design and fabrication of advance elc

Introduction

Micro Hydro Power (MHP) plant is one of the most economical and environmental friendly technology of energy supplement in rural area. MHP fulfills the electrical energy demand in rural area which is not served from national grid due to the higher cost of transmission line. So, all the MHP plants are operated in isolated mode. MHP designers always try to design the MHP at a lower possible cost. Therefore, Electronic Load Controller (ELC) is used instead of oil pressure governor in order to reduce the high cost of governor. MHP is run off river type plant without reservoir at headrace and turbine is operated at constant head and discharge. The plant capacity is designed for minimum discharge available at dry season. There is no meaning of saving the water during light load period. Frequency variation of 2% and terminal voltage variation of 5% from their nominal rated values are generally acceptable in MHP schemes [1].

An Electronic Load Controller (ELC) is an electronic device that keeps the speed of synchronous generator constant at varying load conditions. The generator is driven by unregulated turbine with constant power output and a dummy ballast load is connected across the generator terminals to dump the excess of power generated. When the consumers load changes, frequency of generated voltage changes. The frequency is sensed and compared with the reference frequency and error so obtained is utilized to control power consumed by ballast load so that generator always operates at its full rating, resulting in constant speed [2].

Various types of electronic load controller so far developed are reported in the literature [3]. The ELC developed so far works on frequency balance technique in which the ballast of each phase is fired with same firing angle. So, all ballast loads consume equal power resulting to overloading of synchronous generator for unbalanced loads and oversized generators are selected to overcome this problem. This project deals with the frequency and phase current balance technique of ELC to reduce the overloading of the generator. In the proposed scheme, the thyristor pair is used to control the power consumed by ballast of each phase and the change in load in one phase doesnt affect the dummy power consumption of other phases. The scheme is simulated using Matlab Simulink and the simulation results are presented.

Objective

-To design and fabricate the advance type ELC for micro hydropower plant in which generator terminal currents are balanced even if the consumers loads are un-balanced and thus the problem of generator overloading is eliminated.

Methodology

Step 1: Study the various type of the ELC developed so far and identify their problems.

Step 2: Develop the simulink model of the phase angle control type electronic load controller using frequency balancing technique to study its working principle and identify the problems associated with it.

Step 3: Develop simulation model of advance electronic load controller using phase current and frequency balancing to overcome the problems identified.

Step 4: Select proper microcontroller to implement hardware of the proposed ELC.

Step 5: Develop simulation model of proposed scheme in proteus.

Step 6: Fabricate and test the hardware of the proposed model.

Step 7: Document the research results as a report.

Proposed Scheme

Fig.1 shows the schematic diagram of the proposed scheme. The synchronous generator has an exciter, which provides a constant excitation to produce normal rated terminal voltage at full resistive load and it is capable of generating the reactive power too. It is driven by unregulated turbine with constant power output of 1 p.u. The generator supplies power to the three phase resistive consumer load and the proposed ELC with resistive ballast load in each phase is connected in parallel to the load. When the load of the consumer is changed with unbalanced loading, the frequency and per phase load current changes. The frequency and the load currents of each phase are sensed. Then the firing angles of respective phases are calculated and changed to make constant frequency and balanced generator terminal currents.

Fig. 1. Schematic diagram of the proposed scheme

Modeling of Electronic Load Controller

ELC is used to consume the excess of power delivered by the synchronous generator. The ELC proposed in the scheme consumes power in such a way that the excessive power of particular phase is dissipated in the ballast of the respective phase. So, the generator is not overloaded. In case of no load condition, the ELC can dissipate all active power generated in the ballast load. Fig.2 shows the basic circuit diagram and the control strategy of the proposed ELC.

Fig. 2. Basic circuit of proposed scheme

In this scheme, the three phase generator is connected to a three phase varying load. The frequency of the generated voltage and load currents of all phases are sensed.

Fig. 3. Logic for firing angle generation

Fig. 3 shows the logic behind the firing angle generation for the ballast of each phase. The frequency of the generated voltage is compared with the reference frequency i.e. 50 Hz. The error so obtained is fed to the PI-controller [4]. The PI-controller generates firing angle to balance the frequency of the generated voltage. Moreover, the load current of each phase is fed to the microcontroller. Inside the microcontroller, each phase load current is compared with the rated generator current i.e. 23A for 16KVA generator. This error current of each phase should flow through the ballast of respective phase to balance the generator terminal currents. In order to flow this desired current through ballast, the firing angle is calculated inside the microcontroller using equation (3). Equation (1) and equation (2) give the output voltage and current of the chopped waveform for different values of firing angle for the resistive ballast load [5]. Using data of output chopped current and firing angle obtained from equation (2), the equation (3) has been derived using regression method. Equation (3) gives the value of firing angle alpha for a particular value of current. In this way, the microcontroller generates the firing angle for the ballast of each phase using equation (3) which balances the phase current of the generator. Finally, the firing angle generated by microcontroller for each phase is added with the firing angle generated by the PI-controller. The firing angle so obtained for each phase fires the ballast of respective phase. In this way, both the generator terminal currents and frequency are balanced without causing overload to the generator even in unbalanced load.

(1)

Io== (2)

(3)

Where,

=output rms voltage of the chopped waveform

=input rms voltage

for the thysristor

=output rms current of the chopped waveform

of the ballast

The required value of ballast load resistance is calculated as follows:

Line to line voltage (VL) = 400 V

Generator full load current (Iphase) = = = 23A

Consumer load per phase (Pphase) = 5 KW

Ballast resistance per phase (Rballast)== = 9.45 9

Simulation results

The complete simulation model of proposed scheme is shown in Fig.4. Fig.5 shows the detail of ELC block. The scheme consists of a 16 KVA synchronous generator. The resistance of the dummy load is taken as 9 ohm in each phase. This rating of ballast load is able to consume all the power generated by the generator at no- load case. The excitation voltage Vf of synchronous generator is limited to 2.1 p.u, which is just sufficient to produce 1 p.u. of stator terminal voltage at full resistive load.

The model is simulated with the varying loads. First of all, the model is operated with load of 4 KW in each phase. After 3 sec, the load of R-phase is disconnected. After 6 sec, the load of Y-phase is also disconnected. After 9 sec, the loads of all phases are disconnected from the system. After 12 sec, the loads of all phases are again switched ON. This switching of load is shown in Table-1.

Table-2 shows the balance of active power between the generation and consumption. Table-3 shows the current variables of the system during the simulation period. These tables show the data that are obtained from the responses shown in Fig. 6. This shows that the total power generated by the generator is always equal to the sum of power consumed by the ballast load and consumer load.

The simulation results show that there are some transients because of switching on and off of loads at the interval of 3 sec. The current, power and frequency during the transient are within the acceptable range. The generator current reaches at most to the value of 24.47A during the time of 3-6 sec as shown in Table [3]. This leads to a overloading of generator to the value of about 24% and this is the worst case for the proposed scheme.

Hardware fabrication

Microcontroller

The ATmega32 is a low-power CMOS 8-bit microcontroller based on the AVR enhanced RISC architecture. By executing powerful instructions in a single clock cycle, the ATmega32 achieves throughputs approaching 1 MIPS per MHz allowing the system designers to optimize power consumption versus processing speed. The pin diagram of Atmega32 is shown in the fig. 7.

Fig.7 Pin diagram of Atmega32

Frequency sensor

The frequency sensor senses the frequency of the generated voltage by synchronous generator. In advance ELC, we sense the generator frequency and compare with the reference frequency to generate the error signal for PI Controller. The proteus model and hardware of the frequency sensor is shown in the fig. 8.2. In the circuit of frequency sensor, the voltage output from the transformer gives 6 V rms output before the diode at point A in the figure. Since the negative voltage cannot be fed to the microcontroller so the output voltage from the potential transformer is half wave rectified using diode. Now, this rectified wave is fed to the base of transistor. The transistor works on the principle that when the base voltage is greater than 0.7 V, it turns on and when the base voltage is less than 0.7 V, it turns off, thus generating the square wave pulses. These pulses are then fed to the microcontroller to calculate frequency. The measured frequency of the NEA supply is displayed in LCD as shown in fig. 8.1.

Fig. 8.1 Proteus model of frequency sensorFig. 8.2. pcb of frequency sensor

USART (Universal Synchronous and Asynchronous serial Receiver and Transmitter)

The Universal Synchronous and Asynchronous serial Receiver and Transmitter (USART) is a highly flexible serial communication device. The USART feature of the AVR microcontroller can communicate with another microcontroller (not necessarily another AVR microcontroller), multiple microcontrollers, or a computer using a voltage level shifter or converter. We have used USART of AVR microcontroller in asynchronous mode i.e. UART. Asynchronous is where the microcontroller's clock is not connected together by a wire with the other microcontroller, but they need the same clock beat to process the data on the data line. In advance ELC, we have used this feature of AVR microcontroller because the series Atmega32 of AVR family has only got 3 independent timer module and one timer is used for frequency measurement purpose and remaining two will be used to generate PWM for two phases and PWM for remaining one phase could not be generated by using single microcontroller. So to solve this problem we have used two number of Atmega32 microcontroller with its UART feature to communicate with each other and transfer the frequency sensed by one microcontroller to other microcontroller. Thus, by using two microcontroller we can generate 3 PWM signal one for each phase. The proteus model of it is shown in fig. 9.

