A Short History: The Invisible Series Capacitor

Dec. 12, 2017
The series capacitors and the thyristors made a great team.

Last month I talked about the problem of subsynchronous resonance (SSR) and the dynamic stabilizer as a solution. The stabilizer was basically an early thyristor-controlled reactor (TCR) device that changed the dynamics of the system. To recap, SSR for those who may have missed that discussion. Long transmission lines produced excessive inductive reactance, which impedes the amount of power that can be transferred over the line. To overcome the inductive reactance, series capacitors are installed on the lines, but that introduces another problem. Adding the series capacitors introduces the interaction of electrical and mechanical natural frequencies, which produces a resonance.

This resonance is known as SSR, which, back in the day, had damaged some turbo-generator shafts and had the potential of damaging more shafts unless something was done about the problem. The TCR fed a controlled inductance into the bus damping the SSR forces. This system performed well for the utility, but there are other methods. One of them involves the capacitors themselves and the thyristor valve. Several theorists proposed that a thyristor controlling an inductor in parallel with the capacitor bank would take care of the problem. This idea became the thyristor controlled series capacitor (TCSC). In the late 1980s, Western Area Power Administration (WAPA) formed a joint project with Siemens to develop just such a device. In 1992, the project produced the world’s first continuously variable, 3-phase TCSC installation at WAPA’s 230 kV Kayenta substation in Arizona. I toured this marvel shortly after commissioning and it was an amazing piece of technology and a vast improvement over the dynamic stabilizer I had worked with ten years earlier – technology moves on.

Advancing Technology

The Kayenta TCSC consisted of two 230 kV conventional series capacitor banks, each rated at 165 MVAr with a single-phase impedance of 55 Ohms. One of the two 55-Ohm banks was split into a 40 ohm and a 15 ohm segment. TCRs were installed in parallel with the 15 ohm segment, which provided direct control of the transmission line’s impedance by varying the reactor current and the line current. As a result, the system smoothly controlled the capacitive impedance eliminating any potential SSR. In 1993, Electric Power Research Institute (EPRI), Bonneville Power Administration (BPA), and GE took this device further with the commissioning of the 500 kV TCSC installed at BPA’s Slatt Substation.

The Slatt installation was a 3-phase 500 kV TCSC that consisted of six TCSC modules connected in series on each platform and an advanced control system, which allowed for rapid dynamic modulation of the parallel inductor. This gave a vernier control to the capacitor’s reactance ranging from 1.33 ohms to 16 ohms. This control not only provided SSR dampening, but provided power swing checking, dynamic power control, transient stability control, and overload management. I was also able to tour the Slatt TCSC shortly after its commissioning and saw further advancements of thyristor applications. The TCSC started out to address the SSR problem, but ended up improving the overall series capacitor’s functionality.

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