1.Introduction of metro power supply network
Generally, metro power supply network is connected to utility grid and the AC electricity feeds all kinds of equipment with different voltage classes in metro system after going through transmission and transformation.
According to the different purposes of the loads, the metro power supply network consists of two sub-networks:
● the "traction" sub-network, which is mainly formed by traction substations
● the "lighting & power" sub-network ,which is, by contrast, mainly formed by step-down substations.
Three-level voltages power supply
2Analysis of the power quality in metro power supply network
The traction power rectifier units which deliver DC electricity for the traction vehicles in the metro system are the biggest source of harmonics. Desipte the fact that the harmonic level meet the controllable requirements at the point of common coupling (PCC) due to employing the 24 equivalent pulse rectifier technology, there are still many harmonics present.
As for the "lighting & power" sub-network, there is an increasing use of non-linear equipment such as fans and escalators driven by variable frequency drives(VFDs), rectifiers, invertors and fluorescent lamps. All of these loads produce considerable harmonics, among which are mostly the 3rd、 5th and 7th harmonics.
3 The problems caused by harmonics
In general, harmonics present in the metro power supply network can lead to various problems, which include
● Interference with the reactive power compensation equipment，lowering the power factor or even worse, causing nuisance tripping of the compensation equipment. .
●Creating the resonance of the compensation capacitors which in turn cause the compensation capacitors to overheat or, in extreme cases, breakdown of capacitors
● Causing nuisance operation of the relay protection devices and interfering the power measurement.
In addition to those harmonic issues, it’s also worth noting that the metro power supply network is also complicated, by its very nature, because it consists of two power supply sub-networks with different voltage class and combines DC power supply systems and AC ones.
In view of the above power quality hazards and complicated nature, it’s exetremly important to eliminate harmonics from theirs sources to provide the whole metro power supply network with a safe and reliable working environment .
4 Solution for harmonics in rail transit systems
?Comprehensive treatment for power quality problem at step-down substation in the power and lighting network.
?Centralized treatment at the busbar on the transformers’ low-voltage side
?Using our three-phase four-wire active power filters (HTQF 4L series) which can not only compensate reactive power and also correct three-phase unbalance besides the harmonic filtering
5 Our clients in rail transit system (but not limited to)
?Beijing-Shanghai High-Speed Railway
? Beijing-Tianjin inter-city passenger rail line
6 Case study-Changping Line, Beijing Subway
The Changping Line of the Beijing Subway is a rapid transit line in northern Beijing. The line started construction in 2009. In September 2010, Construction was completed. The line opened on December 30, 2010 and currently runs north from the Xi'erqi station on Line 13 to Shisanling station. It is 31.24 km in length with 11 stations and will connect central Changping District with Beijing subway network.
Our company submitted the tender document on the February 2, 2010 and win the bid after several rounds of evaluations and clarifications of technical specifications and finally signed the procurement contract. The 0.4KV low-voltage active power filters involving in the project include two models , which are HTQF 4L-0.4/250A and HTQF 4L-0.4/60A, with the total of 32 sets of HTQFs deployed in 16 metro stations. Currently, the whole metro project has finished and the HTQFs gain the user’s approval by its reliability and excellent performance.
The picture of a HTQF operationg in one of the switchgear rooms in Changping Line, Beijing Subway