RECOMMENDATIONS
From the
investigation,
the
following recommendations appear
to
me justified:
1.)
To reduce the liability of trouble, by carefully going over all
the
controlling
devices, such
as
relays,
current transformers,
circuit
breaker-operating mechanisms, etc., especially those at or near the gen-
erating stations to ascertain whether they are in perfect condition and
whether they are of the most reliable and safest type now
available,
and where necessary replace them or change them
to the
safest and
most reliable now available for the existing conditions
of operation.
It must be expected, that during the time which many of the controlling
devices have been in operation in the system, advances have been made
in type and design of circuit controlling devices.
The conditions
of
operation have become more
severe, due
to
the
increase of the
size
of the system and especially due to the increasing interconnection of
substations
by
tie
cables.
While
such
interconnection
materially
increases the economy
in the use of the cables,
it
also
increases the
severity and extent of local troubles such
as short
circuits.
Further-
more, many of the controlling devices are not new any more.
While
it is economically not feasible to replace or remodel the con-
trolling
devices
every
few
years
with
every advance
of
the
art,
it
probably
is economically
feasible
to do
so with regard
to
the
con-
trolling devices located in the generating stations proper.
2.)
To study the possibility of intercepting many of the troubles in
their beginning, before they have fully developed into a short circuit.
Cable breakdowns apparently are not always instantaneous, but often
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Report of Charles P. Steinmetz
develop gradually within a time from a few seconds to many days.
A
sufficiently sensitive
differential
relay thus may
discover
a beginning
cable fault, and cut off the cable, before the fault has developed into a
ground or
short.
In a
split conductor
cable, the two parts
of each
conductor
are
so
closely
identical,
that
a
very
sensitive
differential
relay can be placed between them, and
as
a
fault
naturally would
develop
in one of the cable halves
first,
the relay would
act
at the
very beginning of the
fault.
The possibilities and limitations, and in
general the economic
feasibility of the
split conductor
cable, should
thus be investigated.
Similar
results are given by grouping in pairs
of identical cables with
differential
relays between them.
This
latter
arrangement perhaps is somewhat less sensitive and reliable, since with
two separate cables, no matter how
identical they may be, a transient
may occur in the one and not
or a different transient
in the other,
and the
sensitivity of the
differential
relay thus probably has
to be
lowered not to be affected by transients.
On the other hand, the latter
arrangement would not require such extensive replacement of cables.
Systems involving the use of sheath transformers or other schemes for
tripping out on small ground
currents, and
still
other arrangements
for accomplishing the result of operating on an incipient fault, should
be investigated.
It appears that some cable
failures are preceded by a gradual
de-
crease of the insulation resistance, especially while hot, extending over
many days.
Such failures might be intercepted by a systematic testing
of the cables with high voltage direct current, essentially a high voltage
resistance measurement, and the possibility of such should be investi-
gated.
The time for such tests should be chosen immediately after the
peak loads of the day, when the cables are at their maximum tempera-
tures.
I understand that simple devices for getting high voltage direct
current for testing purposes have been developed.
3.)
To cut off the troubles from the generating stations by the
in-
stallation of feeder reactances.
By far the largest majority of troubles leading to short circuit occur
in the feeder cables and beyond them, in the substations, but very few
only
in the generators, and extremely few on the busbars.
The gen-
erators have power limiting
reactors, but no power
limiting reactors
are used in the feeders, and as the result, any short circuit in a feeder
cable, near the generating station,
is practically a short circuit on the
busbars, that is, pulls the voltage of the station section down to nothing,
drops out the synchronous apparatus and thus gives serious and wide-
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Report of Charles P. Steinmetz
spread trouble.
Such short circuits in the feeder cable near the gen-
erating stations, however, may be expected to be more frequent than
short
circuits
in the generating
station
itself, and the
installation of
reliable
feeder
reactors
thus would
eliminate
the majority
of
short
circuits from materially affecting the generating stations, that
is, from
becoming serious.
