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As Combustion Turbines Get Bigger, Black Start Units Follow Suit

As Combustion Turbines Get Bigger, Black Start Units Follow Suit

Conceptually speaking, today’s black start units are not fundamentally different from the vintage units they are replacing. Most still depend on diesel generators to bring an electric power generation plant back online after a widespread outage.

What distinguishes today’s solutions from those dating back to the late 1960s and early 1970s are:

  • The size of the combustion turbines they are expected to restart
  • The voltage of the transmission or distribution systems to which they connect

Fifty years ago, a small 15-megawatt (MW) to 20-MW combustion turbine that connected to a 69-kV transmission system was not atypical. Today, a large 200-MW combustion turbine generating at 345-kV is much preferred.

Given the size of these newer machines, it takes considerable power and specialized experience to design a starting system that both is economical and meets the technical challenges.

Managing Harmonics Is Key

For today's larger units in combustion turbine power generation, the starting systems include a load commutated inverter (LCI), a variable frequency drive that turns the turbine generator into a motor that spins the combustion turbine up to speed. Once its startup work is done, the LCI is removed from the circuit.

LCI start systems have one major downside: They produce harmonics that can overwhelm the power system to the point where the LCI or other electrical equipment can no longer function properly. The key to successful black start design is understanding the effects of harmonics on an LCI and sizing the power system to mitigate them. While some may recommend a sledgehammer approach and overdesign the power system to compensate, we typically prefer a tailored approach.

Rather than rely on a textbook solution, we apply what we’ve learned on more than a dozen similar projects to optimize the amount of black start generation needed to mitigate harmonics. In some cases, that can mean reducing black start generation by as much as half.

Down the road, stored energy (batteries, for example) and other new technologies may become part of the next generation of black start solutions. In the meantime, our engineers go to great lengths to provide customized solutions that are highly reliable, economical and easy to operate. That’s what owners and grid operators want most.

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Chris Ruckman
Written by Chris Ruckman

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