Static Frequency Converter (SFC) based starting will be used for conventional fixed speed Synchronous machines. SFC provides variable output voltage and variable output frequency. The static frequency converter (also named a Load Commutated Inverter, LCI) is a speed governing equipment for providing startup and rotation speed control for large synchronous motors.
SFC is used to reduce the speed of the rotating magnetic field of the stator to a low enough value that the rotor can easily accelerate and lock in with it during one half-cycle of the rotating magnetic
field’s rotation. This is done by reducing the frequency of the applied electric power.
If such a variable-frequency drive unit is included in a motor-control circuit to achieve speed control, then starting the synchronous motor is very easily simple adjust the frequency to a very low value for starting, and then raise it up to the desired operating frequency for normal running.
SFC can quickly drag the motor to the rotation speed according to the control requirement. Meanwhile, it also can control motor operation at the required rotation speed. It has been widely used in pumped storage power plants, gas turbine power plants, and other industrial enterprise.
Applications of Static Frequency Converter:
- Pumped Storage Hydro Power Plants:
Synchronous machines are operated as motors to pump water from the lower reservoir to the upper reservoir and as synchronous generators to produce electric power by using the energy of the water which flows from the upper reservoir to the lower reservoir.
Conventional hydro-pumped storage plants can vary the power in turbine mode, the hydraulic valves that control the water flow through the turbines. In pump mode, however, the power is determined by the pump head and the rotating speed of the turbine. Therefore, the power cannot be controlled in pump mode if the turbine rotating speed is constant.
With a Static frequency converter, the stator frequency of the machine and with that the rotating speed are determined by the converter. This allows to operation of the pump at a controlled rotating speed and therefore to control the power absorbed from the grid depending on the available power. Consequently, the plant can be used for frequency control not only in turbine mode but also in pump mode.
2. Gas Power plants:
In gas-fired power plants, once the engine starts, the compressor does not accelerate fast enough to provide sufficient air to support combustion. To this end, a starter assists gas unit turbines to initially start. The starter system is an advanced frequency converter that converts the synchronous generator into a synchronous motor during the first minutes when the turbine and generator are gathering speed and provide the initial starting power for the gas generator.
Here we will try to understand the function of SFC with a typical example, in a gas-fired power plant.
Thus, at first, after receiving electrical energy from the grid, the generator is used as an electric motor and the gas turbine is started. The SFC converts the AC input to DC power and converts it back into AC power within the frequency range required by the turbine speed in the start-up phase in order to adapt the input power provided to the generator as a starter. In fact, SFC is an AC/DC/AC converter with an input frequency of 50HZ and an output frequency of zero to 35HZ. The start-up operation starts with SFC from zero speed or turning gear and continues to spin at 600 rpm with fuel injection, flame ignition, and acceleration concurrent with mechanical energy production in the turbine until it spins at 2100 rpm and then shuts off. After the turbine reaches the rated speed, the generator assumes its main task, which is electrical energy production.
Components of Static Frequency Converter:
The SFC used will be of High-Voltage large-capacity thyristor converters designed for large-capacity motor starters.
The SFC is basically composed of a rectifier bridge that converts the incoming three-phase AC voltage into a regulated DC voltage and then the DC voltage is then supplied to the inverter bridge to be re-created into AC voltage at a different frequency. The inverter is configured as a current source inverter.
The below figure shows the basic configuration of the static frequency converter.
Where Iu, Iv, and Iw are the machine line currents and id is the current at the DC interface of the SFC.
- Phase Controlled Rectifier:
The first section of SFC is called the rectifier or line-side converter section. Its main purpose is to rectify naturally (diode rectifier) or in a controlled fashion (SCR, IGCT, SGCT, or IGBT), the input supply voltage and frequency into a fixed DC voltage.
The converter section uses silicon-controlled rectifiers (SCRs), gate-commutated thyristors (GCTs), or symmetrical gate-commutated thyristors (SGCTs). This converter is known as an active rectifier or active front end (AFE).
2. DC Link:
The second section of SFC is called the DC link bus, which may include a reactor. The primary function of this section is to maintain a base voltage to be switched by the inverter section.
The DC link uses inductors to regulate the current ripple and to store energy for the motor.
3. Three-Phase Inverter:
The third section of SFC is called the inverter or load side converter section and is designed to switch in a controlled fashion the fixed DC link bus voltage into a variable frequency output. Depending on the current ratings this section may contain GTOs, SCR’s IGBTs, or more recently IGCTs and SGCT’s.
The inverter is a used circuit to convert the DC voltage source remains a source of AC voltage with a certain frequency.
The inverter section comprises gate turn-off thyristor (GTO) or symmetrical gate commutated thyristor (SGCT) semiconductor switches. These switches are turned on and off to create a pulse width modulated (PWM) output regulating the output frequency.
SFC Operation:
The Synchronous machine using SFC operated in two modes.
Synchronous Machine as a Generator:
Conversion of the motor mode to the generator is a must to draw power from the machine. In this process, thyristor pulses of SFC bridges are blocked to stop the power flow from the grid. Field regulation
is then adjusted by the controller in the Static Excitation System and the machine is changed from motor to generator mode.
Synchronous Machine as a Motor:
The initial low speed (150rpm) rotation is called ‘Turning Gear’. This 150 rpm rotation of the machine is called turning gear. ‘Pulse link mode’ is activated by the SFC for this speed. Being low back e.mf the natural commutation for SFC thyristors is not possible hence forced commutation is used. The machine can be kept in turning gear as long as required to stabilize the various parameters.
To get the full speed of 3000rpm inverter output frequency is increased by changing the thyristor firing angle. To increase the speed, the pulse link mode is changed to ‘Synchronous mode’ to get the full speed of 3000rpm. Thyristor commutation is ‘natural’ in this mode.
Process of SFC Commutation:
Three-phase controlled rectifier bridge system is a 3-phase full-wave rectifier that converts a 3-phase AC wave into a wave of DC (direct current).
The rectifier of the SFC is a 3-phase converter with two-quadrant operation, wherein the thyristor is
turned on at intervals of 60°. Therefore thyristor switched on every interval of 60°, then the frequency of the output ripple voltage is 6 times the frequency of the voltage source. The order of conduction of the thyristor into 6 pieces will follow the pattern of T11T12, T12T13, T13T14, T14T15, T15T16, and T16T11.
The inverter of the SFC is fired according to the position of the machine in order to follow the speed of the motor and the rectifier controls the magnitude of the current injected into the machine.
The control strategy of the inverter for synchronous machine starting is divided into two stages. The first stage requires pulse commutation until 10% of the machine-rated speed, the second stage uses natural commutation supported by the voltage induced in the machine windings.
• SFC feeds the stator winding very low-frequency pulses thereby activating the ‘Pulse link mode’.
• Two out of three thyristor legs of Inverter are fired which passes current through two windings at a time. This current produces stator magnetic flux Φi.
• Vector Φi rotates in steps of 60 degrees according to the sequence of the fired pairs of thyristor.
• The SES supplies current to the rotor field winding producing a magnetic flux Φe.
• The torque produced in the motor Tm = K•id•cosϕ•Ψ,
where, K=constant, id =average current in DC link
φ=phase difference between current and voltage
Ψ=motor flux, which is directly proportional to Um/n
Where, Um=Motor voltage, n=speed (revolution).
The torque reaches a maximum value when magnetic flux Φe is perpendicular to flux Φi. It turns the rotor in the direction of the acting force. After the rotor has turned by 60 degrees, the next pair of thyristors is fired (turning vector Φi, by 60 degrees, producing again a maximum torque on the rotor).
I have problem when start gas turbine we get SFC fault and PU VALVE doesn’t block what’s the reason and what meaning PU valve .
Thanks
PU Flow Control Valves are used to reduce the rate of flow in a section of a pneumatic circuit, resulting in a slower actuator speed. Unlike a Needle Valve, Flow Control Valves control flow in only one direction, allowing free flow in the opposite direction.