Zark Bedalov - Practical Power Plant Engineering

Figure 1.4illustrates the latest wiring method with a DeviceNet communication loop clearly indicating that most of the external wiring has disappeared. For DeviceNet and other means of communications (see Chapter 17).

Evidently, this motor needs a cable for its pushbutton station and a DeviceNet loop to a DN module. The loop loops from one starter bucket to the other. That is it. The rest is software.

The specific logic for each drive is now written as software into the processors to receive status from the drives and to feed the decisions of the software logic back to the inputs to start/stop/sequence the drives in accordance with the flow requirements of the conveyors, pumps, etc.

Or if the drive is a VFD‐operated motor, software provides a set point to the drive to increase/decrease its speed to match the plant output at any particular moment. Therefore, the VFDs are not only needed to help the motor to start softly but to also continuously adjust the plant production of a certain product in the plant operation. This could never have been done with relay logic.

What is the difference between a wiring and schematic diagram?

A wiring diagram of a motor is shown in Figure 1.3, complete with all the cabinet terminations. A schematic diagram is the stretched version of the wiring diagram ( Figure 1.4a) and is shown in Figure 1.4b.

As a result of the innovations, the site labor for installing the field control wiring has substantially been reduced. Please do not make a sigh of relief, as yet. Though the operating logic is no longer visible on the above diagrams, you will now have to understand the PLC logic and program the ladder diagrams to make the plant motors function like an orchestra.

Computer program: It is desirable that the engineering company develops a software program that will create schematic/wiring diagrams and cable lists directly from the project load list database by using attributes that automatically get filled on the typical model drawings with the data sourced from the load list. Manual entry to these documents is the biggest source of errors on the project. A small project change must permeate through all the documents. Let the computer enter it for you.

The schematic/wiring diagrams and cable lists are the products of the load list. The diagrams can then be printed for the whole project or for a specific area or MCC.

Some third‐party software programs are available for this purpose. Unfortunately, these were written for the wide market audience to be saleable to every company as one unadjustable product. This third‐party approach unfortunately tends to require a massive manual input, and for that reason, defeats the purpose. In discussions with some users, I was told that the input is overwhelmingly manual and leads to erroneous inputs. It was not efficient, and the software was abandoned.

This author has developed its own program on FoxBASE for that purpose. It is updated for every new project to be project‐specific resulting in minimal manual entry, mostly for cable lengths.

Commissioning of an industrial plant is a bit simpler. The plant operating system can be broken down into smaller subsystems, such as crushing, milling, which could be precommissioned and commissioned totally independently.

In the power plants, the generating units are large operating blocks, which are tested one at a time along with the water or fuel paths (input) and the electricity path to HV switchyard (output), as well as the unit services and operating controls all at the same time. Station services are commissioned separately.

Precommissioning and commissioning of an industrial plant or a power plant are different activities. They must be approached differently in particular if the plants are fully automated. Precommissioning is testing of the equipment such as switchgear, MCC, VFD, or transformers on an individual basis in an energized state, but totally disconnected from the other operating equipment.

Secondary injection: First, the switchgear is meggered and high‐pot tested. Then, the protective relays are tested by secondary injection (simulation) to trip the breakers due to overloads or undervoltages according to the protective relay setting sheets for each breaker. Protective current transformers (CTs) and potential transformers (PTs) circuits are fed to the tester to simulate the operating state. The secondary injection is performed by a three‐phase tester, shown in Figure 1.5.

Circuit breakers: Each breaker in the switchgear can be tested for functioning in its drawn‐in (connected), test and withdrawn position. The switchgear is not energized, but the circuit breakers can be operated because the 125 Vdc control circuits are energized to allow the breakers to function. Furthermore, there may be an additional control circuit at 24 Vac or Vdc used for remote operation and signaling to and from the plant control system. Figure 1.5 Three‐phase tester.In each of the three positions, the switchgear and the circuit breaker leave its mark.In the withdrawn position, one can test the breaker to charge the activating spring and to open/close without affecting the other breakers in the assembly.The breaker test position is a half‐drawn‐out position. In this position, one can fully test breaker in all aspects of control and interlocks, but without affecting the other parts of switchgear assembly.In the connected position, the breaker can be fully tested provided the incoming and the tie breakers are locked and held in a withdrawn position. This test position is very useful in the commissioning (energized) phase of the plant testing that follows the precommissioning.Similar precommissioning activities are carried out on MCCs for each motor or feeder circuit to enable the assembly to be energized and to power motors and feeders for further tests. Each motor is being bumped for its rotation to match that of the pumps or conveyor travels, etc. For this activity, the motors are decoupled from the pumps.Furthermore, the motor branch circuit breakers are also pretested to establish their minimum instantaneous protection settings to suit the motor inrush currents (see Chapter 3for details).

Wiring: During the precommissioning, a lot of simulation will be required to be performed to test the equipment and cable wiring. This includes jumpering the contacts and injecting volts or currents from other sources to command the operation of the switchgear breakers or MCC starters. All the wiring and schematics of the field devices and hard wired safety interlocks must be verified.Wiring diagrams used to be checked during the precommissioning stage too, but these diagrams are now becoming a rarity and often obsolete. As mentioned earlier, wiring diagrams have been greatly simplified by using the communication links, such as Ethernet, DeviceNet, Modbus. The present schematics have all the terminals marked just the same as the wiring diagrams used earlier. It seems to be a trend now. Perhaps not in the industrial projects yet, but, certainly, it is a trend in the large power plants.You may then ask, how do you make cable terminations if you do not have wiring diagrams? That is a very good question. Well, what many contractors now use are the cable tabulation lists showing the terminations from the terminals shown on the equipment A to the terminals on equipment B, but without giving any significance to each wire. The wiremen can swiftly terminate the wires as listed and let someone else think if the list was right or wrong. As a result, not all the signaling is being precommissioned. Some parts of it may be rung out, but not precommissioned to verify the interlocks. It is left to the commissioning group to test it and prove it on the equipment performance basis. Again here, this approach refers to the large power plants and not industrial projects.