DESIGN ANALYSIS OF AN AUTOMATIC PHASE SELECTOR

DESIGN ANALYSIS OF AN AUTOMATIC PHASE SELECTOR ADEDOTUN O. OWOJORI1 Corresponding author, email [email protected] , ABIODUN M. AKINBOLADE2, KAYODE F. AKINGBADE1 1Department of Electrical and Electronics Engineering, Federal University of Technology PMB 704, Akure, Ondo State, Nigeria 2Department of Operation, Ondo State Electricity Board, Akure, Ondo State, Nigeria Abstract: Power instability in Nigeria caused by overbearing demand of power by consumers and lack of proper maintenance of the power system devices among others has brought about the need for alternative power sources such as generators, solar, typical inverters and other alternative supplies which requires one form of switching or the other to achieve phase selection during power failure. This paper gives a design analysis of an automatic phase selector linking available power supplies, that is; switching between a three-phase public utility supply, as a result of total power outage in the public supply to an alternative secondary supply (in this case a Generator and an Inverter system) and back when power is restored. The design adopts the use of a microcontroller-based system interconnected with other hardware components for proper isolation, switching and visualization of switching conditions. The system design is divided into two major part: the hardware which consists of the power supply, sensing circuit, controller or control logic circuit, display and the power electronics switching unit and the software instruction code on the microcontroller unit. The design analysis was first carried out accompanied with computer simulation on a software tool (Proteus 8 Professional, version 8.4) to carry out performance evaluation of the sub-circuits, thereafter, a practical implementation of the design was carried out and tested with the utility power supply using five (5) switches, three of which represents the three-phase primary supply and the other two represents the secondary supply. Keywords: System Automation; power system; phase selector; optocoupler 1. Introduction Nigerians are confronted with issue of power instability needed for continuous services in key sectors of the economy such as research and development (R&D), banking, industrial and health sector hence leading to poor research output, loss, increased cost of production, damages on appliances and even death [1]. These sectors are made up of critical loads that needed to be powered at all times in order to carry out their various processes efficiently [2]. This in turn has made most of these sectors depend on alternative supply of power. It is often noticed that power interruption in the distribution system could be deliberate as a result of curbing unscrupulous customers engaged in Electricity theft [3] or caused by single-phase failure which accounts for 70% of failure on a three-phase system, which leaves the other two phases in normal condition, thus making a change-over switch is important in such a scenario. Alternatively, commercial or industrial outfits makes use of a distributed three-phase system to supply power to various sections or equipment within the industry and in extreme cases of power failures, an alternative (secondary) power supply need to be provided. Fig.1. Manual change-over switch [3]. The introduction of a change-over switch within the three-phase mains and between the mains and the alternative sources shows some challenges in terms of switching smoothly in a timely manner, whenever there is power failure on the main utility [3,4] as shown in Figure 1. These challenges are addressed by the use of an Automatic Phase Selector (APS). The APS has a sensing circuit interfaced with a programmable integrated circuit (PIC) controller or microcontroller [5]. This allows seamless transition between the main and alternative power sources to the load [6,7]. The APS monitors the incoming mains supply voltage (approximately 240 Vrms) and detects when the voltage drops below a certain level or goes off, and cannot serve electrical or electronic appliances [1,8]. At this point of phase failure, a zero-voltage equivalent is delivered to a control circuit which rectifies the zero voltage Alternating Current (AC) to Direct Current (DC) whose output is fed through the sensing circuit to a microcontroller. Simultaneously an alternative voltage source is identified and sent to the microcontroller. The APS’s microcontroller unit compares the voltage between the individual mains component (that is Red, Yellow and Blue phase) and the alternative power sources (that is a solar inverter and generating set) using a comparator circuit to select an available source to power the load via a transistor driver-automatic relay circuit. This concept is similar to a multiplexer circuit design in selecting individual phases [9]. The mains and alternative supply circuit are designed to prevent feedback current into the system and ensure it is in proper synch in terms of voltage and frequency [10]. The auto-selection mode in this system is achieved by stacking a set of relays interconnected in such a way that if one of the relays feeding the load is initially energized; following a phase failure condition, the corresponding secondary winding of the step-down transformer delivers zero voltage which is duly rectified to DC and then fed to the sensing and micro-controller circuit which automatically switches