluctuations in the traction substation. This is especially true when a train switches from one power supply branch to another, resulting in momentary load fluctuations and impacts that can cause abnormal fluctuations in the grid voltage.

2. Nonlinearity: High-speed rail uses AC-DC-AC PWM converter technology to convert power-frequency AC power through rectification and inversion to three-phase AC with adjustable amplitude and frequency to power the traction motors. This is a nonlinear process that inevitably generates a large number of harmonics. Furthermore, the entire train power supply also supplies nonlinear loads such as air conditioning and lighting within the train, which also generate a large number of harmonics.

3. Asymmetry: High-speed rail uses a single-phase power supply system, and the loads on the two power supply branches of the traction grid cannot be consistent. Therefore, for the three-phase grid, this represents an asymmetric load, generating negative-sequence current and causing three-phase imbalance in the grid connected to the traction substation.

High-Speed Rail Power Quality Hazards

High-speed rail power pollution, including harmonics, negative-sequence components, and voltage fluctuations, has a serious impact on the safety and reliability of high-speed rail power systems. High-speed rail power pollution has the following major impacts on the system:

Impact on power grid safety and reliability: High-speed train operation generates a large amount of high-order harmonics that are injected into the traction substation through traction transformers and introduced into the power system. These harmonics, combined with the negative-sequence sources generated by the system's "background load," significantly amplify the third and fifth harmonics within the system's internal power grid during resonance, leading to power grid failures.

Impact on the safe and stable operation of power equipment: Harmonics and negative-sequence components generated by high-speed rail traction loads can damage equipment, shorten its lifespan, or reduce its efficiency. They can even cause malfunctions or performance degradation in equipment such as computers, communications, and power system relay protection devices, potentially leading to over-tripping and escalating accidents or even tripping the main transformer or line. This suggests that power pollution from high-speed rail power grids not only affects the safety and stability of the railway's own local power grid but also impacts external power grids through traction substations. Whether it is the impact on its own power grid or the external power grid, it will ultimately threaten the safety and stability of high-speed rail operations.