single-phase universal motor being connected to the VSI output circuit. Even though the requested starting current is quite high (purple trace), the output phase ...
2nd IEEE ENERGYCON Conference & Exhibition, 2012 / Advances in Energy Conversion Symp
Variable Speed Generating Unit for Stand-Alone Microgrids Marco Lega, Giovanni Lo Calzo, Alessandro Lidozzi, Luca Solero, Fabio Crescimbini University of Roma Tre, Mech. and Industrial Eng. Dept., Via della Vasca Navale, 79 00146 Roma, Italy Abstract—Variable speed operation has been proposed in literature as a solution to efficiency and pollution problems for diesel generating units, as well to improve efficiency and power conversion in rotating systems for renewable energy sources. In general, different topologies of prime-movers, like internal combustion engines, wind turbines and hydro turbines, etc. can be used to move an electric generator at variable speed. In variable speed generating systems, the electric output frequency is disengaged from the mover speed through a power electronic converter. This paper deals with a newly-conceived integralresonant single-loop output voltage control arrangement for a three-phase four-leg VSI, to be used in stand-alone microgrids, for a variable speed generating units using static conversion to provide the constant electric frequency required by the loads. The control structure is discussed and experimentally verified by means of a 25 kVA rated power converter prototype. It is shown that the proposed voltage control allows simplifying the overall control configuration, achieving zero-overhead power factor regulation and fast compensation of load unbalances. Further to that, a complete current limiting strategy is described and tested under different load conditions. Index Terms—Variable speed generating unit, Power Electronic Converters, Four-Leg Inverter, Boost Rectifier, Permanent Magnet Synchronous Generator.
I.
INTRODUCTION
Electrical systems with different aggregation level that have at least one distributed energy resource and associated loads are defined as microgrids when are intended to form intentional islands in the electrical distribution systems. Microgrids are receiving a considerable interest in view of their business and technical structure as a mean of taking full advantage of distributed generation. Within microgrids, loads and energy sources can be, in principle, disconnected from and reconnected to the area or local electric power system with minimal disturbance to the local loads. Any time a microgrid is implemented in an electrical distribution system, it needs to be properly designed to avoid causing problems. There are many places, especially in developing countries, where about 33% of the world's population [1-2] do not still have access to electricity. These regions could be electrified by either extending the grids of the existing power systems or constructing isolated new power systems, being suitably tailored on alternative energy sources. In this case, the isolated power systems are known as standalone microgrids
978-1-4673-1454-1/12/$31.00 © 2012 IEEE
[3]. In general, the extension of the existing grids it is to be preferred; however, the most of the non-electrified regions in developing countries are located in remote and difficult areas, like hilly regions, forests, deserts and islands, which demand huge investments for grid extension. In addition to this, grid extensions are mainly distance dependent whereas hybrid generating systems are generation capacity dependent, hence the break-even distance is related to the generating power demand [4]. For remote places where electricity has not reached yet, it is recommended that decentralized generation, which is mainly based on renewable energy technologies, is perhaps the only efficient way to provide electricity in a modern perspective. Conventional energy sources as diesel gensets are also proposed to provide electric power in remote areas. However, for the same reasons that make difficult the extension of conventional grids, fuel transportation as well maintenance and parts replacement needed for diesel generating units in remote areas are not cost effective. As a consequence, renewable energy sources (RES) are suggested to integrate the conventional energy sources in order to constitute the basis for the electric power generation. The energy for RES is locally available and properly chosen RES can considerably reduce the operating periods of the conventional power sources which result in reduction of the fuel consumption and minimizing the need of maintenance. This in turn results in the improvement of sustainable power generation. In addition, diesel gensets as conventional energy sources are mostly used at fraction of the generating rated power [5]. When gensets are not used at their full load, they are less efficiently utilized and contribute to the polluting role of the environment. Also, increased maintenance is required due to engine fouling, especially in cold ambient conditions. Variable speed operation has been proposed in literature as a solution to efficiency and pollution problems for diesel generating units, as well to improve efficiency and power conversion in rotating system for RES. In general different kinds of prime-mover, like internal combustion engines (ICE), wind turbines and hydro turbines, etc. can be used to move an electric generator at variable speed. In fact, in variable speed generating systems, the electric output frequency is disengaged from the mover speed through a power electronic converter. Variable speed operation has the potential to enhance the overall efficiency of the prime mover that can now work in order to improve the power extraction at the required torque for the loads. As
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an example, Fig. 1 shows experimental results on fuel saving benefits when a variable speed diesel-electric unit with power electronics and permanent magnet (PM) synchronous generator is used instead of a fixed-speed diesel engine with rotor field winding synchronous generator.
