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In the 1940s, a researcher of stability of electrical power systems suggested that the electromechanical oscillations that arise between synchronous generators of systems interconnected by long lines could be damped if the voltage was modulated in phase and proportional to the local oscillations of the frequency. This basic idea, embryo of the PSS, only came to be implemented in the radical decade of the 60´s, when the first excitation systems of generators with thyristors were put into operation.

But the choice of the appropriate signal to be measured and actuated through the new equipment, the power system stabilizer (PSS), on the generator excitation system was a considerable challenge for engineers until the 1980s. At that time , the notable Portuguese engineer F.P. de Mello, who worked in Brazil, then in the GE of the USA, and finally in his own company, PTI, proposed a scheme in which two signals were measured: electric power (Pe) and frequency , or rotation, at the generator terminals (ω), and were combined in order to generate a signal proportional to the accelerating power. These two signals at the time were those that disputed the preference of the manufacturers, and Pe presented good performance in rapid oscillations, but detonated the reactive of the generator in slow transients, whereas ω was the inverse, besides generating problems of noise in the measurement of the signal .

Ontario Hydro engineers (some later founded the Kestrel company in Canada), in 1982 saw in the proposed scheme the possibility of joining the qualities of the two signals. They then increased the cutoff action of the filter that separated the action of the signals into the PSS and put it into an integral structure of the accelerating power. At that time, we faced this problem at Eletrosul (a power generation company in South America that today is ENGIE), in the same group that would later found REIVAX. When I saw the article in IEEE, I realized that the solution would come by. We transformed the power PSS of the “Salto Osorio” Power Plant into this new integral model of the accelerating power. However, I could observe that in rapid generation variations, the filter proposed by Ontario still led to considerable deviation of terminal and reactive voltage. Moreover, the PSS could saturate its output and lose performance at times when it would be most needed.

One day in 1984, I was in the home office, talking to the eng. Nelson Zeni Jr. on the subject, when I was able to establish the exact problem: the PSS could not generate large output variation when the turbine distributor moved on its maximum speed ramp. Nelson was “very sharp” as a good teacher of control system who he was, and began to write. After about 10 pages of equations, he said:

“Hey man, it’s simple!”

In fact, the structure of the new filter incorporated an advance whose constant should be the sum of the delays of the denominator and, in a short time, totally rejected the disturbance caused by the variation in generation.

We implemented and tested the solution with excellent results. Nelson and I wrote an article on “Ramp Tracking” and presented it at a meeting of the IFAC (International Federation of Automatic Control) in Rio in 1985. The only part that I was able to make clear in presenting the article in English, was the response I gave to Chairman Malik, an expert on the topic, when he told me that he had five minutes to complete the presentation:

– “Gosh!”

But Ontario people saw the work, got in touch and then took the idea to be part of a new IEEE PSS model standard: PSS2A and 2B.

When REIVAX North America arrived at Kestrel to validate our product for the market, the people there knew the story, which aided collaborative action.

From the input signals, P and ω, came the name of our PSS, a product that was fundamental for REIVAX to become a respected company worldwide: PWX.


An interesting aspect to be observed in the characters of this story is that the passion for something, naturally led everyone to entrepreneurship.”

Author: Eng. João Marcos Soares, co-founding partner of REIVAX.