Analysis Of The Dependence Between Principal Indicators Of Freight Electric Locomotive Energy Efficiency In Various Operational Modes
Introduction
A significant share of energy consumed by railway transport systems is used to move the trains [1]. In our opinion, it is necessary to study carefully the efficiency of freight electric locomotives in different modes of operation. Modern freight electric locomotives have a multi-engine electric traction drive (up to 16 traction motors) with tractive effort control system on each axle. Therefore, the approach of the Scalable Power Control Technology [2-4], which regulates the quantity of locomotive traction motors (TM) in traction mode, seems quite reasona-ble. In our case, if full locomotive tractive power is not needed within operation, the redundant motors must be automatically switched off [5, 6]. But when additional tractive power is required, the locomotive must use the number of motors sufficient for traction. The goal of this paper is to analyze the principal indicators of freight electric locomotive energy efficiency in different operational modes and searching for de-pendence between these indicators, also elaboration of the locomotive energy effi-ciency control system [7, 8]. As a result, energy consumed by locomotives can be re-duced.
The 2ES5 Electric Locomotive
The double-section 8-axle heavy freight electric locomotive 2ES5 «Scythian» [9] produced by CJSC «Transmashholding» is shown in Figure 1. The mass is 200 tons, the limit speed is 120 km/h, the wheels are solid-rolled. The locomotive is pow-ered by AC 25 kV, 50 Hz network. The nominal tractive power is Pcap = 8×1100 = 8800 kW. Figure 1. Freight electric locomotive 2ES5 «Scythian»In each section, there are four independent power channels. The schematic dia-gram of the electrical circuit of one section is shown in Figure 2 (T – main transform-er, U1-U4 – traction converters, 4qs – rectifier, C – power capacitor, Inv – autono-mous voltage inverter, M1-M4 – traction motors). Figure 2. Schematic diagram of the 2ES5 electric power circuit (for 1 section)
Energy Efficiency Indicators
The energy efficiency of an electric locomotive is characterized by two princi-pal indicators: coefficient of performance (COP) and capacity of utilization coefficient (CUC). The COP of an electric locomotive η can be presented as: (1)where: Pu – power output, kW; Pa – power consumption, kW; Ptr = Pu – tractive power (tangent power), kW; Pp – loss power, kW; Ppb – power of auxiliary loads, kW. The useful tractive power of an electric locomotive (its tangent power) Ptr is: (2)where Ftr – locomotive tangential tractive effort, kN; V – speed, m/s. It is necessary to distinguish between the locomotive's COP in nominal power stationary mode (a precise value of this COP is listed in the technical documentation), and the operational COP. Locomotive's operational efficiency depends on operational modes in different time intervals [10]. The CUC of an electric locomotive γ is the quotient of the useful tractive power Ptr at a given instant, by its available capacity Pcap: . (3)Note, that Pcap in (3) is equal to the number of working TM multiplied by the rated power of one TM: Pcap = NTM * PTM, (4)where PTM is the nominal power of one TM. For example, if all eight traction motors operate, thenPcap = NTM * PTM = 1100 * 8 = 8800 kW,if only four TMs work operate, then Pcap = NTM * PTM = 1100 * 4 = 4400 kW. Note that values of COP (1) and CUC (3) can be expressed as a percentage.
The Operational Experience
There are still significant opportunities to reduce the electrical energy consump-tion when the locomotive operates at incomplete load. As a matter of fact, the freight locomotives of JSC "Russian Railways" mainly work on "round-trip" lines. In for-ward direction, a locomotive drives a heavily loaded train weighing 6000 tons or more, and nearly all the power capacity is used. But in the reverse direction, the lo-comotive drives an empty train which weighs 3-4 times less, and the locomotive power is used only partially. Until the spring of 2016 the locomotives 2ES5 worked in Eastern Siberia, and then on the North Caucasus now. The heavy train experience. There was an experienced ride with 9090 t freight train in February 2018. Ambient air temperature was –23°С. The train went uphill on 9. 1‰ slope between stations S and T. The ride of 21,7 km distance from S to T con-tinued for 43 min. This test ride was successful, the wheels' slip speed and traction motors temperatures were normal. The elevation profile of the section S – T is shown on Figure 3. The plots of speed V and tractive effort Ftr are shown on Figure 4. Consumed power Pa, tractive power Ptr and losses with auxiliary loads are shown on Figure 5. The capacity utiliza-tion coefficient CUC and efficiency COP – on Figure 6. Figure 3.
