Power quality (and monitoring) in railway systems

Electrical compatibility between fixed installations and rolling stock, at power supply level, is paramount when dealing with power quality, system availability and reliability. This article addresses power quality and power quality monitoring on alternate current 25 kV electrification systems, focused on overvoltages generated by near resonance frequency harmonics.

Theoretical background

According to Ohm’s Law, when a voltage is applied to an electrical circuit the resulting current flow is inversely proportional to the circuit impedance. By the same law, if an electric current is injected in a circuit the resulting voltage is directly proportional to the circuit impedance.

By definition the electrical impedance is the measure of opposition that an electric circuit presents to the flow of current. In alternating current (AC) the impedance has two components: resistance and reactance. The resistance component is intrinsic to the materials and shape of electrical elements. The reactance relates to the electromagnetic induction phenomena between electrical elements. While resistance is invariable with frequency, reactance is frequency dependent.

In complex circuits or networks, various electrical elements are connected in series and parallel. Due to the different response of the reactive component of all circuit elements when subjected to signals with different frequencies, electrical circuits present then different values of impedance. The frequency-impedance curve has a non-linear characteristic varying between low impedance frequencies and high impedance frequencies (resonances). The frequencyimpedance curve of a circuit can be determined with specific software tools, given the parameters of all circuit electrical components. In alternative, this curve can be determined experimentally, with appropriate equipment, injecting a variable frequency current in the circuit and measuring the resulting voltage.

In high voltage systems, such as railway networks, it’s very difficult to perform significant changes to the characteristic curve of an existing system. Even when developing new electrifica – tions there are several basic aspects of the system that constrain the shape of frequencyimpedance curve.

As an example, Figure 1 shows the theoretical frequency-impedance curve for the catenary (overhead contact line) sector fed by Fogueteiro Traction Substation (TSS) determined at a distance of 10km from the TSS. This TSS is a 150/25 kV installation with one 20 MVA power transformer in service, located at Sul mainline of the Portuguese rail network, which is a doubletrack line.

In alternating current systems, the voltage signal varies in time sinusoidally at a specific frequency, the fundamental frequency. The 25 kV electrification system operates at the utility frequency, hence its fundamental frequency is 50 Hz.

When a non-linear load is connected to the power system, like modern trains with its power electronic converters, other frequency com – ponents are generated, usually higher than the fundamental frequency. These higher frequency components of the voltage or current signals are designated harmonics.

In AC electrification systems, trains use power electronics to convert the fixed 50 Hz (or 60 and 16⅔ Hz depending on the country and the used system) alternating current signal at pantograph level to a variable frequency signal necessary for efficient train (motor) speed control. In modern trains these power electronic converters also allow the inversion of the energy flow when braking (regenerative braking). The electronic converters generate harmonic currents due to the commutation of the semiconductors at a higher frequency. When a train has multiple converters in service, in order to reduce the harmonic components, the commutation of these converters are synchro – nised at different phase angles (interlacing). With converter interlacing the total harmonic distortion of multiple converters is less than only with one converter. A rule of thumb for this case – if two converters are interlaced, the total harmonic distortion will be close to half of just with one converter. For multiple converters the same rule of thumb can be considered.

Power quality in railway systems

The introduction of modern trains in an existing railway network, or the expansion of the electri – fied railway network with new substations and additional catenaries, requires particular attention regarding the compatibility between these new trains and the electrical fixed installations. On the one hand, the existing infrastructure usually comprises several substations – each one with its own charact – eristic frequency-impedance curve. On the other hand, in the event of one or more power converter outage, modern trains have the possibility of reconfiguring the remaining power converters which will result in different operation modes. Usually, each operation mode has a different pattern of generated harmonic components.

The Euro standard EN 50388 recommends the development of a compatibility study and subsequent lab or tracks tests each time a new element is introduced to ensure the compatibility between rolling stock and fixed power installations. According to this standard, the acceptance criteria for harmonic disturbances is that no peak voltage higher than 50 kV in 25 kV systems shall occur in any point of the network (30 kV peak for 15 kV networks).

In a system with multiple substations and multiple trains (train series), at one point or another, there will be significant harmonic disturbances. It is important to verify the compatibility between rolling stock and electrical fixed installations when introducing a new element but also to keep track of these phenomena over time and assess their evolution.

In most occasions, the overvoltages that result of harmonic distortion close to resonant frequencies are characterised by a significant increase of peak value (crest) and a moderate increase in the RMS value. These overvoltages cause stress to the apparatus internal insulation and reduces their lifecycle. Equipment faults due to internal insulation breakdown and conse quent system outages are frequent when subjected to severe overvoltages over a period of time. High amplitude peak voltages can result in rapid apparatus faults whilst lower amplitude peak voltages have long-term effects. The most sensible apparatus to harmonic overvoltages are high voltage surge arresters and voltage transformers. Low voltage electronic apparatus connected to the catenary through power transformers, such as signalling and telecom systems, are also often affected.

