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Three-Phase OLTC Regulating Transformer (Phasor Type)

Implement phasor model of three-phase OLTC regulating transformer

Library

Flexible AC Transmission Systems (FACTS)/Transformers

Description

This block is used to model a three-phase two-winding transformer or autotransformer using a on-load tap changer (OLTC) for regulating voltage on a transmission or distribution system. Controlling voltage on a transmission system will affect primarily flow of reactive power, which, in turn, will affect the power transfer limits. Although the regulating transformer does not provide as much flexibility and speed as power-electronics based FACTS, it can be considered as a basic power flow controller. This is why it has been included in the facts library. The dynamic performance of the regulating transformer can be enhanced by using a thyristor-based tap changer instead of a mechanical tap changer. As this model is a phasor model which does not implement the details of current commutation from one tap to the next tap, you can use it to model a thyristor-based tap changer and implement your own control system by selecting External control of OLTC in the block menu. You can also use this block together with the Three-phase OLTC Phase-Shifting Transformer Delta-Hexagonal (Phasor Type) block for building phasor models of more complex transformer-based FACTS topologies.

The figure below shows one phase of the three-phase regulating transformer. Each phase consists of main windings 1 and 2 and of a tapped regulation winding.

One Phase of the Regulating Transformer

NtapsN ≤ +Ntaps. The voltage ratios on the two windings are:

Winding 1Winding 2

Voltage regulation is performed by varying the transformer ratio V2/V1 by means of the OLTC. The OLTC can be connected either on winding 1 (left diagram) or on winding 2 (right diagram). The OLTC can select any tap from position 0 (no voltage correction) to maximum tap (position Ntap) yielding maximum voltage correction. The OLTC is also equipped with a reversing switch which allows connecting the regulation winding either in additive or subtractive polarity. The factor multiplying Vnom2/Vnom1 is the voltage correction factor. It is given by:

1/(1 + N ΔU) for OLTC on winding 1
1 + N ΔU for OLTC on winding 2

where

N = tap position
ΔU = voltage per tap in pu of nominal voltage of winding 1 or winding 2.

Negative values of N correspond to the reversing switch position shown in dashed lines (regulation winding in subtractive polarity). The transformer resistance and leakage reactance also vary with the tap position.

Mechanical tap changers are relatively slow devices. The time required to move from one tap position to the next one is usually comprised between 3 and 10 seconds. You specify this mechanical time delay in the block menu.

    Note   OLTCs use additional switches and resistors (or inductors) to transfer current from the outgoing tap to the ongoing tap without interrupting load current. During the transfer, taps are temporarily short circuited through resistors or inductors. The transfer time (typically from 40 ms to 60 ms) is fast as compared to the tap selection process (3s to 10 s). As this block implements a phasor model for study of transient stability of power systems in the range of seconds to minutes, the tap transfer process is not modelled and an instantaneous tap transfer is assumed. A detailed regulating transformer model is provided in the power_OLTCregtransformer example.

Automatic voltage control

The regulating transformer is usually associated with a control system which regulates voltage at the transformer terminals (side 1 or side 2) or at a remote bus. Such a control system is provided in the Three-Phase OLTC Regulating Transformer (Phasor Type) block. You must then connect at the Vm input of the block a simulink signal which is usually the magnitude of the positive-sequence voltage (in pu) to be controlled, but it can be any signal.The control system will adjust automatically the tap position until the measured voltage Vm is equal to the reference voltage Vref specified in the block menu.

The voltage regulator is a hysteresis type regulator. Each time a tap change is required, the regulator sends a pulse either to the Up or Down input of the Tap Changer Controller. Look under the block mask to see how these control blocks are built. The regulator will ask for a tap change if

|VmVref| > DeadBand/2

during a time t > Delay, where Vref, DeadBand, and Delay are parameters of the voltage regulator.

External control of OLTC

If you want to implement your own control system or manually control the tap position, you select External control of OLTC. The Vm input will then be replaced by two inputs labeled Up and Down. Applying a pulse to the Up or Down input will move the tap position upward or downward when the signal is changing from 0 to 1.

Dialog Box and Parameters

The transformer and OLTC parameters are grouped in two categories: Transformer parameters and OLTC and Voltage Regulator parameters. Use the Show listbox to select which group of parameters you want to visualize.

Transformer Parameters

Nominal Power and frequency [Pnom Fnom]

The three-phase nominal power, in VA and the nominal frequency, in hertz.

Winding 1 [V1 R1 X1]

The nominal line to line voltage of winding 1, in volts rms, winding resistance, in pu, and leakage reactance, in pu.

Winding 1 connection

The connection of the three windings 1. Select either, Y (ungrounded wye), Yg (wye grounded), Delta(D1) (Delta connection introducing a −30 degrees phase-shift) or Delta(D11) (Delta connection introducing a +30 degrees phase-shift). See the Three-Phase Transformer (Two-Windings) block documentation for an explanation of the phase shifts introduced by D1and D11 connections. The zero-sequence impedance will vary according to the connection. Zero-sequence impedance is computed assuming that the three-phase transformer bank is built with three single-phase, two-winding units.

Winding 2 [V2 R2 X2]

The nominal line to line voltage of winding 2, in volts rms, winding resistance in pu, and leakage reactance, in pu.