single-phase inverters with the fourth inverter leg being devoted to control the neutral voltage. A carrier based modulation scheme is selected in the experimental section of the paper, according to the work described in reference [8], being such a modulation arrangement very simple to be implemented in both continuous and discontinuous modes of operation. With reference to the system of Fig. 2, the operation and specifications for the three-phase active rectifier converter are reviewed in Section II. Section III is devoted to explain voltage and current controllers for the three-phase four-leg VSI. Simulations and experimental results are provided in Section IV to verify the performance of the designed system. Section V offers conclusions drawn from this work. Output Inverter ( VSI)
Prime Mover
Fig. 1. Fixed-speed vs variable-speed operation for diesel-electric units.
Results show that variable speed systems are most applicable where the peak power system capability is large in comparison to the required average load. In typical existing isolated power systems, fuel savings could be as high as 40% of annual fuel consumption. The benefits in adopting the variable speed solution have been already discussed in several papers [5-7], where some possible configurations for the power electronics are also shown. Efficiency improving, as well overall weight and dimensions reduction in electric power supply units, address to PM synchronous generators as well to engine direct coupling solutions. In fact, the superior performances (i.e. high efficiency, low drop voltage, etc.) in generation mode of operation of PMSM versus other ac machines are largely recognized. Fig. 2 depicts the possible configuration of a variable speed generating unit. The controlled rectifier should be able to force sinusoidal current into the generator when a sine wave PM-machine is used. The output inverter is devoted to supply a three phase grid with neutral connection in order to feed both three phase and single phase loads. As it is easy to understand the variable-speed unit can be used to directly supply both dc and ac loads. Variable speed operation requires the addition of a supervisory controller and of an electronic power converter to provide output voltage at the desired frequency. Voltage Source Inverters (VSI) having a three-phase four-leg power circuit configuration are being increasingly considered for either uninterruptible power supplies or autonomous power generating units being required to supply both three-phase and single-phase electrical loads, in four-wire electric power distribution systems. Having each phase its own voltage control arrangement, suitable voltage control arrangement allows a three-phase four-leg VSI to be operated as three
P M S M
3ph+n grid
Controlled Rectifier
dc-link
Auxiliaries and /or Combined Storage Unit
Variable dc-load
Fig. 2. Variable-speed generating unit schematic view.
II. THREE-PHASE ACTIVE RECTIFIER In consideration of the tight design requirements of variable speed power supply units, electrical machines with Nd-Fe-B permanent magnet excitation are widely recognized to be the most suitable candidates for generator applications. Within the broad category of PM machines several distinct magnetic circuit configurations can be identified, and these include the axial-flux PM machine (AFPM) topology, which in the recent years has drawn substantial interest concerning a number of various machine applications. Adoption of such a particular PM machine arrangement to be used in power supply units should be prompted by the recognition of unique characteristics such as higher torque per volume density and higher efficiency compared to conventional machine topologies [9-13]. In the proposed solution, the PM generator is loaded by an IGBT based controlled rectifier. The boost-rectifier (BR) outer control loop acts in order to regulate the dc-link voltage, whereas the inner control loops are achieved through the decoupled dq-based theory to control d and q axes currents. Being the system strongly non-linear, an adaptive direct-tuning strategy is selected as shown in [14]. Fig. 3 shows a more detailed block scheme of the generating unit. The prime mover is directly coupled to the PM-generator which supplies power to the controlled rectifier. The system is controlled by a DSP (Digital Signal Processor) which acts to regulate the output dc-link voltage (outer loop) and machine currents (inner loops). The DSP
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controls also the prime mover speed according to the dc-link requested power. This task depends on the torque command interface available on the prime mover, which can be mechanical with electric actuator, electrical with analog input signal or digital with the torque command and diagnostic data exchanged on a communication bus (i.e. CAN-bus).
