Excitation systems can be defined as the system that provides field current to the rotor winding of a generator. Well-designed excitation systems provide reliability of operation, stability and fast transient response.
The four common excitation methods include:
- Shunt or Self Excited
- Excitation Boost System (EBS)
- Permanent Magnet Generator (PMG)
- Auxiliary Winding (AUX).
Each method has its individual advantages. All methods use an Automatic Voltage Regulator (AVR) to supply DC output to the exciter stator. The exciter rotor AC output is rectified to a DC input for the main generator rotor. More advanced systems use an additional input to the AVR. This article will explore the construction, function and application for each method and includes diagrams and illustrations for each.
Automatic Voltage Regulator (AVR)
The construction of the AVR vary with the excitation used. All receive input from the stator of the generator when it rotates. AVRs with the capability of receiving a second input to reduce or eliminate internal harmonics caused by load feedback signals are used for non-linear load applications. The two types commonly used are:
- Silicone Controlled Rectifier (SCR) - Senses power level from the stator and determines its firing for the exciter voltage. Can cause troubles when used with non-linear loads.
- Field Effect Transistor (FET) - Senses power level from the stator and translates in to a Pulse Width Modulated (PWM) signal to the exciter. This style of AVR can be used for excitation methods. Non-linear loads do not cause feedback resulting excitation breakdowns.
Shunt or Self-excited
The shunt method is features a simple and cost effective design to provide input power to the AVR. This method requires no additional components or wiring. When problems arise troubleshooting is simplified with less components and wiring to validate.
As the generator is rotated, the stator supplies input voltage to the AVR. In addition the AVR has sensors that monitor the output of the stator.
The AVR supplies power the exciter and is rectified to DC current. The current is induced onto the stator for load output.
The biggest drawback to this system is the AVR is impacted by the load the generator is powering. When the load increases the voltage begins to decrease and the AVR must provide more current to the exciter to support the demand. This pushes the AVR to its limits. If the AVR is pushed beyond it's limits the excitation field collapses. The output voltage is reduced to a small amount.
If a short circuit occurs in the supply to the AVR, the generator will not have an excitation source. This causes a loss of generator power output.
Generators with shunt or self-excited methods can be used on linear loads (constant load). Applications that have non-linear loads (varying load) are not recommend for generators with this excitation method. Harmonics associated with non-linear loads can cause excitation field breakdowns.
Excitation Boost System (EBS)
The EBS system is comprised of the same basic components supplying inputs to and receiving outputs from the AVR. The additional components in this system are:
- Excitation Boost Control (EBC) Module
- Excitation Boost Generator (EBG).
The EBG is mounted on the driven end of the alternator. Physical appearance is the same as a permanent magnet. The EBG supplies power to the controller as the generator shaft rotates.
The EBC control module is connected in parallel to the AVR and the exciter. The EBC receives signal from the AVR. When needed the controller supplies varying levels of excitation current to the exciter at levels that depend on the needs of the system.
The additional power feed to the excitation system supports load requirements. This allows the generator to start and recover the excitation voltage.
This excitation system is not recommended for continuous power applications. It is intended for emergency or back-up power applications. When the generator starts the EBS system is disengaged until operating speed is reached. The EBG is still generating power but the controller does not route it.
System allows for dynamic response, is less expensive and meets requirements for providing 300% short circuit current. Non-linear loads such as motor starting, are improved when compared to the Shunt or Self Excited method.
Permanent Magnetic Generator (PMG)
Generators equipped with permanent magnets are among the most well-known separately excited methods. A permanent magnet is mounted on the driven end of the generator shaft.
PMG supplies isolated power to the AVR when the generator shaft rotates. The AVR utilizes the extra power when supplying non-linear loads such as; starting of motors.
A clean, isolated, uninterrupted 3-phase waveform is produced when the generator shaft is turning.
Some of the benefits of using generators equipped with the PMG excitation method are:
- Excitation field does not collapse allowing for sustained short circuit faults to clear.
- Changing load does not impact excitation field.
- Voltage is created on initial startup and does not depend on remaining magnetism in the field.
- During motor start up excitation field does not collapse because of lack of AVR supply.
The PMG System adds weight and size to the generator end. It is the most commonly used excitation method for applications that use motors that start up and shutdown and other non-linear loads.
Auxiliary Winding (AUX)
The auxiliary winding method has been in use for years. The uses range from marine to industrial applications and are more practical in larger installations.
This method has a separate excitation field, however it does not use a component attached to the driven end of the shaft of the generator. These methods use shaft rotation and a permanent magnet or generator to supply the additional excitation.
An additional single phase winding is installed into the stator. As the generator shaft rotates the stator main windings supply voltage to the AVR as in all above mentioned methods.
The additional single phase windings supply voltage to the AVR. This creates the extra excitation voltage needed when supplying non-linear loads.
For linear load applications shunt, EBS, PMG and AUX excitation methods can be used. Shunt excitation is the most cost effective method.
For non-linear load applications, EBS, PMG and AUX excitation methods can be used. PMG excitation is the most common and widely used.
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