Control Structures of Power Electronics for DPGS

7/27/2019 Control Structures of Power Electronics for DPGS

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Control structures of power

electronics for DPGS

Grid feeding VSI


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Control structures

Because grid interconnectionrequirements are evolving towards thedemand of reliable and controllable power

delivery from DPGS, the control structuresdeveloped for grid connected powerconverters should accommodatealgorithms that would help DPGS to

comply with grid interconnectionrequirements imposed by the transmissionor distribution network operators.


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Grid feeding VSI (grid-tied VSI)

A grid feeding VSI is acting like a current source, synchronizing its outputvoltage with the Grid voltage.

In most of the cases, they inject only active power (at unity PF).

They control the fundamental component of the current, but in the case ofdistorted voltage at the PCC, they may include some additional current

harmonic compensation functions.

The control strategy consists mainly of two cascaded loops:

a fast internal current loop, which regulates the output current;

an external voltage loop, which controls the dc-link voltage.

The current loop is responsible for power quality issues and currentprotection; thus, harmonic compensation and dynamics are the importantproperties of the current controller


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PWM driven voltage source

inverters control

Three main possibilities to structure of VSI

arise:

i) stationary a,b,c reference frame,

ii) stationary , reference frame

iii) synchronous rotating d,q reference

frame.


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Grid converters


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Stationary a,b,c control structure

For stationary abc frame control, the controlstructure is implemented in abc frame, hencegiving the possibility to control independent eachphase current.

Three controllers are necessary in this case,however, the filter and transformer connectionshould be considered in case each phasecurrent is to be controlled individually.

Since in case of star connected load, the sum ofthe three currents has to be zero, additionalcaution has to be payed to this fact whenindividual current control is desired.


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Stationary a,b,c control structure


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Controllers

Hysteresis Controllers and dead beat arenormally employed for current regulation in abcframe.

In addition to this, an adaptive hysteresis band

needs to be implemented in order to obtain aquasi-constant switching frequency.

Also the necessity of high sampling frequencyconstitute the disadvantages when designing

and implementing hysteresis controller. However, its fast dynamics is a strong argumentfor implementing it into a grid connected powergeneration system.


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stationary , reference frame

Compared to a,b,c control structure, the advantage ofimplementing the control in , stationary frame is thatthe number of controlled variables is reduced from threeto two, hence necessitates less computational power

when implemented in digital signal processors or micro-controllers.

In addition, information about filter/transformerconnection is not necessary to be accounted whendesigning the controllers, like is the case for abc

reference frame control. Consequently, independent control of each phase

current is not possible using this control structure.


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Transformation


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Control ,

Due to limitations of PI controllers when

controlling sinusoidal quantities, a new controller

type denoted proportional-resonant (PR) has

gained large popularity lately, especially for gridconnected converters .

Another advantageous feature of PR controller is

the possibility of implementing harmonic

compensator based on generalized integrators ,without interfering with controller dynamics,

achieving a high quality delivered current.


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synchronous rotating dq reference

frame In case of synchronous rotating reference frame, dq

frame, the control is implemented in a reference framewhich rotates with the same frequency as the gridangular frequency.

In order to transform the feedback variables in thisreference frame, the phase angle of grid voltage isnecessary , constituting a disadvantage of this structure.However, the control variables transformed in dq frameappear as dc quantities, hence facilitating easier controland signal processing, e.g. filtering.

In this situation, employment of PI controllers for currentregulation is appropriate, however, as Fig. illustrates,cross-coupling terms and grid voltage feed-forward maybe necessary in order to obtain best results.


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frame


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d,q Control


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controllers

The transfer functions of proportional-integral (PI),proportional-resonant (PR), dead-beat (DB) andhysteresis controllers are possible.

Possibility of implementing the same controller in morethan one reference frame is also addressed by showingthe implementation of PI controller in both dq rotatingreference frame and abc stationary frame.

In addition to the implementation of controllers, differentschemes for harmonic compensation, depending on thereference frame, are possible. It has been shown that it

is less complicated to implement harmonic compensator(HC) in stationary reference frame such abc usinggeneralized integrators (GI) than implementation insynchronous rotating reference frame where several low-pass and high-pass filters need to be used.


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frame


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Current Transfer functions


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Syncronization with the Grid

The synchronization with the grid voltage (aligning the grid voltagevector with one of the d or q axis) is accomplished with a Phase

Locked Loop (PLL), which tracks the grid voltage phase. The PLL

performances affect the entire control loop dynamic behavior and the

quality of the injected power into the MG.

The current control scheme includes two PI (proportional-integral)

controllers for each of the two axis (d, q), the voltage feed-forward

terms, and the cross-coupling elimination terms.

The PI controllers ensure zero error betweenId, Iq andIdref, Iqrefby the

integral action. The reference currents come from the active and

reactive power references.

Thus,Idref is proportional with the output active power, while Iqref is

proportional with the output reactive power:

ref

qd

ddref P

UU

UI

22

and refqd

dqref Q

UU

UI

22


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In the case of grid-feeding inverter, usual the reactive power is set to zero (Qre= 0).

The way the active power reference (Pref) is generated depends of the primary

source type and the DC-side converter topology.

In the case of RES (wind turbines, PV) the objective is to maximize of theextracted power. The maximum power point tracker (MPPT)

accomplishes this task and it can be integrated in the primary source side

or in the inverters control system.

The DC voltage can be controlled from the primary source side or from the

inverters control system. In the second case the control system includes avoltage control loop, also, using a PI controller, which acts onIdref.

The tuning of the controllers is of a great importance, affecting the dynamic

behavior of the current loop and the stability of the control system.

The outputs Ud and Uq are transformed from dq reference frame to abc

reference frame using the reference angle provided by the PLL. Aftertransformation the signals are fed into a PWM signal generator that provides

the PWM pulses for the transistors.


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PLL basis

A PLL is basically composed from the following:

Phase Detector (PD). This block generates an output signalproportional to the phase difference between its two inputsignals.

Loop Filter (LF). This block exhibits low pass characteristicand filters out the high frequency ac components from the PDoutput. Typically this is a 1-st order LPF or PI controller.

Voltage Controlled Oscillator (VCO). This block generatesat its output an ac signal whose frequency varies respect acentral frequency as a function of the input voltage.


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Example of a three-phase PLL

11

p

i

kTs

LF VCO

1

sc

dq

PD

abcua

ub

uc

Ud

Uq

Uf


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Control Structures of Power Electronics for DPGS

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