Fig. 9. UART communication between two microcontrollers.

Zero Crossing Detector (ZCD)

The full wave rectified signal is obtained through center tapped 6-0-6 transformer as shown in fig. 10.1 and fig. 10.3. This rectified signal is fed to the base of n-p-n transistor. When the voltage of base falls below certain voltage level, the capacitor across the resistor discharge. So, the zero crossing is detected as shown in fig. 10.2.

Fig. 10.1 proteus model of ZCDFig. 10.2 ouput of ZCDFig. 10.3. PCB of ZCD

Offset circuit

The op-amp (LM741) has been used in inverting mode as shown in fig. 11.2 and fig. 11.3. The reference voltage to the inverting terminal is given. And the output signal is offset twice the voltage given in the inverting terminal as shown in fig. 11.1.

Fig. 11.1 OutputFig. 11.2 Proteus circuit of offset Fig.11.3 Proteus circuit of offset

Current measure

Current cant be measured directly through microcontroller. So, a transducer called current probe has been used to convert the ac current to equivalent ac voltage which has a gain of 0.1 V/A as shown in fig.12.2. This ac output voltage cant be fed to the microcontroller because the negative signal cant be fed to microcontroller. So, the signal output from the probe is fed to offset circuit which offset this as voltage by 2.5 V. The offset signal is then fed to microcontroller. After sampling the data at an interval of 0.1 milliseconds, 400 different samples are taken & then subtracted from offset value (2.5 V) and difference is divided by gain of probe. The difference is squared and square roots of mean of these differences give the ac current.

Fig. 12.1 NEA supply frequency and fig. 12.2. Current probe

Measured current of 750 W heater

Triac Fire

BTA41 traic has been selected as shown in fig. 13.1 and fig. 13.3.The triac fire optocoupler MOC3021 with a led and photodiac has been used to isolate the higher voltage ac signal from the lower voltage of microcontroller. When the gate signal is fed to input of optocoupler, the led turns on. The photodiac gets short circuited.The ac voltage signal flow through diac to the gate of traic. In this way, triac is biased. The traic stops conducting at zero crossing. In this way, the voltage across load is controlled by firing triac with gate signal at different time intervals as shown in fig. 13.2.

Fig. 13.1 proteus circuit of triac fire Fig. 13.2 output chopped Fig. 13.3 PCB of triac fire

waveform

555 timer

The 555 timer has been used in astable mode as shown in fig. 14.1 and fig. 14.2. After tuning the value of Ra, Rb & C, we get the output pulses of 25.6 KHz as shown in fig. 14.3. that is the reference input clock pulse for the microcontroller to generate the PWM of 100Hz.

25.6 KHz pulse

Fig. 14.1 555 timer in Fig. 14.2. PCB of 555 timerFig. 14.3 output pulse

astable mode

Conclusion

The Advance ELC has reduced worst case overloading from 73% in phase angle regulation type ELC to about 24%. The developed ELC also improved generator terminal current balancing. The developed ELC is better than the existing phase angle regulation type ELC.

References

[1] I.Tamrakar, L.B. Shilpakar, B.G. Fernandes and R. Nilsen, Voltage and frequency control of parallel operated synchronous generator and induction generator with STATCOM in micro hydro scheme, IET Generation, Transmission, Distribution., vol. 1, pp. 743-750, September 2007.

[2] Adam Harvey, Andy Brown, Priyantha Hettiarachi and Allen Inversin Micro hydro design manual Intermediate Technology Publications, 1993.

[3] Prof. I. Tamrakar, Development Stages of ELC for Micro-Hydro Power Plant, E-novation, Vol. 2, pp-1, 2002.

[4] K. Ogata, Modern Control Engineering, fourth edition, 2005.

[5] M.H. Rashid, Power electronics circuits, devices, and applications, third edition, 2004.

DESIGN AND FABRICATION OF ADVANCE ELC

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