DE102022111117A1 - Reactive power control method for decentralized voltage maintenance of a distribution network as well as inverters and operating methods - Google Patents

Abstract

The invention relates to a reactive power control method for decentralized voltage maintenance of a distribution network (2), in particular a low-voltage network, with at least one decentralized power generator (PV1-PV4), with a local network voltage (U) of the distribution network at the feed point (4) of the power generator (PV1-PV4). (2) is recorded, active power (P) is fed into the distribution network (2) and reactive power (Q) is fed in depending on the local grid voltage (U), the local grid voltage (U) being the controlled variable at a voltage limit value (Umax). Local mains voltage (U) is regulated and the reactive power (Q) is used as a manipulated variable.

Description

The invention relates to a reactive power control method for the decentralized voltage maintenance of a distribution network, in particular a low-voltage network, with at least one decentralized power generator, wherein a local network voltage of the distribution network is detected at the feed point of the electricity generator, an active power is fed into the distribution network and a reactive power is fed in depending on the local network voltage. Further objects of the invention form a method for operating an inverter, in particular a grid-following inverter, a decentralized power generator, in particular a photovoltaic system, a computer program, an inverter, in particular a grid-following inverter, for a decentralized power generator, in particular a photovoltaic system, and a distribution network, in particular a low-voltage network with several decentralized power generators.

The power grid, which is used to transmit and distribute electrical energy between electricity suppliers and consumers, is divided into different voltage levels. For example, the power grid in Germany is divided into four voltage levels, which differ in their properties. While the meshed power grid takes care of transport over long distances at the highest and high voltage levels, there is no comparable redundancy in the supply at medium and low voltage levels. In the low-voltage system, the network is supplied with a nominal voltage of 0.4 kV from a central transformer at the transition to the higher-level medium-voltage network, whereby several parallel conductor strands can also be used. This structure enables a clear and cost-effective network structure.

Electricity network operators must adhere to various quality criteria when connecting household customers so that devices function properly and are not damaged. The DIN EN 50160 standard defines the requirements for the voltage quality in the distribution networks. These contain specifications for static voltage quality that must be adhered to to ensure a reliable supply.

One such requirement for voltage quality is static voltage maintenance. In this case, 95% of the 10-minute average values must be within a voltage tolerance of 10% around the nominal voltage, for example 230 V, within a period of one week. However, short-term undervoltages of up to 85% of the nominal voltage are permitted.

Compliance with this voltage band specified by the voltage tolerance requires measures from the electricity supplier, especially if decentralized power generators, such as photovoltaic systems, with a large total output are connected in a network area. The decentralized power generators cannot coordinate with one another with regard to their respective power feed into the distribution network, since there is usually no communication, in particular no data exchange, between individual decentralized power generators in the distribution network. Planning, particularly centrally, a reactive power feed-in that adheres to the voltage band for all decentralized power generators is therefore not possible.

Compliance with the voltage band can be achieved, for example, by feeding in reactive power from the decentralized power generator. With methods used for this purpose, such as the Q(U) method known from the VDE AR-4105 standard, the local network voltage of the distribution network is recorded at the feed point of the power generator, at which it feeds the power into the distribution network. This local network voltage is influenced by the power inputs from the other decentralized power generators that are also in the distribution network and are located along the string coming from the central transformer. This local network voltage corresponds to the voltage of the distribution network at the feed point compared to the reference potential. In addition to the active power, the decentralized power generator also feeds reactive power into the distribution network in order to compensate for a voltage increase caused by the active power feed-in. The strength of this reactive power fed in depends on a predetermined characteristic curve, which specifies the reactive power to be fed in depending on the local grid voltage.

To feed in the active power and reactive power, the decentralized power generator uses its inverter, which is in particular a grid-following inverter. To maintain the voltage, this is operated in such a way that the reactive power is fed in, i.e. H. the reactive power is fed into the distribution network in accordance with the reactive power control procedure. The inverter and/or the decentralized power generator either have control and/or regulation devices specifically designed to maintain the voltage or a variable-use computer or microcontroller on which a computer program designed to maintain the voltage is installed.

A disadvantage of these known methods, however, is that they start feeding reactive power into the distribution network at local network voltages that are still far from violating the voltage band. Due to this early reactive power feed-in, performance-related system and network losses occur, which are also associated with corresponding financial losses. These will increase with an increased expansion of decentralized power generators. Nevertheless, this early reactive power feed-in is necessary with the known methods, since decentralized power generators located at the start of the string would otherwise not be involved in maintaining the voltage. It is necessary that decentralized power generators located at the beginning of the string also participate in voltage maintenance, even though they cannot yet detect local voltage maintenance problems, since the decentralized power generators at the end of the string would otherwise not be able to maintain the local grid voltage at their feed points, which is influenced by all decentralized power generators.

