GR1010420B - An integrated system for voltage and frequency control - Google Patents

Abstract

Integrated system for voltage and frequency control for distribution networks with distributed renewable energy sources and energy storage systems, that can provide the following functionalities: voltage regulation, voltage unbalance mitigation, frequency regulation via virtual inertia provision and primary frequency response, independent of the existence, the type and the composition of the local power generation, including the energy storage unit. The integrated system is implemented in prosumers' four-leg direct current (dc) - alternating current (ac) converters and can provide the above-mentioned functionalities utilizing proper control techniques. In particular, voltage regulation and voltage unbalance mitigation are ensured by the use of a controller, which allows their individual and accurate handling, analyzing the data in the symmetrical components domain. In addition, the frequency control system targets to virtual inertia provision as well as to primary frequency response using a dc-dc converter connected in series at the dc bus of the four-leg dc-ac converter. Furthermore, the frequency control system employs an auxiliary control system to mitigate the power oscillations of the dc-ac converter. Compared to the conventional converters that are available today, the invention can provide the above-mentioned functionalities in smart grids.

Description

DESCRIPTION

Integrated voltage and frequency control system

Scope

The present invention is an integrated voltage and frequency control system (I) used by distributed generation units (DPUs) and/or energy storage units (MAEs) connected to electricity distribution networks (DDN). The voltage and frequency control of the invention can be applied to all DIEs, regardless of the existence, type and composition of the local power generation, including the energy storage unit. The integrated voltage and frequency control system (i) is integrated in four-wire direct current - alternating current (dc-ac) converters (iv) and has four control units that offer the following functions:

1. adjustment of the amplitude of the alternating voltage (ii.a),

2. mitigation of the asymmetry of the amplitude of the alternating voltage (ii.b),

3. regulation of the frequency of the alternating voltage by providing virtual inertia and primary frequency regulation (iii.a),

4. mitigation of power oscillations of the dc-ac converter when adjusting the frequency of the alternating voltage (iii.b).

Description of the problem

The ever-increasing penetration of MDPs into existing DDIEs has significantly reduced their robustness resulting in the emergence of many problems such as over-/under-voltage, voltage asymmetry, frequency variation of the network voltage and power oscillations which need to be addressed.

More specifically, the efficient regulation of the amplitude of the alternating voltage (voltage regulation) is an issue that has been of great concern to the international community. The control of the active power flow at the output of the dc-ac converter of the MDPs and/or the MAEs connected to the DDIE, is one of the most widespread techniques for regulating the voltage within predetermined limits. However, voltage regulation only through active power control has significant drawbacks, as cutting the generated power in NPPs is inconsistent with the European Union's goals for higher NPP penetration rates. Furthermore, if the regulation is achieved through the combined use of MDP and MAE, the use of MAEs must be done in a rational way so as not to reduce their lifetime. For this reason, control schemes have been proposed that combine the simultaneous control of the active and reactive power of the converters to maintain the grid voltages within the permissible limits by making full use of their potential. In these implementations the voltage regulation is usually achieved by controlling the active and reactive power output of the MDP/MAE converters by applying droop curves.

Nevertheless, in these methods either priority is again given to the use of active power over reactive power, increasing the use of MAEs and, by extension, negatively affecting their life expectancy, or a significant increase in the losses of DDIE is observed due to the absorption/ injection of large amounts of reactive power from the inverters. This fact leads to the need to develop an integrated method of voltage regulation that will contribute to the limitation of the voltage within the permissible limits by controlling the active/reactive power of the MDP/MAE converters, prioritizing the available reactive power, limiting the use of MAEs and contributing to the smallest possible increase in network losses.

Also, the asymmetry of the amplitude of the alternating voltage (voltage asymmetry) is one of the most common technical issues found in DDIEs. The problem is mainly due to the random distribution of consumers between the phases of the DDIE, and in recent years it has worsened due to the installation of single-phase substations. Various voltage asymmetry mitigation methods based on power redistribution between phases in three-phase MDP/MAE converters are presented in the existing implementations. One such voltage asymmetry mitigation technique is based on the concept of damped conductance that allows the inverters of the MF/MAE to behave as a constant transverse resistance to the forward and reverse components of the grid voltage, thus changing the active power distribution at the output of converters. However, unbalance cannot be combated if, e.g., the MPAs are not generating or the MAEs are fully charged/discharged, as this technique relies on controlling the amount of active power available at the output of the dc-ac converter . Furthermore, the concept of damped conduction has been combined with the incorporation of a stat curve in the dc-ac converter of the MDP/MAE to simultaneously deal with potential voltage spikes. However, this method leads to conflicting objectives, as there is strong coupling between regulation and voltage asymmetry mitigation controls. According to the above, it is considered necessary to develop an improved voltage asymmetry mitigation method which will work independently of the current voltage regulation control and can be applied even in conditions of zero production of the MDPs or inability to participate of the MAEs (fully charged /unloaded).

