^{1}

^{*}

^{1}

^{2}

^{1}

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 re duction 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 lo w electrical loads.

Regardless of all improvements in renewable energy sustainability, various applications and communities are still dependent on diesel generators (DG) to meet their energy demand. Durability and reliability of DG made this application popular compared to other power sources in remote sites, with electric load oscillations and/or facing difficult weather conditions [

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 [

CVT provides adequate mechanical torque for electrical generator while it is transferring rotational energy from diesel engine. In this paper, we explore a new solution to operate the diesel generator at variable speed using a synchronous generator with a rotating stator [

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.

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 [

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 constant while DE speed reduces to its ideal regime. The new idea has focused on the stator part of PMSG. Thus, obtaining nominal speed for the electrical generator is not limited to rotor speed anymore. Synchronous speed could achieve by controlling the compound speed (Rotor and Stator Speed). If rotor speed reduces due to an ideal mechanical torque from DE, adjustable stator speed could compensate the ideal speed for SG.

Synchronous speed for SG achieves by rotating stator part in a reverse direction of rotor direction using a synchronous motor (compensator motor) installed above the PMSG casing. This method optimizes DE performance by reducing the speed to its nominal regime. Moreover, reducing fuel consumption of DE could also achieve by controlling DE crankshaft power, which mostly connected directly to the rotor of PMSG.

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 [

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 [

η s y n c ( T o t a l ) = η R o t o r + η S t a t o r

Harmonic treatment and low power quality compensation need sophisticated power converters in case of variable speed production. Accordingly, 1800 rpm is

necessary for PMSG to produce 60 Hz frequency [

Synchronous generators are important in power generation and the need for precise modeling is critical during the time of optimization or development research [

{ V a b c s = r s i a b c s + p λ a b c s V q d r = r r i q d r + p λ q d r (1)

Index (s) and (r) are the stator and rotor parameters. r s And r r are the diagonal matrices for a linear magnetic system. To facilitate the calculations of salient rotor synchronous machine, rotor equations transferred to the stator side to keep both machine parts in a same reference frame [

[ λ a b c s λ q d r ] = [ L s L s r L s r T L r ] [ i a b c s i q d r ] (2)

( L s ) Inductance between the stator windings, ( L r ) inductance between the rotor windings and ( L s r ) is the mutual inductance between the stator windings and the rotor windings. It is important to consider the stator windings flux variable by time due to the dynamic style of salient rotor machine.

Stator inductance varies with time while salient rotor type rotating around its axis. In (3) primary flux inductance equations have expanded.

{ λ a a = N a φ a λ a a = N a ( φ q cos θ r + φ d sin θ r ) φ q = F a P q cos θ r , φ d = F a P d sin θ r λ a a = N a F a [ P q cos 2 θ r + P d sin 2 θ r ] (3)

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 ( L a a ) and ( L a b ) could evaluate as (4).

{ L a a = N a 2 [ P d + P q 2 − ( P d − P q 2 ) cos 2 θ r ] L b a = N a 2 [ − P d + P q 4 − ( P d − P q 2 ) cos ( 2 θ r − 2 π 3 ) ] L A = N a 2 ( P d + P q 2 ) , L B = N a 2 ( P d − P q 2 ) (4)

Three-phase stator winding made L s 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 [

{ i ′ j = 2 3 ( N j N s ) i j v ′ j = ( N s N J ) v j λ ′ j = ( N s N J ) λ j r ′ j = 3 2 ( N s N j ) i j i ′ j = 3 2 ( N s N j ) i j j = K q 1 , K q 2 , F d , K d Fourwindingsassumed ontherotoraxis (5)

In summation, by replacing all calculated elements for system inductances and rewriting the primary matrix equation, voltage equation obtains as (6).

[ V a b c s V ′ q d r ] = [ r s + p L s p L ′ s r 2 3 p ( L ′ s r ) T r r + p L ′ r ] [ i a b c s i ′ q d r ] (6)

From Equation (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, [

K s r = 2 3 [ cos θ r cos ( θ r − 2π 3 ) cos ( θ r + 2π 3 ) sin θ r sin ( θ r − 2π 3 ) sin ( θ r + 2π 3 ) 1 2 1 2 1 2 ] (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.