If, starting from an operating point of this method, a further increase in the active power to be fed in occurs, for example in the event of stronger solar radiation contributing to the increase in the performance of the photovoltaic system, then in this known method the local grid voltage also increases.

The uniform parameterization of a Q(U) control of the decentralized power generators, especially their inverters, which is necessary for practical reasons, leads to an unsolvable dilemma between the efficiency and effectiveness of the Q(U) process, since only the voltage is taken into account.

The object of the present invention is therefore to enable voltage maintenance at the feed point by means of reactive power feed-in by decentralized power generators and to reduce the system and network losses caused by the reactive power.

This task is achieved in a reactive power control method of the type mentioned in that the local grid voltage is regulated as a controlled variable to a voltage limit of the local grid voltage and the reactive power is used as a manipulated variable.

By using the voltage limit value as a real reference variable of a control loop, the reactive power fed into the distribution network is not adjusted depending on the voltage using a static characteristic curve, but rather the reactive power feed is used as a reference variable to regulate the local network voltage to the voltage limit value. In this way, it can be avoided that the specified voltage limit value increases as the active power fed in increases, i.e. H. increasing active power feed-in is exceeded. With this reactive power control method, the system and network losses caused by reactive power can be reduced by 70 - 90% compared to the known method, without reducing the effectiveness in terms of integration potential. The integration potential is a measure of the maximum output of all decentralized power generators that can be connected to the distribution network without violating the voltage tolerance or load capacity of transformers and lines. By reducing system and network losses, significant economic savings can also be achieved for the operators of the electricity network and the electricity generator.

The voltage maintenance with this reactive power control method can in particular be carried out without communication. The means of the decentralized power generator used to carry out this reactive power control method cannot be in any data exchange with corresponding means of other decentralized power producers in the distribution network and/or a central regulation or control device in the distribution network.

It is further advantageous if the reactive power is only fed in when the voltage limit of the local grid voltage is reached. It is possible that no reactive power is fed in if the detected local grid voltage is lower than the voltage limit. In this way, the local grid voltage can be maintained at the voltage limit value when it is reached. Participation of decentralized power generators located at the start of the string, without there being a local voltage band violation at their feed points, in the voltage maintenance of decentralized power generators downstream can be limited to times with a generally high power feed into the distribution network. The voltage limit value can form an upper limit of a voltage dead band in which unnecessary reactive power injection can be prevented. The voltage dead band can reduce losses both in the inverter and in the network operator's lines.

In a further embodiment of the invention, the voltage limit value is a variable limit value that is dependent on the active power feed. The voltage limit value as a reference variable can take on a value that changes with the active power fed in. The variable voltage limit value can be determined depending on the active power feed-in, ie the active power fed in, at the feed-in point. In this way, the size of the voltage dead band can be changed depending on the power. By designing the voltage limit value as an active power-dependent variable limit value, the range of the local network voltage in which reactive power is fed into the distribution network can be specified for different active powers to be fed in.

In this context, it is particularly advantageous if the variable voltage limit value is reduced depending on the active power. With higher active powers, reactive power can be fed in at lower local grid voltages than would be the case with lower active powers. In this way, reactive power feed-in in the area of smaller active powers can only take place at significantly higher local grid voltages. The voltage dead band can be reduced by lowering the variable voltage limit depending on the active power as the active power feed-in increases. The reactive power feed-in and the losses associated with it can be avoided in the area of smaller active powers over a larger range of the local grid voltage than in the case of higher active powers.

It is further advantageous if the variable voltage limit value is determined by means of a, in particular continuous and/or monotonically falling, voltage limit value active power feed-in characteristic curve depending on the active power feed-in. With a voltage limit active power feed-in characteristic curve, the values of the variable voltage limit can be easily specified for different active power feeds. With a constant voltage limit value-active power feed-in characteristic, sudden changes in the control process can be avoided. With a monotonically falling voltage limit value-active power feed-in characteristic curve, an increase in the variable voltage limit value as the active power feed-in increases can be prevented, since the voltage limit value becomes smaller as the active power feed-in increases or remains maximally constant over an active power range.

