As Tomahawk discusses the importance of automatic voltage regulation or AVR we will focus on voltage basics, digital excitation technology, paralleling of generator sets, and power system stabilizers.
A majority of the generators Tomahawk will be selling in reference to diesel standby units or portable generators utilizes brushless excited generators. To understand Automatic Voltage regulation it is important to understand that is necessary to control voltage of generator for many reasons the most important being safety. An important part to voltage regulation is generator impedance. The generator’s impedance is the resistance to the flow of electrical current. A generator’s impedance causes a charge in generator output voltage when current flows out of the generator. As no generators today cannot regulate voltage regardless of the load voltage regulators become very necessary.
When a generator applies load increases current the voltage drop will occur across the series generator impedance. This drop in generator voltage with increasing load is the reason for using AVR to adjust generator voltage with changes in load.
There is 3 characteristics of the generator that are important to control the voltage of a generator.
- Generator output voltage can be varied by filed current changes
Generator voltage drop is caused by generator impedance
Generator voltage changes with changes in speed of the prime mover (AKA diesel, LPG, or NG Engine)
As there are various voltages that Generators run at 120v ,240v, 208v, 277v, 480v, 600v AVR’s can be equipped to sense single phase or three phase generator voltages. Some have the capability of doing both. A single phase generator must be fitted with a regulator that senses single phase. A three phase generator maybe set up with either a single or three phase sensing regulator. AVR’s applied to three-phase generators most commonly use the same sensing circuit as single phase generators.
In summary the AVR is brain of the engine and is only as good as the information and voltage it reads. So it is very important make sure the Automatic Voltage Regulator on a generator is always working properly.
Another feature AVR’s have which is on all generators that Tomahawk Power sells is Frequency compensation. Most AVR’s have frequency compensation as a standard feature. There are two forms of frequency compensation: Volts/Hertz and modified volt/hertz. The V/Hz compensation adjusts the excitation for any variations in frequency. The voltage limited V/HZ circuit allows for the minor variations in frequency without compensation until the frequency drops below a preset value.
Frequency variations in the generator system can have a couple of effects. The first is a large load of KW is applied to the generator terminals, the prime mover may slow down; the effect is very apparent in heavily turbocharged engines where a lag exists between the time a load is applied to the engine and when a contribution form the turbo is seen. The second type of frequency variation is caused by the generator requiring more field excitation to maintain rated voltage at a lower frequency, as well as the loss of generator cooling air volume at reduced frequency. The effect is increased heating on the generator field and the exciter. The simple way to prevent excessive heating is to shut down excitation when operating at reduced frequency. If this is not acceptable, then the generator voltage may be reduced in proportion to speed, which results in constant main field excitation with changes in speed.
Voltage regulator stability
The AVR significantly influences generator system stability. During initial start-up of a generator system, it is common to witness generator voltage instability. Most of the time, the solution is to tune the AVR’s stability control until the generator voltage has normalized. Adding an AVR to the system make the generator a closed unit system which requires no outside intervention.
There are 3 factors that affect voltage stability
Gain- the amount of output that can be produced for a small input signal applied. The higher the amplification the greater the gain.
Stability Network-the function of a stability network is to insure that the VR does not correct the system voltage too fast. Causing the machine voltage to oscillate or conversely so slowly that the generator voltage takes a long time to recover.
Field Time constant- the field of the generator has inductance and resistance. The ratio of inductance is L the ration of resistance measure in R. As the generator size grows so does the time constant. Additionally the greater the phase shift, the more difficult it is for the AVR to maintain stable operation.
Here are techniques for adjusting the stability of the voltage regulator
Simple Turning- adjust the gain toward the less stable end typically counter clockwise or the gain can be increased until the field voltmeter just begins to oscillate.
Step Reponses. Another method of determining unit performance is introducing a 12% signal step into the VR set point. The step size should be small enough to keep the AVR from hitting a limit, the positive or negative ceiling level of its output.
Load application/rejection testing. Many technical specs for generator sets require the voltage dip and overshoot to be held with certain limits. There may also be requirements on the time it takes for the voltage to recover to within a specified percent of the final value. Tuning the stability circuit to achieve the technical specification requires an understanding of the various factors that affect the transient performance of the generator set.
Digital voltage regulators have become the norm in the genet industry and All Tomahawk Power Generators use the, Data recording for troubleshooting and commissioning have streamlined the process and made AVR and digital voltage regulation the future of power generation. In summary, the AVR functions to control generator excitation, replace an operator monitoring generator voltage and provide corrections to the excitation. The need for fast accurate voltage control in modern generator and backup generators. Tomahawk makes it essential that we use state of the art voltage regulation systems to properly support the needs of our customers.