Winding 2 connection

The connection of the three windings 2. Select either, Y (ungrounded wye), Yg (wye grounded), Delta(D1) (Delta connection introducing a −30 degrees phase-shift) or Delta(D11) (Delta connection introducing a +30 degrees phase-shift). The zero-sequence impedance will vary according to the connection.

Tapped regulation winding [Rt Xt]

The resistance and the leakage reactance of the tapped winding depend on the tap position. Enter the resistance, in pu, and the leakage reactance, in pu, corresponding to the maximum tap position.

Magnetizing branch [Rm Xm]

The resistance, in pu and the reactance, in pu, of the parallel Rm, Xm branch modelling respectively iron losses and reactive magnetizing currents. Saturation is not modelled.

Initial pos. seq. output currents of winding 2 [Mag Phase]

The initial value of the positive-sequence current phasor (Magnitude in pu and Phase in degrees) flowing out of winding 2. The currents flowing into winding 1 will be computed accordingly. If you know the initial value of currents you may specify it in order to start simulation in steady state. If you don't know these values, you can leave [0 0]. The system will reach steady-state after a short transient.

OLTC and Voltage Regulation Parameters

OLTC on

Select either Winding 1 or Winding 2.

Voltage Step DeltaU per tap

The voltage step per tap, in pu of the nominal voltage of winding 1 or winding 2.

OLTC minimum and maximum tap [MinTap MaxTap]

You normally specify a negative integer corresponding to maximum tap position with the regulation winding connected in subtractive polarity, followed by a positive integer corresponding to maximum tap position with the regulation winding connected in additive polarity. These two integers specify the number of taps and their absolute values are usually the same. However, if you want to limit tap excursion, you may specify two different values.

Initial tap position

Specify an integer N corresponding to initial tap position with MinTap ≤ N ≤ MaxTap.

Tap selection time

The mechanical time delay, in seconds, required for the OLTC to move the taps by one position. Typical values are in the 3s-10s range.

External control of OLTC

The OLTC is usually controlled by the voltage regulator. Check this parameter if you want to implement your own control system or manually control the tap position. The Vm input will then be replaced by two inputs labeled Up and Down. Applying a pulse to the Up or Down input will move the tap position upward or downward when the signal is changing from 0 to 1.

Voltage regulator

This parameter is not visible when the External control of OLTC parameter is checked.

Select on for enabling the voltage regulator. If you select off the OLTC is frozen at the position specified by the Initial tap position parameter.

Voltage regulator [Vref DeadBand Delay]

This parameter is not visible when the External control of OLTC parameter is checked.

Specify the reference voltage Vref, in pu, the dead band of the hysteresis regulator, in pu, and the time delay, in seconds, during which the voltage error must exceed half the dead band for the OLTC to move tap position upward or downward. The dead band is normally adjusted at two times the voltage step specified in the Voltage step DeltaU per tap parameter.

Inputs and Outputs

A B C

The three input terminals connected windings 1.

a b c

The three output terminals connected to windings 2.

Vm

This input is visible only when the External control of OLTC parameter is not checked.

Connect to this input the measured voltage to be controlled by the voltage regulator. This signal is usually the magnitude, in pu, of the positive-sequence voltage measured at the transformer terminals (side 1 or side 2) or at a remote bus.

Up

This input is visible only when the External control of OLTC parameter is checked.

Simulink® input for controlling tap position. Applying a pulse to this input will initiate upward tap changing when the pulse is changing from 0 to 1.

Down

This input is visible only when the External control of OLTC parameter is checked.

Simulink input for controlling tap position. Applying a pulse to this input will initiate downward tap changing when the pulse is changing from 0 to 1.

m

Simulink output vector containing 16 internal signals. These signals are either complex signals (voltage phasors, current phasors or impedances) or control signals. They can be individually accessed by using the Bus Selector block. They are, in order:

Signal

Signal Group

Signal Names

Definition

1-3

VABC (cmplx)

VA (pu)
VB (pu)
VC (pu)

Phasor voltages (phase to ground) at the transformer input terminals A, B, C (pu)

4-6

Vabc (cmplx)

Va (pu)
Vb (pu)
Vc (pu)

Phasor voltages (phase to ground) at the transformer output terminals a, b, c (pu)

7-9

IABC (cmplx)

IA (pu)
IB (pu)
IC (pu)

Phasor currents flowing into the input terminals A, B, C

10-12

Iabc (cmplx)

Ia (pu)
Ib (pu)
Ic (pu)

Phasor currents flowing out of the output terminals a, b, c

13-14

Z (cmplx)

Z1 (pu)
Z0 (pu)

Positive- and zero-sequence complex impedances (R+jX)

15

Tap

Tap

Tap position

16

Ready

Ready

Logical signal generated by the tap changer controller. This signal is used to enable voltage controller action. The Ready signal becomes (1) after the tap selection has been completed, thus enabling a new tap change. The Up and Down pulses to the OLTC are blocked as long as the Ready signal is (0).

Example

See the power_OLTCregtransformer example, which illustrates the use of the Three-Phase OLTC Regulator Transformer (Phasor Type) block to regulate voltage on a 25 kV distribution feeder. The phasor model is compared with a detailed model of regulating transformer. Look under the masked blocks to see how both models are implemented. The detailed model uses switches and three Multi-Winding Transformer blocks, whereas the phasor model uses current sources.

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