Design and Implementation of Induction Generator Controller for Single Phase Self Excited Induction Generator D. K. Palwalia
S. P. Singh
Department of Electrical Engineering Indian Institute of Technology, Roorkee INDIA email :-
[email protected]
Department of Electrical Engineering Indian Institute of Technology, Roorkee INDIA email :-
[email protected]
Abstract: This paper presents design and implementation of Digital Signal Processor (DSP) based induction generator controller (IGC) to regulate the voltage and frequency of single phase self excited induction generator (SEIG) which employs a three phase squirrel cage induction machine, suitable for stand alone power mode employing unregulated turbine such as micro-hydro power generation. A three phase induction machine can be used to generate single phase supply at constant voltage and frequency if the electrical load is maintained constant at its terminals. A prototype model of single phase self excited induction generator is developed such that the load on the SEIG remains constant despite change in the consumer load. The developed prototype model is cost effective, reliable, flexible and capable of network based application for remote configuration with TCP/IP stack. The transient behavior of developed DSP based SEIG-IGC system at different operating conditions such as application and removal of static (resistive and reactive) load is investigated to demonstrate the capabilities of the proposed IGC. Keywords: Self excited induction generator (SEIG), Induction generator controller (IGC), Micro-hydro generation, Digital signal processors (DSP).
I. INTRODUCTION Induction machine generates power when enough excitation is provided and its rotor is driven to a speed higher than that of the stator magnetic field. The required excitation for self excited induction generator (SEIG) is given by connecting appropriate capacitors [1-3] across the terminals. Induced voltage and its frequency in the winding will increase up to a level governed by magnetic saturation in the machine. Induction generators are increasingly being used these days to harness renewable energy resources because of their relative advantageous features. These features include maintenance and operational simplicity, brushless and rugged construction, lower unit cost, good dynamic response, self protection against faults and ability to generate power at varying speed. Also, the induction generator does not require separate DC source and its related equipment like field breaker, rheostat and automatic voltage regulator and therefore requires less maintenance. These advantages facilitate induction generator operation in stand-alone/ isolated mode or in parallel with synchronous generator for supplying local load and in grid mode. In remote locations or hilly areas, electrical energy from local resources can be inexpensive compare to grid connection. A micro-hydro system with unregulated low head turbines, which maintain almost constant input power due to fixed head and discharge coupled with self excited induction generator may be
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one of the most suitable option. Such areas are characterized by sparsely distributed population with electric loads of singlephase. The single phase power supply is preferred over three phase in order to render the distribution system simple and cost effective. Single-phase induction motors can be used as single phase self excited induction generators for single phase power generation [4], but problems can be experienced in determining the size and arrangement of capacitors required to achieve excitation without overloading the windings. In addition, singlephase induction motors are only available for relatively small power outputs. It is possible to use a three-phase induction motor as a single-phase generator with only 20 to 25% power de-rating and this has become the preferred approach to providing a singlephase supply. Three phase induction machines have higher efficiency and lower cost than an equivalent-sized single phase machine. Also, power pulsation like in single phase induction generator under no load does not occur, because the excitation power in this generator is three phase and vibration, noise in the generator caused by this phenomenon can be significantly reduced under load condition as well. However, the use of threephase induction machine for supplying single phase load is an extreme case of unbalanced operation; hence a three phase induction machine would have to de-rated in order to keep the temperature of the machine within allowable limits. Bhattacharya et al. [5] have proposed an unbalanced excitation scheme, known as the 'C-2C' connection for single phase power generation from three phase induction motor. This connection gives single phase 415V output for 415 V delta connected induction machine. Fulkami et al. [6] have proposed a new connection scheme known as ‘Cp-Cs’ connection. This connection gives 220V single phase voltage output for 415 V star connected three phase induction machine. Mahato et al. [7-8] have reported the optimal capacitor for maximum power output for such SEIGs and transient performance of such connection scheme. The self excited induction generator has a major drawback of poor voltage regulation. The generated voltage depends upon the speed, capacitance, load current and power factor of the load. Input power remains constant with unregulated micro-hydro turbine but output power is not constant due to changing load requirement of consumer load. In order to keep SEIG output power constant, a dump load is connected in parallel with the consumer load such that the total generated power is held constant. The amount of power to be supplied to the dump load is decided by induction generator controller (IGC). Various IGCs for SEIG have been reported in literature [9-17].