In addition, the use of power converters to connect NPPs, and in general renewable energy sources (RES) to the grid, has contributed decisively to the reduction of system inertia, thus presenting significant problems of robustness and reliability. In the existing implementations, various methods of providing virtual inertia from the MFD/MAD converters are presented which are based on the calculation of the rate of change of frequency (RoCoF) to determine the amount of active power that the MFD/MAD should contribute to the grid in case of frequency change. However, to measure the network frequency, and ultimately to calculate the RoCoF, Phase-Locked Loops (PLL) are used, which exhibit a slow response and strong oscillations in frequency changes, thus affecting the provision of virtual inertia. This problem is particularly pronounced in weak networks. Therefore, it is necessary to develop an improved method for regulating the frequency of alternating voltage by providing virtual inertia and primary frequency regulation, which will work cooperatively with existing implementations and can be applied even in weak networks. Finally, existing provisions focus on the application of virtual inertia supply control techniques to dc-ac converters, neglecting primary source control. This source is an MAE which is connected to the dc side of the dc-ac converter through a dc-dc converter. It is therefore necessary to develop a complete frequency regulation system, which will include a new dc-dc converter arrangement, its control, as well as the control of the dc-ac converter.

According to Figure 1, the present invention is an integrated voltage and frequency control system (i) comprising the AC Voltage Amplitude Control Units (ii), namely the AC Voltage Amplitude Control Unit (ii.a) and the AC Voltage Amplitude Asymmetry Mitigation Control Unit (ii.b), to limit over/under voltage and asymmetry phenomena, as well as the AC Voltage Frequency Regulation Units (iii), i.e. the Frequency Regulation Control Unit AC Voltage by Providing Virtual Inertia and Primary Frequency Regulation (iii.a) and the dc-ac Converter Power Oscillation Mitigation Control Unit (iii.b) to combat frequency disturbances, especially in low-inertia networks. The integrated voltage and frequency control system (i) can be integrated with four-wire dc-ac power converters (iv). The control implemented by the system is based on the cooperative operation of a four-wire dc-ac converter (iv) and a series dc-dc converter (v).

Among the advantages of the present invention is that the regulation of the amplitude of the alternating voltage is implemented by controlling the active and reactive power at the output of the four-wire dc-ac converter (iv), giving priority to the exploitation of the reactive power. Accordingly, the attenuation of the amplitude of the alternating voltage is implemented using exclusively the reactive power of the four-wire dc-ac converter (iv). Both control strategies are implemented using Proportional-Integral Controllers (Pls), as opposed to conventional controls using proportional controllers, thus leading to reduced losses and injection of smaller amounts of reactive power. Also, by analyzing the data in the field of symmetric components, the proposed system enables separation between the issues of amplitude regulation and mitigation of AC voltage amplitude asymmetry, allowing their individual and targeted treatment. Regarding the regulation of the frequency of the alternating voltage, a new arrangement of dc-dc converter (ν) is proposed, which is connected in series on the dc side in contrast to the classic transverse topologies. The series topology provides the possibility of direct intervention in the current of the dc side and the control based on said current reduces the response times of the system. Thus, the necessary synchronization time of both the PLL and the response of the current control of the four-wire dc-ac converter (iv) is given. Also, the frequency regulation control uses additional measurements on the dc side of the four-wire dc-ac converter (iv) to compensate for grid frequency calculation problems, unlike conventional controls that use PLL only, thus acting as a auxiliary power oscillation mitigation control of the four-wire dc-ac converter (iv).

Brief description of shapes

For an understanding of the invention, the following figures are attached, each of which describes features and functions of the integrated voltage and frequency control system (I) as follows:

<■>Figure 1, location of the integrated voltage and frequency control system (I) at block level. Also imprinted are the AC Voltage Amplitude Control Units (ii), namely the AC Voltage Amplitude Control Unit (ii.a) and the AC Voltage Amplitude Asymmetry Mitigation Control Unit (ii.b), the Control Units of Frequency of Alternating Voltage (iii), i.e. the AC Voltage Frequency Regulation Control Unit by Providing Virtual Inertia and Primary Frequency Regulation (iii. a) and the Inverter Power Oscillation Mitigation Control Unit (iii.b), the four-wire dc- ac converter (iv), which consists of the driving circuit (iv.a) and its electrical circuit (iv.b), and the series dc-dc converter (v), which consists of its driving circuit ( v.a) and its circuit (v.b).

<■>Figure 2, diagram of the circuit of the integrated voltage and frequency control system (i) in three-phase DDIE. Specifically, the driving circuit (iv.a) and the electric circuit of the four-wire dc-ac converter (iv.b) with switching elements Si, S2, ..., S8, the driving circuit (v.a) and the circuit of the series dc-dc converter (v.b) with switching elements SH1, 5H2, SH3 and SH4, the filter circuit at the output of the four-wire dc-ac converter (iv.b) as well as the corresponding filter at the output of the series dc-dc converter ( v.b), where each phase consists of a series resistance (RL,F), a series inductance (LF), a transverse resistance (RC,F) and a transverse capacitance (CF). The MDP is modeled as a voltage source (Vdc), the MAE as a voltage source (Vbαt), while the Thevenin equivalent is used to model a three-phase DCI with voltage sources (νa<g>, vb<g>, V<g> ), impedance Rg+ jωgLg per phase and neutral impedance

RN+ jωgLN, where ωg is the angular frequency of the network.

<■>Figure 3, operation diagram of the integrated voltage and frequency control system (I) at block level. AC voltage amplitude control units (ii) (AC voltage amplitude adjustment control unit (ii.a) and AC voltage asymmetry amplitude mitigation control unit (ii.b)), AC frequency control units are shown (iii) (ac voltage frequency regulation control unit by providing virtual inertia and primary frequency regulation (iii. a) and dc-ac converter power oscillation mitigation auxiliary control unit (iii.b)), the four-wire dc-ac converter (in) (driving circuit (iv.a) and electrical circuit (iv.b)), the series dc-dc converter (v) (driving circuit (v.a) and electrical circuit (v.b)) as well as the auxiliary unit measurement management (ni).

