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Synchronization to the grid

Any power plant commissioning, from small industrial GTG (Gas-To-Gasoline) to nuclear facility, will converge to a very final step before (provisional) commercial operation, sometimes empathized as “first kW”: synchronization to the local or national grid. This is in fact a very critical phase of the installation, where mistakes in pre-commissioning or forced control variables to bypass that last permissive prevention to close the main MV breaker can easily lead to a disaster.

My experience in the first synchronization of the MV generator in an industrial plant
My experience in the first synchronization of the MV generator in an industrial plant (original photo by Francesco Becattini)

As common sense would suggest, synchronization of generator and grid implies that three key parameters of the two waveforms match within a certain window:

  1. Frequency
  2. Voltage
  3. Phase Angle

This condition is verified by a synchro check device, that may be integrated in the main IED protecting the generator as ANSI 25C function and obtained acting on governor and AVR either manually or via an automatic synchronization card, that will send the closing command to the MV switchgear control circuitry finally closing the main circuit breaker (52G).

Check on instrumentation and protection

Before initiating the synchronization sequence, regardless of the fact that all factory test reports, pre-commissioning, and commissioning certificates are available a final check of proper protective functions in the live condition is due.

To perform this key activity all the instrumentation connected to the protection relay including VT via (possibly) primary and secondary injection, and CT via polarity test must be rechecked. Obviously, all other subsystems including prime mover and driven skid shall be fully commissioned, hardwired control loops checked (with special focus on trip signals), communication (soft link) established.

All panels including the MV Switchgear station and excitation panel (AVR) shall be energized and ready.

Open circuit tests will be conducted at first possibly opening the earth switch (89) to test if it has been inserted for safety reasons, checking the expected behavior of voltage and frequency (ANSI 27, 59, and 81) trips, in accordance to the actions listed in the trip matrix, that may include apart from open breaker command lockout relay engage (either electromechanical or logical) and other actions.

In phase 2 earth switch is closed to start the short circuit tests, which will imply a soft ramping of excitation current and measuring armature currents at different field excitation current. An ANSI 87 (differential) protection is simulated changing the ratio of one CT.

In this phase, both proper excitation system functioning and actions of a differential system that is one of the cornerstone protections will usually trigger both a prime mover trip and a fast de-excitation of the generator.

Commissioning live tests on protective system
Figure 1 – Commissioning live tests on the protective system

Phase 3 will involve an unbalance of the three-phase system connecting to the ground one of the phases and obviously removing the shorting link, in this way 51G and 67N protection (via toroidal or ZCT) will be tested.

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Marco Bruschini

Chartered engineer with +10y of experience in electrical rotating and static equipment engineering procurement and installation, for renewables (hydroelectric) and natural gas plants. industrial and O&G markets (onshore and offshore). Genuine interest for energy transition and electrification, with a focus on integration of new energy sources in national grid and digitalization of control system and plant automation, including remote diagnostic.

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