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In this paper, the design and implementation of a DSP based induction generator controller for single phase self excited induction generator using three phase induction machine as a single phase SEIG is given for fixed point operation. Input power remains constant with unregulated micro-hydro turbine but output power is not constant due to changing load requirement of consumer load. In order to keep SEIG output power constant, a dump load is connected in parallel with the consumer load such that the total generated power is held constant. The amount of power to be supplied to the dump load is decided by induction generator controller (IGC). DSP based IGC gives single chip solution to control circuit with fast processing and flexible solution. The developed IGC can be employed for different ratings of SEIG with minor software changes and can also be configured for network based applications for remote operation. The developed controller is tested under application and removal of resistive and reactive loads. II. SYSTEM DESCRIPTION A schematic diagram of the developed DSP based SEIG-IGC system is shown in fig.1 and fig.2. It consists of a three phase star connected squirrel cage induction motor (working as SEIG) driven by a constant power prime mover. Three capacitors are connected, one in parallel(Cp) and two in series (Cs) as shown in fig.1, which have a fixed value to result in rated terminal voltage at rated load. Consumer load and induction generator controller are connected in parallel at generator terminals. The IGC consist of an uncontrolled rectifier, a filtering capacitor (Cf), IGBT based chopper and a series resistive dump load (RD). The uncontrolled rectifier converts the SEIG AC terminal voltage to DC. The output ripples are filtered by filter capacitor. An IGBT is used as a chopper switch. When gate pulse to IGBT is high, the current flows through the dump load and the power is consumed. The pulse width or duty cycle of chopper is decided by the difference of power generation to consumer load. A TMS320F2812 (32-bit, 150 MIPS fixed point digital signal processor) is used for generation of suitable pulse width in accordance with consumer load. The use of DSPs gives flexibility of generating multiple high frequency and high resolution PWM waveforms, fast processing to implement advance algorithms. Multiple features can also be implemented using the same controller. It makes the complete implementation simple and emulates a flexible solution. The future modifications can be realized by appropriate change in software instead of redesigning a separate hardware platform. III. IMPLEMENTATION The SEIG feeds two loads in parallel such that the total power Pout=Pc+Pd is constant, Where, Pout is the generated power of the generator (which must be kept constant), Pc is the consumer load power, and Pd is the dump load power. This dump load power (Pd) may be used for non priority load such as heating, battery charging, cooking etc. Excitation capacitance has
to provide required leading VAR to maintain rated voltage on load at the operating speed for the given induction machine operating as SEIG. The amount of capacitor excitation at no load and rated load may be determined iteratively. The output power of the SEIG is kept constant by IGC. Fig 2 shows the control scheme for voltage regulation of the SEIG. The terminal voltage for feedback is sensed with voltage transducer to achieve a DC value proportional to SEIG output voltage. The voltage transducer converts and linearizes the input AC terminal voltage to equivalent output DC voltage. This voltage is given to ADC input of the DSP TMS320F2812. The TMS320F2812 DSP has 16 ADC input channels with an input range of 0- 3 Volt. The output values are in the range of 0 to 4095 as the TMS320F2812 DSP ADC is 12- bit converter. It reads the input with a sample rate of 0.001 seconds (maximum 0.0004 seconds). The DC output of voltage transducer has peak to peak ripples. A running mean is taken in software to overcome this problem. The sensed voltage is compared with a reference, which is taken as proportional to the rated terminal voltage of the SEIG and may be altered as and when required. The error is scaled (gain) and algebraically added to previous sample time PWM reference level. To initiate the process a initial value of PWM reference is set. As the desired terminal voltage is achieved the error signal becomes zero and PWM reference holds its level. Minimum and maximum level of PWM reference is corresponding to 0% and 100% duty cycle of PWM output for active low configuration. If the PWM reference is at minimum or maximum level and desired voltage is not achieved then either generator is overloaded or dump load required to be adjusted respectively. To signal this, the PWM reference is passed through a saturator which gives the output for preset maximum and minimum values. The output is passed through general purpose digital input output (GPIO) of DSP and out pulse is made high. This may be used for alarming or tripping the circuit. The pulse width references are fed to PWM of DSP. The PWM block have self carrier triangular wave. The carrier frequency is selected according to static switching device. The TMS320F2812 DSP has 6x2 high resolution PWM outputs. The PWM output is then given to IGBT chopper through an opto isolation and pulse driver circuit. IV. RESULTS AND DISCUSSION A prototype DSP based induction generator controller with TMS320F2812 processor has been developed and tested in laboratory under various operating conditions. A three phase 3.73 kW, 400 V, 7.5A, 50Hz, 1440 rpm star connected squirrel cage induction machine is used as a single phase self-excited induction generator. The SEIG is driven by 220 V, 20 A, 5 HP, 1500 rpm shunt wound DC machine used as a prime mover. To generate 1000W at 220V and rated speed, 15 µF and 15µF capacitor of 400 V is connected as CP and CS at the SEIG terminals. A resistive load of 1000 W is connected as dump load. Fig. 3(a) shows the voltage and current transient waveforms of DSP based SEIG-IGC system on application of 300 W resistive loads. In fig 3(b), voltage and current trends are shown on successive application and removal of 400 W resistive load in step of 100 W. On application and removal of load the terminal voltage remains constant. On application of resistive load, main load current increases and dump load current
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Induction generator
CP Consumer Load
Prime Mover CS
Induction Generator Controller
CS
Fig 1. Schematic diagram of Single-phase SEIG
TMS320F2812 Voltage sensor
ADC Ref.