<■>Figure 4, implementation of an auxiliary measurement management unit (VI) at block level.

<■>Figure 5, implementation of a control unit for adjusting the amplitude of the alternating voltage (ii.a) at block level.

<■>Figure 6, implementation of a control unit for mitigating the asymmetry of the alternating voltage amplitude (ii.b) at block level.

<■>Figure 7, implementation of a control unit for regulating the frequency of alternating voltage by providing virtual inertia and primary frequency regulation (iii. a) at block level.

<■>Figure 8, implementation of an auxiliary control unit for mitigating the power oscillations of the dc-ac converter (iii.b) at block level.

Figure 9, implementation of a dc-dc converter (v.a) series drive circuit at block level where d0 is the duty cycle as determined each time by the enabled control unit, e.g., by the Regulation Control Unit of AC Voltage Amplitude (ii.a) or the AC Voltage Frequency Control Unit by Providing Virtual Inertia and Primary Frequency Regulation (iii.a) or the Inverter Power Oscillation Mitigation Control Unit (iii.b), fh-swi switching frequency, orbit auxiliary bit direction and opbit auxiliary operating bit.

Figures 10-16 show the simulation results of the integrated voltage and frequency control system (i). Specifically:

Figure 10, current versus time at the output of the four-wire dc-ac converter (inν) in the rotating frame of reference for (a) forward (I1,d, I1,q), (b) reverse (l2,d, I2,q ) and (c) homopolar (I0,d, I0,q) component (RMS values).

Figure 11, true component voltage profile (V1) (RMS value).

Figure 12, voltage versus time in (a) reverse (V2) and (b) forward (V0) component (RMS values).

Figure 13, active and reactive power as a function of time in (a) forward (Pi, Qi), (b) reverse (P2, Q2) and (c) homopolar (P0, Q0) component (RMS values).

Figure 14, simulation results of virtual inertia supply, (a) output power, (b) dc bus voltage and (c) under-frequency disturbance.

Figure 15, simulation results of virtual inertia supply, (a) output power, (b) dc bus voltage and (c) over-frequency disturbance

Figure 16, power oscillation mitigation simulation results (a) dc-ac converter output power, (b) dc bus current, (c) dc bus voltage, (d) frequency.

In addition, at the end of the figure presentation section, a relevant symbol explanation note is listed.

Detailed description of the integrated voltage and frequency control system (I)

Below is a detailed description of the control units:

<■>adjusting the amplitude of the alternating voltage (ii.a),

<■>mitigation of the asymmetry of the amplitude of the alternating voltage (ii.b),

regulating the frequency of the alternating voltage by providing virtual inertia and primary frequency regulation (iii.a);

<■>mitigating the power oscillations of the dc-ac converter (iii.b).

Figure 2 shows the graphical representation of the circuit, in which the methods of the integrated voltage and frequency control system (i) are applied.

In particular, the control system is integrated in four-wire dc-ac converters (iv.b) that are interconnected in three-phase distribution networks, as in Figure 2. The phase voltages of the network

Va<g>, Vb<g>, Vc<g> are described by (Eq. 1), (Eq. 2) and (Eq. 3), respectively.

Va<g>(t) = Aα<g>.sin(ωg.t) (Eq. 1)

vb<g>(t) = Ab<g>-sin(ωg-t dab) (Eq. 2)

vc<g>(t) = Ac<g>-sin(ωg-t δac) (Eq. 3)

where Aa<g>, Aa<g>, Aa<g>is the amplitude of the alternating voltage of the network in phases σ, b and c respectively, ωg is the angular frequency of the network and δab, Sαc is the phase difference between phases a-b and a-c , respectively.

First, the phase voltages Va, Vb, Vc and the currents Ia, Ib, Ic are measured at the point of interconnection of the four-wire dc-ac converter (in) with the ΔDIE (ac side), as shown in Figure 2. These measurements are entered into the integrated system voltage and frequency control (I) order

to determine the reference currents (I<ref>x,d and l<ref>x,q for (x =1,2,0)) that must be absorbed/

injected by the four-wire dc-ac converter (in) to successfully adjust the amplitude of the ac voltage, mitigate the asymmetry of the amplitude of the ac voltage, and adjust the frequency of the ac voltage (See Figure 1 and Figure 3).

In addition to the above four basic control units, the Measurement Management Auxiliary Unit (MMU) is also included, which is used to carry out the necessary transformations and calculations to perform the system functions. The ladder-level circuit describing the basic functions of the Measurement Management Auxiliary Unit (νi) is shown in Figure 4. More specifically, it takes as input the measurements of the interconnection point ( Vα , Vb , Vc , Iα , Ib , Ic ) and through the transformation of symmetrical components the voltages (Vabc(1<)>Vabc<(2)>Vabc<(0)>) and currents (Iabc(1<)>Iabc<(2)>Iabc<(0)>) are extracted in the normal , inverse and homopolar component,

respectively. Then, the voltages (Vabc(1<)>Vabc<(2)>Vabc<(0)>) are input to respective three PLLs to determine

