Control of Hvdc

8/9/2019 Control of Hvdc

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CONTROL OF HVDC

CONVERTERS AND SYSTEMS


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BASIS FOR SELECTION OF CONTROL

Following considerations influence the selection ofcontrol characteristics:

Prevention of large fluctuation in DC voltage/current

due to variation in AC side voltage

Maintaining direct voltage near rated value

Power factor at the receiving and sending ends

should be as high as possible


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CONTROL IMPLEMENTATION


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Tap changer control It is used to keep the converter firing angles (and ) within the

desired range

They are sized to allow for minimum and maximum steady statevoltage variation

Current limits: Maximum short circuit current is limited to 1.2 to 1.3 times

normal full load current to avoid thermal damage to

equipment

Minimum current limit is set to avoid ripple in the current

that may cause it to be discontinuous or intermittent

Minimum firing angle limit:

In case of a DC fault, the inverter station may switch to

rectification mode. This would result in reversal of power flow

To prevent this, the a minimum value for firing angle is set


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Power control To transmit a scheduled power, the corresponding

current order is determined by:

Iord=Po/Vd

Bridge/converter unit control Determines firing angles and

sets their limits

Pole control

It coordinates the conversionof current order to a firing

angle order, tap changer

control and other protection

sequences


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LIMITATIONS OF MANUAL CONTROL

The DC voltage at either end of transmission line may vary in asudden, unexpected and undesired manner because of

Short circuits or other disturbances on ac system

Faults in converters

It necessitates the rapid grid control to maintain or restore the

desired conditions on the DC side with available range of control

Fig. Shows ideal control characteristics of Rectifier and Inverter,

it is draw by viewing the transmission line from mid point : thegraph of voltage versus current and half line on each side.

Both characteristics are straight lines


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Left hand (rectifier) side

Internal voltage = Vd01Cos

Slope = - (Rc1+ Rl/2)

Right hand (Inverter) side Internal voltage = Vd02Cos

Slope = + (Rc1+ Rl/2)

Nominal operating point : N where

rated current and voltage is Idn& Vdn

Note: Slopes of the lines are drawn on the assumption that at rated current the

voltage drop due to commutation is 8 % and line drop is 9 % of rated voltage and

hence

slope = 8 + 9/2 = 12.5 %


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Let ac voltage at inverter drops by 12.5 % of Vdnthen

Intersection point with rectifier characteristics moves to point Acorresponding to 1.5 Idn

Further voltage drop of 12.5 % moves to point B and increase in currentto 2 Idn

If ac voltage at rectifier drops by 12.5 % of Vdn

while the inverter isnormal

Intersection point with inverter characteristics moves to point Ccorresponding to 0.5 Idn

Further voltage drop of 12.5 % moves to point D and decrease incurrent to nil.

In this Example, dip in ac voltage produces % change in dc current of4 times % change of voltage, such large fluctuations of current cannot be tolerated

The high over currents are undesirable, they lead to arcbacks in

rectifier and commutation failure in inverter


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DESIRED FEATURES OF CONTROL


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CONTROL CHARACTERISTICS

The control characteristics of theconvertor are the plots of thevariation of the direct voltage againstthe direct current.

The Natural Voltage Characteristiccorresponds to zero delay angle, =0. It as characteristic equation

Vd= Vd0Cos (3Lc/) Id

Constant Ignition Angle control

C.I.A.):

It is a similar characteristic which isparallel to the NV characteristic witha controllable intercept is Vd0Cos


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Constant extinction angle control :

The Inverter is usually operated at constant extinction angle.This has the characteristic equation given by

Vd

= Vd0

Cos (3Lc

/) Id


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Constant Current Control CC) :

In a d.c. link it is common practice to operate the link at constant

current rather than at constant voltage. (constant current means

that current is held nearly constant and not exactly constant)

In constant current control,

the power is varied by varying

the voltage. There is an

allowed range of current settings

within which the current varies.


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Actual Control Characteristics :


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Combined Characteristics :


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The margin setting Idmbetween the current setting Ids for theinverter and for the rectifier is usually kept at about 10% to20% of the current setting.

The setting of the convertor operating as rectifier is kept higher

than the setting of that as inverter by the margin setting Idm

The usual operating point for power transfer is the intersectionof the CC control of the rectifier and the CEA control of theinverter.

It also be ensured by proper tap changing that the N.V.characteristic of the convertor operating in the rectificationmode is higher than the C.E.A. characteristic of the inverter, asVdoof the two ends are not necessarily equal


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With convertor A operating as rectifier, and convertor Boperating as inverter, the steady state current under allcircumstances will remain within the upper limit (Ids+ Idm) andthe lower limit Ids.

i.e, the system direct current will not change by more than Idmunder all operating conditions.

By reversing the margin setting Idm, that is making the setting ofconvertor B to exceed that of A, power flow can be

automatically reversed.

Convertor B will then operate as a rectifier and A as an inverter.The reversal of power occurs as a result of the reversal ofpolarity of the voltage.


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CONVERTER CONTROL


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The ideal control system for an HVDC converter should meet the following

requirements Symmetrical firing of the valves under steady-state conditions.

Instant of firing to be decided with regard to permissible values ofcommutation voltage (rectifier) and commutation margin (inverter).

Minimal reactive-power consumption in the converters, subject to the

condition that it is achieved without an unacceptable risk of commutationfailure.