Each train is an independent harmonic current generator. As described before, according to Ohm’s Law, the harmonic current multiplied by the system impedance at the same frequency results in a harmonic overvoltage. The effects of multiple similar trains operating in the same area (catenary sector) are cumula – tive, resulting in higher harmonic overvoltages. By the same principle, when interlacing is lost, all train power converters start to operate at the same phase angle and overlapped, resulting in a considerably higher harmonic distortion.

Over the last decade, a series of equipment faults and power system outages impelled the Portuguese Infrastructure Manager together with Railway Undertakings to study the origin of these faults. Several tests were carried out with rolling stock rigged with on-board monitoring equipment for voltage and current measure – ment. In most occasions the loss of converter interlacing, in random pattern, led to an increment of harmonic overvoltages. Assumption was made that the stress induced by these overvoltages to equipment internal insulation over time caused the occasional faults.

Without proper power quality monitoring systems it’s very hard, not to say impossible, to diagnose the harmonic phenomena and trace its cause. Experience has shown that the record of peak voltage is paramount to assess the quality of the voltage waveform in alternated current railway systems. To reduce stress induced by long duration harmonic overvoltages in catenary equipment and trackside low voltage systems, it is also important to consider the implementation of electric protections triggered by harmonic measurements.

Power quality monitoring system for railway applications

The base requirements set out for the implementation of a permanent power quality monitoring system for catenary in the Portuguese railway infrastructure were the following:

1. Record of voltage and current signal wave – form, RMS, Peak (crest) and Harmonics (single and THD)

2. Protection function capability

3. Remote access for real-time analysis and record download

4. Cheap and scalable system.

Standard power meters are developed for three-phase power systems and oriented for the power quality indicators of those systems. For railway applications, with different power quality indicators, standard power meters lack some of the required functions. The development of a power quality system designed specifically for the set out base requirements was for us the obvious choice.

The developed system is based on standalone rugged modular data acquisition units with analog inputs and digital outputs. Only the necessary features were implemented with the data processing software which resulted in a light power quality monitoring application with direct and simple interface. Real-time data and register are accessible through web browser, linked with the remote monitoring unit over TCP/IP Protocol. The flexibility of the system allows to easily add or remove measurements, data processing features, recorded data and visual elements shown in the web interface. Besides the set specific requirements, this power quality monitoring system is able to assess the conformity with EN 50163 power quality indicators. If required it can also be adapted for three-phase systems and the power quality indicators specified in EN 50160 implemented and others relevant for railway systems, such as voltage unbalance at primary side of substation.

Several stand-alone monitoring units are currently deployed in critical locations of the Portuguese railway network. The power quality monitoring application can be uploaded to as many monitoring units as required, guaranteeing a straightforward scalable system.

A remote protection function was implemented based on peak voltage trend with a negative exponent curve (similar to EN 50163 long term overvoltage curve). When triggered, a digital output is activated and the trip order is sent to the circuit breaker located in the traction substation. This protection enables a reduction of stress to apparatus internal insulation and consequently also a reduction in faults and system outages.

The daily use of the deployed power quality monitoring units allowed us to clearly identify and pinpoint the origin of harmonic disturbances and develop strategies with the railway undertakings in order to reduce or eliminate these occurrences. The recorded data is down – loaded daily from all remote units automatically by a central server and power quality reports are generated with the most relevant information, highlighting disturbances and power quality indicators. These automatic procedures enable a low time consuming daily assessment of catenary power quality.

In the end, the use of adequate power quality monitoring tools is very important to identify disturbances at an early stage and trace its origin, avoiding premature degradation of apparatus, which are costly to replace, and the reduction of system availability and reliability.

The developed system is a scalable low investment solution, designed specifically for railway power quality monitoring that can be easily adapted to other railway networks and their particular characteristics.

Biography

Marco Filipe Santos is an Electrical Engineer specialising in power systems. Marco has worked at REFER since January 2004 as an Electric Traction Engineer and more recently at REFER Enginnering S.A.. Between 2004 and 2006, Marco was mainly dedicated to substation maintenance activities, traction energy management and power quality measurements. Since 2007, Marco has developed work mainly in energy system design, technical studies regarding traction power supply and railway power quality, and has developed simulation software for AC and DC traction systems used for traction power supply design. Marco has performed several electrification studies and collaborated in system specifications for traction power supply of new high-speed connections. Marco is currently Head of the Catenary and Electric Traction Department at REFER Engineering S.A., and has also been a Member of the UIC Energy & Electric Traction Experts Group since 2009.