IMPROVING ENERGY EFFICIENCY OF BELT CONVEYOR SYSTEM

Transcription

1 6 th INTERNATIONAL SYMPOSIUM on POWER ELECTRONICS - Ee 2 NOVI SAD, REPUBLIC OF SERBIA, October 26 th - 28 th, 2 IMPROVING ENERGY EFFICIENCY OF BELT CONVEYOR SYSTEM Dragan Jevtić, Ilija Mihailović, Leposava Ristić, Milan Bebić, Saša Štatkić*, Neša Rašić, Borislav Jeftenić University of Belgrade, Faculty of Electrical Engineering, Belgrade, Serbia *University of Pristina, Faculty of Technical Sciences, Kosovska Mitrovica, Serbia Abstract: The improvement of energy efficiency of belt conveyor systems can be achieved in two ways: first affects efficiency of drive components and second applies different control strategies of multi motor drives on belt conveyors. The paper presents the algorithm developed to generate the reference speed for the system of belt conveyors with remote control on an open pit mine and implemented in control center. Based on principle of reduced energy consumption, a new improved algorithm is proposed in the paper. Simulation results presented in the paper confirm increased energy savings of the bulk material transportation system. Key Words: Belt conveyors (BC), Open pit Mines (OPM), SCADA system, Control strategy, Adjustable speed drives (ASD), Induction motors, Energy savings. INTRODUCTION Energy efficiency is one of the key elements in energy policy of developed countries in the world since it contributes economical prosperity by extending the life time of non - renewable energy sources, especially from fossil fuels. It has been estimated that the widespread energy efficiency improvement with the existing technologies can save 2% of the global energy demand, and another 2% can be saved by preventing waste, i.e., by various conservation methods []. Despite of the type and the organization of the OPM, mining industry presents an unique challenge to the application of electric drives due to very high installed power, complexity of mining equipment and demanding environmental conditions, which all together with the high cost of downtime emphasize the need for reliable and robust technologies. In recent times, very long BCs have been built with lengths of several dozen kilometers. In facilities and systems where BCs are used, their installed power is almost always a significant portion of the total installed power, meaning their consumption dominates within the total consumption of energy. Because of this, the issue of energy savings in BCs is naturally of significant interest. The paper presents the new BC system in an OPM, which transports overburden from the excavator to the spreader with the system of five BCs, with all aspects for energy efficiency improvement considered and applied in its realization. The belt drive of BC station is with a belt wih of 2 mm and has installed power of 4 MW, meaning the entire system has installed power of 2 MW. Control of the BC system as a whole in the view of improved energy efficiency is possible only if the remote control is utilized. The paper presents the applied algoritm for generating reference speed of the BC system as the function of instantaneous capacity, applied in the control center. The paper further investigates additional possibilities to improve energy efficiency of the BC system with modifications of the presented algorithm. 2. ENERGY SAVINGS IN VARIABLE SPEED BELT CONVEYORS In order to increase energy savings in the system of BCs, different control strategies are applied of multi motor drives on BCs, based on reference speed control as the function of instantaneous capacity. Bulk material which is transported by a BC can be distributed along the length of the belt in various ways depending on how the material is deposited onto the conveyor. The quantity of material which is transported within a unit of time, meaning the capacity, can be expressed with the general formula T Q = A v() t T () where Q is capacity, A(t) is instantaneous cross section area of material at the reference point, v(t) is current speed of the belt and T is period of observation. The instantaneous quantity of bulk material which is being transported using a BC depends on the operational mode of the system within which the BC is used. In a large number of cases, this quantity is variable and most often the instantaneous cross section area of material on the belt is less than the rated value. Since BC often operates at a decreased capacity, the same quantity of material can be transferred in two ways: with a constant rated speed and smaller cross section of material on the belt, or with rated cross section area of material, but at a lower than rated speed. It has been shown in [2] that most often in practice A(t)<A r, meaning that if the speed is modified according to (2), the BC could operate at a lower than rated speed:

