Subsynchronous relay

A subsynchronous overcurrent relay system for providing protection from sustained subsynchronous current in a power system having a normal synchronous operating frequency and wherein said subsynchronous current is of a lower frequency,

• input circuit means for connection with said power system for receiving from said power system a subsynchronous current,
• first filter means connected across said input circuit means for passing synchronous current at said normal frequency and for blocking subsynchronous current, said filter network means comprising a dual tuned block-pass filter for passing synchronous current and blocking said subsynchronous current and including a parallel resonant network having an inductance connected in parallel with a capacitor, the parallel resonant network being connected in series with a series inductance, and the series combination of the parallel resonant network and the series inductance being connected across said input circuit means, and
• second filter network means connected in series with detector means, the series combination of said second filter network means and said detector means being connected across said input circuit means and in parallel with said first filter circuit means for blocking the synchronous current and for passing a predetermined range of subsynchronous current and for detecting the predetermined range of subsynchronous current, said second filter network means comprising a synchronous blocking filter network including an inductance connected in parallel with a capacitance for passing said predetermined range of subsynchronous current and for blocking said synchronous current, and said detector means comprising a first high-set detector and a second low-set detector both connected in series and connected in series with said second filter network means for respectively operating when said subsynchronous current is within said predetermined range, said detectors having upper and lower response limits for the subsynchronous current for providing, after predetermined delays to allow for natural damping of the subsynchronous current, a change of state of an associated electrical contact means when the subsynchronous current is greater than the limits.

A subsynchronous overcurrent relay system for providing protection from sustained subsynchronous current in a power system having a normal sixty hertz synchronous operating frequency and wherein said subsynchronous current is of a lower frequency,

• input circuit means for connection with said power system for receiving from said power system a subsynchronous current,
• first filter means connected across said input circuit means for passing synchronous current at said normal frequency and for blocking synchronous current, said filter network means comprising a block-pass filter with a resonance frequency of approximately thirty hertz for passing synchronous current and blocking said subsynchronous current, said block-pass filter comprising a parallel resonant network including an inductor connected in parallel with a capacitor, the parallel resonant network being connected in series with a series inductor, and the series combination of the parallel resonant network and the series inductor being connected across said input circuit means, and
• second filter network means connected in series with detector means, the series combination of said second filter network means and said detector means being connected across said input circuit means and in parallel with said first filter circuit means for blocking the synchronous current and for passing a predetermined range of subsynchronous current and for detecting the predetermined range of subsynchronous current, said second filter network means comprising a synchronous blocking filter including an inductor connected in parallel with a capacitor, for passing said predetermined range of subsynchronous current and for blocking said synchronous current, and said detector means comprising a full wave bridge rectifier having alternating current input terminals connected in series with said blocking filter and having direct current output terminals connected to a first high-set detector in series with a second low-set detector for respectively operating when said subsynchronous current is within said predetermined range, said detectors having upper and lower response limits for the subsynchronous current for providing, after predetermined delays to allow for natural damping of the subsynchronous current, a change of state of and associated electrical contact means when the subsynchronous current is greater than the limits, said detectors comprising relay windings for operating said electrical contact means.