Insensitivity to normal variations in voltage and frequency of the AC supplynetwork.

Some degree of prediction of the optimum instant of firing in the inverters,

based on actual network voltage and direct current, subject to the conditionthat it is achieved without an unacceptable risk of commutation failure.

Current-control characteristics with sufficient speed and stability margin tocope with changing reference values and disturbances.

Continuous operating range from full rectification to full inversion.


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FIRING ANGLE CONTROL :

The firing instant for all the valves are determined at ground

potential and the firing signals sent to individual thyristors

by light signals through fibreoptic cables.

While a signal pulse is adequate to turn on a thyristor, thegate pulse generator must send a pulse whenever required ,

if the particular valve is to be kept in a conducting state.

There are two basic firing schemes,

Individual phase control (IPC).

Equidistant pulse control (EPC).


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INDIVIDUAL PHASE CONTROL

The firing instants are determined individually for each valve. In which constant delay (or extinction) angle is maintained for all the

valves in the steady state with respect to the earliest firing instant(i.e. the voltage crossing).

Phase control is a process in which forward current conduction ismaintained in a valve by varying the time period of cycle for shortperiod.

The generation of pulses at a valve is determined or known from thezero crossing of commutation voltage.

In rectification the constant delay angle is normally determined froma negative-feedback control loop, involving the set current and theactual monitored current.


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To maintain safe inverter operation with minimum reactivepower requirements the individual firings require:

a continuous calculation of the available voltage integral for

commutation

a continuous calculation of the required voltage integral for safecommutation.

Optimum firing is achieved when the results of these two

coincide.

The relationship governing the commutation process relies on

the fact that the time integral of the commutating voltage,

i.e. the voltage integral, is equal to the overall voltage

change produced by the commutating current, ic.


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This can be achieved as follows:

under commutation

In the integration such a relationship can be expressed as

From above equation the integral of voltage lies betweent1=

and t2 = - and hence phase control of system can be

achieved


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The pulses in individual phase control can be generated by two

methodsConstantcontrol

Inverse cosine control

Constant control Method :

This method also called delay control method

The firing pulses are obtained individually by determining thezero crossing of the commutation voltage.

The commutation voltages are produced or generated by usingvoltage transformers

In this scheme the gate pulses are generated at same delaytime


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The value ofis minimum at the instant when zero crossing of

commutation voltage.

The delay angle can be controlled by controlling the voltage.

The delay angle at rectifier is generally determined with

negative-feedback control loop, involving the set current and

the actual monitored current.

In inverter the delay angel is controlled with minimum reactive

power requirements for safe operation.

The schematic diagram of Constantcontrol is as follows:


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Inverse Cosine control :

In this method the commutation voltages obtained are phase

shifted by 900

The zero crossings of sum of two voltages gives the firing

pulses for a particular valve that is considered

The delay angle depends on ac system voltage amplitude and

shape and is also proportional to inverse cosine of voltage.

The voltage across the bridge varies linearly with controlvoltage Vc

The essential requirement in this scheme is the maintenance

of phase shift by 900. The schematic diagram is as follows:


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EQUIDISTANT PULSE CONTROL

In this system the pulses are generated at regular intervals of time byusing phase-lock oscillator.

The main components in this system are voltage controlled oscillatorand a ring counter.

The pulses of a particular frequency proportional to control voltageare passed to the ring counter from a voltage controlled oscillator.

The train of pulses is fed to a six-stage ring counter in which only onestage is on at a time, the ON stage is stepped cyclically frompositions 1 to 6 by the oscillator pulses.

As each ring-counter stage turns on, it produces a short pulse at theoutput (once per cycle).

Therefore the complete set of six output pulses normally occurs atsuccessive intervals of 60


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The STOP pulses are also obtained from the ring counter but

two stages later (e.g. the START pulse for valve 1 is from stage1 and the STOP pulse for valve 1 is from stage 3, normally

120 later).

One oscillator and one ring counter per bridge constitute the

basic control hardware.

The various control modes only differ in the type of control loop

which provides the oscillator control voltage, Vc.

There are three methods in equidistant pulse control

pulse frequency control

pulse phase control

pulse period control


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Pulse Frequency Control :

In this method, the frequency is determined by the control voltage Vcwhich is related to error in the quantity (current, extinction angle ordc voltage) being regulated.

The frequency in steady state operation is equal to pfo m

where ppulse number fo is nominal frequency

PFC system has integral characteristic and hence it is used infeedback mode for stabilization.

At firing instant tn of the pulse is determined from the following

equation:

Where V1bias voltage V3proportional to system period


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In steady state

and

since

in steady state the gain K1can be chosen as


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It does not has frequency correction, Ainsworth proposedfrequency correction according to the following block diagram


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Pulse Period Control :

This is similar to PFC, in which VCis summed with V3instead ofV1. The firing instant tnof the pulse is determined from thefollowing equation:

The frequency correction scheme is obtained by eitherupdating V1in response to the system frequency variation orincluding another integrator in CC or CEA controller


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Pulse Phase Control :

Firing pulses generated according to the following equation

For Proportional current control, the steady state can bereached when the error or Vcis constant.

To reduce the error to zero it is recommended that a slowalpha

control with feed back signal taken from the measured delayangle.

This signal in addition to the signal derived from the frequencyerror is used to control the voltage V1

Control of Hvdc

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