Auto Mains Failure System Auto Mains Failure System is Employed for Auto start up of the DG when the Mains Supply fails . This is a very common system. Here we will see that what is the basic concept in AMF? What hardware is required to set up AMF and how PLC Programming is done. First of all let us understand what is AMF? Generally all the major industries/companies/institution have the DG for power back up , but when main power is cut off , someone has to go to start the DG and this takes time and also a man has to be kept for this purpose. So to eliminate this process PLC panel is installed to Auto start the DG when main power fails. Working is as follows:when main power goes , a signal is received to PLC and after a delay DG starts . When the Main supply fail signal is received by PLC then after a delay time, output for ACB/Contactor of DG is ON and DG starts automatically. Also the ACB/Contractor of main transformer if sent OFF Command. When main power comes then the signal is again sent to PLC and PLC off the ACB/Contactor of DG and after 2 seconds it on the ACB/Contactor of main transformer and after 30 seconds it off the DG. DG is sent OFF command a little late so that the load beard by DG for a long time can be compensated by running at no load for some time. NOTE :- In no Condition the ACB/Contactor of both DG and Main transformer should be ON. Sufficient interlocks will be provided. So this is the main concept in DG AMF system. In a simple system there are generally 4 inputs and 4 outputs. Inputs :- 1. DG ACB/Contactor close feedback. 2. Transformer ACB/Contactor close feedback. 3. Transformer Voltage Available. 4. DG Voltage Available. Outputs :- 1. DG start. 2. DG Stop 3. DG ACB/Contactor close. 4. Transformer ACB/Contactor close.
Feature of PLC based AMF system Conventional AMF systems are relay based. These systems are having lot of operational issues arising because of large and complex wiring, which make sit prone to faults. This is why, of late more and more AMF panels are equipped with PLC for safety interlock and operations. In PLC based system all the logic and interlock wiring will be replaced by internal PLC logic program. Generator Protection – Generator will be tripped during LLOP and HWT. Change Over time – The generator breaker will close within 3-6 seconds after generator has started. Advanced Phase Failure Sensing – In case of the EB supply phase fails or falls below a set threshold limit then automatically the generator gets started and supply will be restored. Flickering Buffering– At the point of loss of EB supply, the system will wait few seconds to confirm that the supply is going to stay out before stating the generator. This prevents the operation of generator during flicker (momentary power failure) Multimode selection - The system works in three modes: Auto mode, Manual mode and Emergency mode. Power management System – PLC based AMF system can be incorporated with Power Management System. Load sharing can be provided. Also DG can be started based on the load. Remote Monitoring - SCADA system can be provided to the system which enables controlling and monitoring of the system at a remote station. Future expansion – PLC based AMF system makes future expansion more easy, fast and less expensive , as new system can be incorporated with existing system. Less maintenance – As PLCs and other hardware are designed for harsh and rough condition, chances of occurring faults are minimized, so maintenance is reduced.
Features of a PLC (Programmable Logic Controller) • •
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Flexibility: One single Programmable Logic Controller can easily run many machines. Correcting Errors: In old days, with wired relay-type panels, any program alterations required time for rewiring of panels and devices. With PLC control any change in circuit design or sequence is as simple as retyping the logic. Correcting errors in PLC is extremely short and cost effective. Space Efficient: Today's Programmable Logic Control memory is getting bigger and bigger this means that we can generate more and more contacts, coils, timers, sequencers, counters and so on. We can have thousands of contact timers and counters in a single PLC. Imagine what it would be like to have so many things in one panel.
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Low Cost: Prices of Programmable Logic Controllers vary from few hundreds to few thousands. This is nothing compared to the prices of the contact and coils and timers that you would pay to match the same things. Add to that the installation cost, the shipping cost and so on. Testing: A Programmable Logic Control program can be tested and evaluated in a lab. The program can be tested, validated and corrected saving very valuable time. Visual observation: When running a PLC program a visual operation can be seen on the screen. Hence troubleshooting a circuit is really quick, easy and simple. Operating speed:PLC operation speed is faster than the relay. Speed PLC scan time is determined by its in unit of milliseconds. Simplifies the control system components: The PLC also have counters, relays and other components, so it does not require components such as additional. Use of relays requires counters, timers or other components as additional equipment.