The elevation profile of the section S – T (Start – Terminus) Figure 4. The locomotive speed V (red line) and tractive effort Fт (blue line) when going uphill Figure 5. Consumed power Pa (blue line), tractive power Ptr (red) and losses with auxiliary loads Pp + Ppb (green) Figure 6. Efficiency COP (red line) and capacity utilization CUC (blue)Let us turn to the analysis of energy consumption. The full energy Еa con-sumed by locomotive was 6349 kWh. The useful tractive work Аtr of 5197 kWh was calculated as an integral of tractive power Рtr. The distributions of consumed energy and useful tractive work, depending on the locomotive capacity utilization modes, are shown by bar charts (histogram) on Figure 7, 8. We see that during the ride, most of the energy was consumed when the locomotive was used at full capacity. The average value of the locomotive efficiency (1) is: The time for went uphill was Δt = 43 min = 0,717 h. According to (3) and (4) we can find average value of CUC: The regular experience with empty train. It was observed that locomotive power is most fully used in acceleration modes and on the uphill with heavy train. But on the flat railway with empty train, its tractive power is used in part. As an example, let consider the ride on flat railway. The train weighs 2192 t. A 108 km ride takes 142 min. The locomotive consumed energy Еa was 3017 kWh. The useful tractive work Аtr was 2174 kWh. The distribution of consumed energy, de-pending on the locomotive capacity utilization modes, is shown by bar charts (histo-gram) on Figure 9. Bar chart of useful tractive work is placed on Figure 10. The average value of the locomotive efficiency (1) on the flat way: Let's find the average value of CUC. The time for ride was Δt = 142 min = 2,37 h. The average value of the CUC (3) calculate as Let us compare bar charts for heavy uphill ride (Figure 7, 8) and regular ride on flat railway (Figure 9, 10). Efficiency COP and usage of locomotive power CUC is greatly less on a flat railway with a light train. After considering these examples, let's try to find the way to increase efficiency in different operational modes.
Locomotive Efficiency Control Method
Because the locomotive 2ES5 has data recorder on board, a lot of experi-mental data was collected in different operational modes. Let's show energy effi-ciency η of electric locomotive in traction mode, depending on its useful tractive power Рtr (Figure 11). Figure 11. Energy efficiency of an electric locomotive in traction mode as a function of its useful tractive powerThe analytical dependence of the electric locomotive efficiency on the tractive power was found by statistical processing (look at red line on Figure 11): . (5)Coefficients a, b equal with confidence interval 0,95: a = 1,055 (1,054, 1,056); b = 564,2 (563,1, 565,3). Dependence of electric locomotive efficiency (coefficient of performance) η on capacity utilization coefficient γ is found by dividing formula (5) by locomotive capacity Pcap: η = η(γ) =, (6)where c = b / Pcap = 564,2 / 8800 = 0,0641. Results and dependencies (5), (6) allowed us to develop algorithm of in-creasing locomotive efficiency: Discrete-Adaptive Control system (DAC). This control system distributes tractive effort to optimum quantity of traction motors, and redundant motors are switched off. Switched-on motors are using al-most full power, and they have high efficiency. To determine the optimum quantity of motors in the traction mode, we choose capacity utilization coefficient γ as a cri-terion. The efficiency control system was checked by computer simulation on 108 km under the same conditions as real ride from above in this paper. Traction mo-tors quantity regulator was added in detailed rolling stock computer model. The re-sults are shown in Fig. 12, 13 (compare with figures 9, 10). In computer simulation of the DAC algorithm, with the same executed useful work, the energy consumption from the catenary decreased from 3017 to 2623 kWh, or by 13%. An average locomotive efficiency ηaver reached 84%, compared to 72% efficiency for the sample ride on flat way with empty train.
Conclusions
- Indicators of freight locomotive efficiency in different operational modes were analyzed.
- Dependence of electric locomotive efficiency on the tractive power was found by statistical processing of experimental data.
- A locomotive efficiency discrete-adaptive control system is represented. This control system increases the locomotive efficiency by distributing tractive ef-fort to optimum quantity of traction motor.