2 A() t vt () = vr, (2) Ar where A r is rated cross section area of the material on the belt and v r is rated speed of the belt. The transport of material at a lower than rated speed would naturally lead to a decrease in the amount of energy needed to conduct transport [2-6]. The needed force F for transport of a quantity of mass m of material is: F() v = m g μ() v, (3) where g is gravitational acceleration, μ (v) is friction coefficient. Friction is generally a function of speed. The dependency of friction on speed depends on the type of motional resistance, i.e., from the construction of the transportation device, in our case of the BC. To establish this dependency, one can measure the power for operating the BC with no load P nl at a constant speed of v, which can be expressed using (4): Pnl () v = mb g μ() v v (4) where m b is equivalent mass of the belt during no load operation. Fig. shows the results of measuring the power on two BCs at various speeds when there was no material on them. The installed power of the drive is 3x MW, the belt wih is 2 mm and the lengths of the conveyors are dissimilar. Fig. shows the no-load power in percentages of the installed power as a function of speed in [%]. The power in the speed range from 5 to % differed by 6 to 2% of the installed power depending on the length of the BC. The dependency of no-load power on speed in Fig. is practically linear, which means that friction does not depend on speed. That could be expected because the maximum speed is a relatively small 5.8 m/s, all bearings on the BC are rolling-element and at the time when the measurements were taken the equipment was brand new. at the rated speed and material is deposited onto it so that the instantaneous capacity is constant, but less than rated, meaning that the cross section area of material on it is A < A r. The second transporter in the series has a speed determined according to (2), so that the cross section area of material on it is approximately A r. Fig. 2. Two BCs in a series connection - cross section of material on a belt The necessary power of the first BC, assuming that friction does not depend on speed, is: Plo = A L γ μ vr + mb g μ vr (6) The necessary power for the second BC is: Plo2 = Ar L γ g μ v+ mb g μ v (7) The first term addend in (6) and (7) corresponds to the power necessary for transport of materials and in both cases is equal, which can be confirmed if the speed in the first addend of (6) is replaced by (2). The second term corresponds to the power for overcoming motional resistances of an empty transporter which depends on the speed as shown using the measurement results displayed in Fig.. The conclusion is that if the speed is adjusted according to (2) for the transport of a certain quantity of material, a savings of energy will be achieved based on the decreased power necessary for driving the belt. From this short analysis we can derive that the power on the second BC from the example is less by the value provided by: Plo Plo2 = mb g μ ( vr v). (8) The presented analysis supported by measurements conducted with an empty belt confirms the expected savings of energy for transport of bulk material with reduced speed. 3. APPLIED CONCEPT OF BC SYSTEM CONTROL A simplified communication network configuration of the BC system is shown in Fig. 3. Fig.. No-load power of a belt drive as the function of speed, BC lengths - 849m, 2-25m Under the assumption that the cross section area of material on the belt is the same along the entire length of the conveyor, the speed is constant and friction does not depend on speed, the necessary power P lo for driving a BC with a length L equals: Plo = ( mbm + mb ) g μ v = ( A L γ + mb ) g μ v (5) where m bm is mass of the material on the belt. For the purpose of attaining a better understanding of the achieved energy savings, suppose that we have two BCs of the same length L in a series which transports material as shown in Fig. 2. The first transporter operates Fig. 3. Communication network of a BC system The supervisory control system is located within the control center (CC) alongside the PC with monitors in which SCADA is implemented. Within the control center is also the PLC, which controls the operation of the BC system (as a whole), and the remaining necessary equipment. The PLC and PC are connected via Ethernet 2

