Reliability is one of the most important aspects we should be focused on especially when it comes to Plant Maintenance. Frequent shutdowns in plant could be caused by poor maintenance management and lack of advance tools to keep the system running.
For a plant working 24/7, most of the running equipments should be treated critically and monitored regularly. However, Redundant Systems are used to save the plant from shutdowns if failure occurs. Aside from back-up power we should always thought of back-up systems that will manually or automatically take over the operations in the event of fault or alarm.
In this article, I will share with you the redundant systems that I have encountered by working in Maintenance Department and Engineering Design.
Table of contents
1. Uninterruptible Power Supply
2. PLC CPU and Communication Module
3. Power Supply Unit
4. Diesel Engine
5. Redundant Loads
Uninterruptible Power Supply
Uninterruptible Power Supply or UPS is an electronic type of equipment that serves as a back-up power supply to some critical loads. Critical loads are defined by your plant operation. However, equipments such as Drives, Programmable Logic Controller, Power Supply Unit and Softstarter are the common loads of UPS that I have encountered and designed.
It consists of Rectifier that will convert the AC supply to DC. It’s supported by battery banks that are computed based on UPS Rating. Also, an inverter that will convert the DC voltage to AC as an outgoing supply to load.
UPS can be redundant especially on critical load or equipments need to run 24/7. In case of power failure or fluctuations, both UPS shall take over and keep system running based on battery ampere-hour ratings.
Redundant UPS has individual output to supply a common load. This will increase the reliability of your supply to critical loads. They are synchronized as they work in parallel operations. When one UPS is shutdown by failure or by preventive maintenance. The other UPS can take over the operations as back up power. Therefore, your UPS shall be sized to carry the all connected loads individually.
PLC CPU, Power Supply and Communication Module
Most conventional PLC design consist of a CPU, communication module, power supply module and Input/Output modules. However, for plants that are working 24/7 with critical loads to be controlled. System must be backed up with another controller that will take over in case of failure.
Those modules mentioned above can give you different types of faults that can shutdown the operations. It’s one of the reasons why some plants required redundant system for controllers. In some manufacturers, they call it “hot stand-by” where another CPU and communication module will take over the operations when an error to primary controller occurs. Secondary controller consists of the same program and configuration as the primary controller. However, it always comes with a redundancy module and a specific cable. This system have a separated rack with the same modules installed and have communication configuration based on manufacturers recommendation. Therefore, selection of modules must be based on manufacturers recommendations. When unsure, you can always contact technical support from manufacturers.
This will give you time to check and troubleshoot any failure to PLC or communication after an automatic swap to secondary CPU.
Power Supply Unit
Based on what I have encountered, Power Supply Unit or PSU in 24V output mostly supplies PLC Modules, control circuits, OIT, HMI, etc.
A failure in Power Supply Unit will make a big impact to operations or shutdown a system. That is why redundant power supply is used. It comes with a redundancy power supply module and two power supply units. Both power supply unit will give voltage to load through a redundancy module.
When one power supply unit fails, the other power supply unit will be left on giving supply to load. It means, both power supply shall be sized and can handle the full load capacity of loads individually.
Diesel Engine or DG is commonly used as back-up power supply to a certain load or whole system in the event of normal power failure. In most cases, one DG is enough to supply a certain load. However, in system with many critical equipments and require a continuous operations, redundant DG is used.
Based on my experience, our Diesel Engines used in plant work in parallel operations. A synchronization panel will take care of the parameters to synchronize the two DG. DG controller has coordination with plant undervoltage sensor that will give signal to start the DG after a predetermined time. Two DG will start automatically and will work in parallel through synchroscope.
The advantage of redundant DG as back-up supply is to ensure that if one DG Fail to start in auto mode, we can manually start the other DG. Means, they can work alone to supply critical loads. However, we should always ensure that both DG will work in case of emergency by doing weekly no-load or load test.
Load test can be done by synchronizing two DG and normal power to work in parallel. Therefore, synchroscope for two DG and another for normal power shall be provided.
Aside from redundant back-up for supply and controller, critical loads must have redundant system too. It will be useless for back-up power and controller if your critical loads don’t have redundant system in case of failure to equipment. Therefore, redundant system to loads were designed for reliability of operations. Below are the redundant loads that I have encountered:
Medium and Low Voltage Transformers and Switchgear
Transformer is an electrical machine that transfers electrical energy from one circuit to another. It works with the principle of electromagnetic or mutual induction.
As experienced in one of the plants I’ve worked with. They used two Transformers to supply one Switchgear as shown in the photo above. It uses three air circuit breakers (two as Mains and one as Coupler) for power coordination.
It means, when one Transformer is shutdown by failure or preventive maintenance, one transformer can supply all loads thru coupler. However, both doesn’t work in parallel and coupler must be open when both transformer will supply their assigned loads.
Blowers and Fans
Blowers and fans could be critical in some plants just like what we have in one plant I’ve worked with. As they need this for cooling, exhausting, firing, etc. which are essential for operation and safety.
Therefore, it’s advisable to have redundant system to enable swapping in the event of maintenance or failure. It could be manually or automatically started as they have permissive signals need to satisfy.
Compressors are the main supply of pressurized air as need in most plants. In our plant, it works with a lead-lag principle where in there will be lead compressor. It carries the initial air pressure requirement and lag compressor will sustain the required pressure if lead compressor reached maximum setting.
You can manually assign which compressor will be your lead. However, it’s preferred to have a cycle of lead operation in each compressor available in your system.