hvdc class after minor1

8/7/2019 HVDC class after minor1

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High Voltage DC TransmissionHigh Voltage DC Transmission

Dr.Dr. SukumarSukumar MishraMishra

IIT DelhiIIT Delhi


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Representation for Stability studies

In stability program, the ac network equations are represented interms of +ve sequence quantities.

This imposes a fundamental limitation on the modelling of the dc

systems, in particular commutation failures cannot be predicted

accurately.

Notwithstanding the above limitation, models of various degrees

of detail have been effectively used to represent dc systems in

stability studies.

Some of the early efforts to incorporate HVDC system models

into stability programs used detailed representation.In recent years

the tend has been toward simpler models.


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Functional block diagram of an HVDC SYSTEM MODEL


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Representation for Stability studies Simpler models are adequate for general purpose stability studies

of systems in which the dc link is connected to strong parts of theac system.

For weak ac system applications & for multiterminal dc systems

require detailed models.

So, the trend is to have flexible modelling capability with wide

range of detail.

The required degree of detail depends on the purpose of the study

and the particular dc system.


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contd

Each dc system tends to have unique characteristics tailored to

meet the specific needs of its application.

Therefore, standard models of fixed structures have not been

developed for representation of dc system in stability studies.

So, three categories, as :

A. Simple model

B. Response or performance model &

C. Detailed model with flexible modelling capability.


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contdB. Response models :

For general purpose stability studies the dynamics of dc lineand pole control may be neglected.

The pole control action is assumed to be instantaneous and

the lines are represented by their resistances.

Many control functions are represented in terms of their net

effects, rather than their actual characteristics of the

hardware.

Different features included in a typical response type model

are:

(1). Converter & line equations

(2).Current control order with limits

(3).Control actions during ac faults

(4).Commutation failure checks

(5).Power/current order modulations


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(1). Converter & line equations :

The three modes of control are:

Mode 1: Rectifier on CC ; Inverter on CEA .

Mode 2: Inverter on CC ; Rectifier on CIA .

Mode 3: Rectifier on CIA ; Inverter on constant- control .

In this case T/F taps are not adjusted as they are no fast enough to be

effective during the period of interest.


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(2)Current control order with limits:

Iord is determined so as to provide either current control or power

control as desired. Constraints are imposed on the current-order level to keep the

current within the range of min. & max. limits

The maximum current is determined by VDCOL as shown.

The VDCOL may be given a time delay to assist in riding throughac system faults.


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(3)Control actions during ac faults :

It is necessary to have adequate representation of the actions of

control during the faults.

If the ac voltage on either side falls below a certain value for

longer than a specified time, the direct-current order is set to zero.

A ramp limit restricts the rate of decrease of direct current.

The line is shut off when the current falls below a specified

minimum value.The direct current is restored after the ac voltage

recovers to an acceptable level.

If the voltage recovers before the direct line current has reached its

minimum value, the direct current is restored to its original value

immediately


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(3).Control actions during ac faults :

contd.

If the voltage recovery occurs after the after the line has been shutdown, the recovery has been delayed by a specified time.

After this, the direct line current is restored to its original value at a

specified maximum rate.

The alternative recovery procedures.

(i). The current is increased by controlling rectifier , with the

inverter firing angle fixed at 900

.When the current reachesIdesired- Im,the inverter extinction angle is ramped down to a

specified value.

(ii). The current is increased with a maximum possible dc voltage.


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(3).Control actions during ac faults :

contd.

Option (i) ensures ,

during recovery maximum reactive power is drawn

from the ac system and may be used to control the ac over

voltages.

Option (ii) ensures,

maximum possible power is transmitted through the

dc link.

The mode of operation & dc blocking and deblocking sequences

are system dependent.

The optimum sequence is established by experimentation.


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Shutoff and recovery sequence

Current control block diagram


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(4).Commutation failure checks :

The dc model usually includes a logic for shutting of the dc linefor commutation failure, detected by monitoring commutation

voltage or converter margin angle.

(5).Power / Current order modulations: Dynamics of controls used for ac system stabilization are

represented accurately, consistent with the representation used

for other forms of stability controls. Eg: PSS


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The dq0 transformation

The stator phase currents (ia,ib,ic) in new variables (id, iq) as :

id = kd [iacos + ibcos(-2/3) +iccos(+2/3) ]

iq = -kq [iasin + ibsin(-2/3) + icsin(+2/3) ]

The constants kd., kq are arbitary and their values may be chosen

to simplify numerical coefficients in performance equations.

If kd & kq = 2/3, for a balanced sinusoidal conditions, the peak

values of id & iq are equal to the peak value of the stator

currents.


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For balanced condition,

ia = imsin stib = imsin (st - 2/3)

ic = imsin (st + 2/3)

id = kd [imsin st cos + imsin (st - 2/3) cos(-2/3)+ imsin (st+2/3)cos(+2/3) ]

= kd (3/2) Imsin (st- )

Similarly,iq = -kq (3/2) Im cos (st- )

i0 = 1/3 (ia + ib + ic)


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The transformation from the abc to dq0 in matrix form is:

The inverse transformation is :

These transformations also applicable to stator flux

linkages and voltages.


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Design of a variable structure current

regulator for weak AC/DC systems

For weak AC/DC systems the modulation of DC power for stabilization results in transient changes of both frequency and

amplitudes of the alternating voltages on the converter bus bar.

This transient modulation of the commutating voltages, and theassociated shiftings of the voltage zero crossings, activates the

converter controls.

This may lead to instability and collapse of the DC link due to

repetitive commutation failures.