3 to the PLCs on the BC stations. In this application, on each BC station, there is four FCs which are connected to the PLC through a ProfiBus network. A speed regulator implemented into the PLC on the belt conveyor station can be PI or PID. Strong differential action should be avoided due to the elasticity of the belt. The output signal of the speed regulator, as the reference torque, is forwarded to all frequency converters (FCs) realized with DTC algorithm. The algorithm for generating the reference speeds of speed-controlled BCs as the function of instantaneous capacity has been implemented into the PLC within the CC. At the input of the speed reference generator are the instantaneous value of the material cross section measured at the end of the second BC and the instantaneous vales of speed of each BC (achieved with SCADA). Speed control of a BC requires one to possess data on the quantity of material which is deposited onto the belt, meaning the instantaneous capacity must be known. The instantaneous capacity is: dv Qt () = = At () vt () = At () vconst At () (9) The speed of the belt onto which the material is deposited should be modified in accordance with (2) in order to achieve the defined criteria of speed control. However, the instantaneous capacity changes quite frequently and sporadically. This means that the speed should be increased and decreased in the same manner as the instantaneous capacity changes. These dynamic processes would be unfavorable for the mechanical assemblies of a BC, especially for the belt, and could lead to increased energy consumption, i.e., exactly the opposite of what this method for control the operation of the BC is trying to achieve. Because of the fact that the instantaneous capacity changes, and those changes cannot be predicted, the control algorithm must be such that the belt speed is adjusted to the conditions at the beginning of the conveyor, i.e., at the location where the instantaneous capacity is measured. The algorithm for generating the reference speed of the belt drive is defined as follows:. The theoretical belt speed v t (t), meaning its reference speed, is calculated on the basis of the equation (2), from which () is derived. Ain vt = vin() t () Ar In () A in (t) and v in (t) are the instantaneous value of cross section of incoming material and the instantaneous speed of previous belt. The actual reference speed of the belt drive v ref (t) is calculated on the basis of () according to (2) under the conditions defined by (): dvt and vt( t) ( t), () = c ( vt ()) t + ( t ), (2) where t is the moment when both conditions defined by () are acquired and c is constant with the dimension of s When the conditions from () are not fulfilled, the actual reference speed is determined on the basis of (3), = ( t2) + k ( t t2) (3) where t 2 is the moment when at least one of the conditions from () ceases to be valid and k is deceleration. A block diagram of the described algorithm is provided in Fig. 4. When a BC drive operates with variable speed then the P2 switch is in position. During the period when the quantity of material coming onto the conveyor increases, the reference speed of the drive is determined according to (2), and at that time the drive accelerates. In this manner the cross section of the material on the speed controlled belt increases, meaning it gravitates towards A r. When the quantity of incoming material decreases, the reference speed is calculated on the basis of the relation (3), i.e., the speed decreases with a deceleration k. The speed adjustment range is limited, minimum speed should be 5% of the rated speed; the maximum speed is set at 25%, dependant on the capacity of excavator and working conditions. The speed adjustment range from 5% to % of the rated speed is considered in the paper. vin Ain A r vt v r d c c k AND Fig. 4. Algorithm for generating the reference speed of the belt The constant k determines deceleration of the drive which should confirm with characteristics of the drive construction. An abrupt deceleration unfavorably affects all mechanical assemblies, couplings, bearings, the belt, etc. BC drives with a route which does not traverse an incline normally use braking with a resistor and chopper in the DC circuit. There are two dominant constraints of the system which should be taken into account while determining the value of constant k: to avoid spillage of material over the belt and to avoid abrupt deceleration which may lead to the activation of the electric braking system. Considering all the listed conditions and constraints, a value of -. [p.u.] for constant k is empirically determined during the system commissioning. It is applied in the algorithm for generating the reference speed of the system of five BCs in an OPM to suite all conditions, but it is not the optimal one considering energy consumption. The results of measurements performed on the BC3 are given in Fig. 5. It can be noticed from presented results then the cross section of material on the belt is sometimes greater than % which is the maximum theoretical value. Even, when the value of the cross section reaches 6% of the theoretic value, transport can still be conducted without spillage [7]. The first two BCs, BC and BC2 are bench conveyors which operate with constant rated speed. BC3 and BC4 are connecting conveyors and operate with variable speed, as well as BC5 which is a dump-side conveyor. To further improve the algorithm for generating reference speed, modifications concerning deceleration k are analyzed. 3

