The so-called energy efficiency optimization of the power generation systems, power transmission systems, and energy consumption systems we mentioned in the previous chapters are actually to solve two problems. One is to determine how many devices to use, and the other is to allocate the load rate of each device.

3.1 The Four Components of the Energy-Saving Control System

In order to solve these two problems, it is necessary to collect on-site process data and equipment operation data, which requires sensors to complete this task. Commonly used sensors include: pressure, flow, temperature, speed, torque and other sensors, and then the process data, energy parameters and equipment operation data measured by the sensors are sent to the controller. Common controllers include: PLC programmable controllers, DCS distributed controllers or other controllers. Through the analysis and calculation of the software program in the controller, the switch control signal is sent to the actuator to realize the start and stop switching of the equipment, send the analog control signal to the actuator to control the load distribution of each operating equipment. Commonly used actuators include: governors, frequency converters, clutches, valves, etc. In order for the operator to have an intuitive understanding of the process status and equipment operation, it is necessary to use a touch screen or a computer. Use the touch screen software to program the touch screen, or use the universal configuration software to program the PC. After completing the software programming, these operating data can be displayed on the screen for monitoring. At the same time, data input windows and buttons should be left on the screen to realize the operator's modification and control of process parameters and control parameters, as well as start and stop of equipment.

Simply put, the energy-saving control system includes four parts:

  1. (1)

    Sensors,

  2. (2)

    Actuators,

  3. (3)

    Controller and software,

  4. (4)

    Touch screen and programming software (or PC and configuration software).

3.2 Several Structures of Energy-Saving Control System

3.2.1 Single Controller Structure

For the situation where the energy-saving control system is relatively small and the equipment is relatively concentrated, one controller (such as PLC) can generally be used for energy-saving control.

For example, the pum** station for secondary water supply, 2–5 sets of 3–15 kW water pumps, are small in size and are generally placed in the basement of a building, occupying a very small space. For such a pum** station energy-saving control system, it can be installed in 1–2 control cabinets on site. As shown in Fig. 3.1, the pressure sensor at the inlet and outlet of the pipeline, the flow rate sensor at the outlet of the pipeline are installed on the pipeline. The air circuit-breaker, intermediate relay, AC contactor, power sensor, frequency converter, PLC programmable controller are installed inside the cabinet, and the touch screen is installed on the cabinet door.

Fig. 3.1
A photo of a secondary water supply pump station. The setup includes P 1, P 2, F 1, F 2, F 3, and F 4, a control panel, and a large storage tank. The pumps are aligned in a row, with pi** and electrical connections.

Secondary water supply pump station

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The control structure of this small energy-saving system is shown in Fig. 3.2.

Fig. 3.2
A schematic of a small energy-saving control system. It depicts connections between a human-machine interface, a programmable logic controller, and network components via R J 45 cables. The P L C connects to various input and output modules.

A small energy-saving control system

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Figure 3.2 is a block diagram of an energy-saving control system for a small pum** station.

In Fig. 3.2, the HMI is a touch screen, and it is connected to the PLC through the programming network port RJ45. PLC has 3 analog input signals, pum** station input pressure P1, pum** station output pressure P2 and pum** station flow F1. PLC reads the data of power synthesis module W1, voltage, current and power data through RS485 communication port, and the four of PLC Digital output DO and 4 analog output AO control the start-stop and frequency of 4 frequency converters. 4 frequency converters drive the motors of 4 water pumps.

3.2.2 Multi-Controller Structure

The same pum** station, for a medium or large pum** station energy-saving control system, for example, a water plant pum** station with a daily water supply of 650,000 tons and a circulating water pum** station in the steel industry. As shown in Fig. 3.3, it is much more complicated. Each pump may require the installation of flow rate sensors and pressure sensors, as well as vibration sensors and temperature sensors, so that the working status, fault diagnosis and energy efficiency of each pump can be monitored online at any time. The volume of the pump is large, and the footprint of the pipeline is also large. The sensors are scattered and installed in different motors, pumps and relevant parts of the pipeline.

Fig. 3.3
2 photographs of pum** stations with large-scale industrial pumps installed in a facility. The stations include colored pumps, pi**, control panels, and surrounding infrastructure.

Pum** stations

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Figure 3.3 water plant pum** station and steel mill circulating water pum** station.

For a large-scale energy-saving system, such as a thermal power plant, a hydropower station, or a large pum** station, it may involve multiple workshops, multiple workstations, or multiple large-scale equipment. If there is a certain distance between workshops, workstations, or large-scale equipment, the network monitoring of multi-PLC and multi-PC can be realized by using Profibus-DP bus or industrial Ethernet.

3.2.2.1 Profibus-DP Fieldbus

The Profibus-DP bus uses optical fiber transmission to realize long-distance communication at a high communication speed. Since the optical signal is not subject to external electromagnetic interference, the anti-interference ability of the bus can be improved.

