11-11-2016, 11:34 AM
Functions and Performance
Requirements of Excitation Systems
. The function of an excitation system are >To provide direct current to the synchronous generator field winding >To perform contron and protective functions essential to the satisfactory operation of the power system
. The performance requirements of the excitation system are determined by
a) Generator considerations:
> supply and adjust field current as the generatior output varies within its continous capability > respond to transient disturbances field forcing consistent with the generator short term capabilities
- Rotor insulation failure due to high field voltage
- Rotor heating due to high field current
- Stator heating due to high VAR loading
- Heating due to excess flux (volt/Hz)
b) Power system considerations:
> contribute to effective control of system voltage and improvement of system stability
Types of Excitation Systems
Classified into three broad categories based on the excitation power source:
1. DC excitation systems
2. AC excitation systems
3. Static excitation systems
1. DC Excitation Systems:
. utilize dc generators as source of power; driven by a motor or the shaft of main generator; self or separately excited
• represent early systems (1920s to 1960s);
lost favor in the mid-1960s because of large size; superseded by ac exciters
• voltage regulators range from the early non-continuous rheostatic type to the later system using magnetic rotating amplifiers
Figure .1 shows a simplified schematic of a typical dc excitation system with an amplidyne voltage regulator
self-excited dc exciter supplies current to the main generator field through slip rings
. exciter field controlled by an amplidyne which provides incremental changes to the field in a buck-boost scheme
. the exciter output provides rest of its own field by self excitationAC
2. AC Excitation Systems:
• use ac machines (alternators) as source of power
• usually, the exciter is on the same shaft as the turbine-generator
• the ac output of exciter is rectified by either controlled or non-controlled rectifiers
• rectifiers may be stationary or rotating
• early systems used a combination of magnetic and rotating amplifiers as regulators; most new systems use electronic amplifier regulators
2.1 Stationary rectifier systems
• dc output to the main generator field supplied through slip rings
• when non-controlled rectifiers are used, the regulator controls the field of the ac exciter; Fig. 2 shows such a system which is representative of GE-ALTERREX system
. Static Excitation Systems: >all components are static or stationary
>supply dc directly to the field of the main generator through slip rings
>the power supply to the rectifiers is from the main generator or the station auxiliary bus
3.1 Potential-source controlled rectifier system:
• excitation power is supplied through a transformer from the main generator terminals
>regulated by a controlled rectifier
>commonly known as bus-fed or transformer-fed static excitation system
>very small inherent time constant
> maximum exciter output voltage is dependent on input ac voltage; during system faults the available ceiling voltage is reduced
3.2 compound-source rectifier system:
. power to the exciter is formed by utilizing current as well as voltage
• achieved through a power potential transformer (PPT) and a saturable current transformer (SCT)
• the regulator controls the exciter output through controlled saturation of excitation transformer
• during a system fault, with depressed generator voltage, the current input enables the exciter to provide high field forcing capability
An example is the GE SCT-PPT.:
3.3 Compound-controlled rectifier system:
• utilizes controlled rectifiers in the exciter output circuits and the compounding of voltage and current within the generator stator
•
• result is a high initial response static system with full "fault-on" forcing capability
An example is the GE GENERREX system.
Control and Protective Functions
A modern excitation control system is much more than a simple regulator
It includes a number of control, limiting and protective functions which assist in fulfilling the performance requirements identified earlier
Figure 8 : illustrates the nature of these functions and the manner in which they interface with each other
any given system may include only some or all of these functions depending on the specific application and the type of exciter
control functions regulate specific quantities at the desired level
limiting functions prevent certain quantities from exceeding set
if any of the limiters fail, then protective functions remove appropriate components or the unit from service