Performance Optimization of Diesel Generators Using Permanent Magnet Synchronous Generator with Rotating Stator

One of the solutions to reduce fuel consumption of diesel generators (DG) is to adapt the rotational speed to mechanical torque of the crankshaft. When load power decreases, a reduction in both mechanical torque and rotational speed of the diesel engine will maintain the combustion efficiency near the levels of the nominal regime. Accordingly, the generator itself should operate at a variable speed which normally requires power electronics converters. In this paper, we are exploring a new generator concept that uses a stator rotating in opposite direction to the rotor such as the relative velocity between the two components remains constant when diesel engine slows down. The stator itself is driven by a compensator synchronous motor (CM) such as the relative velocity of the rotor is constant, eliminating as such sophisticated power electronics. The model developed for the synchronous machine with a rotating stator is based on Park’s transformation. This new concept was modelled using MATLAB software. Experimental analysis has been conducted using a 500-kW diesel GENSET equipped with a permanent magnet synchronous generator (PMSG). The numerical and experimental results are in good agreement and demonstrate that fuel consumption is reduced with a rotating-mode stator for PMSG during low electrical loads.


Introduction
Regardless of all improvements in renewable energy sustainability, various ap-DOI: 10 [7]. The variation of diesel engine speed results in poor power quality production. Electronic devices and electric apparatuses are sensitive to and affected by poor power quality and high harmonics [8]. Therefore, it is critical to control the DE speed to produce a constant frequency and voltage. Fuel governor is a mechanical device that adjusts the fuel injection to maintain the nominal speed when load power and associated torque vary. However, this controller accuracy is limited, and the adaptation of DE speed is relatively slow during electrical load oscillations [9]. Consequently, the diesel engine runs at fixed speed at all regimes with its best fuel efficiency at nominal power. When DE operates at different regimes than nominal, its efficiency is reduced, sometimes significantly. Especially at low regimes, poor fuel efficiency and high maintenance cost affect significantly DG operation [10] [11] [12]. Specific fuel consumption (SFC) rate for DE is also affected by other operation parameters such as appropriate maintenance, loads and ambient temperature [13]. Therefore, a major inconvenient of operating DE at fixed speed is that its efficiency is optimal at nominal power but decreases sharply at lower regimes [14]. One solution to improve operation efficiency of the DE is to adapt its rotational speed to the load power (torque of the generator) such as to operate at maximum efficiency at all regimes [7].
This means that is necessary to reduce the rotational speed of the DE at lower regime and, consequently, have a diesel generator that can operate at variable speed.
The "classic" solutions to adapt the DE speed to required torque from the generator is to use power converters [15] [16] or variable speed gearbox [12] [17].  [13].
In the paper, we will present the new system, then develop a model for the synchronous machine with a rotating stator based on dq-transformation. Numerical modeling with MATLAB and experimental analysis conducted for a 500-kW diesel generator with rotating stator demonstrate the fuel consumption reduction during low electrical loads.

System Description
The major aim of this project is to reduce diesel engine speed at low regimes to maintain higher fuel consumption efficiency. In the meantime, the synchronous generator needs to rotate at its synchronous speed to produce high-quality power whatever the load. In conventional GENSET system, the diesel engine crankshaft connected to the rotor of the SG runs at 1800 rpm to produce 60 Hz frequency [17].
Inevitably, this research has developed the electrical generator in a way to compensate the ideal speed for itself. In this new SG concept, an innovative rotating-mode stator design has introduced to keep total synchronous speed con- This interesting idea decreases maintenance fee by reducing mechanical stress on DE crankshaft system, extend diesel-engine life and optimize greenhouse gases (GHG) emission profile [21]. Figure 1 illustrates the GENSET schema using non-stationary synchronous stator. One side of SG has coupled with compensator motor and the other side has fixed to DG (crankshaft). In this project CM rotate at anticlockwise direction to compensate the required speed for PMSG.

Rotating-Stator Concept
Based on the explanation before, this research proposes a rotatory stator for a PMSG structure and simultaneously tries to follow the electrical standards limitation regarding harmonics, voltages and currents. In this application, PMSG has developed by a series of separate bearings installed on the external body of the stator end and it was tried to make it rotatory with minimum friction. Regarding conventional SG structure, stator is fixed with machine casing [22]. But in this research both rotor and stator are rotating around the same axis. Moreover, the stator windings and connections remain the same as the conventional one. A pulley has attached at the end of synchronous machine casing and connected to the stator. Therefore, by applying dynamic force on the pulley, stator starts to rotate at our ideal direction and speed. 75-kW synchronous motor (compensator motor), is mounted on the body of 500-kW generator and coupled by the stator pulley using timing-belt. In this study, CM has adjusted to rotate the stator in the reverse direction of rotor rotation (Rotor has already coupled by the crankshaft of DG). The compensator motor is supplied by the PMSG production during low load connection. Thus, the small fraction of GENSET production is consumed by itself (compensator motor). Finally, this technology helps DE to reduce the speed to its ideal regime whereas SG total speed kept constant by the help of CM. GENSET application is one of the major challenges in electric production [23].
Variable speed stator technology for SG, leads this application to achieved synchronous speed without any interference on DE performance or using power converters. In this new structure, both rotor and stator rotate around the same axis but in different directions. Stator speed rotation ( s η ) is in a reverse direction of rotor speed rotation ( r η ) to compensate the necessary speed for producing synchronous speed. Therefore, we can write the primary assumption equation as follow:   necessary for PMSG to produce 60 Hz frequency [16]. Non-Static stator technology operates in a wide range of speed and increases system durability by eliminating complicated mechanical gearbox or electrical converters.

