Supply voltages of railway systems

Voltages and frequencies are defined by two standards: EN 50163 and IEC 60850.

Electrification System	Lowest Non-permanent Voltage [V]	Lowest Permanent Voltage [V]	Nominal Voltage [V]	Highest Permanent Voltage [V]	Highest Non-permanent Voltage [V]
DC (mean values)	400	400	600	720	800
	500	500	750	900	1000
	1000	1000	1500	1800	1950
	2000	2000	3000	3600	3900
AC (r.m.s. values)	11000	12000	15000	17250	18000
	17500	19000	25000	27500	29000

The frequency of the 50 Hz electric traction system is imposed by the three phase grid. Therefore, the values stated by EN 50160 are applicable. The frequency of the 16,7 Hz electric traction system (except for synchronous- synchronous converters) is not imposed by the three phase grid.

	Systems synchronous connection + Interconnected system		Systems no synchronous connection + Interconnected system		Systems connected to the railway + Interconnected grid
50 Hz	50 Hz ± 1%	for 99,5% of a year	50 Hz ± 2%	for 95% of a week
	50 Hz + 4% / - 6%	for 99,5% of a year	50 Hz ± 15%	for 100% of the time
16,7 Hz	16,7 Hz ± 1%	for 99,5% of a year	16,7 Hz ± 2%	for 95% of a week	16,7 Hz + 2% / - 3%	for 100% of the time
	16,7 Hz ± 6%	for 100% of the time	16,7 Hz ± 15%	for 100 % of the time

AC or DC traction

It doesn’t really matter whether you have AC or DC motors, nowadays either can work with an AC or DC supply. You just need to put the right sort of control system between the supply and the motor and it will work. However, the choice of AC or DC power transmission system along the line is important. Generally, it’s a question of what sort of railway you have. It can be summarized simply as AC for long distance and DC for short distance.

AC systems always use overhead wires, DC can use either an overhead wire or a third rail. And the return circuit is via the running rails back to the substation. The running rails are at earth potential and are connected to the substation.

It is easier to boost the voltage of AC than that of DC, so it is easier to send more power over transmission lines with AC.

DC, on the other hand was the preferred option for shorter lines, urban systems and tramways. Apart from only requiring a simple control system for the motors, the smaller size of urban operations meant that trains were usually lighter and needed less power. But it needed a heavier transmission medium to carry the power and it lost a fair amount of voltage as the distance between supply connections increased.

This was overcome by placing substations at close intervals – every three or four kilometers at first, nowadays two or three on a 750 V system – compared with every 20 kilometers or so for a 25 kV AC line.

Voltages distribution overview

This are some examples of nominal supply voltages and installed power of different types of rolling stock:

And an overview of Europe railway electrification map with different supply voltages:

References

• openrailwaymap
• http://www.railway-technical.com/infrastructure/electric-traction-power.html
• EN 50163 (2007), EN 50163: Railway applications. Supply voltages of traction systems, IET
• TODO change to actual section –> ** **Plaquin, B. (n.d.). 4.1 Railway gauges. In Rolling Stock in the Railway System (2020th ed., Vol. 1, pp. 260–273). essay, Trackomedia.