Install an optical fiber module (OLM: Optical Link Module) on a DP network segment to connect each PLC and PC with a DP bus interface to the OLM, as shown in Fig. 3.4. The OLM module converts the DP electrical signal of the device into an optical signal and transmits the signal using an optical cable. The electrical signals between the OLM fiber optic modules are electrically isolated. The OLM module automatically recognizes the communication speed of the DP bus, from 1.6Kbps to 12Mbps. Through the fiber optic module, the total length of the communication distance can be greatly improved. Using glass fiber, OLM fiber optic modules, the communication distance between them can reach 10–15 km.

Fig. 3.4
A schematic diagram of Profibus D P bus optical fiber transmission. The network includes sensors, actuators, and governors connected to multiple P L Cs via D P bus and optical link modules, O L M. The P L Cs connect to a personal computer, P C 1 for control and monitoring.

Profibus-DP bus-optical fiber transmission

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In Fig. 3.3.4, 3 PLCs and 1 PC are connected together with Profibus-DP field bus. 1#PLC is in an independent workshop and uses an OLM module alone. 2#PLC and 3#PLC are in the same workshop, relatively close to each other. They are arranged in a DP network segment, and share an OLM module. The PC in the control center is far away from the two workshops and uses an OLM module alone. Various sensor signals enter three PLCs respectively, and the control signals of the three PLCs are sent to the actuators in the two workshops respectively.

3.2.2.2 Industrial Ethernet

Selecting PLC and PC with industrial Ethernet interface can make use of the existing Ethernet equipment and structure to realize the networking and information transmission of the energy-saving control system, which is very convenient, as shown in Fig. 3.5.

Fig. 3.5
An industrial Ethernet star network structure. It involves connections between sensors, actuators, and governors to multiple P L Cs, 1 hash P L C, 2 hash P L C, and 3 hash P L C through an ethernet switch. The P L Cs communicate with a central P C or P G for system control.

Industrial Ethernet - star network structure

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In Fig. 3.3.5, 3 PLCs and 1 PC form a star network structure. The PN port on the PLC in the figure is the Ethernet interface, and the DP port is the Profibus-DP field bus interface.

3.2.2.3 Notes on DP Fieldbus

3.2.2.3.1 Address Occupied by Repeaters

Beginners should pay attention to the use of RS485 repeaters. The two ends of the RS485 repeater are electrically isolated. In the DP bus, although the DP address is not arranged for the RS485 repeater in the hardware configurationof the PLC device, but it needs to occupy the DP address.

In the DP bus, when there are more than 32 connected devices, it should be divided into several sections with no more than 32 devices, and the sections are connected with RS485 repeaters; as shown in Fig. 3.6.

Fig. 3.6
A schematic of a D P bus network. It includes multiple P L Cs, 1 hash P L C, 2 hash P L C, and 3 hash P L C connected via D P bus lines. Each P L C connects to C P Us, R S 485 communication modules, and other peripheral devices. It provides the detailed configuration of the bus system.

DP bus

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In Fig. 3.6, since there are 33 devices occupying DP addresses, which exceeds the maximum allowable number of DP addresses of 32 in a network segment, it is necessary to add an RS485 repeater to divide the DP bus into two network segments, so that the total number of devices in each network segment does not exceed 32. The added RS485 repeater also occupies the DP address, there are 35 DP devices in Fig. 3..3.6.

For each section of the bus, the terminal resistors of the bus connectors at both ends should be set to “ON”, and the rest should be set to “OFF”. The maximum distance allowed between two repeaters is 1 km.

3.2.2.3.2 The Total Length of the PG Cable

Connect several programming computers PG to the DP bus through “connecting cables”, as shown in Fig. 3.7. In the figure, PG1 and PG2 are connected to the bus using a single cable connection, which is the “connecting cable”.

Fig. 3.7
A schematic diagram depicts a computer P G or P C connected to a D P bus. The setup includes 3 P L Cs, each with P S and C P U, P C 1, P G 1 and P G 2. The P Gs are connected to the D P bus.

Computer PG or PC connected to DP bus

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If you use PROFIBUS bus cables to make your own “connecting cables”, you need to pay attention to the length requirements of these “connecting cables”. When the communication speed is equal to or greater than 3Mbps, please use standard connecting cables. In a network segment of the DP bus, there are certain requirements for the maximum total length of these “connecting cables”, the length of each segment and the total number. Please refer to the instructions of the relevant PLC manufacturer. Cables that are too long can cause communication failures.

3.2.2.3.3 Fork Problem of DP Bus

The workshops of an enterprise are scattered, and some workshops are distributed in the opposite direction of the main workshop. At this time, attention should be paid to the layout of the DP bus. Assume that the distribution of the workshops is shown in Fig. 3.8.

Fig. 3.8
A schematic diagram illustrates a communication problem. The setup includes multiple P Cs, P L Cs, and a main factory area connected via R S 485 repeaters to a D P bus. The diagram highlights potential points of failure or interference in the communication network.

Communication problem

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In Fig. 3.8, one water source workshop is in the south of the main factory area, and the other water source is in the east of the main factory area. With the design method in Fig. 3.8, there will be no problems in the “main factory area” and “2# water source” workshops. But there will be a problem in the “1# water source” workshop, the communication is often disconnected, and there is no data in the “1# water source” workshop. The main reason is that the RS485 repeater (1) is in the middle of the DP bus network, and the fork is too long. It's like a “connection cable” that is too long causing communication problems.

