Figure 8–5: ROUND TRIP DELAY & CLOCK OFFSET COMPUTATION FROM TIME STAMPS
Filters are used in the phase locked loop to assure stability, to reduce phase and frequency noise. This is well known tech-
nology. The primary feedback mechanism shown in the Loop Block Diagram is phase angle information through the well
known proportional plus integral (PI) filter. (The Z in the diagram refers to a unit delay, and 1 / (Z – 1) represents a simple
digital first order integrator.) This loop is used to provide stability and zero steady state error. The filter is similar to the one
described in reference , except in our application there is no need for an adaptive bandwidth. Also, we have extended
the filter to include a frequency deviation input to provide frequency tracking.
A PI filter has two time parameters that determine dynamic behavior: the gain for the proportional term and the gain for the
integral. Depending on the gains, the transient behavior of the loop can be underdamped, critically damped, or over
damped. For this application, critically damped is a good choice.
This sets a constraint relating the two parameters. A second constraint is derived from the desired time constant of the
loop. By considering the effects of both phase and frequency noise in this application it can be shown that optimum behav-
ior results with a time constant for the main loop of about 10 seconds.
The primary loop uses a single input that is switched back and forth between the two sources of phase angle deviation
which are; a) that computed from timestamps in the ping-pong algorithm and b) that computed from current measurements.
This approach makes the current information the primary source, when it is available. This makes the system immune to
changes in communications delays, as long as current information is available. The rules for switching are:
Phase angle deviations from both current information and ping-pong information are always computed. The ping-pong
algorithm has a wider range of validity, and is used to help decide which source of phase angle information is to be
used by the filter.
L90 Line Differential Relay
8 THEORY OF OPERATION
8.1.12 PHASE LOCKING FILTER
GE Power Management