Operation of Sine-wave Voltage Source Inverter in Hybrid Genset Based Autonomous Power System R Seliga, E Ernest, K Paciura, N L Brown Cummins Generator Technologies Ltd, UK, email:
[email protected]
source. In this kind of hybrid generating system additional power comes from renewable energy source (i.e. wind, PV solar, fuel cell), engine waste heat recovery (WHR) module or battery storage bank where in each of those subsystems critical part is DC/AC power converter [4], [5]. Common feature of those supplementary sources is present of DC/AC sine-wave voltage source inverter (SVSI) which is coupled to WFSG. This creates a challenging task to try to achieve optimal operation of AC-coupled power sources. Commonly used control method for parallel operation of WFSG is based on frequency and voltage adjustment represented by droop characteristic to achieve active power or reactive power sharing, respectively. In a typical genset WFSG output voltage amplitude control is executed by automatic voltage regulator (AVR) and brushless excitation system (BES) when output voltage frequency is controlled mechanically by engine speed governor [2], [6]. Such arrangement results in a slow dynamic response, in contrast to inverter inherited fast dynamic operation. Careful consideration is required when autonomous power system containing genset and SVSI is designed [7], [8]. This paper describes some results of development and practical implementation of hybrid generating system. In section 2 topology of the system, its main function blocks and modes of operation are presented. SVSI structure and its operation are discussed in section 3. Section 4 shows selected results of experimental tests performed on the system under different modes of operation.
Keywords: power generation, autonomous micro-grid, hybrid system, synchronous generator, sine-wave DC-AC converter.
Abstract Nowadays local autonomous micro-grid supply systems are mainly based on generating set (genset), where wound-field synchronous generator driven by diesel engine feeds power to distributed loads. Such systems often operate in remote locations where genset fuel supply is limited and costly. Commercial availability of renewable energy sources with power converters make them attractive to integrate with genset to improve its efficiency but introduce interesting challenge how to operate those different energy sources. A hybrid power system consisting genset and supplementary power source coupled to autonomous AC micro-grid through DC-AC inverter has been developed and researched. The paper focuses on DC-AC inverter control development and presents achieved system performance test results validated experimentally in laboratory environment.
1 Introduction Throughout the years stand-alone distributed generation (SDG) power systems were based on diesel engine driving wound-field synchronous generator (WFSG). Depending on the application this kind of generating set (genset) operate in Stand-by (SB) mode to supply critical load in case of main distribution Grid failure or in Prime-power (PP) mode as main source of power to utility loads [1], [2]. Genset common operation future is that engine and generator run in fix-speed mode typically of 1500 rpm or 1800rpm to provide required 50Hz or 60Hz AC voltage to load, respectively. Especially in PP mode fix-speed operation requirement leads to low engine fuel efficiency and faster engine wear out when long-lasting light loads are fed from genset [3]. It can be found [2] that one commonly used solution to reduce those drawbacks is paralleling different size gensets and switch them on/off when load demand changes. Although, this enables some fuel savings in most cases alternative solutions are more beneficial. Since the beginning of XXI century commercial availability of alternative energy systems (PV, wind, fuel cell, energy storage) made them attractive not only as Grid-tied sources, but also as supplementary power sources for autonomous AC micro-grids powered by diesel gensets. Increasing fossil fuel cost and demand to reduce genset fuel consumption led to solutions where genset is assisted by alternative power
2 Hybrid Power System Topology Topology of hybrid generating system is shown in Figure1. Stand-alone genset, consisting a diesel engine, which drives WFSG is assisted by Power Module (PMO) to supply various loads connected to local autonomous AC-bus configured as 3phase 4-wire micro-grid. PMO consists of SVSI, which can be powered by supplementary energy source (SES) such as PV panels, wind generator, etc or energy storage (ES) containing battery bank. This kind of hybrid system can supply loads with the following modes of operation. 1) Genset only operation (Mode_1): diesel engine (DE) drives WFSG delivering active power Pg and reactive power Qg to micro-grid connected loads. DE governor controller maintains engine fix speed and generator AVR regulates AC-bus voltage rms value trying to keep it within the limits when loads are changing. Total system output active power Pt is equal to Pg and output reactive power Qt is equal to Qg. In this mode
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PMO is disconnected from micro-grid and is in OFF stage as a result of not sufficient energy level available from SES. 2) Hybrid load sharing operation (Mode_2): genset runs and is connected to micro-grid as well as PMO is connected to micro-grid and is in ON stage. Loads are fed from genset and PMO in which both subsystems are delivering active and reactive power to AC-bus. In this case total output active power Pt demanded by loads is shared by both subsystems and is expressed by
Important part of analyzed hybrid generating system is power module PMO. Its main subsystem is DC/AC sine-wave voltage source inverter (SVSI), which simplified circuit diagram and control structure is presented in Figure 2. SVSI power circuit consists of three individual half-brides (T1, T2), LC filters (Lf, Cf) on AC output and common DC-bus (Cdc) connected to SES. Each pair of (T1, T2) and (Lf, Cf) filter components are controlled individually to produce three sinusoidal voltages (U, V, W) with reference to common neutral (N) shifted by 120O, respectively. All SVSI output phase can be connected to or disconnected from Micro-grid AC-bus by 3-pole contactor So. SVSI hardware layout example of one of earlier development is shown in Figure 3.
where kP is ratio factor for active power sharing. Similar way total output reactive power Qt demanded by loads is expressed by (2)
Supplementary Energy Source or Energy Storage
= + (1 − )
where kQ is ratio factor for reactive power sharing. This mode is primarily used when SES has sufficient enough over the time energy Es level which allows PMO to support genset in periods of time when high load demand happens. If Es level is low PMO can still operate but its primary function is to supply reactive power Qi to AC-bus to reduce reactive power Qg drawn from WFSG by loads. 3) PMO only operation (Mode_3): with sufficient Es level available from SES and light loads connected to micro-grid genset can be disconnected from AC-bus and set to OFF stage. Both active Pt and reactive Qt powers demanded by loads are delivered only by SVSI fed from SES.