ON the next relay that delivers power to the load. The microcontroller unit also provides a visualization interface with an LCD unit, on which the selected phase can be displayed. Unlike conventional circuits where transformers are used to step down the power supply [11], this system utilizes a direct supply of five sources; three connected to the three-phase supply and the other two are supplies from solar inverters and generator. These were represented by five electrical switches from a single-phase power system which serves as sources of power to the system as shown in Figure 2. The direct supply implies the system is lethal having direct supply of 240 Vrms and as such the design should be handled carefully with correct cabling specifications. The design was simulated on Proteus 8 Professional V8.4 and hardware developed to be low cost and easily adaptable to houses, offices, hospitals etc. faced with constant power outages. Fig.2. Five electrical switches representing the three-phase mains and two alternative power supply The system consists of four main parts namely; the voltage sensing circuit, controller (which is the brain of the system) shown in Figure 3, transistor driver, and electrical switching devices (Relay). The circuit connection is divided into power lines and signal lines where the power lines is also subdivided into AC mains and DC power supply. A portion of the input power line is rectified and fed to the sensing unit, controller unit and switching unit, while the other portion is fed directly to the load via an electrical switch control as shown in Figure 4. The signal lines are used to link small voltage signals from the sensing unit to the microcontroller and between microcontroller and the display unit. Figure 4 illustrates how the various unit interconnect each other and are independent of load connected. Fig.3. Microcontroller unit Fig.4. Block diagram of an automatic phase selector 2. ExperimentAL SETUP 2.1. Sensing Unit The voltage sensing circuit utilizes a sensor or electronic configuration to detect input supply into the system. The supplied voltage is preprocessed to allow control operation, which involves conversion from an Alternating Current (AC) to Direct Current (DC). A direct connection from the supply to the control circuit requires careful selection of components to operate the system. An approximate input voltage of 240 Vrms at 50 Hz frequency is expected from mains power supply, hence an appropriate choice of diode is required to ensure the current and power rating are not violated. In full-wave bridge rectifier configuration shown in Figure 5, the IN4007 diodes are used having a maximum peak reverse voltage (VRRM) of 1000 V and average forward current (IF) of 1 A. Considering the rectifying circuit (1) From equation (1) an approximate peak reverse voltage (VRRM) of 680 V is required, of which the peak voltage is 340 V. A capacitor is introduced to the circuit whose estimates is expressed in equation (2) to filter out the ripples. An external 9 V supply voltage was also introduced to the circuit via the microcontroller terminal. This helps to nullify the use of a voltage regulator capable of withstanding the high DC voltage from AC mains. Moreover, a voltage regulator present on the Arduino board makes it easier to convert the 9 V supply to 5 V. (2) Fig.5. Full-wave bridge rectifier configuration with electronic sensing 2.1.1. Electronic configuration circuit An electronic configuration uses the concept of voltage division to supply signal voltage to the microcontroller circuit as expressed in equation (3). The DC voltage output is obtained with the help of fixed and variable resistors connected in series to set a detectable voltage range on the microcontroller and the presence of a Zener diode ensures that the voltage does not exceed the Zener voltage. (3) With the configuration and resistor parameters chosen, the output voltage will be far lesser than the input DC voltage output. This voltage however will be constrained by the IN4732 Zener diode which has a power and voltage rating. 2.1.2. Sensor A typical sensor (optocoupler) as shown in Figure 6, on the other hand, was deployed for real-life circuit development. The optocoupler is an electronic component capable of transferring electrical signal between two isolated circuits such as a high voltage and low voltage circuit. Fig.6. A typical optocoupler circuit The component with augmented circuit configuration operates based on infra-red transmission at the primary side and detection at the base of the secondary side of the optocoupler. The secondary side connected to a low voltage (Vcc) gives out an output proportional to the secondary voltage as expressed in equation (4), once it detects an infra-red transmission. (4) 2.2. The Controller Unit In both sensing cases, possible fluctuation may occur warranting the use of the ADC input pins on the Arduino expressed in equation (5). The Arduino processing unit shown in Figure 3, consists of the ATMEGA 328 microprocessor on which the firmware is stored which supports clocking and reset functions, memory and I/O registers [1]. The controller unit operates based on instruction codes (in software) to sense the input from the Analog input pins to actuate signals sent to the base terminal of the transistor driver circuit. (5) 2.2. Switching unit The switching circuit operates with a voltage supply of 5 VDC biasing the collector