4 Speed, Material cross section [%] Material cross section at the 2. belt, A in Material cross section at the 3. belt, A out Speed of the 3. belt Speed reference of the 3. belt Fig. 5 Characteristic values of the third belt 4. IMPROVED SPEED REFERENCE GENERATOR WITH CONSTANT DECELERATION The transfer point between two belt conveyors is considered: the instantaneous value of cross section of incoming material A in (t) has the rated speed v r, while the instantaneous value of cross section of outgoing material, i.e. at the next belt in the queue, is A out (t) and has the variable speed v ref (t), which is the result of the applied algorithm. According to (9), the conservation of capacity may be expressed as: Ain vr = Aout. (4) Based on (4), the expression for the theoretical speed () and the expression (2) for determination of v ref (t) in the case of increasing material on the belt, i.e. in the case when conditions defined in () are fulfilled, it can be derived that = vt Aout = Ar ; < vr, (5) Aout = Ain (); t = vr. In accordance with the applied algorithm, the reference speed will take the value of theoretical belt speed, i.e. v ref (t) will be proportional to the increase of material on the belt. As the consequence, the cross section of material on the belt will have its rated value, i.e. the belt will be completely filled. In such a case, the average power and the average speed will have their minimum values under the given constraints, therefore, the maximum energy savings will be achieved. When the reference speed reaches its maximum value, i.e. rated speed v r, the belt will continue to convey the material with this speed. The material at the output will follow the shape of the material at the input, and the belt will not be completely filled, as it was before. At the moment when at least one of the conditions, defined in () is not fulfilled, the speed of the material at the output, i.e. v ref (t), will start to change based on (3). In other words, when A in (t) start to decrease, the v ref (t) will also start to decrease. In order to keep the condition A out (t) A r fulfilled, following expressions can be written in accordance with (4) and (3), Ain vr Ar, (6) v ( t ) k ( t t ) ref 2 2 Ain vr Ar ( t2) Ar k ( t t2). (7) 4243 Ain( t2) vr It can be concluded from (6) and (7), that the absolute value for the constant k must be less than the rate of change of input material cross section, i.e. ΔAin k (8) Δt where Δ Ain = Ain ( t2 ) Ain and Δ t = t t2. If the outgoing material A out (t) continues to run with the theoretical belt speed v t (t), i.e. if the sign of equivalence in (8) is applied, the material cross section on the belt will be maximum (A r ), but the electrical braking may be activated in accordance with the Newton's law. Therefore, the constant k value should be determined in accordance with (8) to avoid spillage of material and the activation of electrical braking. While determining the value of constant k, one should know that the reduction of the absolute value of constant k will lead to the reduced potentials for the electrical braking activation, but also, the difference between A out (t) and A r will be increased. Consequently, the belt will not be completely filled and the equivalent energy savings will be lower. When reference speed of BC is the result of the applied algorithm with constant deceleration k = -., the BC drive starts to decelerate always when the condition dv t (t)/ < is fulfilled, consequently reducing the range of BC operation with the theoretical belt speed v t (t). The objective of this research work was to extend the range of BC operation with the theoretical belt speed v t (t) but to avoid activation of electric braking. The best results, obtained in accordance with technical characteristics of the system, are achieved with constant deceleration k =.52 and with the modification of the algorithm shown in Fig. 4 by changing () into (9). dvt.52 and vt( t) ( t) (9) While analyzing the