I would recommend that 0.9 ohm, or at least 0.7 ohm feeder reactors
(5.2%
to 4%
for
a 300 ampere
line)
be
installed, and
the
circuit
breakers be
set to cut
off as quickly
as possible,
in case
of a short
circuit
in the feeder.
I believe such a feeder reactance would
in no
way
adversely
affect
the
operation
of
the
substations,
but
it would
limit
the
short
circuit
to
about
100,000 KVA.
If
then
the
circuit
breakers can be made to open
this
short
in
less than a second, the
station voltage will be only a
little affected during the short, due to
the great sluggishness of the turbo-alternator
fields, and immediately
come back to practically normal, so that
it may be expected that no
synchronous apparatus will be dropped out, that
is, the trouble limited
to the short circuited feeder cable and
its substations.
If, however, the
short circuit holds on for several seconds, an appreciable voltage drop
must be expected
in the generating stations, and
at least some of the
synchronous apparatus supplied from this generating station would be
dropped out.
The advantage of feeder reactors thus not merely consists in limiting
the short circuit current and thereby the voltage drop and
in general
the shock on the system, but, by permitting to set the circuit breakers
for a materially shorter time limit,
it also greatly reduces the duration
of such short circuit and thereby correspondingly reduces the
liability
of dropping synchronous apparatus and spreading the trouble beyond
the feeder
directly involved.
4.)
Install a power limiting busbar reactance between the two sec-
tions of Fisk Street Station, so as to tie the three station sections
:
Fisk
Street A, Quarry Street and Fisk B, together into a ring.
This should
increase the synchronizing power between
these
stations.
It
should
also guard against the system being cut into two parts out of synchron-
ism with each other, in case that a short circuit
at the busbars of an
intermediary section (Quarry Street or Fisk Street B), drops the volt-
age of this section to zero and thereby destroys its synchronizing power.
The same size of reactance as now used, of about 1.75 ohms, would
be recommended.
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4
Report of Charles P. Steinmetz
5.)
I should recommend strongly to endeavor to change the present
connection between the Northwest Station and the rest of the system,
which now consists of six cables to Fisk Street B; and to connect the
Northwest
Station by
cables and
power-limiting
reactances
to
Fisk
Street A, as well as Fisk Street B, thus making a second ring, between
Fisk Street B, Northwest Station and Fisk Street A, that
is, have the
entire system of four station-sections tied together into a double ring,
by five power limiting reactors.
At present, due to the absence of power limiting reactances and the
low resistance
(.3 ohms)
of the tie cables between the Northwest Sta-
tion and Fisk Street B, these two stations are practically on the same
busbars.
This imposes too
severe a duty on the controlling devices
such as circuit breakers, of these two stations, and a trouble in one of
these two stations is equally severe in the other, that
is, a short circuit
on the busbars of either station puts both stations out of service and
thereby involves too large a part of the entire system, as borne out by
the experience of September
18th.
As the present
six
tie cables between Northwest
Station and
Fisk
Street B, are also used as, feeder cables to intermediary substations, in
the proposed
ring connection and
installation of
reactances,
careful
study must be given to make the arrangement such as not to interfere
with the economical use of these tie cables as feeder cables to the inter-
mediary substations.
Possibly three (or even two)
of these cables may
be used between Northwest Station and Fisk B;
three
(or even two)
between Northwest
Station and Fisk A;
these three
(or two)
cables
brought together at the station end to a short auxiliary bus and a power
limiting reactance installed between
this auxiliary bus and
the main
bus of the station.
This would divide the power limiting reactance into
halves, one at each end, thus giving a total of four new reactances, two
at the Northwest
Station, one
at Fisk A, and one
at Fisk
B,
for the
interconnection with the Northwest Station.
Each of the reactors then
would be about .875 ohms (half the size of the present power limiting
busbar reactors)
.
Such an arrangement may require a slight increase
of excitation of the synchronous converters in the substations connected
to these tie cables, to keep their voltage by giving the current a slight
lead.