Z -1
+ -
Gain
Ver(t)
+ +
Saturation
PWM
GPIO
Alarm Pulse Driver & Isolation
IGBT Cf
R RD
Induction Generator Controller Fig 2. Schematic diagram DSP based induction generator controller for SEIG decreases so that power transfers from dump load to consumer load and SEIG experiences constant load on it. The IGC is a nonlinear system and feeds harmonics in the SEIG. With application of resistive main load, power dissipating in dump load reduces therefore less harmonics are generated and SEIG voltages and currents are close to sinusoidal. Capacitive current also decreases due to less reactive burden. Fig 4(a) shows the voltage and current transient waveforms of application 200 W, 0.8 pf load. Fig 4(b) shows the voltage and current trends on application and removal of 200 W 0.8 pf load. On application and removal of load the terminal voltage remains constant. When an inductive load is connected to the SEIG, the voltage will decrease and the IGC respond by reducing the dump load. The reduced load causes an almost instantaneous frequency
increase due to reduced slip and an additional gradual increase in frequency due to rising prime mover speed. The increase in frequency will depend on power factor of the load and the degree of saturation of the SEIG. As modern induction machines tend to be highly saturated, the frequency regulation with variation in load power factor is quite small. A small increase in frequency results in a significant reduction in magnetizing current. In addition, extra VARs are produced by the excitation capacitors due to reduced impedance. The IGC is a non-linear system and feeds harmonics in the SEIG. Table I summarizes the main load, dump load, terminal voltage, frequency variations, voltage THD. The good voltage and frequency regulation is observed. There is a slight increase in terminal voltage as the voltage THD in the system increases.
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(a)
(a)
(b)
(b)
(c)
(c)
Fig 3:- Voltage and current at (a)Application of 300 W resistive load (b) Removal of 300 W resistive load (c) Voltage and current trends at application and removal of 400 W restive load in step of 100 W
Fig 4:- Voltage and current at (a)Application of 200 W 0.8 pf lagging load (b) Removal of 200 W 0.8 pf lagging load (c) Voltage and current trends at application and removal of 200 W 0.8 pf lagging load.
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A slight increment in frequency is observed for loads of lagging power factor. Three phase induction motor working as a single phase induction generator is principally behave as unbalanced generator for reactive loads. An increase in voltage and current THD is observed for reactive loads on SEIG.
VII. REFERENCE [1] [2]
Table 1 Main load (W) 0 200 400 600 800 1000 200W 0.8pf lag 500W 0.8pf lag
Dump Load (W) 1000 800 600 400 200 0 800 500
Terminal Voltage (V) 219.1 217.3 218.6 219.3 219.6 220.8 218.6 219.3
[3]
Frequency Voltage (Hz) THD (%) 49.16 9.8 48.23 12.4 48.37 11.3 49.20 10.4 49.80 10.1 49.95 9.6 50.03 10.6 51.14 9. 3
[4] [5] [6]
[7]
V. DISCUSSION In this paper a digital sample based controller have been developed which check and update for required pulse width with every sample. The sample time is taken as 0.001 second (maximum 0.0004 second) which results in speedy response. This is simple as compare to a PI controller based application. The PI controller has to tuned for proportional (Kp) and integral gain(Ki) for dynamic responses. DSP based IGC gives single chip solution, thus reduces the hardware complexity and increases the reliability. A single DSP TMS320F2812 can control as many as 12 SEIGs or even more. It can also be configured for network based control. The boards like eZDSK91C111 Ethernet Daughter Board for TMS320F2812 with TCP/IP Stack facilitate network based applications for remote configuration, high speed data exchange and distributed control. Also, use of such digital signal processors would result in low cost controller design. Although, TMS320F2812 DSP costs around $30 but DSPs like TMS320C241, TMS320C242 or TMS320C243 is less than $5. The speedy response, high performance, multiple feature implementations on single processor and flexible solution are other added advantages with DSPs. VI. CONCLUSION A DSP based induction generator controller has been developed for three phase induction motor working as a single phase self excited induction generator. The developed IGC system acts as the voltage and frequency regulator for the SEIG. The voltage and frequency regulation have remarkable improvement in context to SEIG. There is significant improvement in the magnitude of frequency deviation at full load rejection. The duration of voltage transients on application and removal of static and dynamic load is found satisfactory. The IGC behaves as a non-linear system and feeds harmonics in the SEIG. The voltage THD of SEIG is found to be satisfactory. It has also been observed that there is a slight increase in frequency to compensate for lagging power factor loads.
[8]
[9] [10]
[11] [12] [13] [14]
[15] [16] [17]
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