operation of the excavator at the observed ECS system on OPM, it has been noticed that the shape of deposed material on the belt has a sinusoidal component with the period which can be few minutes, depending of the excavator way of operation. The mean value of the period of sinusoidal component is found out to be approximately 4 minutes. This time dependency of the incoming material can be expressed as ( ) (( 2 π / ) ) A t = A ± A sin T t (2) in av sin sin By assigning A sin = ±.25 [p.u.] and A av =.75 [p.u.] in (2), the range of the input material variations is set between.5 and [p.u.], in accordance with the range of speed variations (from 5 to % of rated speed). Results of simulations performed on the model of BC3 [4], for the incoming material as a function of time given with (2) and for different values of period of sinusoidal component T sin, are presented in figures 6, 7 and 8. Small variations of A 3in around the maximum value are due to lagging of actual BC speed to reference speed, which follows the shape of material, with required acceleration and deceleration - without braking. It can be concluded from the presented results that in cases when period of the sinusoidal component of the incoming 4

5 material is equal or greater than 5 min, the observed BC will run with theoretical speed v t (t), i.e. the constant deceleration k = -.52 will not be applied. In cases when period of sinusoidal component is T sin 5 min, the constant deceleration k = -.52 will be applied, when at least one of conditions defined by (9) is not fulfilled. It can also be seen that in cases when sudden discontinuity of incoming material happens, the electric braking of BC drive will not be activated. 5 BC3 speed reference BC3 speed Fig. 6 T sin = 2 min - BC3 characteristic values BC3 speed 4 reference 6 BC3 speed Fig. 7 T sin = 4 min - BC3 characteristic values BC3 speed 4 reference 6 BC3 speed Fig. 8 T sin = 5 min - BC3 characteristic values Additionally, if we observe the case when period of sinusoidal component of incoming material is T sin = 4 min, a comparative analyses can be made between two cases: A) k = -. and B) k = Results of simulations for average power consumption and consumption of energy per cubic meter of transported material for the same period of steady state operation of the BC3 are given in Table. TABLE. Simulation results (steady state operation) case P av [MW] W [kwh/m 3 ] A B Results presented in Table. show that with the modified algorithm for generating reference speed of BCs in the BC system, additional savings of 5.5% can be achieved. This improvement is made regarding two dominant constraints of the system. 5. CONCLUSION The paper presents control strategy applied in the control center of the ECS system on OPM. The system consists of five BCs with adjustable speed induction motor drives. The applied algorithm for generating reference speed of the belt is designed with the goal to reduce energy consumption. The improved algorithm compared to the applied in the control center at the OPM is developed and tested on the detailed mathematical model of the drive system with the rubber belt. The results of simulations indicate additional energy savings. Beside the presented modifications in conventional realization of BC system control, the authors of the paper are considering other approaches to increase energy savings, such as fuzzy logic. The authors of the paper certainly hope that their contribution will reduce the energy consumption and improve the efficiency of the mining process leading. 6. REFERENCES [] B. Bose, "Global Warming: Energy, Environmental Pollution, and the Impact of Power Electronics", Ind. El. Magazine, IEEE, vol. 4, pp. 6-7, 2. [2] W. Daus, et al., "Raw Coal Loading and Belt Conveyer System at the Nochten Opencast Mine", Braunkohle Surface Mining vol. 5, p. 2, 998. [3] B. Jeftenić, et al., "Energy efficiency in transportation of bulk material with frequency controlled drives", EPE- PEMC 2, Ohrid, Macedonia, 2, pp. T [4] B. Jeftenić, et al., "Optimal Utilization of the Bulk Material Transportation System based on Speed Controlled Drives", The XIX Int. Conf. on Electrical Machines ICEM 2, Rome, Italy, 2, pp. -6. [5] F. Petrich, "Optimization of mining technology in the Lusatian mining area", Surface Mining - Braunkohle and Other Minerals, vol. 55, pp. 6-76, 23. [6] Shirong Zhang, Xiaohua Xia, "Modeling and energy efficiency optimization of belt conveyors", Applied Energy, vol. 88, pp , 2. [7] B. Kolonja, D. Ignjatović, B. Jeftenić, "The application of frequency converters for the regulation of belt conveyor drives in surface mining", Transport & Logistics, vol. 5, pp. -27, 23. 5