1. Introduction

2. Railway System Modeling

Figure 1. Schematic diagram of BSSs, TPSSs, and utility buildings.

Figure 1. Schematic diagram of BSSs, TPSSs, and utility buildings.

2.1. Bulk Supply Substation (BSS)

The electricity supply for the MRT Line 2 is provided by the utility through three bulk supply substations (BSS) located at Jinjang, Kuchai Lama, and Universiti Putra Malaysia (UPM). These BSSs are connected to the 132 kV power grid, and the supply voltage is stepped down to 33 kV through two 50 MVA transformers before being delivered to the traction power supply substation (TPSS). Figure 2 presents one of the BSS feeders for the MRT Line 2. Tables 1 and 2 show the key parameters of the BSS transformers and earthing transformers. These parameters are included in the ETAP simulation model.

Figure 2. Single line diagram of a BSS with transformers.

Figure 2. Single line diagram of a BSS with transformers.

Table 1. Parameters of BSS transformers.

Table 1. Parameters of BSS transformers.

Parameters	BSS transformers
	Jinjang and Kuchai Lama BSS	UPM BSS
Power Rating (MVA)	50	40
Voltage level (kV)	132/33	132/33
Positive sequence impedance (%)	12.5	12.5
Zero sequence impedance (%)	10	10
X/R ratio	45	45
Impedance tolerance (%)	±7.5	±7.5
Tap range	±10	±10
Vector group	YNd1	YNd1

Table 2. Parameters of earthing transformers.

Table 2. Parameters of earthing transformers.

Parameters	Earthing transformers (ET)
	Jinjang and Kuchai Lama BSS	UPM BSS
Power Rating (MVA)	160	160
Voltage level (kV)	33/0.433	33/0.433
Positive sequence impedance (%)	4	4
X/R ratio	1.5	1.5
Impedance tolerance (%)	±10	±10
Neutral grounding resistor (NER) (A)	900	900
Vector group	ZNyn11	ZNyn11

2.2. Traction Power Supply Substation (TPSS)

There are total of 25 TPSSs along the MRT Line 2, with six of them located in underground stations. Each TPSS consists of two rectifier transformers and two auxiliary transformers, as shown in Figure 3. Tables 3 and 4 shows the setting of the auxiliary and rectifier transformers used in this study. The rectifier transformers step down the voltage from 33 kV to 0.585 kV and a 12-pulse model rectifier converts the 0.585 kV AC supply to a 750 V DC supply for the traction loads. On the other hand, the auxiliary transformers step down the power from 33 kV to 0.4 kV for the auxiliary loads such as air conditioning systems, lifts, escalators, and other systems that consume electricity within the railway stations. The TPSSs provide a 750 V DC supply to the rolling stocks via the third rail and the return current will flow back to the source via the rail tracks. The rolling stock parameters for the train are listed in Table 5.

Figure 3. Schematic diagram of TPSS for MRT Line 2.

Figure 3. Schematic diagram of TPSS for MRT Line 2.

Table 3. Parameters of auxiliary transformers.

Table 3. Parameters of auxiliary transformers.

Parameters	Auxiliary Transformer
Rating (MVA)	0.16, 0.63	0.75, 0.8, 1, 1.25	1.5, 2
Voltage level (kV)	33/0.433	33/0.433	33/0.433
Zero sequence impedance (%)	80	80	80
X/R ratio	1.5	3.5	6
Impedance tolerance (%)	± 10	±10	±10
Tap range (%)	±7.5	±7.5	±7.5
Vector group	Dyn11	Dyn11	Dyn11

Table 4. Parameters of rectifier transformers.

Table 4. Parameters of rectifier transformers.

