Читать книгу Practical Power Plant Engineering - Zark Bedalov - Страница 27

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 Circuit breakers: Each breaker in the switchgear can be tested for functioning in its drawn‐in (connected), test and withdrawn position.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1.5 Three‐phase tester.In each of the three positions, the switchgear and the circuit breaker leave its mark.In the withdrawn position, one can test the breaker to charge the activating spring and to open/close without affecting the other breakers in the assembly.The breaker test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connected position, the breaker can be fully tested provided the incoming and the tie breakers are locked and held in a withdrawn position.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 3 for 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.

 Transformer oil: Transformers oil is tested for its dielectric strength several days prior to energizing. New oil should have a strength of >65 kV/cm, while older oils must demonstrate the insulating strength of >60 kV/cm. If the strength is lower than those desired, the transformer oil must be purified by the heating and filtering equipment to exhaust the moisture before energizing. Oil samples will be taken from the transformer during installation a week before energizing.

Figure 1.6 presents an actual handover chart of a power plant from construction, through precommissioning, commissioning, and reliability run (RR) to operation and ownership transfer.

The precommissioning checks on a larger piece of equipment, for instance, a large hydroelectric generator, is a relatively complex endeavor. A large number of interlocks must be simulated, much of them from the software. Some precommissioning can be done and must be done, such as unit trip logic and emergency stops and safety trips. In order to make the simulations more manageable, control functions are usually broken down into a number of sequential steps. These critical offline tests are performed before the online tests are attempted in order to minimize any unforeseen inadvertent operation. During this process, the unit is tested through a restricted logic to allow the checks on part of the logic and then proceed to the next ever larger step. The rest is left for commissioning. Since there are too many interlocks to be dealt with, there is also a fear that some of the simulation jumpers may be forgotten and left behind. Jumpered contacts left behind hide unreal bypassed conditions.