SG modeling carries on by presenting two independent mechanical inputs. ( w r r ) which assumed as rotor speed and ( w r s ) which assumed as stator speed are two independent angular velocities for PMSG. ( w r t ) is total angular velocity which leads SG speed to achieve ideal frequency.

w r t = w r r + w r s − StatorandRotorRotateintheSameDirection + StatorandRotorRotateintheReverseDirection _{ }

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.

{ V q d 0 s r = r s i q d 0 s r + w r t λ d q s r + p λ q d 0 s r V ′ q d r r = r ′ r i ′ q d r r + p λ ′ q d r r ( λ d q s r ) T = [ λ d s r − λ q s r 0 ] (8)

By expanding the dq matrix calculation, the synchronous equation release as (9).

{ v q s r = r s i q s r + w r T λ d s r + ρ λ q s r v d s r = r s i d s r − w T λ q s r + ρ λ d s r v 0 s = r s i 0 s + ρ λ 0 s v ′ k q 1 r = r ′ k q 1 i ′ k q 1 r + ρ λ ′ k q 1 r v ′ k q 2 r = r ′ k q 2 i ′ k q 2 r + ρ λ ′ k q 2 r v ′ f d r = r ′ f d i ′ f d r + ρ λ ′ f d r v ′ k d r = r ′ k d i ′ k d r + ρ λ ′ k d r (9)

In addition, the values of leakage flux are also calculated by expanding matric [ λ q d 0 s r V ′ q d r r ] as (10).

{ λ q s r = L l s i q s r + L m q ( i q s r + i ′ k q 1 r + i ′ k q 2 r ) λ d s r = L l s i d s r + L m d ( i d s r + i ′ f d r + i ′ k d r ) λ 0 s = L l s i 0 s λ ′ k q 1 r = L ′ l k q 1 i ′ k q 1 r + L m q ( i q s r + i ′ k q 1 r + i ′ k q 2 r ) λ ′ k q 2 r = L ′ l k q 2 i ′ k q 2 r + L m q ( i q s r + i ′ k q 1 r + i ′ k q 2 r ) λ ′ f d r = L ′ l f d i ′ f d r + L m d ( i d s r + i ′ f d r + i ′ k d r ) λ ′ k d r = L ′ l k d i ′ k d r + L m d ( i d s r + i ′ f d r + i ′ k d r ) (10)

As a final step before deploying these equations into MATLAB block diagrams, flux leakages and currents are illustrated as (11). ( w r e f ) is the base electrical angular velocity to obtain primary inductive reactance during system simulation.

T e = ( 3 2 ) ( P 2 ) ( λ d s r i q s r − λ q s r i d s r ) (11)

This research has used automatic voltage regulation (AVR) to control the output terminal voltage of the alternator while it is connected to the loads. The principle of this excitation system is to follow the reference flux and adjust itself with it to avoid any voltage drop during load variation. However, the objective of simulation is to develop mechanical torque and governor system, where two different mechanical inputs are fixed to illustrate the effect of rotor and stator rotation.

As for total mechanical torque ( T m t ) of synchronous machine, dual PI controller system has used to reduce the errors for total angular velocity ( w r t ). 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 [

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.

In full order modeling of SG, internal excitation produces fluxes (9) and (10) in rotor windings respectively [

Variables | Nominal Values |
---|---|

x d | 1.79 Ω |

x d d | 0.169 Ω |

x d d d | 0.135 Ω |

T d 0 d | 2 |

T d 0 d d | 2.3 |

x q | 1.71 Ω |

x q d | 0.228 Ω |

x q d d | 0.2 Ω |

T q 0 d | 3 |

T q 0 d d | 3.3 |

w B | 50 (HZ)*2*pi |

w r r | 51.5 (HZ)*2*pi |

w r c | 8.5 (HZ)*2pi |

H 1 | 2 |

H 2 | 0.86 |

R a | 0.01 Ω |

P g 0 P g 1 | 0.625 W 0.617 W |

U g 0 | 0.884 V |

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, ( L s ) 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

Simulation of PMSG with developed governor accomplished using dq-projection. This projection reduces three-phase complicated equation into dq0-rotating axis. These difficulties are due to the mutual inductances of Rotor-Stator and variation of inductances by time. MATLAB dq0 to abc block diagrams have used to convert dq variables into three-phase voltages and currents.

