PMG, EXCITATION, AVR & Power generation phenomenon in Generators

PMG:

The PMG (Permanent Magnet Generator) is a system which is used for secondary exciting. The PMG provides stable and reliable electric energy for AVR regardless the generator’s terminal voltage. The generator with PMG excitation system can provide 300% rated current during short-circuit, which occurs for 5–10 seconds.

The most common function of a diode is to allow an electric current to pass in one direction (called the diode’s forward direction), while blocking it in the opposite direction (the reverse direction). As such, the diode can be viewed as an electronic version of a check valve. This unidirectional behavior is called rectification, and is used to convert alternating current (AC) to direct current (DC).

EXCITER & EXCITATION:

DC supply is required to give to the field winding of a generator. This DC supply is obtained from various sources. Supply of DC power to the field is called excitation. There are two types of excitation.

• Separately excited generator

• Self-excited generator

In a separately excited generator, DC supply to the generator field is made available from a separate source which has no connection with the generator’s own generated supply. This type of generator is not used in the power plants. 

In a self-excited generator, DC supply to the field is temporarily given from the other source. Once the voltage is built up in the generator, this source is changed into the generator’s own generating supply. In this system, field supply is obtained either from an excitation transformer or from an exciter mounted on the generator shaft.

Brush less excitation system:

In this system, a small generator, called exciter, is mounted on the generator shaft. Exciter generator arrangement may be with brush-like static excitation system or brushless. In this arrangement, exciter generator is used which is a small generator having rotating coil and stationary field. This arrangement makes the excitation system brushless. No brush is required to feed the field current to the main generator. 

In exciter field of the exciter generator is stationary and the coil is rotating. Voltage generated in the rotating coil is rectified through a set of rotating diode bridge. Rotating diodes are mounted on an insulated base. Rectified output of this rotating diode bridge is connected to the field winding of the main generator (rotating) through cable which passes through drill way made in the shaft of generator. Supply to the stationary field of the exciter generator is obtained from an automatic voltage regulator (AVR). A separate source or a permanent magnet generator (PMG) is used for getting power supply for AVR. 

Power plant equipents efficiency calculation

AUTOMATIC VOLTAGE REGULATOR (AVR):

AVR system is used to control the generator out put voltage, irrespective of any load on the generator. By raising or lowering the field current (excitation), the output voltage of the generator can be raised or lowered respectively. Raising or lowering of field current is done with the help of an AVR. When load on a generator increases, its output voltage drops. By raising the field current, voltage can be raised to the normal level. Like this when, the load on generator reduces, the voltage of generator increases. In this case, field current is required to be reduced.

Other function of AVR are

• It controls the power factor of generator.
• It limits the stator current and rotor current
• It also limits the load angle.
• It detects failure of any diode.

Phenomenon of Power generation in Turbine & Generators:

Calculation of Power generation in Steam Turbines

Step-I -Start Up:

• After reaching the rated pressure and temperature of steam, turbine will be given run command.

• Turbine will speed up as per start up curve and reaches rated speed.

• At the rated ideal speed, turbine will consume minimum steam to maintain only speed.

• At rated speed of Turbine , alternator (4 pole) also will rotate with 1500 rpm.

• At this speed PMG will generate AC three phase voltage.

• This three phase AC voltage is made to fed to AVR.

• In AVR, this AC voltage will convert into DC through rectifier and is again fed to AC exciter field, which is stationary.

• AC voltage generated from rotating coil of the AC exciter is rectified through rotating diodes and this DC voltage is fed to field winding of main generator through cable which passes through drill way made in the shaft of generator.

Step-II- Load Raising:

• Once Generator reaches 1500 rpm AVR is made ‘ON.’

• After AVR is made ON, rated voltage of generator is bulid up at its terminals.

• Then Generator breaker is closed to share the auxiliary load of the plant.

• After closing the Generator breaker, the generator gets synchronized with DG.

• After synchronizing DG is made “OFF” and STG will share home load (plant auxiliary load).

• In order to share the extra load on turbine, power is to be pumped to external grid.

• In order to raise the load, a load “rise” command is given to Wood word governor through DCS or through local panel.

• This “rise” command for a particular load is calculated in terms of percentage of droop in Governor & based on this a reference speed will be generated. (For example for 20 MW,6300 RPM & 4% droop Turbine  has reference speed  =6300+6300 X 4% = 6552 RPM) 

• This calculated reference speed signal will be go to HP valve for opening the HP valve

• After opening the HP valve torque on the turbine rotor will increase (speed will not increase as turbine is connected with grid frequency).

• This torque on the turbine rotor will transfer to reduction gear box (RGB) and also on generator rotor.

• Due to this higher torque stress will produce on stator winding, which causes to increase of current on main generator, as voltage and speed are constant.

• This high current and voltage are compared and MW signal will be generated

• This rise and drop in current and voltage will be sensed by AVR through CT/PT situated at generator output bus bars, so AVR maintains rated voltage by giving more/less excitation to AC exciter. Thus the process continues.