Another possibility, which might be more convenient, would be
to
install
normal
feeder
reactors
at
each
end
of
each
of
the
tie
cables, and install the rest of the required reactance in the substation,
arranged so that the substation or the individual converters in the sub-
stations can tap the feeder cable
at either side of
this reactance,
de-
pending on from which station
it intends to take the power.
That
is,
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Report of Charles P. Steinmetz
5
two busbars may be used in the substation, connected together by the
power limiting reactance, the one connected to the one, the other to the
other generating station, and the converters arranged so that they can
be thrown on either of the two busbars.
Possibly a still better arrange-
ment may be devised.
Until reactors are installed between the Northwest Station and the
rest of the system,
I would recommend, whenever there
is any serious
trouble in the Northwest Station, or in Fisk Street B, which connects
with
it, to immediately open all the tie lines between these two stations,
and synchronizing them together again
after the trouble
is perfectly
cleared.
6.)
Install in each station section, as permanent busbar instruments,
as many
suitable synchronoscopes
as there are other
station
sections
(three at present) , for the purpose of continually indicating the phase
difference and the frequency difference of the station section from all
other station sections.
If by some trouble a station section has broken
out of synchronism with the rest of the system,
it appears practically
impossible without the assistance of a synchronoscope, to control the
steam supply in this station section so as to have it promptly drop back
into
synchronism.
With
a
synchronoscope,
however,
indicating
the
speed difference of the
station from the next adjoining
station, with
which
it
is out of synchronism (though
still tied with
it by the react-
ance), merely the ordinary action of synchronizing will bring the sta-
tion quickly back into synchronism.
Such synchronoscope would
also
indicate the phase
difference
be-
tween adjoining stations due to the power flow over the busbar react-
ances, and thereby permit most economical load
control.
Such
syn-
chronoscope would be very valuable
in
case
of trouble,
in showing
what happens :
whether the stations are out of synchronism, or hunting
against each other, or steady but with excessive power flow over the re-
actors, etc.
It is possible that a more sensitive type of synchronoscope
will have to be designed, than the present one.
7.)
For the present, until 4, 5 and 6, have been carried out I would
recommend, in case the voltage of a station section disappears, as result
of a short circuit at or near the
station, and
if the voltage does not
promptly come back
after the clearing of the short
circuit,
to open
the power limiting reactor or reactors which connect this station section
with the rest of the system, and thereby
isolate
it.
Then, as soon
as
the voltage has
recovered,
the
isolated
station should
again be
syn-
chronized
in with the rest of the system.
If, after thus
isolating the
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6
Report of Charles P. Steinmetz
generating station in which the trouble has occurred, the voltage does
not promptly recover,
it means that the individual generators of this
station have broken out of synchronism, and the quickest way of restor-
ing service probably
is, to disconnect the generators from each other,
and synchronize them again.
8.)
A special study should be made of the method of operation
of the substations, that
is,
their connection
to
the generating system
and to each other, to get the most satisfactory compromise beween
re-
liability and economy; that
is, to use the feeder copper most econom-
ically, and at the same time in case of trouble reduce to a minimum the
interference of the substations with each other, and of the generating
station sections with each other through the substations.
The question then requires consideration, whether and how
far
tie
feeders between substations are
to be used, and how they should be
controlled and protected.
Whether every important substation should
receive power from two generating station
sections, and whether and
what protection
in this case can be afforded against
interference
be-
tween the two substation sections in case of trouble in any generating
station
section.
Or whether each substation should be fed from one
generating station section only, but adjacent substations connected to
different generating station sections, so that in case of a substation shut-
ting down by trouble in the generating section feeding
it, the adjacent
substation can maintain service,
etc.
Also, the question of the control of the converters in the substations
should be investigated, whether the A.C. circuit breakers might be set
somewhat higher; whether the D.C. reverse current relay may not be
given
a time
limit and
its
setting
increased;
whether
a
D.C. power
limiting resistance might be considered,
etc.