of the phase signals (θx , where x=1, 2, 0) for each symmetrical component. Then, using the Park transformation (M/S Park) and the phase signals (θx), the voltages (Vx,d and Vx,q) and the currents (Ix,d and IξΛ) of each symmetrical component (x = 1, 2) are calculated , 0) in the rotated reference frame dq0. Then, from the voltage values V1,d, V2,d, V0,d, the voltages V1, V2 and V0 are determined, respectively, using Moving Average Window filters. Finally, the calculated values of voltage and current in the dqO frame are used to determine the instantaneous value of active ( P1, P2, P0 ) and reactive ( Q1, Q2, Q0 ) power in the forward, reverse and homopolar components, respectively. Additionally, the magnitudes V1, I1, P1 and Q1, which are calculated in the Measurement Management Auxiliary Unit (vi) as well as the frequency (f), which is calculated in the AC Voltage Frequency Regulation Control Unit by Providing Virtual Inertia and Primary Frequency Regulation (iii.a) (See relevant section) are registered locally on a storage medium and/or sent in real time to the DDIE administrator. These data can be used further for the analysis and evaluation of the dynamic behavior of the DDIE.

After performing the necessary calculations and transformations, the quantities determined in the Measurement Management Auxiliary Unit (vi) are entered into the AC Voltage Amplitude Control Units (D) and the AC Voltage Frequency Control Units (iii), as shown in Figure 3. In the AC Voltage Amplitude Control Units (ii.a) and AC Voltage Amplitude Asymmetry Mitigation (ii.b) are determined, based on the control methods incorporated in the integrated system

voltage and frequency control (y), the reference currents (!f'd and lf<r>q(x = 1, 2, 0)) (It is noted that for

the l[<e>d<f>determined by the Voltage Regulation Control Unit (ii.a) applies that = /™ ) for each symmetrical component to be absorbed/injected by the four-wire dc-ac converter (iv.a ). It is noted that the final reference current of the right component on the d axis (

l[<ef>d), is determined as the sum of the corresponding reference current of the Control Unit

Setting the Amplitude of the Alternating Voltage (ii.a) (/^ ) and the reference current of the Unit

AC Frequency Control (iii) ( ). In addition, during the process of its adjustment

amplitude of the alternating voltage determines the amount of active power ( P[<ef>) to be absorbed/injected by the MAE. The required partition ratio ( d0 ) is determined from this value. The division ratio ( d0 ) and the corresponding energy direction control signal (orientation bit - orbit) are transmitted to the driver circuit of the series dc-dc converter (v.a) (Figure 9); according to the energy direction control signal (orbit) the Unit AC Voltage Amplitude Control (ii.a) indicates to the series dc-dc converter (ν) whether the active

power ( P[<e{>) must be injected (orbit= 1) or absorbed (orbit= 0) by the MAE. In the driver circuit of the series dc-dc converter (v.a), according to the value of the orbit, the conduction direction of the switching elements of the series dc-dc converter (v) is determined (Table 1), in order to achieve the charge/discharge of MAE. More details on the Voltage Control (ii, ii.a, ii.b) and AC Frequency Control (iii, iii. a, iii.b) Units are listed in the following sections.

Finally, the reference currents (/^ and ifq for (x =1,2,0)) resulting from the integrated

voltage and frequency control system (i) are passed to the four-wire dc-ac converter (iv), as shown in Figure 1. There, the output currents as well as the pulsation of the dc-ac converter are determined by its drive circuit (iv.a) , which uses conventional closed-loop current controllers and conventional pulsing methods.

AC Voltage Amplitude Control Unit (ii.a)

The block diagram describing the implementation of the AC Voltage Amplitude Control Unit (ii.a) is shown in Figure 5. More specifically, the positive component of the voltage (Vi), the value of the positive component voltage in d-axis (V^d), the instantaneous value of active (Pi) and reactive power (Qi) in the right component (as determined by the Measurement Management Auxiliary Unit (vi)) are input to the AC Voltage Amplitude Control Unit (ii. a), where the control of Figure 5 is used, to restore the voltage amplitude ( Vi ) within the permissible limits. The following is a brief description of the voltage regulation control implemented by the AC Voltage Amplitude Regulation Control Unit (ii.a) of the invention.

The voltage regulation control implemented consists of Mode #1 and Mode #2. OL two modes are separated by a small bandwidth (db), where the control remains inactive to avoid oscillations and unwanted switching between modes. It is emphasized that the bandwidth (db) is determined by the accuracy of the measuring devices, without however affecting the performance of the voltage regulation control.

More specifically, the circuit of Mode #1 restores the voltage ( Vi ) within predetermined limits by prioritizing the use of reactive power (Qi) over active power (Pi). This function is activated through the signal ( ASM ), either when the voltage (Vi) exceeds the maximum allowable voltage limit ( V[™<x>), i.e. if an overvoltage is observed, or when the voltage (Vi) is lower

from the lower allowed voltage limit ( ), i.e. if undervoltage is observed. In the case where a violation of the allowable voltage limit is observed, i.e. ASM= 1, the voltage regulation is initially selected only by managing the available amount of reactive power (Qi) of the four-wire dc-ac converter (in) (Regulation Circuit #1). The reference reactive power (Q^<f>), to be absorbed/injected by the four-wire dc-ac converter (in) to regulate the voltage, is determined by the analog-integral controller (PI) used to zero the error between the voltage ( Vi ) and the reference voltage (Vr ?/)■It is noted that Vref= Υ™<x>- 0.5 -db when considering overvoltage effects, while Vref= 0.5 -db for undervoltage effects.

