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TP-6200 10/12 Appendix 203
Frequency Setpoint or Cut-In Point
The point at which the underfrequency unloading begins
to take effect is adjustable, allowing the system to be
tailored for each application. Because the
characteristics of the engine have the largest effect on
the system’s performance, the engine’s response
should determine the unloading point. The unloading
setpoint is the frequency below which the excitation
control will reduce the voltage so that the engine may
begin to recover.
The cut-in point, or frequency setpoint, should be set
0.5--3.0 Hz lower than the normal steady-state band of
operation. If the engine normally operates within a very
narrow range of speeds close to the nominal, a setpoint
of 0.5 to 1.0 Hz below nominal should be suitable. If the
engine normally operates over a wide range of speeds,
the setpoint may need to be 2.0--3.0 Hz from the
nominal. The underfrequency unloading function can be
eliminated by setting the cut-in point below the minimum
expected operating frequency.
Example
A 90 kW load is applied to a 100 kW, 60 Hz generator set
driven by a turbocharged diesel engine with an
electronical control module (ECM). The speed drops
10% and takes 20 seconds to recover to at least 59.5 Hz.
The voltage, meanwhile, drops from 480 to 460 and
recovers to 480 within 15 seconds. Therefore, some
underfrequency unloading should be provided. A good
starting point would be a frequency setpoint or cut-in of
59 Hz. A slope of 15 volts per-cycle-per-second is
appropriate as well. If after these adjustments the speed
recovers very quickly, in about 5 seconds, but the
voltage drops below 440 volts, the slope should be
reduced to 12 volts per cycle. More adjusting may be
required to get the most desirable compromise between
speed and voltage.
Three-Phase Sensing
Three-phase sensing describes how the excitation
control or voltage regulator determines the condition of
the alternator output voltage. Early types of regulators
sensed the voltage on just one phase of the alternator.
Single-phase sensing is not uncommon today as most
alternators are designed to produce balanced, equal
voltage on all three phases. If the loads applied to the
generator set including no load are equal and balanced,
the output voltage on each phase will be nearly equal.
However, in some applications, individual phases may
have unequal or unbalanced loads. In these cases, the
output voltages will not be equal on each phase. In
general, the phase with the greatest load will have the
lowest voltage while the phase with the least load will
have the highest voltage. This is true regardless of the
type of sensing used in the regulator system. A
single-phase sensing excitation controller will keep the
voltage of the sensed phase at the voltage adjustment
value. A three-phase sensing system will average the
three phases and hold the average to the adjustment
setting. The average is the sum of the voltages of three
phases divided by 3.
As stated above, three-phase sensing does not
eliminate the unequal voltage phenomenon.
Three-phase sensing balances the inequality of voltage
between the phases to the desired value. In other
words, if a system with unbalanced loads uses a
single-phase control feedback, the voltage on the
sensed phase would be at the setpoint while the other
two phases would vary by their proportional loads. For
example, if the sensed phase had rated load while the
two other phases were only loaded at half the rated
value, those two phases would have higher-than-rated
voltage which may be undesirable. If a three-phase
sensing feedback were utilized, the phase with rated
load would be regulated to a voltage slightly below the
rated voltage while the other two phases would be
slightly above the rated voltage (but lower than in the
previous case). The sum of the three, divided by 3,
would be equal to the regulation setpoint.
In a single-phase system, line-to-line voltage is held
equal to the line-to-line voltage adjust setting. In a
three-phase system, the average of the three line-to-line
voltage is regulated to the voltage adjust setting. In
some cases, it may be desirable to keep one phase at a
particular value. Modify the voltage adjust setting higher
or lower accordingly for any unique requirements for the
particular application. Each of the individual phase
voltages is available in Menu 11, Voltage Regulator.