LCC Quick Notes, Generation Turbine On Line Regulation Testing
Generation steam turbines in the US will likely be subject to proving frequency control readiness in the future as NERC (National Energy Reliability Council) advances toward European standards in governor testing. Properly functioning turbine governors are the number one asset in fighting grid frequency upsets, but proving their efficacy involves some special governor features. Having supplied systems for the UK's National Grid, where on line testing has been required since 1995, LCC is familiar with these features and the testing process.
All generation steam turbines with the exception of thermally limited nuclear plant units are expected to serve in a grid stability function at an assigned regulation rate. This can be thought of as proportional frequency control. Prior to synchronization, turbine governors work as speed controllers. Once synchronized, turbine governors as supposed to work as proportional frequency controllers, but proper operation is very difficult to prove and many older steam turbine governor systems have little, if any, practical frequency control to contribute to the grid. Although load control centers may switch tie lines, start peakers, and ramp load setpoints of units under their control, these operations take on the order of minutes to execute. A governor frequency correction on the other hand can be performed in seconds. This is why NERC has an interest in maximizing operative governors on the grid.
In 2002 some innovative engineers at load control centers came up with a means of determining a particular generation unit's lack of governor frequency response by observing the unit net electrical output immediately following a large system frequency upset. The engineers noticed that some units performed as expected, i.e., if the grid frequency dropped suddenly, the net electrical output of the unit's generator immediately increased according to the assigned regulation. Many units were also observed to "deadline" through disturbances, providing no corrective actions whatsoever. Figure 1 illustrates correct and non-compliant unit governor responses. These findings led to the issuance of corrective action letters to the non-compliant unit operators.
In 2007 after major blackouts, the focus turned to means of proving governor functionality on a routine basis as has been the case in the UK. NERC issued PRC-018 to address the requirement (www.nerc.com/files/PRC-018-1.pdf). Under PRC-018 testing proof must be supplied upon NERC demand. While the "deadline response" to grid frequency upsets identifies obviously inoperative governors, the new testing standards go the step further to prove that the governors are regulating to the percentage required and stand ready to respond to grid upsets. Some power operator associations have resisted this testing, falsely claiming that technology does not exist currently to support the program. One needs only to look at the now sixteen years of testing in the UK to debunk this excuse. LCC's Series 2 Euro Governors have had the capability since 2005.
(2) Frequency Test Feature Implementation
With the experience of designing and installing steam turbine governors for generation units on the UK grid which are mandated to include built-in on line regulation testing, LCC has implemented the methods and instrumentation required. The basic method of test is to introduce a variable level offset summed to the real time governor speed feedback as received from electronic speed probes. This pretty much eliminates older mechanical-hydraulic governor systems from compliance which do not utilize electronic speed feedback, but for many additional reasons these systems should be replaced at once.
The offset magnitude to be summed is established by a external voltage -10 vdc to +10 vdc steering signal such that the full range of required frequency response may be tested. Safety interlock mechanisms prevent accidental test injection during normal operation. With this type of system the governor regulation efficacy can be proven under any pre-existing operating conditions with a reasonable load ceiling (not operating at or close to 100% load). By recording net load changes with both positive and negative frequency offsets the true regulation of the turbine governor is established. In some instances the testing will uncover lower or higher regulation rates than designed or specified. In these cases the testing program provides a calibration adjustment to bring the governors into correct operation,
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