Parameters	2.3 MVA Rectifier Transformer	3.5 MVA Rectifier Transformer
Rating (MVA)	2.3/1.15/1.15	3.5/1.75/1.75
Voltage level (kV)	33/0.585/0.585	33/0.585/0.585
Positive sequence impedance (+Z${}_{pri}-Z_{sec}$) (%)	6	6
Positive sequence impedance (+Z${}_{pri}-Z_{ter}$) (%)	6	6
Positive sequence impedance (+Z${}_{sec}-Z_{ter}$) (%)	12	12
Zero sequence impedance (+Z${}_{pri}-Z_{sec}$) (%)	80	80
Zero sequence impedance (+Z${}_{pri}-Z_{ter}$) (%)	80	80
Zero sequence impedance (+Z${}_{sec}-Z_{ter}$) (%)	80	80
X/R ratio	10	10
Impedance tolerance (%)	±10	±10
Tap range (%)	±7.5	±7.5
Vector group	Dd0y11	Dd0y11

Table 5. Rolling stock setting of MRT Line 2 train.

Table 5. Rolling stock setting of MRT Line 2 train.

Parameters	Configurations
Train weight (ton)	218
Number of axles, n	4 (M-T-T-M)
	M: Motor car T: Trailer car
The total length of train (m)	90
Area of train (m${}^2$)	11.0408
Rolling resistance	0.019292685 + 0.000370932110091743v+ 6.8421x10${}^6$$v^2$
Acceleration limit (m/s${}^2$)	1
Deceleration limit (m/s${}^2$)	1.1
Minimum voltage (V)	500
Maximum voltage (V)	900
Minimum speed for regeneration (km/h)	3

2.3. Case studies

This study investigates the impact of rolling resistance, track elevation, and track curvature on the energy consumption and braking energy recovery of trains traveling between two stations, labeled as Station NB1 and Station NB2 as shown in Figure 4. The rolling stock parameters, tractive effort, and braking effort data are obtained from the manufacturer’s specifications for the MRT Line 2.

Figure 5 depicts the train traveling under different track scenarios. Segment A represents a straight track where the train needs power to overcome rolling resistance and acceleration forces. In Segment B, the train requires additional power to overcome rolling resistance, acceleration forces, and gravitational forces. Segment C represents a curved track where the train needs extra power to overcome curve resistance. Table 6 summarizes the forces that the train needs to overcome under different track scenarios.

Figure 4. Station NB1 and NB2 modeled in ETAP-eTraX.

Figure 4. Station NB1 and NB2 modeled in ETAP-eTraX.

Figure 5. Track scenarios involve different civil alignment parameters.

Figure 5. Track scenarios involve different civil alignment parameters.

Table 6. Tractive forces need to be overcome by train in different track scenarios.

Table 6. Tractive forces need to be overcome by train in different track scenarios.

Track segments	Forces need to be overcome by train
A	Acceleration force, F${}_a$, $rollingresistance,F_{rr}$
B	Acceleration force, F${}_a$, $rollingresistance,F_{rr}andgraderesistance,F_{gr}$
C	Acceleration force, F${}_a$, $rollingresistance,F_{rr}andcurveresistance,F_{cr}$

There are a few assumptions made in this simulation study as follows: Numbered lists can be added as follows:

• The weight of the trains chosen is based on the full weight with passengers and standees of six passengers/$m^2$.
• The train is traveling in an open-air environment.
• A typical train scheduled is used, whereby the headway time and dwell time are 109 seconds and 40 seconds respectively.

Headway time is the time interval between two successive trains arriving at a station. Typically, the railway operates with 3 minutes intervals during peak hours (from 7 a.m. to 9 a.m. and from 5 p.m. to 7 p.m.) and 6 minutes intervals during off-peak hours. Therefore, this study simulated headway time interval from 2 minutes to 6 minutes to observe the impact on the energy consumptions and the recuperation of braking energy.

In dynamic analyses, the electrical distribution network of Jinjang BSS is modeled to investigate the variation in headway time. This network stretches from Damansara Damai to Kampung Baru North which consists of 8 TPSSs and 15 stations. Figure 6 and Figure 7 illustrate the track data, including elevation and track curvature from Damansara Damai to Kampung Baru North, respectively.

The speed profiles and the speed limits of the train travels from Damansara Damai station to Kampung Baru North station are as shown in Figure 8. The speed limits are set in the range of 70 km/h to 110 km/h according to the curvatures and elevation of the tracks. As depicted in the figure, the train’s speed is constrained by the designated speed limit throughout the simulation.

Figure 9 illustrates the power consumption and the speed profile of MRT Line 2. It can be noticed that the power is a negative value when the speed decreases. The negative power indicates that the energy is being transmitted back to the power grid. Figure 10 shows the relationship between power consumption, speed limit, and the speed profile of a single train for different driving modes such as acceleration, cruising, coasting, and deceleration.

Figure 6. Track elevations of MRT Line 2.

Figure 6. Track elevations of MRT Line 2.

Figure 7. Track curvatures of MRT Line 2.

Figure 7. Track curvatures of MRT Line 2.

Figure 8. The speed profile of a single train of MRT Line 2.

Figure 8. The speed profile of a single train of MRT Line 2.

Figure 9. The power consumption and speed profile of a single train of MRT Line 2.

Figure 9. The power consumption and speed profile of a single train of MRT Line 2.

Figure 10. Power consumption, speed profile, and speed limit of a single train travel between 2 stations.

Figure 10. Power consumption, speed profile, and speed limit of a single train travel between 2 stations.

3. Formulation of Train Dynamic Load Flow

Figure 11. Overview of dynamic load flow calculation.

Figure 11. Overview of dynamic load flow calculation.

4. Results and Discussion

4.1. Case studies for train’s energy consumption under different speed limits

Figure 12 shows the train’s energy consumptions when the train’s speed limit varies from 70 km/h to 100 km/h, for the distance between Station NB1 and NB2 spanning from 1 km to 3 km. The train’s energy consumption increases as the train’s speed limit increases.

Figure 12. Energy consumptions of train between Station NB1 and NB2 for different speed limits under different distances.

Figure 12. Energy consumptions of train between Station NB1 and NB2 for different speed limits under different distances.

For the distance of 3 km, the energy consumption of the train at the speed of 110 km/h is 16.95% higher than that of 70 km/h. On the other hand, the train’s energy consumption between the different speed limits for a short train travel distance such as 1 km, is not significant because the maximum speed that the train can attain is confined by the distance between the two stations. Table 7 shows the maximum speed that the train can attain for different distances and speed limits. It can be observed that the minimum distance between two stations for the train to reach 110 km/h is 3 km.

Table 7. Maximum speed of a train under different distances and speed limits.

Table 7. Maximum speed of a train under different distances and speed limits.

		Attainable speed before a deceleration in needed (km/h)
	Speed limits	70km/h	80km/h	90km/h	100km/h	110km/h
Distance between 2 stations
1km		70	80	85.6	85.6	85.6
1.5km		70	80	90	95.9	95.9
2km		70	80	90	100	103.8
2.5km		70	80	90	100	109.2
3km		70	80	90	100	110

4.2. Case studies for train’s energy consumption under different levels of elevation

Figure 13 shows the results of the train’s energy consumption under different elevation levels between Station A and B from -20 m to + 20 m with the distance between the stations from 1 km to 3 km. The range of elevation is set according to the maximum and minimum track elevation of MRT Line 2. It can be observed that the greater the elevation between Station NB1 and NB2, the greater the train’s energy consumptions. This can be explained by the fact that the greater traction force must be applied to overcome the gravitational force for the steeper inclination to keep the train at a constant speed. This is consistent with Equation (5), where the higher the elevation, the larger the gradient resistance force, $F_{gr}$.