Then, from the reference reactive power (<(>2<r>f ) the reference current in the q-axis is calculated (l[<ef>q

), while the active reference power ( P[<εi> ), and by extension the reference current in the d-axis ( l™d ), in Setting 1 is equal to zero.

This process continues until either the voltage (\Λ) at the junction point is regulated or the maximum available reactive power Qmax is reached, which is determined by (Eq. 4):

(Ex. 4)

where 5^ is the apparent power of the four-wire dc-ac converter (in) and Pi is the active power managed by the four-wire dc-ac converter (in) at the time when the control takes place.

In the case where the four-wire dc-ac converter (In) reaches the maximum amount of reactive power (Qmax) and the voltage (Vi) is not set within the permissible limits then the control of the active power (Pi) of the four-wire dc-ac is also required converter (in), via Regulating Circuit #2. Thus, through the control signal ( RSP2 ), Regulation Circuit #2 is activated and the reference active power ( P[<e{>) to be absorbed/injected by the four-wire dc-ac converter (in) is determined using of the analog-integral controller (PI) (as in Setup #1). Then, taking into account the active reference power (P[<ef>) and the current (/dC) on the dc side of the four-wire dc-ac converter (in), the required voltage difference (AVdc) to be applied to the busbar is calculated dc from the series dc-dc converter (v), in order to achieve the charge/discharge of the MAE. The voltage difference ( AVdc ) is translated through an analog term into the required division ratio (d0). It is noted that, in addition to the division ratio ( d0 ), during the detection of phenomena of violation of the voltage limits, the auxiliary direction control signal (orbit) is created, according to which the AC Voltage Amplitude Adjustment Control Unit (ii.a) declares in the series dc-dc converter (v), whether the calculated active power (Pi) should be absorbed [orbit= 1) or injected [orbit= 0) by the MAE. Next, the reference current in the axis d ( /^ ) is calculated, while the reference current in the axis

q ( I^η) is adjusted to a new value, so that the four-wire dc-ac converter (in) absorbs/injects the maximum amount of reactive power (Qmax) (according to (Eq. 4)) and at the same time they are not violated its functional characteristics.

This process continues until either the voltage (\Λ) at the junction point is regulated or the maximum available active power (Pmax) of the four-wire dc-ac converter (in) is reached, which is determined by (Eq. 5):

Pma* =VP/ (Eq- 5)

where 5^ the apparent power of the four-wire dc-ac converter (in) and pf the nominal power factor of the four-wire dc-ac converter (in).

After operation #1 is completed, the AC Voltage Amplitude Control Unit (ii.a) goes into stable mode. Therefore, possible different grid operating conditions may lead to unnecessary power absorption/injection from the four-wire dc-ac converter (in). To avoid such cases, Mode #2 is activated via the [AS/J] signal, either when the voltage (Vi) drops below - db (in case where overvoltages were previously present), or when the voltage (Vi) is greater than the value

db (in case there were previously undervoltage effects). During Operation #2,

priority is given to the active power control (Pi) of the four-wire dc-ac converter (in) in order to avoid the possible unnecessary use of MAEs. Thus, when ASA2 = 1, the active power (Pi) of the four-wire dc-ac converter (in) is first controlled according to Setting #2 until the voltage (Vi) is adjusted or the active power (Pi) is zero. In the latter case, via the control signal ( RSPi ), the process goes back to the regulation of the voltage (Vi ) by controlling only the reactive power (Oi) (Setting #1) to zero, unless during voltage (Vi). As in Mode #1, so in Mode #2, the reference currents ( l™d and l[<e>q ) are calculated in a similar manner.

AC Voltage Amplitude Metering Unit inc i!LM

The implementation of the Alternating Voltage Amplitude Asymmetry Mitigation Control Unit (ii.b) at block level is shown in Figure 6. In more detail, the homopolar (Vo,d, Vo,q) and the inverse ( \/2 ,d, ViA) component of the voltage in the rotating frame of reference dqO (as determined by the Measurement Management Auxiliary Unit (vi)) are input to the AC Voltage Amplitude Asymmetry Mitigation Control Unit (ii.b), and using proportional controllers (Proportional Controllers - P) with gains Bo and B2 calculate the reference currents that the four-wire dc-ac converter (in) must absorb/inject in the homopolar and reverse components ( /^ and ifq, respectively). It is noted that the gains Bo and B2 simulate the damped susceptibility in the homopolar and inverse components, respectively. Furthermore, according to the asymmetry mitigation control the d-axis reference currents ( IQJ and ffd ) are assumed to be zero.

In this way, the MAE/MAE system for asymmetry mitigation only adjusts the reactive power output of the four-wire dc-ac converter (in) in the forward (Qo) and reverse (Ch) component reducing the use of MAE as it avoids active power absorption/injection, and it is ensured that anti-asymmetry can be addressed even when the MAEs/MAEs are not available (zero output, fully charged/uncharged MAEs). Finally, it is noted that the contribution of each unit depends to a large extent on the value of the damped susceptibility (Bo and B2). More specifically, a high value of susceptibility (Bo and Bi) leads to an increase in the output currents of the four-wire dc-ac converter (in), which in turn lead to a higher compensation of voltage asymmetries.