including both an inner current loop and an outer voltage loop should be avoided due to the difficulties in powerfactor regulation. In addition, a double loops control arrangement is intrinsically slower than a single loop voltage control whenever load step changes must be compensated.
. Fig. 3. Prime mover side block scheme of the proposed generating unit
A. Starting Procedure The starting procedure for the prime mover side generation unit is arranged in order to automatically wake up each power electronic system as the engine is started. In this case, the BR acts as master continuously monitoring the PMSM speed, which is initially equal to zero. As soon as the prime mover starts to run the BR also starts the power switches modulation by keeping saturated the reference voltage vector. The proposed starting strategy is suitable to start the BR as a diode rectifier. During this starting phase only the current loops are enabled, so the prime mover is not loaded by the BR except for the power electronics losses which are negligible at low speed, low dclink voltage and near zero currents. Once the generator desired minimum speed ( min ) is reached, the BR enables the voltage loop control bringing the dc-link voltage to the final operating value. In order to avoid torque jogs, a suitable soft-start procedure has been implemented; the dc-link voltage reference linearly increases from the initial dc-link voltage to its final value. After that, the BR sends a run command on the CAN-bus to the output VSI, which enables the output voltage regulation and the phase voltage reference, is suitably increased from zero to the final value. III. THREE-PHASE FOUR-LEG VOLTAGE SOURCE INVERTER Fig. 4 shows the three-phase four-leg VSI including the required output filter. In order to arrange the proposed control strategy a number of VSI output quantities need to measured, i.e. the three output currents Iph_A, Iph_B, Iph_C, and the three phase-to-neutral output voltages, Vph_A, Vph_B and Vph_C, as schematically shown in Fig. 4. In power supply applications such as either uninterruptible power supplies or autonomous power generating units, the VSI has to be able to provide sinusoidal output voltages whenever operated under load conditions having any given value for the power factor. A control architecture being
Fig.4. Three-Phase Four-Leg VSI with current and voltage measurement sensors and output filter.
However, the use of a single loop voltage control imposes adoption of suitable current-limiting control strategies, devoted to manage the VSI output current for VSI selfprotection against over-current conditions in the load circuit. In the following a brief description of the proposed voltage control arrangement is given. A. Integral-Resonant Voltage Control For the three-phase four-leg VSI the output voltage control system would be arranged by using a integral-resonant (I+R). In order to control the duty-cycle of each phase the first-harmonic voltage controller handles both the sinusoidal voltage reference, and the actual voltage measured at the terminals of the output filter capacitors. The output voltage higher harmonic components would have their own reference values set to zero, in order to achieve sinusoidal voltage waveform. As VSI operation under nonlinear loads need to be accomplished, harmonic compensators are required in the control algorithm. As a pure-resonant control structure would totally be ineffective with respect to the output voltage dc component, in the proposed control algorithm an integral action ki / s is added in order to compensate the dc component of the VSI output voltages, thereby forcing to zero the dc-current injection.
