Voltage Regulation and Load Sharing
DC generator Voltage Regulation
It is very important that before the power is distributed to any system, that it is having a constant voltage, the process of maintaining a constant voltage is called as voltage regulation.
Constant voltage supply is the most important factor for the efficient operation of electrical equipment in an airplane. The voltage produced in a generator depends on the rate of change of flux, more the rate of change if flux, more is the voltage produced a from the generator.
Hence the voltage output of a generator, can be controlled by changing the speed of rotation or the strength of the field current which changes the field strength or flux.
If the voltage increase or decrease varying the speed of the generator is not a practical solution to keep a constant output voltage, therefore field current is changed as it can be conveniently controlled.
The most simple voltage regulator can be a rheostat connected in series with the field circuit. If the rheostat is set to increase the resistance in the field circuit, less current flows through the field winding and the strength of the magnetic field in which the armature rotates decreases. Consequently, the voltage output of the generator decreases. If the resistance in the field circuit is decreased with the rheostat, more current flows through the field windings, the magnetic field becomes stronger, and the generator produces a greater voltage.
BASIC VOLTAGE REGULATOR
Carbon Pile Voltage Regulator
Carbon has a granular surface and the contact resistance between two carbon faces that are held together depends on the actual area of contact and the pressure with which the two faces are held together.
When a number of carbon discs or washers are arranged in the form of a pile and connected in series with the shunt field of a generator the field circuit resistance can be varied by increasing or decreasing the pressure applied to the ends of the pile and changes in generator output voltage can be obtained.
The pile unit is housed within a ceramic tube which, in turn, is enclosed in a solid casing, or more generally, a finned casing for dissipating the heat generated by the pile. The number, diameter, and thickness of the washers which make up the pile, varies according to the specific role of the regulator. Contact at each end of the pile is made by carbon inserts, or in some types of regulator by silver contacts within carbon inserts
CONSTRUCTION OF CARBON PILE VOLTAGE REGULATOR
Functions of each component are as follows:
Compression screw: the means of setting up compression on the pile and compensating for erosion of the pile during its life.
Spring plate and armature: this compresses the pile to its minimum resistance position.
Voltage coil: contains a large number of turns of copper wire and, with the core screw, forms an electromagnet when connected across the generator output.
Magnet core: concentrates the coil flux; it is also used for voltage adjustment during servicing.
Bimetallic washers: providing temperature Compensation
Under static conditions of the generator system, the carbon pile is fully compressed and offers minimum resistance and since there is no magnetic ''pull" on the armature, hence the resistance in the generator shunt field circuit is minimum and the air gap between the regulator armature and electromagnet core is maximum.
As the generator starts operating, its output voltage also increases which is applied to the voltage coil and the resulting field establishes an increasing "pull" on the armature until and equilibrium is established between magnetic force and spring-control force of the carbon pile.
If the generator output voltage increases the voltage across the voltage coil also increases pull of the armature towards itself thus reducing the compression of the pile, which increases the air gap between the discs to increase its resistance. The increase in the piles resistance reduces the field current and the field strength, thus reducing the the generator output.
If the generator output voltage is decreased the voltage across the voltage coil is also decreases the pull of the armature thus increasing the compression of the pile, which reduces the air gap between the discs to decrease its resistance. The decrease in the pile resistance increase the field current and the field strength, thus increasing the generator output.
NOTE : When the generator voltage output is increased, the carbon pile regulator resistance increases due to increase in the air gap between the discs, however the air gap between the carbon pile regulator and the voltage coil is decreased due to the expansion of the carbon piles. Similarly when the generator voltage output is decreased, the carbon pile regulator resistance decreases due to decrease in the air gap between the carbon discs, however the air gap between the carbon pile regulator and the voltage coil is increased due to the compression of the carbon piles.
WORKING OF CARBON PILE VOLTAGE REGULATOR
Vibration Contact Regulator / Three Unit Regulator
The Vibrating Contact Regulator are sometimes also called as Three Unit Regulators since they are made up of the following three units
Reverse Current cutout Relay
However sometime the regulator can consist of just the Voltage and Current Regulator.
THREE UNIT REGULATOR
The Voltage Regulator consist of a voltage coil connected in parallel with the generator output, hence same voltage flows through the armature and the voltage coil. It consist of a set of contact which open or close depending upon the generator output and a fixed resistance. There are two paths for the current to flow.
When the output voltage of the generators is constant the current flow through the contact points and the contacts remain closed to allow current to flow into the field windings as shown by the green arrows. Following the least resistance path.
When the generator output voltage increases beyond the specified range, the voltage coil strength increases which open the contacts and this introduces the resistor into the field windings and the current flows through the resistor as shown by blue arrows, thereby reducing the field excitation current, and subsequently reduces the generator output.