PWM
(1)
Cdc
= + (1 − )
3 Power module structure
Figure 2: Sine-wave voltage source inverter topology and basic control structure.
Figure 1: General diagram of hybrid power system. All three described hybrid system modes of operation lead to different requirements for power module control algorithms. In next section topology of SVSI and its control structure is briefly presented. Figure 3: Example of sine-wave voltage source inverter hardware layout.
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Pt = 42kW and Qt = 20kVAr lagging. CH3 of oscilloscope shows WFSG phase U output current and CH4 shows corresponding phase voltage on WFSG output terminals.
3.1 Implementation of Control Methods of controlling SVSI connected to AC microgrids are widely presented in various publications. It can be found in [9], [10], [11] that those inverters are controlled the way that allows achieving two main modes of operation: voltagecontrolled source inverter or current-controlled sources inverter. To meet proposed system operation requirements described in Section 2, similar approach was taken during development and it was based on work described in [14], [15], [16]. As presented in Figure 2 output voltages vf of each SVSI phases (U, V, W) are controlled individually by applying inner current loop and outer voltage loop both with PI regulators to achieve high quality sinusoidal voltage vi which feeds loads connected to AC-bus. In PMO operation Mode_3 SVSI works as voltage source and reference block REF sets control of voltage vf outer loop with its vPI regulator. Then output from vPI sets reference to current if inner loop with its iPI regulator, which output through PWM modulator block controls driving circuit DRV of half-brides IGBT switches (T1, T2) for relevant phase. When Mode_2 PMO operation is required SVSI works as current source connected in parallel to WFSG. Reference block REF sets iPI regulator to control current if amplitude and phase with reference to AC-Bus voltage vi to achieve active power Pi and reactive power Qi sharing with generator, both SVSI and WFSG feeding micro-grid loads. To allow synchronised change between different system modes of operation phase-lock loop control block PLL was implemented. Although each phase of SVSI is regulated independently from each other their shared common controller CON based on DSP-FPGA architecture with sensing circuits and CAN-Bus for external SVSI communication. Example of CON hardware implementation is shown in Figure 4.
Figure 5: System current and voltage waveforms in Mode_1 (genset only) operation. Example of system hybrid operation in Mode_2 is presented in Figure 6. Total load power has stayed the same and SVSI control was set to provide equal active power sharing between genset and POM and no reactive power feeding from SVSI. It can be seen that inverter phase U output current ii (CH2 waveform) is almost in phase with AC-Bus phase U voltage (CH1 and CH4 waveforms). This means that only active power was supplied by SVSI. It can also be observed that amplitude of WFSG output current (CH3 waveform) was reduce when compared with CH3 in Figure 5 and load sharing was achieved.
Figure 4: Example of sine-wave voltage source inverter controller hardware implementation.
4 Experimental Results System performance was validated during laboratory testing. Parameters of experimental system comprising of enginegenerator set with 200kVA power rating and SVSI fed from independent power source of 50kVA rating. AC-Bus voltage was configured as 400V line-to-line and 50Hz. Figure 5 shows voltage and current waveforms captured when system was running in Mode_1 (genset only) at steady-state condition supplying symmetrical 3-phase loads of total power
Figure 6: System currents and voltages waveforms in Mode_2 (hybrid load sharing) operation. Transient load change from 42kW to 22kW in Mode_2 is presented in Figure 7. SVSI operation was set to deliver constant power Pi to the loads and it can be observed from CH2 waveform that amplitude and phase of output current ii
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has not changed. Power Pg delivered by WFSG dropped and this is reflected in WFSG output current CH3 waveform changed.
Figure 8: System currents and voltages waveforms captured during transient mode (from Mode_2 to Mode_3) changed operation. Figure 7: System currents and voltages waveforms in Mode_2 operation captured during transient load changed.
5 Conclusions
One of most interesting example of preformed laboratory tests is illustrated in Figure 8. In this case hybrid power system was supplying symmetrical 3-phase loads of total power Pt = 22kW and Qt = 20kVAr lagging when transition from Mode_2 to Mode_3 was preformed. During this kind of event it is expected that POM will start to provide total load power Pt as genset will get disconnected from AC-Bus. It is also expected that SVSI will transition smoothly from current-controlled sources operation to voltage-controlled sources operation to maintain stiff sinusoidal voltage feeding the micro-grid loads. It can be observed from waveform illustrated in Figure 8 that hybrid power system operation was achieved. In instant moment when genset has been disconnected from AC-Bus WFSG output current ig drops to zero (CH3 waveform) and at the same time total load current starts to be drawn from SVSI output which illustrated CH2 waveform. It is also noticeable SVSI output voltage (CH4 waveform) overshoot slightly and it took around 80ms to stabilise this voltage with the limits.
The hybrid power generating system operating in autonomous micro-grid application has been presented. The system architecture presented incorporates diesel engine genset and auxiliary power module POM, which consists sine-wave voltage source inverter SVSI, both subsystem supplying local loads connected to distribution AC-Bus. The work presented in this paper focuses on SVSI operation as voltage-controlled source inverter or current-controlled sources inverter. The desirable operation has been achieved, where the experimental results preformed on laboratory development system illustrate stable and robust SVSI operation under transient system configuration change and dynamic load sharing condition. SVSI can operate as current or voltage source to supplement genset operation in autonomous microgrid power system.
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