B. Current Limiting Algorithm As early discussed, in order to achieve a fast-response voltage control in the proposed VSI voltage control arrangement, the current control inner loop is avoided and a single-loop voltage control is accomplished. However, in order to deal with transient over-current conditions in the load circuit, VSI modes of operation with current limiting must be implemented without affecting the voltage regulation dynamic. In the proposed algorithm a current limiting action is included to modify the voltage reference
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values being utilized in the voltage control loops for regulating the VSI output voltage amplitude. To this purpose, three current levels are defined, being two of them related to a hysteretic band current control, whereas the third current level is fixed by considering a selected “off & restart” current threshold value. The current limiting approach herewith in the following discussed can be applied to either single phase or three phase inverter without modifications. C. Hysteretic Current Limit As soon as the sampled current is higher than a selected upper threshold iLup, the voltage reference is forced to zero by the gain kph_x. On the other hand, whenever the phase current iLx falls below the lower threshold iLdn, the voltage reference is set to its actual value. This comparison can be simply done using the absolute value of the measured current as shown in (1) in order to limit both positive and negative wave. Finally, phase voltage reference for the generic phase x can be written as in (2). 0, if ( abs (iLx ) iLup ) k ph _ x 1, if ( abs (iLx ) iLdn ) Vph _ x _ ref k ph _ x Vph _ pk sin(0 t x )
(1)
Network). Both PM-generator and active rectifier and VSI are water cooled. Prime Mover
PM Generator
AC/DC Converter
VSI 4‐legs 400V, 50Hz 3ph + n Three‐phase linear & non‐linear loads
Single ‐phase linear & non‐ linear loads
Fig. 5. Experimental set up.
The 20kW boost rectifier prototype is shown in Fig. 6, from top to bottom are in evidence the sensing board for measures acquisition, the DSP control board, the driver board and the power board. The IGBT modules were directly assembled on the 200 m copper power PCB. In order to improve reliability and efficiency, electrolytic capacitors have been avoided and film caps were selected. Total dc link capacitance is achieved by the combination of two bulk capacitors, which are mainly intended for voltage ripple limitation, with three PCB mounted square box shaped capacitors, properly chosen and located for IGBTs turn off over-voltage limitation.
(2)
where x a, b, c . The proposed method allows obtaining the current limit feature independently on each phase which is interested for single phase loads. D. Off and restart operation If the electrical time constant of the load is faster than the voltage loop dynamic, the hysteretic current mode may fail since the output voltage is not reduced quickly enough. The proposed solution is based on selecting an additional current threshold, iLor, which is greater than iLup. Whenever the phase current becomes equal to iLor the control action immediately forces the controller output duty-cycles to 50%, resets all the voltage controllers and finally enables a new fast start-up procedure. Such an additional feature allows that the hysteretic current limit works properly, avoiding any damage to the power converter. IV.
EXPERIMENTAL RESULTS
In order to validate the proposed system an experimental set up was accomplished by using the autonomous power generating unit having the schematic layout shown in Fig. 5. An internal combustion engine (Geminiani PowerPack 415P4) driven permanent-magnet generator (Lucchi gSAE4) is used to supply the three-phase IGBT boost-rectifier. The boost-rectifier dc output provides the dc input voltage to a three-phase four-leg VSI being devoted to supply a fourwire output circuit having both three-phase and single-phase loads. The control algorithm is implemented on a similar proprietary DSP-based control board. The communication with the ICE and outer systems is accomplished using the CAN-bus (Controller Area
Fig. 6. Prototype of the 20 kW Boost-Rectifier.
The DSP control unit has a built-in floating-point core being operated at 150 MHz which allows running the whole BR control algorithm in less than 40 s. Being the switching frequency set to 15 kHz, the control program can run synchronously with the PWM sampling time. The measured AC-DC system efficiency at rated operating condition results as shown in Table I. TABLE I – OVERALL SYSTEM EFFICIENCY @ RATE POWER
AC-DC power stage
Ia [Arms] 33.07
Vdc [V] 746.1
Pdc
m
br
[kW]
(PMSM)
(BR)
19.91
95.02
97.54
Experimental tests have been carried out in order, also, to validate the starting phase procedure for the whole generating unit. Initially both dc-link voltage and output voltage are equal to zero. As soon as the ICE starts the dclink capacitors are charged until the ICE speed reaches its idle value, which is set close to 950 rpm. In order to turn the
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power electronic converter on, the speed must reach the desired minimum operating speed, which is set close to 1500 rpm. After that, the boost-rectifier enables the voltage loop control and raises the dc-link to the final operating voltage. Finally, the output inverter is enabled and when the soft-start routine is accomplished the whole GEN-SET is ready to follow load requests.
