IEEE Power & Energy Magazine - January/February 2014 - 39

BSUs
as we have already explained, bsUs are units that do not
require off-site power to start. generally, these fall into four
categories:
✔ Hydroelectric units. these units can be designed for
black-start capability and have fast primary frequency
response characteristics.
✔ Diesel generator sets. diesel sets usually require
only battery power and can be started very quickly.
they are small in size and useful only for supplying
the power needed to start larger units. they generally
cannot be used to pick up any significant transmission
system elements.
✔ Aeroderivative gas turbine generator sets. this
type of gas turbine typically requires only local battery power to start. these units can usually be started
using remote commands and can pick up load quickly.
✔ Larger gas turbines operating in a simple cycle
mode or steam turbine units. these units are not
in themselves black-start capable but are coupled
with on-site diesel generator sets to make the plant a
black-start source. the diesels are started and used to
energize plant auxiliary buses and start either a gas
turbine or a steam turbine. a gas turbine is generally
quicker to bring online. the time to restart and available ramping capability are generally functions of
how long the unit was off-line.
Figures 1, 2, and 3 show examples of hydroelectric, diesel,
and small gas turbine generators that could be used as bsUs.

Load Frequency Control
when only a portion of the system is lost and is being
restored using tie lines to a larger power system, load frequency control is not generally a large concern. the outside
system generally has the capacity to absorb changes in load
without significant frequency deviations.
when restoration without external resources is required,
however, load frequency control is of critical importance.
during the restoration process, the black-start generating unit will typically be used to pick up large induction
motors associated with a larger power plant such as boiler
feed pumps and forced-draft and induced-draft fan motors.
the frequency of the black-start system will be controlled
by the speed governor of the turbine driving the black-start
synchronous generator.
standard practice for units operating in parallel in multimachine power systems under normal conditions is to operate all turbine speed governors in a droop-governing mode.
this provides a stable sharing of the electric system load
among all units. the proportional characteristic of the droop
speed governor control, however, results in a steady-state frequency errors remaining in the system. automatic generation
control (agC) has the form of a pure integral controller and
will follow the primary frequency control action of the speed
governors to remove this undesirable steady-state frequency
january/february 2014

error. typical steady-state regulating droop (r) for speed
governors is 5% using a system frequency base of 50 hz or
60 hz and a power base equal to the turbine Mw rating.
during a black-start event, agC will not be operating, however, it is imperative that system frequency regain its scheduled
value following the start-up of motors or pickup of other loads.
this frequency control should be automatic, since the crew in
charge of the bsU will be operating under extreme emergency
conditions, which can lead to undesirable operating errors.
the automation of the frequency control process can therefore
be carried out by the prime-mover speed governor of the bsU,
operated in a constant frequency or isochronous control mode.
in this pure integral control mode, the steady-state frequency
error is zero because of the resetting characteristic of the pure
integral control. Most if not all modern diesel engines, gas turbines, and hydraulic turbines are furnished with digital speed
governors with which a selection of either a droop or isochronous operating mode can be carried out by means of a simple
change in command.
once the system has more than one generating unit
online, all speed governors should be operated in a droop
control mode, unless it is decided in the restoration plan that
one of the largest units should operate in isochronous control
to maintain the control of the system frequency. as noted
above, agC would be disabled under this extreme condition.
in summary, the preferred control mode for speed governors
associated with bsUs is isochronous or constant frequency control. when additional units are added, the preferred control mode
for speed governors is droop control mode. in some cases, it may
be preferable to keep one large unit in isochronous control mode.
Units should not be operated in parallel with more than one unit
in isochronous control mode.

Voltage Control
Control of voltage is obtained through the generator's excitation system. the excitation system must be operated in automatic control with the automatic voltage regulator (aVr)
in service. the system voltage will be a function of the
generator terminal voltage. the generator scheduled voltage may therefore need to be adjusted throughout the restoration process, as load is picked up, and also coordinated
with any changes in transformer tap positions. such adjustments should be an integral part of the restoration plan. the
changes in voltage that will occur with the starting of large
motors or the pickup of large blocks of load require that the
excitation system respond in a rapid, well-tuned manner.

Motor Starting
Motor starting is a concern during black-start restoration. a
bsU's primary function is generally to start up the auxiliary
load of a larger next-start unit. this auxiliary load is made
up of lighting and motor load used, for the most part, in the
start-up of steam generators and fuel systems. the motor
load is made up of a large number of small- and mediumsize motors and a few large motors ranging anywhere from
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