This model well represents PMSG performance especially during balance load. ( i 0 ) in dq0 matrix placed for unbalanced condition. Constant k q , k d and k 0 are valued as 2/3, 2/3 and 1/3 respectively to alleviate the numerical coefficient of dq0 matrix. This model controls the PMSG output using its internal excitation system. AVR system adapts required magnetic field in rotor field winding during load fluctuation. For the second part of this research, 307-kW load has connected to the PMSG. Rotor and stator speeds were kept constant at the same speed of the first scenario. Load current increased however; it is important to mention that voltage remains constant.

Full order model of synchronous machine compensates the transient time of SG behavior by neglecting the effect of stator inductive current on itself. U d d d and U q d d are the stator damper winding voltages to repel the stator negative effect on itself and the effect of load variation on SG production.

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.

Variables | Nominal Values | Variables | Nominal Values |
---|---|---|---|

Diesel Engine | Generator | ||

Cooling | Radiator Package | Voltage L-L | 480 V |

Fuel | 3 Stage Filter | Power | 50 KW |

Governor Type | Programmable Electronic Engine Control | Apparent Power Compensator Motor | 625 KVA |

Starting-Charging | Battery Charger | Stator Speed (Timing Belt) | 0 - 255 rpm |

Generator | Output HP | 100 HP | |

Excitation | Self-Excitation | Frequency | 60 Hz |

Number of Poles | 4 | Motor Speed | 1785 rpm |

Frequency | 60 Hz | Number of Poles | 4 |

RPM | 1800 - 1545 | Starting Method | Inverter |

Over Speed Capability | 150 | Power Termination | |

Alignment | Pilot Shaft | Bus Bar Circuit Breaker | |

Voltage Regulation | 1/2% Steady State 1% (No load to Full load) | Control Panel | Digital O/I Module Load Share Module |

Results below are the PMSG electrical outputs while using rotatory stator. This technology helps DE crankshaft to be more flexible during low electric load. DE speed decreased however, PMSG outputs using rotatory stator are near to the conventional one. Three-phase Fluke multimeter power quality and analyzer device have used to measure PMSG outputs. This technology is tested, and results are indicating in

For the second scenario of this project, CM speed kept constant but resistive load increased.

The proposed section is the experimental validation using fixed and Non-static stator mode for PMSG. Modeling and experimental results are compared in

The objective of this study brightly shines in this part as the fuel consumption illustrates significant improvement during low loads connection.

This research has proved significant fuel saving by rotating DE at its most efficient speed during low loads connection.

A = Analysis E = Experiment Load 307 KW | E r r o r | E r r o r | ||||
---|---|---|---|---|---|---|

A | E | % | A | E | % | |

Voltage Harmonic (%) | 5.9 | 6.2 | 5.08 | 8.1 | 7.6 | 6.17 |

Current Harmonic (%) | 1.41 | 1.5 | 6.38 | 1.27 | 1.3 | 2.36 |

Frequency (HZ) | 60 | 59.8 | 0.3 | 60 | 60.87 | 1.45 |

P. F | 0.96 | 0.98 | 2.08 | 0.96 | 0.98 | 2.08 |

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.

The authors declare no conflicts of interest regarding the publication of this paper.

Mobarra, M., Issa, M., Rezkallah, M. and Ilinca, A. (2019) Performance Optimization of Diesel Generators Using Permanent Magnet Synchronous Generator with Rotating Stator. Energy and Power Engineering, 11, 259-282. https://doi.org/10.4236/epe.2019.117017