Variable Speed Integrated Generating Set an

Paper accepted for presentation at 2003 IEEE Bologna Power Tech Conference, June 23th-26th, Bologna, Italy

Variable Speed Integrated Generating Set an Emerging Technology for Distributed Power Generation Wlodzimierz Koczara, Warsaw University of Technology, Poland Nazar Al-Khayat, Robert Seliga, Jawad Al-Tayie, NEWAGE-AVKSEG, Stamford Lincolnshire UK Abstract--A variable speed integrated generating set consists of diesel engine, axial flux permanent magnet generator, and power electronic converter. DSP Sharc processor controls output threephase sinusoidal voltage and adjusts speed according actual load. Load tests of 15 and 40 kVA units confirm high quality of output voltage for a case of symmetrical and non symmetrical loads.

The radial gap machine has magnets positioned around the circumference of a cylinder. Sometimes the cylinder is long and has a small diameter (rod-shaped), and sometimes the cylinder is short and has a large diameter. For variable generation system, power electronic converter is used to convert variable-frequency variable-voltage output Index terms--Variable speed, Variable Speed Integrated Genset to a constant voltage constant frequency output [1] - [8]. This (VSIG), distributed generation, DSP processor, permanent paper will discuss the axial flux generator and power electronics converter topology in details. Emphasis will be magnet generator, diesel engine, power electronic converters. given to the recently developed Variable Speed Integrated Genset (VSIG).

I. INTRODUCTION II. SYSTEM DESCRIPTION Distributed generation is a modern tendency to decrease costs of energy supply and to improve the quality of the output voltage. Conventional power generating sets producing 50 Hz (or 60 Hz) ac voltage consists of reciprocating diesel engine and synchronous wound field generator. Above 5kVA, the most popular prime mover is a diesel engine. A great deal of diesel engine innovation and new technology application are currently emerging in the automotive diesel sector. This innovation leading to a desirable attributes for gen-sets that are smaller, lighter, more efficient and have lower acoustic, vibrational, and thermal output. Light-duty (car and small truck) transportation diesel engines are usually operated at relatively high allowable rotational speeds (3600– 5000 rpm) for at least short time periods, often utilize sophisticated controls, are designed for high power-to-weight ratio, and employ turbochargers. Competition in the automotive and light truck market (60–250 hp) coupled with increasingly stringent environmental regulations being applied to all sizes of diesel-powered vehicles [11]. Such developments in the automotive diesel sector include a number of technological advances that currently being applied to diesel engines used for industrial applications, including engines that power portable generators. The prime mover of the gen-set is directly coupled to the electrical generator. Possibilities for this alternator include an ac induction machine, a permanent magnet (PM) brushless machine, and a switched reluctance (SR) brushless machine. A PM generator using high-flux rare earth permanent magnets has a specific power approximately three times that of the induction motor and 33% higher than the SR generator. Axial gap machines generally take the form of a “pancake” design with a rotor that contains the PMs sandwiched between two stators that contain coils of wire, which would produce voltage and currents when the rotor is mechanically driven.

0-7803-7967-5/03/$17.00 ©2003 IEEE

The overall block diagram of overall variable speed (VS) power generation system is shown in Fig. 1. A permanent magnet generator PMG, driven by a prime mover En, is producing variable frequency and amplitude ac voltage which is feeding power electronic converter IN. The converter IN produces constant frequency three-phase sinusoidal voltage, which supplies a load L. A system controller SC is used to control and monitor the VS power generating set. The output voltage and load demand set a reference speed of the engine. When no load is applied then the engine En operates with minimum speed set by signal MS. When load increases then the power calculation unit PC is producing reference speed signal EC that orders to adequate speed assuring a balance between power demand and power delivered by the engine. There are three independent phase voltage outputs U-N, V-N, W-N for single phase loads and three phase output (line to line voltage) U-V, V-W, W-U.