Once the output voltage drops to the specified range, the contacts close (by a spring mechanism) and the resistor is bypassed, allowing full excitation current back into the field.
The on/off cycle repeats hence they are called as vibrating contact type. However they cannot be used for high current application like the carbon pile voltage regulator since the constant making and breaking of the contact can cause sparks.
VIBRATING CONTACT TYPE VOLTAGE REGULATOR
The current regulation is achieved in a similar way, i.e. by controlling the field current. However the current coil is connected in series with the generator output.
When loads are high, the voltage output may be insufficient to open the contacts. The result is that the output will continue to increase until the maximum rated current is reached. At this point, the current regulator contacts open and the resistor is connected into the field windings. As shown by the blue arrows.
The accuracy of this type of regulation depends on the resistor value and spring tensions. In the event of high rotor speed and low electrical load on the generator, the output could exceed the specified system voltage despite the field being supplied via the resistor.
VIBRATING CONTACT TYPE CURRENT REGULATOR
Reverse Current Cutout Relay
The reverse current cutout relay can be used along with the voltage and current regulator or it can also be used as in independent unit as a protection device. Under normal conditions the current flows from the generator to the busbar. However if the generator fails there is a possibility the the current can flow the bus bar to the generator causing it to work as a motor thus damaging the engine drive. Hence Reverse current cutout relay is used.
It consists of two coils Voltage coil connected in parallel with the generator and the Current coil connected in series with the generator.
Under normal operation conditions the current flows in the current and voltage coil, since the direction of current flow is same the magnetic field produced by them is also similar and they assist each other thus keeping the contacts closed as shown by the green arrows.
In the even of the generator failure or engine failure no voltage will be generated by the generator, Since the bus bar is at a higher voltage then the generator the current will flow from the bus bar to the generator as shown by the blue arrows. As we can observe that the current from the bus bar will be opposite direction so the flux now produced by the current coil will be in opposite direction to that of the voltage coil. Hence the contact will now open thus cutting the generator of the bus bar.
The contact points are normally open as if the generator is offline it will prevent the reverse flow of current to the generator.
These are generally used with voltage and current regulator as this will prevent its unnecessary operation, when current and voltage vary with load or speed of the generator.
REVERSE CURRENT CUTOUT RELAY
Solid State (Electronic) Voltage Regulator
There are many types and configurations of electronic voltage regulators. The figure below consists of three transistor (NPN) TR1, TR2 and TR3, two diodes D1 and D2, alternator switch, field relay, a zener diode Z, resistor R1, R2 and a variable resistor RV1.
When the alternator switch is closed the supply from the bus bar or the battery is given to the field relay coil and the relay is closed. This supplied current to the base of transistor TR2 and through the voltage dividing network R1, R2 and RV1, the network along with the the zener diode Z establishes the system operating voltage.
With Power applied to the base of transistor TR2 it is switched on and the current flows to the emitter collector junction. The amplified output of the emitter flows to the base of transistor TR3, thereby switching it on and current is supplied to the field of the alternator and to the ground via transistor TR3.
When the generator reaches the preset operating value the zener diode Z breaks down allowing the current to flow through TR1 and thus switching of transistor TR2 and transistor TR3. So the current to the field is cut off.
The purpose of diode D1 is to provide path so that current can fall at a slower rate whenever the transistor TR3 is switched off.
When the alternator output voltage falls the zener diode Z cease conduction and the current again starts flowing through the transistor TR2 & TR3 and restore the current to the field winding.
This sequence of operation is repeated and the alternator output voltage is maintained to a present operating value.
The operating value can be adjusted by the variable resistance RV1.
SOLID STATE VOLTAGE REGULATOR
DC Generator Pralleling and Load Sharing
Most modern aircrafts are multi engine and hence have multiple power sources. It is desirable that the generators driven by each engine are operated in parallel thereby ensuring that in the event of failure of of an engine or its generator, there is no interruption of the primary power supply.
Parallel operation required equal sharing of load by the generators so that their output voltages are as near equal as possible under all possible under all operating conditions. Since voltage is the only factor to be considered for DC supply distribution system circuit diagram of paralleling of DC is fairly simple.
The method commonly adopted is called as "Load Equalizing Circuit". The generators are interconnected on their negative side, via a series "Load- Sharing" or "Equalizing" loop containing equalizing coil (Ce) each coil is a part of the individual voltage regulator. The resistance R1 and R2 are connected to the negative side of the generators.
Under balanced condition the voltage drop across each section will be same, hence the net voltage drop will be zero and no current flows through the equalizing coils.