It is further advantageous if the voltage limit value-active power feed-in characteristic curve has a power threshold value above which the dependency between the active power feed-in and the voltage limit value changes. By using a power threshold, the voltage limit-active power injection characteristic can be divided into at least two different regimes. In these regimes, the active power-dependent change behavior of the voltage limit value can differ from one another. The power threshold can be in the range of 20% to 80%, in particular in the range of 50% to 70%, preferably in the range of 55% to 65%, particularly preferably 60%, of the nominal power of the power generator.

In this context, it is particularly advantageous if the variable voltage limit value assumes a constant value when active power is fed in below the power threshold value. In this way, a voltage dead band of constant size can be specified for active power feeds below the power threshold value. The constant value can be in the range from 106% to 110%, in particular in the range from 107% to 109%, preferably in the range from 107.5% to 108.5%, particularly preferably 108%, of the nominal voltage of the distribution network.

In a further embodiment of the invention, the variable voltage limit value drops, in particular linearly, with active power feed-in above the power threshold value as the active power feed-in increases. As the active power feed-in increases above the power threshold value, the voltage dead band can shrink in this way, so that the reactive power feed-in occurs at increasingly smaller local grid voltages.

The variable voltage limit value preferably assumes a minimum value when the active power feed corresponds to the nominal power of the power generator. The nominal power of the power generator can be the nominal active power that the power generator can achieve at its maximum. When the nominal power is reached, the maximum possible active power can be fed in by the power generator. The minimum value that the variable voltage limit value assumes at the rated power can be a predeterminable voltage offset below the constant value that the variable voltage limit value assumes for low active power feeds below the power threshold value. The minimum value can be in the range from 103% to 107%, in particular in the range from 104% to 106%, preferably in the range from 104.5% to 105.5%, particularly preferably 105%, of the nominal voltage of the distribution network.

According to a further embodiment of the invention, it is proposed that a maximum Reactive power is specified, up to which the reactive power to be fed in can be increased. Up to this maximum reactive power, the variable voltage limit can be prevented from being exceeded. The reactive power fed in can reach saturation from the maximum reactive power. The maximum reactive power can correspond in particular to a maximum shift factor of 0.90. After the maximum reactive power has been reached, the control can remain active, but can initially no longer influence the manipulated variable and thus the local grid voltage. Only after a drop in the local grid voltage or a reduction in the active power feed-in, in particular due to influences outside the control system, can the control system influence the local grid voltage again.

In this context, it has proven to be advantageous if, once the maximum reactive power is reached, this is fed in as reactive power when the voltage limit value is exceeded. Once the maximum reactive power has been reached, the reactive power control method cannot cause a further increase in the reactive power feed-in, especially when the local grid voltage and/or active power feed-in increases. When the maximum reactive power is reached, the decentralized power generator can continuously feed the maximum reactive power into the distribution network, regardless of a further increase in the local grid voltage.

A further embodiment of the invention provides that only inductive reactive power is fed in. In this way, there is no need to feed in capacitive reactive power. In this way, an undesirable increase in voltage due to capacitive reactive power can be excluded.

In an advantageous embodiment of the invention, a control loop without a stationary setpoint deviation is used. A control loop without a stationary setpoint deviation cannot have a control deviation or control difference between the controlled variable and the specified reference variable in the steady state. In particular, this can be achieved with a control loop that has an integrating control behavior. Such a control loop can contain at least one pure integrator. In particular, the control loop can be equipped as an I, PI or PID controller.

According to one embodiment of the invention, the control takes place in discrete time steps. Nevertheless, the regulation can also take place continuously.

ermittelt werden. Dabei gibt Q_t+1 die einzuspeisende Blindleistung zum Zeitpunkt t+1 an, welche aus der eingespeisten Blindleistung Qt zum Zeitpunkt t, einem das zeitliche Einschwingverhalten angegebenen Gewichtungsfaktor K und der Differenz aus der zum Zeitpunkt t gemessenen lokalen in Netzspannung U_mess,t und dem Spannungsgrenzwert U_max ermittelt wird.The reactive power feed-in can be done in particular using the equation $${\text{Q}}_{\text{t}+1}={\text{Q}}_{\text{t}}+\text{K}\cdot \left({\text{U}}_{\text{measuring}\text{,t}}-{\text{U}}_{\text{Max}}\right)$$

be determined. Q _t+1 indicates the reactive power to be fed in at time t+1, which consists of the reactive power Qt fed in at time t, a weighting factor K indicating the temporal transient behavior and the difference from the local mains voltage U measured at time t _{, t} and the voltage limit U _max is determined.