Power Unit for Pu9uurnc pk Frequency and Alternating Voltage with Virtual Inertia Supply and Primary Frequency Regulator fiii.a)

The implementation of the AC Voltage Frequency Regulation Control Unit by Providing Virtual Inertia and Primary Frequency Regulation (iii.a) at block level is shown in Figure 7. In more detail, the grid frequency (/g) is used as an input to circuit. The value of the frequency (/ι) is calculated from the derivative of the angle Θ1 obtained from the PLL of the right component. The frequency value (/j) is compared to a past value (Δ/j<-1>) with a certain time-delay tx, in order to calculate the RoCoF. Then the value of RoCoF is input to an analog controller (P) with gain H in order to calculate the required injected/sucked amount of active power ( APc<H>nt). In addition, to the above calculated power (APc<H>nt) can be added an additional amount of power (APc<H>nt) which is proportional (gain D) to the difference between the calculated (/i) and the nominal frequency (/g ) of the network, thereby simulating primary frequency regulation. It is emphasized that by inputting the final power value (AP0) to an analog controller (P) of gain J, the current is determined

inertia (/^ ), which is added to the reference current of its Regulation Control Unit

Amplitude of the Alternating Voltage (ii.a) ), thus composing a new reference value (l"d<f>) (Figure 3).

Next, the required voltage difference ( AVdc ) to be applied to the dc bus by the series dc-dc converter (v) is calculated so that the MAE contributes to the virtual inertia supply and the primary frequency regulation. Finally, the voltage difference ( AVdc ) is translated through a proportional term into the required division ratio ( d0 ). As the division ratio ( d0 ) as the signal of the series dc-dc converter driver circuit (v.a) is always a positive quantity, its absolute value is sent. FI direction of the power flow is recognized by the sign of the calculated voltage difference (AVdc) and transferred to the driver circuit of the series dc-dc converter (v.a) using an auxiliary direction signal (orbit, 1 for positive value and 0 for negative value). When the RoCoF becomes zero, the control stops working.

Auxiliary Control Unit for Measurement of Oscillations of the dc-ac Converter (iii.b)

Figure 8 shows the implementation of the Converter Power Oscillation Mitigation Auxiliary Control (iii.b), which can be applied in cases of weak networks. The auxiliary control works cooperatively with the main AC Frequency Regulation Unit by Providing Virtual Inertia and Primary Frequency Regulation (iii.a) and is based on the use of measurements (Vdc, /dc) on the dc side of the four-wire dc-ac converter ( IV) to address the problems of frequency measurement through PLL and control of the four-wire dc-ac converter (IV) in weak networks. According to Figure 8, the current (/dC) on the dc side of the four-wire dc-ac converter (in) is used as the input to the circuit. The price of electricity

(/dC) is compared to a past value (/^ ) with a certain time-delay tx, in order to calculate the rate of change of the current. The error is then fed to an analog-to-integral controller (API) so as to determine the voltage to be applied by the series dc-dc converter (v) to the dc bus. Also, by means of an analog controller (P) of gain J, the output of the analog-integral controller (PI) is converted into a reactive current component Ii' I which is then added to the reference current of the Amplitude Control Unit

of the Alternating Voltage (ii.a) (/^ ), thus composing a new reference value 3). The action time supervision of the method in question is carried out based on the RoCoF calculated in the AC Voltage Frequency Regulation Control Unit by Providing Virtual Inertia and Primary Frequency Regulation (iii.a). When the error is zeroed, the current (/dC) at the bus of the dc side of the four-wire dc-ac converter (in) returns to its pre-transient value, at which point the auxiliary stabilization control ceases to operate.

With this particular implementation, the mitigation of power oscillations is achieved, between MDP and network, partially uncoupling the PLL, which is vulnerable to transient effects. This reduces the negative effects of desynchronization/slow response until the network balances and the frequency stabilizes. It is worth noting that the series dc-dc converter (ν) is specifically designed for this logic implementation to act directly on the dc side current (/dC), which is a direct result of the PLL error. The specific logic is mainly aimed at avoiding rejection of MDPs and ensuring smooth power flow in the upcoming low-inertia networks.

Quantification of the complete system voltage and frequency control (I)

Below are the results obtained from the simulation of the functions analyzed above (AC voltage amplitude regulation, AC voltage amplitude asymmetry mitigation, virtual inertia supply and primary frequency regulation, as well as power oscillation mitigation). The results concern the response of the integrated voltage and frequency control system (I) in a network with a strongly resistive character (Rg> Lg).

Initially, the treatment of the problem of overvoltage and asymmetry of the amplitude of the alternating voltage is considered. Regarding the overvoltage problem, both modes (Mode #1 and #2) of the AC Voltage Amplitude Control Unit (ii.a) are considered during the simulation run. More specifically, initially the operation of a network is simulated in which an overvoltage problem is detected, which leads to the activation of Operation #1, according to which the amounts of reactive (Qi) and/or active power (Pi) that must be absorbed by the four-wire dc-ac converter (in). After the voltage (Vi) is adjusted within the permissible limits, a change is introduced in the operating conditions of the network which leads to a new equilibrium state of the system. Because of this particular change, it is necessary to enable function #2, in order to avoid unnecessary absorption of active (Pi) and reactive (Qi) power by the four-wire dc-ac converter (in).

