Practical Power Plant Engineering. Zark Bedalov

Figure 1.7 Plant interlocks, hard wired and software.

And if it still does not work, then the schematic must be corrected and the circuit rewired. Often, an interposing relay may not be seating properly. There are so many ways to fail?

      The wiring tabulations, mentioned earlier, are generally not being updated. The schematics are updated in red and issued “As Built.” The commissioning approach, though relatively cumbersome, avoids all the simulations to be done and saves the time as the work is concentrated on fixing faulty wiring and wrong logic lines only, and not on all the wiring and all the logic. This approach may work providing the unit was being fully tested under all the operating conditions of loading, ramping, and stopping to expose the functioning of all the instrumentation, the transmitters, and indicating instruments. The transmitters may work correctly for certain loading and operating sequences, but rapid changes in the load may expose their weaknesses.

      In my judgment, the following are typical commissioning problems:

Here is an example of unpredictable nature of certain field devices. A large hydro generator sometimes fails to transit from the synchronous condenser mode to the generator mode when requested. Having spent days to review the field devices that control this activity, the focus shifts toward three‐level switches operating in a tube controlled by an orifice. The switches may work well for days, but then suddenly fail to provide a transition signal. After days of trying and figuring out many situations, it is finally correctly concluded that the water in the tube is too turbulent at certain conditions and water pressures and affects the operation of the switches. Several new orifice sizes were tried and eventually a proper size is selected and placed. The trial period to prove a particular orifice may take weeks. Also, the same orifice size used on Unit 1 may not be the right one for Unit 2, due to varied operating conditions.

      Naturally, all the changes implemented on Unit 1 in the wiring, control logic, and unresponsive field devices are immediately updated on the next unit if it is available, thus, insuring faster commissioning and release to operation.

      In an industrial plant, the commissioning is based on a system‐by‐system basis grinding, flotation, conveying, leaching, etc. Switchgear, transformers, MCCs, and motors are tested together under partially energized conditions. Each system is semiautonomous and gradually energized to prove it operates in proper sequence for starting and shutting down under specific conditions. The systems are being commissioned until the whole plant is made operational and ready for starting up from the control room and it is correctly represented on the control system monitors.

      Some say that commissioning is the slowest game in town. It is 90% logistics and preparation to position right people to the right places, keep them focused on the task with proper tools, and to prepare the safe environment and operating conditions for the test. The actual test may take only a few minutes to show a change in state on the instruments, or that something had moved or rotated. Conveyors start moving in sequence, grinders start grinding, and pumps start pumping and filling the tanks. Level switches regulate the speed of the drives to prove the set points established by the control system. The automation starts directing the process.

      For the commissioning to work well, one has to have a good cooperation of the mechanical, process, and controls engineers and excellent means of communications in the field. Why are the electrical engineers most often assigned to be the commissioning engineers? Well, I think because the electrical engineering is the hardest part of the project to understand. Electricity is the blood flow of the plant. It connects all the pieces in an invisible way that experienced electrical engineers can understand.

1.3.3 Reliability Run

RR is also shown in Figure 1.6. This is a big commissioning event as the unit enters the operation and starts producing electricity. Typically, RR is applied to power plants but not to the industrial plants. RR lasts anywhere from 7 to 30 days. A period of 30 days is the most common RR period. For the plant to enter an RR, it means it has been fully commissioned and proven to be operational, with some minor listed deficiencies that do not affect the production. These can be fixed during the RR or later. Successful RR leads to ownership transfer and a start of a one to two years warranty period.

      During an RR, the owner's operators take over the plant operation in the presence of the suppliers in supervisory role. This is also a phase of practical training for the operators. The operators follow the directives of the dispatch center and load the machines accordingly in terms of MW and MVARs. The intent is to operate and expose the plant to all the operating modes and transitions without restrictions and as often as possible. As more and more automation and supervision is added to the plants the commissioning and RR tests get more and more involved.

      Each plant owner may dictate different constraints for RR. The rules may also depend on the unit performance during the commissioning. If the plant has been failing often, owners may impose more stringent conditions. In general, the unit must operate 30 days, 24 hours a day without a major failure that would cause the unit to reduce its capability to carry load. If that happens, the RR is restarted from the beginning. For instance, a failure of a pump with a successful automatic transition to the healthy pump will not be a cause to stop the RR, but considered a successful operating action.

      It is much easier to commission a plant if you had followed it through the design and construction into commissioning. Sometimes a commissioning engineer may be invited to do a commissioning on a plant that he/she may not be familiar with. There were a number of cases like this. One of difficult ones was in Lahore, Pakistan, where we were invited to conduct a commissioning and RR test after it has failed in 12 earlier attempts over a period of two years. It was a 5 × 30 MW thermal plant.

      During the RR, the plant was supposed to operate flawlessly for seven days without a single alarm while being tested under all the operating conditions. In addition, there was a specific test during the RR while all the plant units were running, called “Islanded Test.” An unexpected three‐phase fault is arranged on the HV line, connected to the plant. To pass the test, the plant had to separate itself from the grid, shutting down four units, while one unit was supposed to be left running and maintaining the plant station service load.

      Well, this time, it worked well, and the plant passed the test. We knew that this time, we had to take a different approach. We modified the protective relay settings and readjusted the governor transfer functions. Every short circuit is different, and the units are required to conform accordingly. Perhaps we were just plain lucky. It is hard to tell. This time the protective relays operated selectively and the new governor settings acted correctly.

1.3.4 Power Plant Grid Tests

The power plant or a unit connected to a HV grid is often subjected to a number of grid tests that must be completed to confirm its compatibility with the grid and to operate from the dispatch center. Here is a list of some of the grid tests: