SUBMITTED BY: A.KALYAN(09TQ1A0210)

Variable frequency transformer

A new power transmission technology has recently been made available which is simple and reliable.

This technology is called VARIABLE FREQUENCY TRANSFORMER

A VFT has been in comercial operation since 2004 and has prooven to be a reliable power transmission technology

There are presently two VFT’s in comercial operation. They are

1) Hydro-Quebec’s Langlois substation, Canada.2) Laredo VFT substation,texas.

VARIABLE FREQUENCY TRANSFORMER

The variable frequency transformer (VFT) is a controllable, bi-directional transmission device that can transfer power between networks of two grids under some or all operating conditions

The two grids need not be synchronous. While primarily designed to move power across an

asynchronous interconnection. the VFT has also shown itself to have remarkable stabilizing benfits

The VFT is not only compatible with other devices but in some cases enhances the performance of nearby existing equipments.

What is vft..?

The VFT is composed of a rotary transformer , a torque motor and an associated drive and control system.

Vft technology

The ROTARY TRANSFORMER is composed of a stator that is very much like a stator of hydro generator.

There are laminations of steel stacked inside a stator frame. Winding bars are configured into a three phase , four pole

arrangement. The rotar also contain three phase ,four pole winding. One grid is to the winding , while the other grid is connected

to the rotor. The net effect is the circular transformer has been produced ,

with winding separated by an airgap.

construction

In order to make the connections to the rotor and the rotor to turn freely a slip ring arrangement is necessary.

The VFT contains a device known as the COLLECTOR. The collector consist of three phases of brushes and large

copper slip rings The number and size of the element of collector that the full

rating of the machine current can be transferred continuously through the full range of speeds in either direction ,including zero speed.

Also on the shaft of the VFT is a TORQUE MOTOR.

Cont..

This motor is used to align the rotor with respect to the stator and maintain the rotation necessary bridge the difference in the frequency of the two grids.

A motor and drive system are used to adjust the rotational position of the rotor relative to the stator, thereby controlling the magnitude and direction of the power flowing through the VFT.

Cont..

Consider VFT connected to two asynchronous grids. The two grids shall be called as grid-A ,grid-B. when both the grids are operating at exactly same

frequency ,say 50HZ,the rotor will stop. If grid-A frequency slightly increases to 50.1Hz,while grid-

B’s frequency remains constant. The rotor of VFT will turn to allow for this difference.

Because it’s four pole machine , it will be rotate at 3rpm. Now the condition where grid-A frequency slow down to

49.9Hz and grid-B still at 50Hz.the rotor will now be turning in the opposite ditection at 3rpm.

The existing VFTs designed for the North American market have a maximum operating speed of 90 rpm yielding a three hertz maximum frequency delta.

operation

Performance factor (PF) : The formula established to evaluate the VFT performance

factor was defined in the contract and is shown in (3).

Where MWh[real] is energy through the VFT and MWh[sheduled ] is Energy scheduled on the VFT.

After the first three years of commercial operation, the official performance factor of the Langlois VFT was 99.7%.

OPERATIONAL EXPERIENCE

Improved HVDC Performance : More than one potential VFT customer has studied the

possibility of placing a VFT in parallel with an existing back-to-back HVDC.

Studies have shown that a number of advantages could be realized with such a hybrid interconnection.

Advantages include increased system stability, additional short circuit strength, improved commutation, low power level operation ,improved damping, and with the VFTs optional black start feature, one grid could help black start the other

Cont..

Improved Range of Phase Angle Regulator : The three VFTs are effectively in parallel with three PARs, all

moving power between two different control areas. The result of paralleling the PARs is that the phase angle

between the two systems will be tightened through an additional impedance path.

it actually improves the performance characteristics of the closest PAR

Cont..

The power flow through a VFT behaves according to the same rules as any AC circuit.

As in the case of phase angle regulator (PAR) control of voltage angle can be an effective means of controlling power flow between the two points.

Where,V=sending end & receiving end voltages(δ− σ) =difference in voltage angleX=impedence between two points

Power flow

Cont..

Because of VFT’s unique construction simply turning on one of the transformer winding with respect to another can introduce of difference in phase angles there by effective power flow.

The angle introduced in the rotor with respect to stator by the torque motor is propotional to amount of torque applied to shaft.

Electrical angle between rotar and stator shown in figure.

One-Line diagram of a Variable Frequency Transformer Interconnection.

Continuous and no fixed set change points Response for stability purposes Simple model for power system use HVDC alternative Can transfer power at different frequencies More control of the real power flow

advantages

Limits on maximum power flow capability More lossy especially in reactive power losses Works at low kV range so it needs step up/down

transformers

Disadvantages

The VFT has proven as a variable transmission technology. It allows an ac connection to be made between asynchronous

ac grids for reliable and precise power transfer. It has helpful in stabilizing characterstics and does not

produce any hormonics or tersional threats. The operation of VFT does not rely on grid strength for

operation and works well in weak grid conditions. Therefore ,VFT represents an alternative technology that may

be used in INDIA as the view of electric power as a bussiness emerges.

conclusion

THANK YOU