III. THE ENGINE OPERATION The engine En, is producing torque Ten that results in rotational speed (S). The power P en, delivered to the permanent magnet generator PMG, by the engine En, is proportional to the engine torque T en and the speed (S). When the speed (S) is expressed in rpm and torque in N·m then power Pen = Ten S (π/30)

(1)

The selection of the engine is very important in VSIG set. The torque – speed characteristics and inertia of the system all play a critical role in system response and performance.

Sp n

En Engine

V

IN AC/AC

PMG Generator

Converter

L W

Load

N

Pen4

Pen3 MSFC

Pen2

MS EC Engine control

Engine power Pen

U

Pen5

Pen1 SC AC/AC

DSP SHARK & FPGA

Spadmin

controller

Spconst 50Hz

Spadmax

Engine speed (rad/s) Fig. 3. Power of Diesel engine as a function of speed. PC

whereas for constant conventional speed genset the maximum available power

Power calculation and reference speed setting

Power reference

Pconsmax = Pen3 = Spconst Ten3

Fig. 1 Block diagram of overall variable speed power generation system.

Typically diesel engine torque curve is flat over a wide speed range (Fig. 2).

Engine torque Ten (Nm)

T en3

T en2

T en4 MSFC

T en5

T en1 Spadmin

Sp const 50Hz

Sp admax

Engine speed (rad/s)

Fig. 2. Torque of Diesel engine as a function of speed.

For the maximum speed Spadmax the available power (Fig. 3) is equal to Penmax = Spadmax Ten4

(3)

Hence, comparing to conventional operation with constant speed Spconst we gain more power form the engine in ratio kp = (Spadmax Ten4)/(Spconst Ten3)

(4)

The most frequently applied speed of conventional gensets is 1500 rpm. Industrial engines have a limited speed range and performance optimised (for fuel consumption) around the synchronous speed of the genset. On the other hand in automotive diesel engines have wide speed range but limited durability for continuous high speed mode of operation. For distributed generation long life cycle is important. In variable speed system for a low load demand, the engine speed is close to ideal and engine torque is limited to T en2. For further increase in load the response along load torque will change from Ten2 through Ten3 to Ten4 matching the load demand to minimum fuelling consumption. VSIG method of operation is ideal for varying load. Majority of load profiles have transients peaks of power representative of motor starting or in rush conditions. The maximum power demand is short and results in higher engine speed mainly governed by the system dynamics. The average load cycle has a mean significantly below the maximum rating of the genset. Therefore, high speed mode of operation is short and can be argued as it will not impact the longevity of the engine.

IV. THE AXIAL FLUX PERMANENT MAGNET GENERATOR

(2) The axial flux generator (“torus”) [5]-[8], dedicated to the variable speed system, is a very simple machine and consists of two external rotor discs and stator placed between the two discs.

On the rotor plates are fixed permanent magnets. A simplified flux paths, through generator, is shown in Fig. 4.

and produces fixed frequency and amplitude high quality output voltage. The converter is designed to operate in autonomous or grid connection mode. A topology of the ac/ac converter, operating autonomously is shown in Fig. 6.

R O T O R D IS C S

Rep AC/DCp

S

Udc1

S

IN-U

IN-W

IN-V

U

V

W

En PMG N

N

Ren AC/DCn

S

S

Udc2

N

STATOR

Fig. 4. Simplified diagram of flux paths through axial flux generator.

RADIATOR AND FAN

POWER ELECTRONIC CONVERTER GENERATOR

En Engine

P M G

PEC

FRAME

Fig. 5. Construction of VSIG set.

The axial flux permanent magnet generator (using high power rare earth magnets) is very short, light in weight and efficient machine when compared to conventional wound field machines. Reduction in weight about 3 - 10 times than conventional generators Designed by NEWAGE-AVKSEG and coined as VSIG generator, the current range is 5 to 100kVA. The VSIG generator is directly coupled to the engine crank shaft and replaces the engine flywheel. Fig. 5 shows a schematic layout of the system.

V. POWER ELECTRONIC CONVERTER The power electronic converter is fed from the permanent magnet generator by variable amplitude and frequency voltage

PMG rectified voltage and dc bus voltage

Fig. 6. Topology of the 3 phase 4 wires AC/AC converter.