Proposed SG Model Using dq-Projection
Synchronous generators are important in power generation and the need for precise modeling is critical during the time of optimization or development research [24]. The main purpose of investigation on PMSG model using park transformation is to adapt this existing model with rotating-mode stator. Existing synchronous machines in MATLAB power system library propose conven- where ( ϕ ) and ( λ ) are magnetic flux through winding surface and flux leakage.
(P) is permeance, the path of flux (opposite of magnetic resistivity). Thus, once again for calculating stator winding inductances in a specific position and time and by simplifying equations ( aa L ) and ( ab L ) could evaluate as (4).
Three-phase stator winding made s L elements 3 × 3. In a stator inductance matrix, all diagonal elements are belonging to the self-mutual inductances and leakage plus mutual. On the other hand, off-diagonal inductances are mutual inductances between two different sets of windings. They are negative due to 120-degree phase difference between each axis and regarding flux effect of one winding into another [24]. Back to equation (2), all matric elements of inductances calculated from (3) and (4). In this step, all parameters like voltage, current, flux and the inductance of the winding leakage and their resistance should refer to the stator side. Considering the number of stator ( s N ) and rotor ( r N ) windings, Equation (5) illustrate the transformer equations.   (6) all calculations transferred to the stator side, but these equations are getting more complex since all phases quantities and matrix inductances are not independent from each other and they are variable with time. dq0 transformation (park transformation) is a projection to simplify the equations by transforming stator quantities from stationary reference frame to rotating dq0 reference frame, [24]. This study has used dq0 transformation that rotates at synchronous speed just for stator components because rotor components are already located at reference frame. Equation (7) To avoid unwilling harmonics from PMSG production, providing constant 60 Hz frequency (1800 rpm) for SG is necessary. Rotating stator technology aims to achieve this speed by combining rotor and stator speed either in the same or opposite direction. Since SG rotor speed, reduced due to adapt DE speed with demanded load, another concept introduced for SG stator to rotate around the rotor axis but in reverse direction to compensate the total speed. Figure 4 indicates the developed stator structure for SG.
SG modeling carries on by presenting two independent mechanical inputs. In this simulation for 500-kW PMSG, the stator speed has fixed to rotate at 255 rpm in opposite direction of rotor rotation. Thus, in this case the rotor speed could decrease to 1545 rpm which helps directly the DE crankshaft to reduce its speed. However, the total speed of synchronous machine has remained constant at 1800 rpm. Finally, by transferring the above equations to the reference frame using dq0 transformation, and by simplifying the calculations, voltage equations in the reference frame are calculated as (8). The first part is related to the ohmic voltage, the second part is related to the total speed rotating voltages which include two different inputs, and the third part is related to the transformer voltage.
By expanding the dq matrix calculation, the synchronous equation release as (9).
In addition, the values of leakage flux are also calculated by expanding matric As a final step before deploying these equations into MATLAB block diagrams, flux leakages and currents are illustrated as (11). ( ref w ) is the base electrical angular velocity to obtain primary inductive reactance during system simulation.  Figure 5 indicates a schematic view of GENSET system using rotatory stator.

Numerical Modeling for Developed GENSET
Synchronous machine simulation starts with dq-transformation and continues by adding internal excitation and mechanical governor system into it. Excitation system is important in synchronous machine due to production of flux by passing current through rotor field winding. The aim of As for total mechanical torque ( mt T ) of synchronous machine, dual PI controller system has used to reduce the errors for total angular velocity ( rt w ). The aim of first controller is to track DE and CM speed and reduce the possible errors by considering reference speed. DE mechanical torque out-put may vary to reach its appropriate regime. Thus, the second PI controller follows registered command for CM to rotate it at the speed which leads PMSG to reach 1800 rpm.
However, in this simulation DE speed is fixed at 1545 rpm and rotates in a clockwise direction. In continue, this simulation introduces another independent in-put to compensate the require speed for SG (synchronous motor (CM)).
CM speed is valued at 255 rpm and rotates in the anticlockwise direction.
Therefore, developing governor system simulation to have a compound speed, leads SG to control total angular velocity either with CM or DE. Synchronous machine simulation carried out by projecting a-b-c three phase onto the d-q axis.
One feature of dq-projection is the ability to specify the d-q axis speed to be any that is convenient for the user [24]. Two separate dq0 block diagrams for current and voltage are placed after dq-transformation block to produce three phase power. It is important to mention here that three phase loads are consist of purely resistive panel plus CM. Small fraction of SG production consumes by CM itself. Figure 6 indicates the conditions on which the governor system produce appropriate m T . A conditional switch with three in-puts, placed to meet different regimes. First and third in-puts are for both rotor and stator speed while second in-put fixed at 1800 rpm. The function of this switch is based on total system speed.
If system speed is less than 1800 rpm then, logic switch passes third input to compensate the speed. Otherwise, switch block passes first input to reduce total SG speed. Figure 7 shows swing equations for SG, while system work under normal condition. Therefore, the relative position of rotor and magnetic axis is fixed. In full order modeling of SG, internal excitation produces fluxes (9) and (10) in rotor windings respectively [22]. Therefore, fluxes cross stator windings to produce currents. Figure 8 shows field and quadrature current using dq0 transformation. By implementing park transformation in a balanced system, field current ( d i ) and quadrature current ( q i ) carry the same performance and characteristic of three-phase SG system during different conditions.