Conversely, when $F_{gr}$ is negative or when the train is moving in a downward direction, it will cause energy consumptions to decrease because the downhill helps the train to reach its targeted speed in a shorter time and the train must apply a brake to slow down the speed of the train. When the train applies the brake, the train regenerates the braking energy and deliver to the grid. Besides, it is also noted that the train’s energy consumptions in Figure 13 increases as the distance between the two stations increases because the train journey time from Station NB1 to NB2 increases. The energy consumptions of the train with the track elevation of +20 m at the distance of 3 km between the two stations is 227% higher than that of -20 m. The energy consumption becomes negative for the elevation of -20 m and the distance between two stations of 1 km because the traction power is recuperated from the regenerative braking to the grid.

Figure 13. Energy consumptions of train between Station NB1 and NB2 for different track elevations under different distances.

Figure 13. Energy consumptions of train between Station NB1 and NB2 for different track elevations under different distances.

4.3. Case studies for train’s energy consumption under different track curvature

Figure 14 shows the train’s energy consumption when the bend radius of the track increases from 100 m to 8000 m, with the total distance between Station NB1 and NB2 changing from 1 km to 3 km. These bend radii are obtained from the design parameters of MRT Line 2. The result shows that the effect of bend radius on the energy consumptions is not significant because the curvature force is much smaller as compared to the acceleration force and rolling stock resistance. The difference of energy consumptions of the train for the bend radius of 100 m and 8000 m at the distance of 3 km between the two stations is 4.4%.

Figure 14. Energy consumption results of train transit between Station NB1 and NB2 for different track curvatures under different distances.

Figure 14. Energy consumption results of train transit between Station NB1 and NB2 for different track curvatures under different distances.

4.4. Case studies for recuperation of braking energy under different civil alignment parameters

The recuperation of the train’s braking energy under different civil alignment configurations is as shown in Figure 15. The recuperation of the train’s braking energy is obtained by comparing the energy consumptions of the train with- and without the RBE recovery. The 3 km distance between two stations is selected because it has the highest energy consumptions.

Figure 15. Regenerative braking energy recovery results of train transit between Station NB1 and NB2 under different speed limits, elevations, and track curvatures.

Figure 15. Regenerative braking energy recovery results of train transit between Station NB1 and NB2 under different speed limits, elevations, and track curvatures.

It can be seen in Figure 15 that the changes in elevation and track curvature have a little effect on the recuperation rate of the braking energy. The recuperation rate of the trains when traveling downwards is almost the same compared to that of the train traveling upwards. However, a substantial difference in the recuperation rate of RBE is observed when the speed limits varied from 70 km/h to 110 km/h. This is because when the speed of the train is high, the momentum of the train is high, hence resulting into a high recuperation of RBE.

4.5. Case studies for variation of headway time

Figure 16 shows the total energy consumptions of the MRT Line 2 system for the trip of a single train travels from Damansara Damai to Kampung Baru North for the case with- and without RBE recovery. It can be seen that the train traveled without the RBE is 2.3 times higher than that of the train traveled with RBE. In other word, the energy consumption of the train without RBE consume 132% higher than that of the train with RBE.

Figure 16. Energy consumption of train travel from Damansara Damai Station to Kampung Baru North Station.

Figure 16. Energy consumption of train travel from Damansara Damai Station to Kampung Baru North Station.

The results from the previous case studies were obtained using a 1.82-minute headway time, as provided by the railway operators. To cater to the varying passenger demands during peak and off-peak periods, train schedules with different headways are implemented. Consequently, it becomes necessary to examine the RBE amount under various headway conditions. As can be seen from Figure 17, it is evident that the 3-minute headway time exhibits the highest RBE amount, with a difference of 2.2 kWh compared to the 6-minute headway time. This can be attributed to the high synchronizing rate between the arrival and departure trains, which enhances the natural exchange of RBE between the trains within a station.

Figure 17. Recuperation of braking energy of MRT Line 2 at different headway time.

Figure 17. Recuperation of braking energy of MRT Line 2 at different headway time.

5. Conclusion

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