BASIC PRINCIPLE OF LOAD SHARING
If the generator no. 1 is sharings a larger share if the total load then generator 2, this implies that the voltage generated by generator no 1 will also be greater than that of generator no 2. Hence the voltage drop across R1 will also be greater than the voltage drop across R2, this will allow a equalising current Ie to flow through the equalising coils in such a manner that the voltage produced by generator no 2 will be increased and the voltage produced by generator no 1 will be reduced. Thus maintaining the a balanced load sharing condition.
The above example is the basic load sharing principle, however in the actual aircraft systems the load sharing circuits are used with the voltage regulators.
When they are used with carbon pile voltage regulators, the equalising coils are wound on the same magnetic core of the voltage coil. If we consider the same case as above i.e. the generator no 1 is sharing more load then the generator no 2, the equalising current will flow in the same direction to that of the voltage coil. Both current will assist each other which will increase the pull on the armature, which will increase the carbon pile resistance and decrease the field current thus reducing the voltage generated by generator no 1. While in case of generator no 2 the current will flow in opposite direction and reduced the carbon pile resistance,which will increase the field current and thus increase the voltage generated by generator no 2.
CARBON PILE VOLTAGE REGULATOR WITH EQUALISING COIL
In case of vibrating contact type voltage regulator the construction is slightly different. The circuit does not contain the resistance, they consist of two paralleling relay, the coil of which are connected to the individual generator armature. Hence under balanced condition thee current in each coil of the paralleling relay is equal but opposite in direction.
An additional coil Eq i.e. equalising coil is connected within the voltage regulator in such a manner that under balanced load condition the current flowing through the voltage coil and equalising coil are in the same direction.
If we consider the same case as above i.e. the generator no 1 is sharing more load then the generator no 2, more voltage is flowing in the voltage coil of generator no 1. Since the same current flows through the equalising coil and in the same direction it opens the contact which allows the resistance to be introduced in the circuit and function in a similar way to the vibrating contact redulator. Hence the field current is reduced which reduces the voltage produced by the generator.
AC Voltage Regulation
The Voltage Regulation for AC system differ from those of DC system as the AC output cannot be directly given to the field or the voltage regulator. The field of any generator is fed with DC supply, if AC supply is fed to the field the electromagnet will keep on changing its polarity as the direction of current through them will keep on reversing.
There are two types of AC system in use one is constant frequency system and other is variable frequency system also known as frequency wild system. Most modern aircraft are constant frequency system.
Frequency Wild Generator Voltage Regulation
Voltage regulator consist of a network of magnetic amplifiers or transducers, transformers and bridge rectifiers interconnected. The field excitation is controlled by two factors one is the load current and other is the error between the line voltage desired and the actual voltage obtained.
The voltage regulation network provides sensing of error voltages and necessary re-adjustment of excitation current.
It will be noted from the diagram that the three- phase output of the generator is tapped at two points; at one by a three-phase transformer and at the other by a three-phase magnetic amplifier . The secondary winding of one phase of the transformer Is connected to the a.c. windings of a single phase "error sensing" magnetic amplifier and the three primary windings are connected to a bridge "signal" rectifier.
The d.c. output from the rectifier is then fed through a voltage sensing circuit made up of two resistance arms, one (arm ''A") containing a device known as a barretter the characteristics of which maintain a substantially constant current through the arm, the other (arm "B") of such resistance that the current flowing through it varies linearly with the line voltage
FREQUENCY WILD VOLTAGE REGULATION
The two current signals, which are normally equal at the desired line voltage, are fed in opposite directions over the a.c. output windings in the error magnetic amplifier. When there is a change in the voltage level, the resulting variation in current flowing through arm "B" un- balances the sensing circuit and, as this circuit has the same function as a d.c. control winding, it changes the reactance of the error magnetic amplifier a.c. output windings and an amplified error signal current is produced.
After rectification, the signal is then fed as d.c, control current to the three-phase magnetic amplifier thus causing its reactance and a.c. output to change also. This results in an increase or decrease, as appropriate, of the excitation current now to the generator rotor field winding, continuing until the line voltage produces balanced signal conditions once more in the error sensing circuit.
Constant frequency Voltage Regulation
The regulation of the output of a constant-frequency system is also based on the principle of controlling field excitation.
The circuit is comprised of three main sections
Voltage error detector
The function of the voltage error detector is to monitor the generator output voltage, compare it with a fixed reference voltage and to transmit any error to the pre-amplifier. It is made up of a three- phase bridge rectifier connected to the generator output, and a bridge circuit of which two arms containing gas-filled regulator Lubes and two contain resistances.
The inherent characteristics of the tubes are such that they maintain an essentially constant voltage drop across their connections for a wide range of current through them and for this reason they establish the reference voltage against which output voltage is continuously compared.