Simulation Results
Modeling and simulation of PMSG using Non-Static stator have been carried out. Two different mechanical inputs are implemented. One input control the rotor speed and the other one, adjust stator speed. This modeling adds both values (mechanical inputs) and considers them as one for a PMSG mechanical prime mover. Results below released from MATLAB software using per unit system showing the three-phase voltages, currents, power, etc. Two different stage of loads applied in this project respectively. Initial speed of machine has fixed at 1500 rpm, as reference speed. The total speed of PMSG at rpm form is the summation of two mechanical inputs. Regarding the value of inductances in previous section, ( s L ) change as the rotor rotates (salient rotor). Therefore, the stator inductance values change by time. For the first scenario, the generator has connected to the 196-kW load power and the results are released in Figure 9 and

Experimental Setup
This technology tested using developed 500 kw Caterpillar diesel generator and all measurements are extracted from the precise three-phase electric instrument and electronic weighing scale.   Figure 13. Developed GENSET using compound speed.  Figure 14 and Figure 15 by connecting PMSG into 196-kW and 307-kW loads respectively. It is important to mention that the compensator speed has kept constant during experiment. No sophisticated gearbox or power converter has used regarding electric outputs treatment.

Results
For the second scenario of this project, CM speed kept constant but resistive load increased. Figure 15 indicates the graph and the parameters of developed PMSG while connecting to the 307-kW load. Frequency and harmonics have shown a very small change by comparing with 196-KW load. AVR system increases the internal excitation of the rotor winding to compensate the load augmentation. Therefore, system voltage kept almost constant as the load increased.

Experimental Validation
The proposed section is the experimental validation using fixed and Non-static stator mode for PMSG. Modeling and experimental results are compared in Table 3. The goal of this part is to evaluate the impact of the non-static stator on the PMSG out-put. However, it is important to show the optimization of DE consumption using rotatory stator technology. This optimization has achieved while PMSG products, kept at its standard range. Voltage, frequency and other PMSG results are recorded while the prototype ran in its steady state condition.

Fuel Measurement
The objective of this study brightly shines in this part as the fuel consumption illustrates significant improvement during low loads connection. Figure 16 indicates the fuel optimization of developed GENSET. Fuel rates are considered in liter per hour. By comparing bar charts, the amount of fuel saved for the first load (196-kW) is larger than the second load (307-kW). However, new stator concept has used in both scenarios. During load augmentation, more current absorbed from demand side. Thus, AVR system applied more voltage in rotor winding to meet the energy demand. Whereas, this condition put more stress on DE crankshaft. Inevitably DE governor open fuel hatch more than before to increase the power. Finally, DE crankshaft can rotate the generator rotor at the same speed as it rotates for the first load (mechanical torque increased).

Fuel Efficiency
This research has proved significant fuel saving by rotating DE at its most efficient speed during low loads connection. Figure 17 approximates the fuel consumption of 500-kW GENSET based on the electrical load at which the generator is connecting to. This experiment was repeated twice for a developed PMSG by connecting 196-kW and 307-kW RL-load into it. By the data released from Figure 16, fuel saving is more significant while developed PMSG, connected to 196-kW-RL-Load. Therefore, fuel saving during low load connection for 500-kW generator is achieved by decreasing mechanical torque on DE crankshaft. It is important to mention that investigation on real fuel saving is always depends on the load impact for an application. This technology could also reduce fuel consumption in another way by replacing smaller diesel engine rather than high capacity DE for the same electric demand. Smaller diesel engine could apply for these kinds of applications due to reduction of mechanical load.

Conclusion
Developed PMSG stator using compensator motor is proposed. The principle, simulation and structure of 500-kW GENSET are discussed in detail. This study proves DE fuel optimization, by developing PMSG structure based on rotating mode-stator. Moreover, this development does not have significant negative consequences on the electrical output. This system shows good dynamic durability due to elimination of sophisticated gearbox, high reliability without using power converters, reducing greenhouse gases by optimizing fuel consumption and high steady-state performance.