Discussion
of Recommendations
While recommendations 1) to 3) should greatly reduce the frequency
of troubles or keep them out of the generating system by isolating or
localizing them by the feeder reactors,
it obviously
is not possible to
absolutely guard against the occasional troubles in the generating sys-
tem, such as short circuits.
But as soon as the trouble
is cleared as by
the opening of the circuit breakers, in a second or a few seconds, the
system should immediately return to normal, and to begin to pick up
again
the
load which
the
short
circuit dropped.
The most
serious
feature of the troubles of September 18th, May 19th, and October 22nd,
in my opinion, was that with the clearing of the short circuit, the sys-
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[[PDF_PAGE:13]]
Report of Charles P. Steinmetz
tern did not promptly come back to normal voltage, but in a large part
of the system
(Fisk
Street B and Northwest)
the
voltage remained
practically zero for about a quarter of an hour after the trouble had
been cleared.
It appears, as the result of the momentary short circuit
the stations had broken out of synchronism with each other and were
not able to pull back
into synchronism, but kept
drifting
past each
other
indefinitely,
short
circuiting
each
other and
thus
keeping
the
voltage down to practically zero.
In these very large power systems,
it
is essential
for the
safety of
operation to limit the possible local concentration of power, by divid-
ing the system by power limiting reactors.
To
fulfill their purposes,
these reactors must be fairly large, and the value of 1.75 ohms used
in the power
limiting busbar
reactors of the Commonwealth Edison
Company of Chicago,
is by no means too high.
Necessarily, however,
these power limiting reactors also
limit the synchronizing power
be-
tween the station sections.
Thus if in a station section as Fisk Street A,
which
is connected by one power
limiting reactor to the
rest of the
system, full load of 60,000 KW is suddenly thrown off
as by a short
circuit at the busbars dropping out the synchronous machines
in the
substations
while
full
steam
supply
is
still
on,
the
synchronizing
power coming over the power
limiting reactor
is
insufficient to hold
the station in step, and the station breaks synchronism and speeds up.
Whether synchronous operation
is preserved
or synchronism broken,
depends on the relative speed, with which the synchronous machines in
the substations drop out, the turbine governors shut off steam and the
alternators speed
up.
The synchronous machines
in the
substations,
carrying load on the direct current side and feeding back on the alter-
nating
side, probably would drop out very quickly, while the turbine
governors must take an appreciable time to reduce the steam supply,
and the alternators speed up
rapidly;
at
full voltage and
full steam
supply, a
little over a second
after the load has dropped
off, the
sta-
tions would have speeded up
so
as to have broken synchronism with
the
rest of the system,
in spite of the maximum synchronizing power
exerted over the power limiting reactor.
In reality obviously the load
cannot drop
off instantly but would hold on an appreciable time, and
the governors would immediately begin
to
cut
off steam;
but on the
other hand, the station voltage has dropped under short circuit, and
is
below normal, and with
it the synchronizing power
(which goes with
the square of the voltage)
.
Tying the station section by power limiting
reactors to two other station sections (as by installing a reactor between
Fisk
Street A and
B,
thus completing ring
connection)
would
give
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[[PDF_PAGE:14]]
8
Report of Charles P. Steinmetz
ample synchronizing power, at full voltage, to keep the station section
in synchronism with the
rest of the system, even
at no load but
full
steam supply,
so that
it could break out of synchronism only
if the
short circuit lasts
sufficiently long to demagnetize the alternator fields
and thereby drop the voltage.
Therefore
it is recommended to tie all the stations by power limiting
reactors into ring connection.
If a short circuit occurs at or near the busbars of a station
section,
it necessarily drops the busbar voltage to
zero.
It takes, however,
a
number
of seconds
for
the
short
circuit current
to
demagnetize
the
alternator
fields, and
if therefore the short circuit
is opened quickly,
the
alternator
field magnetism
is
still
there,
at
least
partly, and the
station voltage thus comes back instantly,
at
least partly.
If then the
station section has
sufficient synchronizing power against the adjacent
section,
it
is probable that it would remain in synchronism, no further
trouble would occur, and
it would probably catch again many of the
synchronous machines receiving power from
it.