Figure 10 shows the symmetrical components of the current in the rotating dqO reference system. More specifically, Figure 10(a) shows the positive component currents (h,d, k,q), while Figures 10(b) and 10(c) show the reverse component currents (l2,d, h,q) and homopolar (fo,* lo,q) component, respectively. Then in Figure 11 the right component voltage profile (\Λ) is shown. Figure 12 shows the voltage profiles of the reverse (V2) and the forward (Vo) component. Finally, Figure 13 shows the active and reactive power absorbed/injected by the dc-ac converter (in) to regulate the voltage (Figure 13(a) - magnitudes in the right component Pi, Qi) and to limit the asymmetry (Figure 13(b) and 13(c) - magnitudes in the reverse (P2, Q2) and homopolar (P0, Qo) component, respectively).

More specifically, at the start of the simulation, it is assumed that an overvoltage problem occurs in the network. From Figure 10(a) it can be seen that from time t = 2 s, the current h,q begins to increase, as the voltage Vi exceeds the maximum allowable voltage limit ν'™<*>, as verified by Figure 11. Then, as the current h,q increases, it can be seen that the absorbed reactive power (Qi) from the four-wire dc-ac converter (in) also increases (Figure 13(a)), until it reaches the maximum possible value (Qmax ) at time t = 3.7 s. It is noted that the current h,d remains at 0 until time t = 3.7 s, since the voltage regulation control gives priority to the available amount of reactive power. From Figure 11 it is observed that the absorption of the maximum possible amount of reactive power (Qmax) is not sufficient to efficiently regulate the voltage (Vi) within the permissible limits. For this reason, at the time t = 3.7 s the charging process of the MAEs starts to limit the overvoltage, through the active power absorption process (Pi). This can be seen both from Figure 10, where the current h,d increases, and from Figure 13(a), where the amount of active power (Pi) increases. Also, from the same figures it is confirmed that as the absorbed active power (Pi) increases, the amount of reactive power absorbed is readjusted, so as not to violate the operating characteristics (Qi—Qmax) of the four-wire dc-ac converter (in). As the simulation continues, it is easily seen from Figure 11 that the voltage (Vi) is regulated within the allowable limits at time t = 6.7 s. To keep the voltage (Vi) within the permissible limits the four-wire dc-ac converter (V) continues to absorb the amounts of reactive (Qi) and active power (Pi) as determined by the control until there is some change in conditions operation of the network.

At the time t = 12 s, a change occurs in the operating conditions of the network which leads to a reduction of the voltage (Vi) below the -db value. This change results in the activation of Function #2 of the AC amplitude control. More specifically, Figure 10 and Figure 13(a) show the continuous decrease of current h,d and active power (Pi) of MAEs, which results in the increase of voltage (Vi) (Figure 11). Since the voltage (Vi) at the junction point is not regulated and the current h,d is set to zero, the process proceeds to reduce the current IiL, and consequently the reactive power (Qi), until the voltage (Vi) is regulated at the time t = 18.6 s. After the voltage (Vi) is successfully set, the control is disabled and the four-wire dc-ac converter (In) absorbs only the necessary amount of reactive power (Qi) to keep the voltage (\Λ) within the permissible limits, avoiding unnecessary use of MAEs and the unnecessary absorption of reactive power (Qi), which leads to an increase in network losses.

Regarding the mitigation of the asymmetry from Figure 10(b) and 10(c) it can be seen that the control is activated at the time t = I s, where the increase of the currents h,qκ ai/o,q is also observed. The increase of currents h,q and lo,q leads to the reduction of the voltages in both the reverse (V2) and the homopolar (Vo) component, and by extension the asymmetry, as verified by Figure 12(a) and 12(b) . Furthermore, from Figure 13(b) and 13(c) it is verified that the reactive power (Q2, Qo) acts as the only means of mitigating the asymmetry, since throughout the simulation the active power of both the inverse (Pi) as well as the homopolar component (Po) remains zero. It is worth emphasizing that the mitigation of the asymmetry was achieved for voltage levels of reverse and homopolar component below 1 %. Furthermore, according to Figures 10 - 13, it appears that the voltage regulation control does not affect the asymmetry mitigation control.

In the second series of simulations, the application of AC frequency regulation control in a weak network is considered. The results of the simulations are presented in Figures 14 - 16. In more detail, Figure 14 presents the results of the simulations concerning the AC Voltage Frequency Regulation Control Unit by Providing Virtual Inertia and Primary Frequency Regulation (iii.a). At time 0.8 s, the grid frequency decreases from 50 Hz at a rate of -1 Hz/s until time 1.3 s where it balances at 49.5 Hz. During grid frequency reduction, the active output power (Pi) of the four-wire dc-ac converter (in) increases almost stepwise, thus providing virtual inertia to the grid. Also, at the same time, an increase in voltage (I/dc) is observed on the dc side of the four-wire dc-ac converter (v), which is realized by discharging the MAE through the series dc-dc converter (v). Similar conclusions can be drawn for the reverse case of increasing the network frequency presented in Figure 15. It is noted that during the simulation the primary frequency regulation is disabled, in order to consider only the provision of virtual inertia.

Finally, Figure 16 shows the results of simulations on a weak network in which the responses obtained by applying and ignoring the operation of the DC-AC Converter Power Oscillation Mitigation Auxiliary Control Unit (iii.b) are compared. In this case, a high RoCoF is considered, i.e. -6 Hz/s. The RoCoF of the network starts at time 0.8 s and lasts until time 1.3 s, where the frequency of the network balances at 47 Hz. It is evident that the auxiliary control acting on the disturbance current (/dC) in the dc bus reduces the disturbances in the output power of the four-wire dc-ac converter (in).