There are two solutions, depending on the distance between 1#PLC and 2#PLC in the main factory area, method 1, as shown in Fig. 3.9.

Fig. 3.9
A schematic diagram of Solution 1 for a communication problem. The setup includes multiple P Cs and P L Cs connected through R S 485 repeaters to a D P bus. The diagram suggests a configuration to mitigate the communication issues by adding additional repeaters and adjusting connections.

Solution 1

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Putting the bifurcated RS485 repeater (1) on one end of the DP bus can avoid communication problems. Method 2 is shown in Fig. 3.10.

Fig. 3.10
A schematic diagram for solution 2. The setup includes multiple P Cs and P L Cs connected through R S 485 repeaters to a D P bus. This solution proposes an alternative configuration to address communication issues, including different repeater placements and connections.

Solution 2

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The communication line between the “main factory area” and “1# water source” workshop can use “2 pairs of shielded twisted pair” cables, and add 2 RS485 repeaters, and the cost will increase.

3.2.2.3.4 DP Bus Communication Speed and Distance

When using DP bus communication, the longest cable length allowed by a network segment is related to the communication speed. When the longest distance is exceeded, an RS485 repeater is required to extend the communication distance. Beginners need to pay attention to these details, otherwise the entire DP bus communication problems may arise.

One PC and two PLCs with DP ports are shown in Fig. 3.11. The terminating resistors on the DP plugs at both ends of the DP bus need to be set to “ON”, and the DP plug in the middle is set to “OFF”.

Fig. 3.11
A schematic diagram depicts one P C and two P L Cs with D P ports connected to a D P bus. The setup includes a power supply, C P U, and bus length labeled for each device. The diagram illustrates the connection layout and communication distances for the devices on the D P bus.

One PC and two PLCs with DP ports

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The maximum length of a bus cable allowed by a DP bus is related to the communication speed of the DP bus. If the communication speed is 187.5Kbps, the length is 1 km; if the communication speed is 3Mbps, the length is 100 m.

If the optical fiber module (OLM) is used to convert the electrical signal of the DP bus into an optical signal, the communication distance between two optical fiber modules can reach 10-15 km.

3.3 The Four Key Points of Industrial Bus and Industrial Ethernet Applications

Taking Siemens’ industrial automation PLC product, S7-300 as an example, the description is as follows:

1. The hardware structure of industrial bus and industrial ethernet communication

Industrial bus: one communication line connected in series.

  1. (1)

    MPI bus: This bus is the bus that comes with all Siemens S7-300PLC CPU modules, and it is not open to third parties. Equal but opposite electromotive forces approximately cancel each other, so the twisted pair has a certain ability to resist electromagnetic interference.

  2. (2)

    It is necessary to pay attention to the number of PCs in an MPI bus. According to the author’s experience in a project many years ago, when more than 3 PCs are used in an MPI bus network, sometimes the PCs that are added after appearing cannot be connected. It is not normal, and it will be normal again after turning off one unit. The current situation has not been checked, and this is only for readers’ reference.

  3. (3)

    Profibus-DP bus: some Siemens S7-300PLC CPU modules have their own bus. CPU modules without DP interface need to insert a dedicated DP module on the guide rail. This bus is open to third-party profibus devices, and the medium is 1 pair twisted pair wires (2) or optical fiber. When optical fiber is connected, it needs to use optical fiber module to connect, and the transmission distance can be very long. Because the optical fiber is not affected by the electromagnetic field, it has strong anti-electromagnetic interference ability.

  4. (4)

    Siemens industrial Ethernet (one communication line connected in series or star radiation structure):

  5. (5)

    Profinet Industrial Ethernet: 4 pairs of twisted pairs (8, namely network cables) or optical fibers, need to use switches for connection, open to third parties, when using optical fibers, it has strong anti-electromagnetic interference ability and long transmission distance.

2. After the hardware is set up, use Siemens STEP7 software to configure the PLC hardware:

  1. (1)

    For MPI and DP buses, assign an MPI or DP address to each PLC connected to the bus, which cannot be repeated.

  2. (2)

    For industrial Ethernet, assign a URL to each PLC connected to the Ethernet, which cannot be repeated.

  3. (3)

    Download it to all PLCs in the bus or network, so that the PLCs can know each other which address PLCs exist.

3. In the hardware configuration stage, define the automatic transmission of data between PLCs, without programming in the PLC program:

  1. (1)

    Define the address and quantity of data to be transmitted to each other, define the sender PLC, define the receiver PLC, define the sending address, define the receiving address, define the number of sent data, define the number of received data, and realize data transmission without programming.

  2. (2)

    Download the hardware configuration information to all PLCs on the bus or network.

4. Program with Siemens STEP7 software to realize data transmission between PLCs.

In the program, write the data transmission module, define the address of the PLC to send data, the amount of data to be transmitted, and the address of the receiving PLC, to realize the mutual data transmission between PLCs. This data transmission method is clear and clear. Online inspection and monitoring are more convenient.