when the induction motor is connected to the VSI. The bottom side of Fig. 9 shows the detail of Fig. 8 when the induction motor is approaching the end of the start-up phase (-B-) and the control switches to normal mode operation. Fig. 10 shows the experimental verification of the proposed soft and fast restart procedure using an induction motor with a rated power very close to the VSI rated power. At first the induction motor is suddenly connected to the VSI output circuit by a manual breaker. After that, the control algorithm acts to reduce the output voltages due to the high phase current working in hysteretic current limiting mode.
Fig. 7. Prototype of three-phase four-leg VSI.
As shown in Fig. 7, the three-phase four-leg VSI was arranged in the form of a 25 kVA rated power prototype being controlled by a proprietary control platform based on the TMS320F28335 Digital Signal Processor. In such a VSI prototype the input capacitance was set at Cdc = 450 F, whereas the output filter was arranged with Ln = 650 H and Cf = 4.5 F. Concerning the “current manager” being used in the control algorithm of the VSI prototype, current values such as iLup 44 A, iLdn 36 A, iLor 50 A were
Fig. 9. Detail of initial start-up and end of start-up of an induction motor suddenly connected to the VSI output filter. Van (VSI line-to-neutral voltage), Vdc (dc-link voltage), Ia_vsi (VSI phase current), Ia_br (input boost-rectifier current).
selected.
However, one of the three phase currents increases above iLor enabling the fast off and restart procedure. Thereby, the inverter voltages are increased starting from zero until they reach their final values again. During this time the hysteretic current algorithm limits the current to the desired maximum value.
Fig. 8. Behavior of the hysteretic current limit considering the whole system: VSI and BR. From top to bottom, VSI output phase A voltage (300 V/div), dc-link voltage (200 V/div), VSI phase A current (30 A/div), boostrectifier phase A current (20 A/div).
Fig. 8 shows experimental traces taken from a transient loading test where a three phase, 8kW rated power, induction machine load was suddenly connected to the VSI output terminals (highlighted part -A-) being the hysteretic current limiter enabled. As soon as the induction motor is connected to the VSI output circuit, the control algorithm detects the over-current and acts to limit the VSI output current keeping it bounded within the hysteresis band. The highlighted part -B- of Fig. 8 shows the system behavior when the induction motor starting phase is almost completed and the VSI control algorithm switches to normal VSI mode of operation. The top side of Fig. 9 shows the zoom of the highlighted -A- part of Fig. 8 which represents the instant
Fig. 10. Induction motor start-up behavior, from top to bottom, VSI output phase A voltage, dc-link voltage, VSI phase A current.
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Experimental traces resulting from VSI full load operation under a three-phase balanced resistive load are depicted in Fig. 11.
restart procedure is presented. The proposed VSI output voltage control is suitable for application in autonomous power generating units being required to supply both threephase and single-phase electrical loads in four-wire electric power distribution systems. REFERENCES [1]
[2]
[3] [4] [5] Fig. 11. System voltages and currents at full load.
On the other hand, Fig. 12 shows experimental traces resulting from the starting transient of a 1 kW rated power single-phase universal motor being connected to the VSI output circuit. Even though the requested starting current is quite high (purple trace), the output phase voltage (yellow trace) decreases few tens of volts and it is recovered in less than 50 ms.
[6]
[7] [8] [9]
[10]
[11] [12]
[13]
Fig. 12. Single-phase universal motor start-up transient.
V.
[14]
CONCLUSIONS
A newly-conceived integral-resonant single-loop output voltage control arrangement for a three-phase four-leg VSI, to be used in stand-alone microgrids, is discussed and experimentally verified. The proposed strategy allows compensating both output voltage harmonics and dccomponent by using a single integrator and a multi-resonant voltage loop. Further to that, a complete current limiting strategy based on hysteretic current limiting and off &
[15]
[16]
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