The implementation and integration of the integrated voltage and frequency control system (I) in four-wire dc-ac converters (IV) was presented in detail and its efficiency was confirmed through simulations. It was demonstrated that the integrated voltage and frequency control system (I) is able to regulate the amplitude of the alternating voltage with the minimum possible use of MAEs positively affecting their lifetime. In addition, it manages to mitigate the asymmetry of the AC voltage amplitude even in cases where the voltage levels of the reverse and DC components are below 1 %. Finally, with regard to the frequency control systems of the alternating voltage, these can provide in a short period of time virtual inertia to the network and pre-smooth possible disturbances in the output power by frequency adjustment, while also of the four-wire dc-ac converter (in).

Claims (10)

1. Integrated voltage and frequency control system (i) in distribution networks with distributed generation and storage units, which includes a series direct current - direct current (dc-dc) voltage converter (v) and is integrated into four-wire alternating current - direct current (dc-ac) converters (iv) of self-producers, Characterized by a. control unit for adjusting the amplitude of the alternating voltage (ii.a), b. control unit for mitigation of the asymmetry of the amplitude of the alternating voltage (ii.b), c. control unit for regulation of the frequency of the alternating voltage by means of provision of virtual inertia and primary regulation of frequency (iii.a), d. auxiliary control unit for mitigating the power fluctuations of the dc-ac converter (iii.b). 2. The AC voltage amplitude control unit (ii.a) according to claim 1, characterized by: a. checking the available amount of reactive power of the four-wire dc-ac converter (iv) in the correct component (Q1) in priority, b. control of the flow of active power to the correct component (P1), when the available amount of reactive power (Q1), is not sufficient for efficient voltage regulation. 3. The AC voltage amplitude asymmetry mitigation control unit (ii.b) according to claim 1, characterized by exclusive control of the available amount of reactive power of the four-wire dc-ac converter (iv) in the reverse (Q2) and the common ( Q0) component. 4. The AC voltage amplitude control unit (ii.a) according to claims 1 and 2, characterized by: - Obtaining the phase voltages (Vα, Vb, Vc) and currents (Vα, lb, lc) at the junction point of the four-wire dc-ac converter (iv) and calculating the voltage (V1) and current (I1) in the right component . - Zeroing the error between the reference voltage (Vref) and the voltage (V1) using controllers and determining the reference amounts of reactive (Q1<ref>) and/or active (power) that should be absorbed/injected by the four-wire dc- ac inverter (iv). - Calculation of the reference currents of the correct component in the rotating reference frame dqO ( Ι<VR>1,d, l<ref>1,q) and inputting them into a conventional controller to determine the final current of the correct component ( I<ref>1) which must be absorbed from/injected to the distribution network by the four-wire dc-ac converter (inν) to adjust the amplitude of the alternating voltage. 5. The AC voltage amplitude asymmetry mitigation control unit (ii.b) according to claims 1 and 3, characterized by: Obtaining the phase voltages (Va, Vb, Vc) and currents (Ia, Ib, Ic) at the junction point of the four-wire dc-ac converter (in) and calculating the voltages (V2and V0) and currents (I2and I0) in reverse and the homopolar component. Calculation of the reference currents of the inverse and co-polar component in the rotating reference frame dq0 ( I<ref>2,d, I<ref>2,qand I<ref>0,d, I<ref>0,q) in proportion to the voltages ( V2 and V0) exploiting the concept of damped susceptibility ( B2, B0 ) and introducing them into conventional controllers to determine the final currents ( l<ref>2, l<ref>0 ) to be absorbed from/injected into the distribution network from the four-wire dc-ac converter (in) to mitigate asymmetry of the amplitude of the alternating voltage. 6. The control unit for adjusting the frequency of the alternating voltage by providing virtual inertia and primary frequency regulation (iii.a) in the alternating current network, according to claim 1, characterized by: a. control of active power injection as a function of grid frequency (fg), b. control of the available energy of the storage unit, c. control of the dc voltage level (Vdc), through the series dc-dc converter (v). 7. The auxiliary control unit for mitigation of power fluctuations of the dc-ac converter (iii.b), according to claim 1, characterized by: a. control of the injection of active power as a function of the current (Idc) of the dc bus, b. control of the available energy of the storage unit, c. control of the level of the dc voltage (Vdc), through a suitable signal in the series dc-dc converter (v), d. mitigation of the current (I*) in the dc bus. 8. The control unit for regulating the frequency of the alternating voltage by providing virtual inertia and primary frequency regulation (iii.a) according to claims 1 and 6 characterized by: - Obtain voltage and current at the connection point of the storage unit. - Receive voltage (Vdc) and current (Idc) on the dc yoke. - Calculation of the network frequency of the right component (f1) by means of the measured quantities according to claim 4. 9. The auxiliary control unit for mitigating power oscillations of the dc-ac converter (iii.b), according to claims 1 and 7, characterized by: - Obtain voltage and current at the connection point of the storage unit. - Receive voltage (Vdc) and current (Idc) on the dc yoke. - Calculation of the network frequency of the right component (f1) by means of the measured quantities according to claim 4. - Calculation of the rate of change of dc bus current to compensate for grid frequency calculation problems arising from phase-locked loops (PLLs). 10. The integrated voltage and frequency control system (i) of claim 1 can be applied to all distribution networks, regardless of the existence, type and composition of the local power generation, including the energy storage unit.