Vdc

V4

V1

Sp adm in

Engine speed Sp [rad/s] Sp

admax

Fig.7. Generator rectified voltage and dc bus voltage.

The converter produces three phase sinusoidal voltage to supply both three phase and single phase loads. Moreover, a full asymmetry of phase load is possible. The neutral point is produced by two rectifiers-boosters Rep and the Ren. Rep rectifiers the generator voltage and boosts it to the demanded Vdc1 positive voltage in reference to neutral point N. The Ren produces in similar way a negative voltage Vdc2. Fig. 7 shows the boosting operation of the Rep and the Ren as a function of the generator speed. The rectified voltage V1 – V2 is boosted to fixed dc voltage V dc. The independent control of the positive and negative voltage permits to keep symmetry positive and negative voltage what is needed when single phase or asymmetry of load results in different (in time) load for negative and positive source of voltage. Three independent inverters IN-U, IN-V and IN-W provide three independent sinusoidal voltages U-N, V-N, W-N. Such topology assures decoupling of interaction of one phase load on the other. Delivered, high quality sinusoidal voltage, fulfil the very severe requirements of voltage symmetry. In case when a source supplies single and three-phase loads,

especially when single phase loads are asymmetrical and nonlinear, there is asymmetry in line-to-line voltage and sum of the single phase is not equal to zero. Hence, in proposed independent control such asymmetry is significantly reduced. Such option is not possible in conventional fixed speed synchronous generators with controlled field. The stiff neutral point, and three simple dc/ac converters are important advantages of the proposed power electronic topology.

Figure 8, shows the system output for unbalanced conditions, current waveform on phase U and V. First phase is loaded by 4.5 kW, second 2.4 kW but third has no load. All three phase voltages are fully symmetrical and not distorted in spite of 100% of load asymmetry.

Rectified generator current

Output voltage

VI. DSP CONTROL There are three main functions of the DSP system (Fig. 1). First is the control of the ac/ac converter, second to produce reference speed signal and third – monitoring, diagnosis and external communication. Control of the ac/ac converter is related to control: generator current, dc link voltage, three ac output voltages and currents. The digital controller is built using 32 bit floating point processor and gates programmable FPGA unit. The control concept of the power system is to adjust speed and load torque to power demand [2]. The variable speed system has to combine high quality power source and fuel economy especially in island operation. This is a main difference to variable speed generation system powered by wind. Usually wind turbine driven generator systems, connected to grid, operate on principle to gain maximum power but quality of energy is assured by high power grid. DC link voltage

VII. EXPERIMENTAL RESULTS A 19 and 40kVA VSIG sets were built and tested by NEWAGE-AVKSEG. A durability testing of more than 1500 hour, with different types of loads, confirms high reliability of the system and high quality of output voltage. Fig. 8 shows an ascilogram of three phase voltage waveform and load currents of 19kVA genset. V o lta g es 1 0 0 V /d iv

Speed

Fig. 9 Transient state conditions of the 40 kVA genset during step load from 7 to 19 kW.

The response of a 40 kVA VSIG genset is shown in Fig. 9. The oscilogram shows transients produced by step load from 7 to 19 kW. The speed increases and output ac voltage after short time recovers its amplitude. A rectified generator current reaches, for short time, its maximum value and when dc voltage recovers to its reference value the rectified current drops to a newer value.

C u r r e n ts 2 0 A /d iv

VIII. SUMMARY

Fig. 8. Three-phase voltages and non-symmetrical load current of the 19 kVA genset.

The VSIG set has many advantages over non-integrated synchronous speed conventional system. VSIG set is lighter in weight, smaller in size, lower in cost of ownership and mechanically and electronically integrated for optimum system response. The use of light weight axial flux PM generator contribute significantly to improvement in power density and system dynamics. Power electronic converter developed for VSIG is a robust controller for wide speed range. Engine speed is adjusted by measurement of load current at the output of the inverter and the engine torque is monitored by the current in the dc bus. Output voltage is independent of engine speed and can be tuned for tight regulation at any output frequency.

VI REFERENCES 1.

2.

3.

4.

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6.

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