genutzt werden.When using a variable voltage limit value U _max (P) that depends on the active power feed-in, the formula can be used $${\text{Q}}_{\text{t}+1}={\text{Q}}_{\text{t}}+\text{K}\cdot \left({\text{U}}_{\text{measuring}\text{,t}}-{\text{U}}_{\text{Max}}\left(\text{P}\right)\right)$$

be used.

genutzt werden, bei welcher nur eine induktive Blindleistung, d. h. bei Verwendung des Verbraucherzählpfeilsystems eine Blindleistung mit positivem Wert, eingespeist wird.If only inductive reactive power is to be fed in, the formula can be used $${\text{Q}}_{\text{t}+1}=\text{Max}\left({\text{Q}}_{\text{t}}+\text{K}\cdot \left({\text{U}}_{\text{measuring}\text{,t}}-{\text{U}}_{\text{Max}}\left(\text{P}\right)\right)\mathrm{,0}\right)$$

can be used, in which only an inductive reactive power, ie when using the consumer counting arrow system, a reactive power with a positive value, is fed in.

In a method for operating an inverter of the type mentioned at the beginning, in order to solve the above problem, it is proposed that a reactive power feed into a distribution network for decentralized voltage maintenance is regulated in the manner described above, which results in the advantages described in connection with the reactive power control method.

In order to solve the above task, the computer program of the type mentioned is proposed to include commands which, when the program is executed by a computer, cause it to carry out the reactive power control method described above, which results in the advantages described in connection with the reactive power control method.

The computer program can be executable by a computer that controls the decentralized power generator, in particular an inverter of the decentralized power generator. The decentralized power generator, in particular its inverter, can have detection means for detection the local mains voltage. The computer program controlling the decentralized power generator can read out the detected local grid voltage and/or cause the detection means to detect the local grid voltage.

To solve the above-mentioned problem, a computer-readable medium is also proposed on which the previously described computer program is stored, which results in the advantages described in connection with the computer program.

In the case of an inverter of the type mentioned at the outset, it is proposed to solve the above-mentioned problem that it has means, in particular a control device, for decentralized voltage maintenance in the manner described above, in particular for executing the computer program described above.

The features described in connection with the method according to the invention for operating an inverter can also be used individually or in combination in the inverter. The same advantages arise that have already been described.

A control device can specify a control loop in terms of circuitry, in particular include a hardware implementation of the control loop, such as a controller. The control device can have a memory for storing a voltage limit value or the voltage limit value-active power feed characteristic curve. The control device can have a readout device for reading out the active power-dependent value of the variable voltage limit from the voltage limit-active power feed characteristic curve.

As an alternative or in addition to a control device, the means for maintaining the voltage can also be a computer or a programmable computer chip installed in the inverter, on which the previously described computer program is stored and can be executed to maintain the voltage.

The inverter can have a detection means for detecting the local grid voltage.

In the case of a distribution network of the type mentioned at the beginning, in order to solve the above problem, it is proposed that this has means, in particular control devices, for decentralized voltage maintenance in the manner described above, in particular inverters of the type described above.

The features described in connection with the reactive power control method according to the invention for decentralized voltage maintenance and/or the inverter according to the invention can also be used individually or in combination in the distribution network. The same advantages arise that have already been described.

Each of the decentralized power generators can be assigned a means for decentralized voltage maintenance. This means that each of the decentralized power generators can contribute to maintaining voltage locally and decentrally. The resources and in particular the decentralized power producers cannot exchange data with each other. In particular, the means for decentralized voltage maintenance can be part of the decentralized power generator.

A control device can specify a control loop in terms of circuitry, in particular include a hardware implementation of the control loop, such as a controller. The control device can have a memory for storing a voltage limit value or the voltage limit value-active power feed characteristic curve. The control device can have a readout device for reading out the active power-dependent value of the variable voltage limit from the voltage limit-active power feed characteristic curve.

As an alternative or in addition to a control device, the respective means for voltage maintenance can also be a computer or a programmable computer chip installed in the respective decentralized power generator, on which the previously described computer program is stored and can be executed for voltage maintenance.

• 1: den Strang eines Verteilungsnetzes mit mehreren dezentralen Stromerzeugern,
• 2: den Regelkreis des erfindungsgemäßen Blindleistungsregelungsverfahrens,
• 3: eine Spannungsgrenzwert-Wirkleistungseinspeisungs-Kennlinie und
• 4: das Regelungsverhalten der Blindleistungsregelung bei steigender Wirkleistungseinspeisung.