The output side of the bridge is connected to an "error" control winding of the preamplifier and then from this amplifier to a "signal" control winding of a second stage or power amplifier. Both stages are three-phase magnetic amplifiers. The final amplified signal is then supplied to the field windings of the generator.
CONSTANT FREQUENCY VOLTAGE REGULATION
The output of the bridge rectifier In the error detector is a d.c. voltage slightly lower than the average of the three a.c. line voltages ; it may be adjusted by means of a variable resistor (RV1)to bring the regulator system to a balanced condition for any nominal value of line voltage.
A balanced condition the bridge circuit concerned is obtained when the voltage applied across the bridge (points "A" and "B) is exactly twice that of the voltage drop across the two tubes.
Since under this condition the voltage drop across resistors R1 and R2 will equal the drop across each tube, then no current will flow in the output circuit to the error control winding of the pre-amplifier
If the a.c. line voltage should go above or below to fixed value, the voltage drops across R1 and R1 will differ causing an unbalance of the bridge circuit and a flow of current to the "error" control winding of the pre-amplifier.
The direction and magnitude of current flow will depend on whether the variation, or error line voltage, is above (positive error signal) or below (negative error signal) the balanced nominal value, and on the magnitude of the variations.
When current flows through the "error" control winding the magnetic flux set up alters the total flux in the cores of the amplifier, thereby establishing a proportional change in the amplifier output which is applied to the signal winding of the power amplifier.
If the error signal is negative it will cause an increase in core flux, thereby increasing the power amplifier output current to the generator exciter field winding.
For a positive error signal the core flux and excitation current will be reduced, thus, the generator output is controlled to the pre-set value which on being attained restores the error detector bridge circuit to the balanced condition.
Load Sharing of AC Generators
The concept of load sharing or Paralleling of generators is different for AC generators as compared to the DC generators. Since in DC generators only factor to be considered is the Voltage, however for AC generator both the volatge and frequencies are to be consisdered
Frequency Wild System Load Sharing
The concept of load sharing or paralleling of generators does not all to the frequency wild systems as the frequency is different and two different generators cannot be connected together.
Constant Frequency System Load Sharing
In an AC system there are two kinds of load that exists in any system
Real Load Sharing
The real load is the actual working load output in kilowatts (kW), real load is directly related to the input power to the generator. So the real load sharing control must be applied to the engine by adjusting the torque at the output drive shaft.
The real load sharing is controlled by the real relative rotational speed of the parallel generators, which in turn is controlled by the constant speed drive units.
Current transformers sense the Real Load distribution at the output of each of the paralleled alternators. When current flows through these transformers, voltage is induced in them and a current will flow in the Load Sharing Loop. Each of the current transformers, which are connected in series with each other in the loop, has an Error Detector wired in parallel with it.
REAL LOAD SHARING CIRCUIT DIAGRAM
Suppose in balanced condition a current of 5 A flows from the output of the current transformer. Since all the current in all the transformers is constant, no current will flow through the error detector.
If the drive unit of the No 1 alternator increases its torque output, due to which the speed of rotation of the armature will increase and thus the rate of change of flux would increase too. Due to the increase in the rate of change of flux the output of the generator would increase and it will take a bigger share of the load than the other two alternators which will decrease by a proportional amount.
Suppose the output of the No. 1 alternator current transformer has increased to 7 amperes so this will mean that the output of the No. 2 and 3 transformers will decrease by 1 ampere each to 4 amperes so that the average current flowing in the circuit is still 5 amperes.
According to Kirchhoff’s first law the difference between each current transformer and the average current will be pushed through the error detectors in opposite directions for the load controller 1 and in the same direction for the load controller 2 and 3.
This signal, when amplified, will be sent to the speed governors to tell the CSDU for the No. 1 Gen to reduce torque (speed) and the CSDUs for the No. 2 and 3 Gen to increase torque (speed) until the current in each transformer is once again equal and the real load is once again balanced.
Reactive Load Sharing
The Reactive Load sharing is the vector sum of inductive and capacitive current and voltage in the system and is expressed in kilovolt-amperes reactive (kVAR). It is also known as "Wattless load".
The reactive load sharing is controlled by exciter field current just like the DC generator voltage regulation.
REACTIVE LOAD SHARING CIRCUIT DIAGRAM
The sensing of out of balance loads by the current transformers is the same as in the case of real load, however the reactive load needs to be passed on to the error detector by the help of mutual reactors.
The mutual reactor is a phase shifting transformer which ensures that the error detector only detects that part of the current which is 90° out of phase with the voltage (reactive load).
The error signal is then amplified and correcting signals are sent to the generator field circuit to increase the voltage on the low voltage generator and reduce the voltage of the higher voltage generator to balance the reactive load.