In today's railway projects, the electromagnetic compatibility (EMC) between equipment of various systems must be ensured. Traditionally, traction power supply has been one of the primary sources of electromagnetic interference (EMI), which causes trouble in the electronic signaling equipment that detects low level ac currents in the rails to confirm the absence of trains.

EMI is characterized by conductive harmonic currents of a very wide frequency range in the traction return rails. The magnitude of harmonic currents that fall within the operating frequency band of the signaling equipment must be estimated in order to assess the effects of these currents, taking into account the specified immunity levels.

The power supply contractor for the Mass Transit Railway (MTR) Tseung Kwan O Extension project in Hong Kong is required to conduct an assessment of the harmonics in the 1500 V DC traction power supply. PB was retained by the contractor to provide project management, programming and technical support with regard to EMC, so we have fully undertaken this responsibility.

Figure 1: DC Network Model Where: VH1 is the harmonic voltage source representing the harmonic spectrum generated by the rectfier unit in traction substation (TSS) #1 ZTSS1 is the source impedance of TSS #1 ZOHL1 and ZRAIL1 are the overhead line and railimpedance between TSS #1 and EMU #1 ZEMU1 is the equivalent impedance of EMU #1, and so on.

The Challenge

Electric multiple units (EMUs), which are 8-car trains, will be running scheduled train service in the railway network. By applying circuit theory to the complete network as shown in Figure 1, we calculated the harmonic currents in the rails.

It was necessary to simplify the network by considering one EMU at a time. Analyzing such a complete DC network would have been too difficult and onerous because:

Methodology

We took the following steps in the assessment to estimate
harmonics in DC traction power supply:

Step 1: Determine Harmonic Voltage Spectra. Basing our investigation on 12-pulse rectifier units, we took a number of equipment failure and imperfections scenarios into consideration, including:

Having known the full load current, its corresponding overlap angle and firing angle, we replicated the rectifier output voltage time-varying waveforms for the six scenarios. We then used Fourier analysis to obtain harmonic voltage spectra up to the highest operating frequency of the signaling equipment.

Figure 2: Simplified Model of Traction Substation and Single EMU Where: IH1 is the current flow in the closet EMU that is derived from the simplified model in figure 2 IHX is the current flow in the Xth EMU in the same electrical section

Figure 3: Circuit for Weighting Factor Estimation

Figure 4: Identify the Worst Instant from the Headway Chart

Figure 5: "S" Bond between EMU and Transition Busbar (TBB)

Step 2: Estimate Harmonic Currents Flowing through an EMU and the Aggregate Effect of Several EMUs. Once the harmonic voltage spectra were evaluated, harmonic currents flowing in the traction power supply system could be estimated. Figure 2 shows the simplified model in which an EMU was assumed to be very close to the TSS such that the TSS source impedance, overhead line and return rail impedance were ignored. When high frequency harmonics were considered, the varying DC characteristics of EMU under different driving modes could be neglected. Thus, the equivalent impedance of EMU is attributable only to the line filter.

A weighting factor (Mx) that represents the harmonic current contributed by the xth EMU as a ratio of that from the closest EMU can be established by using simple current divider formulae on the circuit diagram, as shown in Figure 3.

Only one electrical section on each side of the subject TSS was taken into account. The harmonic current returning from sections farther away was insignificant because of considerable rail and overhead catenary wire impedance.

We estimated the maximum traction return current flow in the rails under normal train service by identifying the instant of the largest number of EMUs within two adjoining electrical sections from the train service headway charts. This instant is considered the worst case in terms of harmonic current in the return rails. The headway chart in Figure 4 shows a snapshot when an EMU is closest to a TSS that has the maximum number of EMUs within the electrical sections on both sides. The distances between the TSS and individual EMUs were taken from the headway chart for determining the respective weighting factors.

Step 3: Determine Harmonic Current Flowing in Return Rails. As specified by the signaling contractor, the immunity levels of various trackside signaling equipment were defined as the maximum permissible in-band harmonic currents for the rails. Track circuits were identified as the most vulnerable type of signaling equipment under influence of harmonic currents.

On the Tseung Kwan O Extension project, two types of track circuits are used-CVCM of Alstom and FTG-S of Siemens. (CVCM and FTG-S are names of the track circuits models supplied by the two manufacturers.) The "S" bond of FTG-S track circuits and the impedance bond of CVCM track circuits were the subjects of concern. For the "S" bond, both the balanced and unbalanced currents were considered; whereas for the other signaling equipment concerned, only balanced current would be of interest. We found that the magnitude of unbalanced harmonic current through the "S" bond depends on the relative positions of the first train axle, the transition busbar through which the traction current returns to the TSS, and the "S" bond. Figure 5 on page 96 shows a possible scenario of the three elements.

The CVCM track circuits are located in a remote section beyond the site boundary of the power supply contract. For this reason, we need only to consider the effect of harmonic current generated from the closest traction substation that reaches the first CVCM track circuit at the site boundary, which is at least one electrical section apart (more than 1 km, or 0.6 mile, in distance). Please refer to the headway chart in Figure 4. The current magnitude is relatively smaller than that flowing into FTG-S track circuits because of considerable attenuation in overhead line and rail.

While conducting a sensitivity test, we observed that unbalanced current increases when the EMU is getting nearer to the "S" bond. A similar sensitivity test was performed with the "S" bond located at the right side of the transition busbar showed that current unbalance decreases with the transition busbar being farther away from the "S" bond.

Summary of Findings

The estimated harmonic currents within the 3-decibel bandwidth of the signaling equipment were compared with the corresponding maximum permissible current limits. We found that the harmonic currents at two of the FTG-S operating frequencies will likely exceed the permissible limits specified. Therefore, Mass Transit Railway Corporation was advised to take care when selecting the frequency of FTG-S track circuits in designing the track-bonding plan of the Tseung Kwan O Extension.

Other harmonics generated by the traction power supply equipment in the interested frequency ranges are within the limits of the interested signaling equipment and have quite a generous safety margin. It should be noted, however, that any change in EMU filter design may increase the harmonic currents significantly, especially where line filter inductance is concerned. Such a change may also have an effect on a much broader range of signaling frequencies.