terminal of the five NPN transistor and the primary side of the relay module. Depending on the sensed signal, the microcontroller triggers required driver circuit whose output via the collector is fed to the relay module. The secondary side of the relay directly connects the input AC power supply and at the contacting terminal, they are all interlocked. This stage requires care as two or more different sources intersecting each other could cause an explosion. In other words, the effectiveness of the microcontroller, transistor and the proper operation of the relay module must be ascertained. The interlocked terminal is connected to a test load that selects only one supply based on availability and priority settings, while others are switched OFF or made inactive. Diodes are externally incorporated in the primary side of the relay module to prevent inductive spikes or flyback current from destroying the transistors in relay driving circuits. Figure 7 shows an example of a three single-phase switching configuration assuming the mains supply are grouped as one while the other two are for generating set and solar inverter unit. Fig.7. Three single-phase switching configuration circuit 3. Results and discussion The sample automatic three single-phase selector configuration was tested in simulation mode with three single phases power supply and later expanded and deployed for hardware implementation design. The switching process perfectly mimics the control flow chart in Figure 8. It demonstrates priority placed on the mains power supply ‘PHCN’ now BEDC as against the alternative supplies. The generator or solar inverter supply automatically switches ON whenever the mains supply goes OFF with a switching delay of 5 secs. The following tests results were summarized in Table 1. Table SEQ Table \* ARABIC 1. Test and Result analysis Tests Results Switching OFF the mains supply The switching circuit or relay attached to the generator terminal triggers to switch ON the load Assuming there is no power from the generator demonstrated by an OFF switch, the relay attached to the solar inverter automatically triggers to switch ON the load Switching OFF the generator power supply The circuit automatically switches back to the mains supply based on priority set on it and hence triggers the relay attached to the mains supply to switch ON the load The situation changes when power from both mains and generator are OFF as the relay attached to the solar inverter triggers and switches ON the load Switching OFF of the solar inverter supply The circuit automatically switches to the mains supply based on priority, hence triggering the relay attached to the mains to switch ON the load. In a situation when the mains power supply is OFF the generator automatically switches ON via its relay triggering and supply the output load Switching OFF all the supply inputs. The output load goes OFF and the monitoring unit displays no supply Switching ON all the supply inputs The control circuit automatically selects the mains supply based on priority hereby triggering the relay attached to the mains while others are disregarded by the control circuit to avoid short of any form by the input supplies Timing of the delay circuit from the microcontroller The circuit allowed a timed delay of within ±5 secs tolerance Reset switch This is an external button used to free the memory of the control unit is all processes hereby creating enough space in the memory. The implemented APS simply extends the workings of the flow chart by splitting the mains supply into three independent sources whereas the algorithm remains the same by selecting the red phase as priority in the switching order. This device can as well be implemented by adopting the finite state machine technique. Fig.8. Automatic Phase Selector Flow Chart Fig.9. Automatic phase selector simulation circuit In other to test the performance of the system, Switch R1-Rn, where n is three, are introduced to control the red phase, yellow phase, blue phase, switch G and S are introduced to control the generator and solar input power supply. The public supply was used connected from three separate terminals to represent the different phases. At the output of the system, a 60 W bulb was connected as load and the same procedure based on the phase selector algorithm was used as described in Table 1 and shown in Figure 8. An OFF state represents an outage while an ON state represents availability of power supply. 4. CONCLUSION This paper had illustrated how to analyze, design and construct a microcontroller-based automatic single-phase selector from three single-phase sources in the simulation stage and additional two alternative sources (generator and solar inverter) in anticipation of the implementation stage design. This design adopts the use of low-cost materials which made it easily affordable to residential building, industries and particularly hospitals and ensures the right components and cable size were chosen. The design analysis was first carried out to test the performance of the sub-circuits, thereafter, practical implementation of the design was carried out with the utility power supply to switch a 60W bulb. REFERENCES [1] Ihedioha, A. C., Design and implementation of a microcontroller based automatic three phase selector, IJARIIE, vol. 3, no. 1, 2017, p.115- 122 [2] Adedokun J. A., Oladosu, K. 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