In order to assure proper operation of the DC link and minimize

voltage fluctuations throughout the network it is best to maintain the

converterAC bus voltage constant during the modulation period.


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To overcome the mentioned difficulties, coordinated active and

reactive power modulations using input signals from both the

converter stations have been used

A Kalman filtering technique to minimize the system measurement

requirements, and observer based on the Kalman filtering approachis used for controlling AC/DC systems.

Both optimal and modal control of the rectifier current regulators

have been undertaken to damp electromechanical oscillations ofweak AC/ DC systems.

However, none of the above schemes possesses disturbance

rejection properties, nor are they insensitive to plant parameter

variations or the non-linear loads at the AC/DC interface buses.


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A new approach based on sliding mode control and variablestructure systems theory for the design of the rectifier current

regulator can be used to get above said benifits.

The same technique can also be applied to the reactive power

control schemes using a static VAR compensator or gamma

modulation regulator.

The sliding mode control constrains the system motion to a state

trajectory and provides a robust, realizable control scheme.

The problem of designing the controller is to determine a suitable

switching surface that yields a desired control input forcing the

instantaneous states to sliding modes.


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The variable structure current regulator has a normal DC currentregulator along with a control signal derived from speed, voltage

or load angle deviation signals using sliding mode theory.

A two-terminal integrated AC/DC system model


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The synchronous generator supplying power through an ACtransmission line and an HVDC transmission link is represented

by an equivalent voltage source E behind the transient reactance

xd!.

The DC transmission link modelled in this study is a two-terminal

one, having one controlled rectifier and one controlled inverter as

links to the AC system. Shunt capacitors are used at both the

rectifier and inverter ends to provide reactive power support.

A static VAR compensator is either connected to the rectifier or

the inverter bus to provide adequate damping for oscillations ofthe AC/DC system during transient disturbances.

A non-linear load is connected at the rectifier bus and its active

and reactive power are non-linear functions of the load bus

voltage.


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System model :Using a simplified generator model of a voltage behind the transient

reactance, the following equations hold good for the integrated AC/DC

system at the generator:


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System model :


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System model :

For the DC transmission link, constant current control is assumedat the rectifier and constant extinction angle control at the inverter.

The DC power modulation for the improvement of AC/DC system

dynamic performance is implemented by adding a modulating

signal to the reference current of the converter current controller.


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The model of the converter current controller with the auxiliary

stabilizing control u1 derived from the generator speed deviation

signal and using a feedback gain K .

HVDC link current control with active power modulation by speed


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HVDC link current control with a variable structure control


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The equation for the rectifier firing angle control is

given by:


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The DC line is represented by an equivalentRL circuit yielding adynamic first-order response for the DC current:

Rac and Lac are the resistance and inductance, respectively, of the DC

line.


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Static VAR compensator model :

Reactive power control with variable structure control

A static VAR compensator (SVC) with thyristor controlled reactor

(TCR) is considered for reactive power modulation control.

SVC is either placed at the rectifier or the inverter end depending on

the system requirements and the effective short-circuit ratio.


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The primary function of the SVC is to control the reactive powerand stabilize the commutating bus voltage.

The auxiliary stabilizing signal us is added to the main controller to

improve the dynamic performance of the AC/DC system.

The dynamic equation for the susceptance presented to the converter

bus is

Qs is the reactive power rating of the SVC and o the initial firing

angle (0 o )


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Sliding mode control of the HVDC link:

variable structure systems (VSSs) are used to enhance the dynamic

performance of the AC/DC system.

VSS controllers possess several attractive features, for example, fast

response, good transient performance, and insensitivity to variations

in plant parameters and external disturbances.

Further, the sliding mode control, which constrains the system motion

to a state trajectory, provides greater robustness than classical control

schemes.


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To design a variable structure current regulator for the HVDC link,the system equations (discussed earlier) are linearized about an

operating point to yield,

and u1 is the auxiliary control for the current regulator.

The constants g1 to g8 are obtained by linearizing the system eqns.


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The switching hyperplane is defined by the equation

where parameters cl to c3 are chosen to achieve the desired system

performance.

A non-linear combination of state variables may be used, provided

that the desired dynamic performance can be obtained with the

Chosen form.

The necessary and sufficient conditions for the existence of a sliding

mode on the switching surface require, in the vicinity = 0,

that


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Eliminating x4

from switching hyperplane (with ), the

Reduced state equation in a matrix form

The characteristic equation of this reduced system is


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Assuming the above characteristic equation has one eigen value at

zero and two other eigen values at 1 and 2 (which can be fixed as

desired),


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The expression for auxiliary control u1 is taken as

Thus,


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Hence, if feed back gain parameters 1,2,and 3 are chosen to be

and the dynamics ofx3 are neglected, then the condition

yields the following set of inequalities:

Where,


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Further, the simplification pIdc

= 0 can be used without much effect

on the controller design, thus neglecting the dynamics of the DC line

(its time constant being small). With this simplification, the system

equations are :

The switching hyperplane is


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The characteristic matrix with one pole at zero of the reduced system

is

Proceeding in the same way as before, we obtain the values of c1 and c2


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Instead of the rotor angle deviation , the voltage deviation at the

rectifier bus can be used as a feedback signal for the DC link current

controller design:

The expression for Vs is

The gains 1 and 2 are obtained in a similar way:


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However, if a coordinated current control at the rectifier and a

modulation at the inverter is required, the change in is given by

Also, in a similar way, if a static VAR controller is used at the

inverter end, the change in the susceptance at the inverter end isgiven by

These equations are to be incorporated in the linearized equations of

the system to give the coefficients g1, g2 . . . . . g12 of the coordinatedControl scheme.

hvdc class after minor1

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