If, however, the short circuit lasts long enough to materially demag-
netize the alternator fields, then at the clearing of the short, the voltage
does not immediately come back,
as the
field magnetism would
first
have to build up.
Without voltage there obviously can be no synchron-
izing power, and the station
section thus probably
drifts out of syn-
chronism with the rest of the system.
With the load being released by
the dropping out of the synchronous machines in the substations, before
the governors can
cut
off steam,
the turbo-alternators will probably
have speeded up and operated their emergency steam cut
off, as with
full steam and no load
this takes only about
~L~y2 to 3 seconds.
The
machines then slow down
until again put on governor
control.
As
there are necessarily very great differences between the individual ma-
chines in the speed at which they trip the excess speed cut
off, in the
time required to reach this speed, and
in the rate of slowing down,
it
is obvious that when governor control
is restored, the individual ma-
chines and the station section as a whole probably are
far from syn-
chronism with the
rest
of the system.
It
therefore
is hardly
to be
expected that they would promptly drop
into synchronism but rather
would continue indefinitely to
drift out of synchronism with the
rest
of the system.
Two
alternators
or
stations, thrown
together
out
of synchronism,
that
is, differing in frequency from each other, will promptly, that
is,
practically instantly, pull each other into step, that
is, the slow machine
[[END_PDF_PAGE:14]]

[[PDF_PAGE:15]]
Report of Charles P. Steinmetz
speeds up and the fast machine slows down,
if their frequency differ-
ence was low enough.
This, however, would, with
turbo-alternators,
require a frequency difference not much exceeding one percent, and
it
is not probable that
the unloaded
station,
idling on
the
governors,
would be so close in frequency to the loaded station, especially as the
frequency difference in normal steam governing of these turbo-alterna-
tors is 4% between no load and full load.
If the frequency difference
is
greater, the two
stations continue
drifting past each
other
out
of
synchronism, but steadily a power transfer between them tends to pull
them nearer together, so that, even with an initial frequency difference
of 5%,
they
should
pull
each
other
into
synchronism,
in
about
a
minute, more or less.
However, this synchronizing power
is so small,
only a fraction of 1% of the rated load, and much less than the exci-
tation or friction and windage losses
in the machines, so that the
in-
evitable variations
in the steam supply when governing
at no
load,
probably are much larger and overshadow
this synchronizing power.
That
is,
it has only a theoretical
interest, and practically the stations
would indefinitely drift past each other out of synchronism; with the
fluctuations of steam supply,
etc., sometimes coming nearer together,
sometimes drifting farther apart,
etc., until at some time they happen
to
drift close enough together
within \%
so as to pull each other
in
step.
The characteristic of this drift out of synchronism is that the fluctua-
tions of current, etc., are constant, and not gradually decreasing, as in
hunting oscillations, and the frequency or period of fluctuation
is
ir-
regular.
This seems to agree with the observations.
It appears then :
if a station section has dropped out of synchronism
by
a
short
circuit
or
other
trouble,
as
indicated by
its
voltage not
coming back promptly at the clearing of the short, then
it is not prob-
able that this station
section will pull
itself into synchronism within
reasonable time, but regular synchronizing appears necessary.
Either the reactor or reactors connecting this station section should
be opened, thus isolating this
station and allowing
it to again build
up
its voltage, and then
it should be synchronized
in again with the
rest of the system.
Or, without opening the reactors, the
station may be synchronized
into the system by controlling the steam supply in correspondence with
the indications of synchronoscopes connected between this station and
the adjacent stations.
[[END_PDF_PAGE:15]]

[[PDF_PAGE:16]]
10
*
Report of Charles P. Steinmetz
Similar
drifting
past
each
other,
out
of
synchronism, may
also
occur, as the result of a disturbance such as a short
circuit, between
the turbo-alternators of one station, especially
if they are machines of
different types, as in the Northwest Station.
Or a single machine may
break out of synchronism, while the other machines in the same station
stay in step
especially if the off machine
is of different type, as No.
11 in Fisk Street.
As there are no power limiting reactors used between
the machines of the same station, the quickest way of restoring syn-
chronism would probably be, in the first case, to open the circuit break-
ers of the individual machines and then synchronize them again ; in the
latter case, to open the circuit breakers of the
off machine, and syn-
chronize
it
again.