Description
Background:
The Integrated Generator Control and Protection Unit (GCPU) is designed for use on
DC Power Generating and engine starting system of ALH (Advance Light Helicopter).
ALH uses twin Generators in independent and / or parallel mode operation with one
integrated GCPU required for each Generator. This unit is designed for power rating
6KW, rated voltage 28V, rated load current 200A, rated speed range from 8000 rpm to
12150 rpm.
Summary:
The requirements of this unit is to regulate the Generator output to deliver constant
28V DC output as well as protecting the Electronics equipments running through the
generator by disconnecting the generator from the load bus.
Functional Description:
The Starter / Generator (S/G), for which GCPU is designed, works on two modes:
a) Starting mode: In this mode S/G acts as a motor by drawing power from external
power unit or internal batteries and drives the engine gas generator until engine
gets started up.
b) Power generation mode: In this mode, S/G is driven by engine through the engine
accessory gear box and supplies power to the main bus bar on which battery floats.
Hence the GCPU also has two modes of operation:
a) Starting Mode function: This mode of GCPU supports S/G as motor
(Starter).
b) Generating mode function: this mode of GCPU supports S/G as Generator
STARTING MODE FUNCTION
(i). FIELD WEAKENING
In the starting mode, the starter generator is used as a motor to drive the engine. At
low rpm of starter, torque is sufficiently high to drive the engine. As speed
increases torque decreases and at high speed of starter its torque may not be
sufficient to drive the engine. Hence field weakening function of GCPU is required
to provide the sufficient torque at higher starter speed. This Field weakening is
obtained by providing pulse width modulated (PWM) shunt field current. The
modulator output is high for longer period of armature current greater than 300Amp.
The ON time decreases gradually for decrease in armature current due to higher
rpm which maintains the high torque at higher speed. (Fig. 1)
GENERATING MODE FUNCTION
(i). REGULATION
The most basic of the GCPU functions is that of voltage regulation. Regulation of
any kind requires the regulation unit to take a sample of a generator output and
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compare that sample to a known reference. If the generator's output voltage falls
outside of the set limits, then the regulation unit must provide an adjustment to the
generator field current. Adjusting field current controls generator output.
This unit regulates the Generator voltage within the specified limit (28V DC +1.9V/-
2.2V) under no load to full load and at various speed (rated speed) conditions. This
is obtained by Pulse Width Modulation (PWM) control circuit, which adjusts the duty
cycle of generator field current. The potentiometer is used to adjust the regulation
voltage requirement by varying the amount of sample of generator voltage being
fed to PWM circuit. (Fig. 2)
(ii). LOAD SHARING
This provides equal load sharing between two Generators running in parallel by
cross sensing of equalizing coil voltage proportional to their load currents.
(iii). CURRENT LIMITING
Under this function, unit ensure the decrease field current in order to limit the
Generator current below the specified limit. Generator Voltage drops when
generator current exceeds 300A +30/-0A across equalizing coil.
(iv). DIFFERENTIAL VOLTAGE SENSING
This function of a control system is designed to ensure all generator voltage values
are within a close tolerance before being connected to the load bus. If the output is
not within the specified tolerance, then the generator contactor is not allowed to
connect the generator to the load bus.
Unit connects the Generator to load bus by energizing the line contactor only if the
Generator voltage is higher than bus bar voltage by 0.5 V (minimum).
(V). GENERATOR VOLTAGE PROTECTION
a) OVER VOLTAGE PROTECTION
The overvoltage protection system compares the sampled voltage to a reference
voltage. The overvoltage protection circuit is used to open the relay that controls
the field excitation current. Thus disconnects the Generator and GCPU from the
bus bar. (Fig. 3)
b) UNDE R VOLTAGE PROTECTION
Under voltage function ensures that in case of DC under voltages the Generator
and GCPU are disconnected from the bus bar. (Fig. 3)
c) FALSE TRIPPING
To avoid the false tripping due to under voltage when battery is ON and Generator
is not running, the unit should not trip for generator voltage 0 to 2 V -0/+3V. (Fig. 3)
(vl). GENERATOR OVER CURRENT PROTECTION
In case of over loading, GCPU trips off the Generator after a time delay inversely
proportional to the level of current. Integrated GCPU trips the generator within 3 to
5 sec if generator current reaches 400A. (Fig. 3)
(vii). REVERSE CURRENT PROTECTION
If the generator cannot maintain the required voltage level, it eventually begins to
draw current instead of providing it. This situation occurs, for example, if a
generator fails. When a generator fails, it becomes a load to the other operating
generators or the battery. The defective generator must be removed from the bus.
The reverse current sensing function monitors the system for a reverse current.
Reverse current indicates that current is flowing to the generator not from the
generator. If this occurs, the unit disconnects the Generator from bus bar by deenergizing
the line contactor. (Fig. 3)
(viii). FIELD OVER EXCITATION PROTECTION:
When one generator in a paralleled system fails, one of the generators can become
overexcited and tends to carry more than its share of the load, if not all of the loads.
Basically, this condition causes the generator to produce too much current. If this
condition is sensed, the overexcited generator must be brought back within limits,
or damage occurs.
Circuit senses the duty-cycle of the field and generator voltage. Generator gets
tripped off if 100% duty cycle of the field voltage occurs at 29.5 V ± 0.5V of
generator voltage after time delay of 30 ms ± 5 ms. (Fig. 3)
starter Contactor
Control from Engine
Starting System
zzy- •Q'D
Starter
- s i Contactor
Coil
V
PWM
CONTROLLER
Relay
Rl
FILTER
Transistor
Vref
Diode A
Feedback
Load Bus
]9^ Coil Line
-x' Contactor
Engine
Battery

CLAIMS:
(1) Integrated GCPU is indigenously developed by ASERDC, HAL Lucknow, to control
and protect the starter generator in ALH.
(2) 'Field weakening feature' maintains positive accelerating torque at higher speed in
the starting mode of S/G as motor through Pulse width modulated shunt field current.
(3) 'Voltage regulation feature' regulates the Generator voltage within the specified limit
(28V DC +1.9V/-2.2V) under no load to full load and at various speed (rated speed)
conditions.
(4) 'Load sharing feature' provides equal load sharing between two generators running
parallely
(5) 'Current limiting feature' ensures the current level within the specified limit (300 Amp
+30/-0A) through current limiting function.
(6) "Differential voltage sensing feature' connects the generator to load bus only if the
generator voltage is higher than bus bar voltage by 0.5V
(7) 'Protection feature' disconnects the Int. GCPU & the generator from bus bar in case
of over/under voltage, over current, reverse current and over excitation.