Further details and advantages of a reactive power control method according to the invention, a method according to the invention for operating an inverter, a computer program according to the invention, an inverter according to the invention and a distribution network according to the invention will be explained below by way of example using the exemplary embodiments of the invention shown schematically in the figures. It shows:

• 1 : the strand of a distribution network with several decentralized power generators,
• 2 : the control circuit of the reactive power control method according to the invention,
• 3 : a voltage limit value-active power feed-in characteristic curve and
• 4 : the control behavior of the reactive power control as the active power feed-in increases.

In 1 a distribution network 2 is shown in the form of a low-voltage network, which is connected to the higher-level medium-voltage network 200 via a central transformer 100. Starting from the central transformer 100, the distribution network 2 extends as a strand 3.

Along this strand 3, the distribution network 2 has several decentralized power generators PV1 to PV4 arranged between the start of the strand 3.1 and the end of the strand 3.2, which are in particular photovoltaic systems. These decentralized power generators PV1 to PV4 generate electricity at various points in the distribution network 2 and, together with this, feed active power P into the strand 3 of the distribution network 2 via their respective feed points 4.

Without correction, this additional and generally not constant power feed into the distribution network 2 would lead to significant location-dependent fluctuations in the voltage of the distribution network 2. The voltage quality required to ensure a stable power supply for the end consumers also located in the distribution network 2 could not be maintained because the local voltage of the distribution network 2 is not maintained in a voltage band predetermined by predetermined voltage tolerances, typically +/- 10%, around a nominal voltage could become.

In order to be able to keep the local voltages within the limits of the voltage band, the decentralized power generators PV1 to PV4 record the local network voltage U of the distribution network 2 at their respective feed points 4 in order to achieve decentralized voltage maintenance on this basis. The individual decentralized power generators do not exchange data with each other or with a central control device of the distribution network 2. To achieve this decentralized voltage maintenance, the decentralized power generators PV1 to PV4 feed not only an active power P into the distribution network 2 at their feed points 4, but also a reactive power Q dependent on the local grid voltage U.

2 shows a control circuit 1 with which the reactive power control method according to the invention for decentralized voltage maintenance of the distribution network 2 is carried out. The control circuit 1 is described below as the control circuit of the decentralized power generator PV4, although the other power generators PV1 to PV3 of the distribution network 2 can also have control circuits 1 designed in the same way. The local mains voltage U is used as a controlled variable, which is recorded as described above and regulated by the control circuit 1 to maintain the voltage. In addition to the local mains voltage U as a controlled variable, the control circuit 1 uses a voltage limit value U _max as a reference variable to which the local mains voltage U is to be regulated.

In the next step, a control deviation E is determined from the local mains voltage U and the voltage limit value U _max and used as an input variable of the control device 6. Based on the control deviation E, the control device 6 determines the reactive power Q, which is fed into the distribution network 2 by an inverter of the decentralized power generator PV4 and thus serves as a manipulated variable of the control circuit 1.

The control device 6 can use the control deviation E, which corresponds to the difference between the local grid voltage U and the voltage limit U _max , to determine whether the local grid voltage U is smaller than the voltage limit U _max . If the local network voltage U is below the voltage limit U _max , the control device 6 can ensure that no reactive power Q is fed into the distribution network 2. In this way, the reactive power Q is only fed in when the voltage limit U _max of the local grid voltage U is reached.

Along the controlled system 7 of the control circuit 1, the fed-in reactive power Q causes a change in the local grid voltage U through the control behavior 8. In addition to the influence of the reactive power Q fed in by the decentralized power generator PV4, the active power feed-in and reactive power feed-in from the other decentralized power generators PV1 to PV3 and others also influence Influences the local grid voltage U at feed-in point 4 of the decentralized power generator PV4. These influences on the local grid voltage U, which are not directly caused by the decentralized power generator PV4, are summarized in the disturbance behavior 9 of the controlled system 7, which affects the local grid voltage U as a disturbance variable Z.

Both the disturbance variable Z and the fed-in reactive power Q combine to cause a change in the local grid voltage U at the feed-in point 4 of the decentralized power generator PV4. This changed local mains voltage U is then recorded and compared again with the voltage limit value U _max during the next run of the control circuit 1. This reactive power re This can be achieved either in discrete time steps or as a continuous-time control.

So that after the control loop 1 has settled over time, there is no longer a stationary setpoint deviation, that is, that the local mains voltage U in the regulated state assumes the voltage limit value U _max that serves as a reference variable without any deviation, the control device 6 can have an integrating control behavior. For this purpose, the control device 6 and thus also the control circuit 1 contains a pure integrator. It can be designed as an I, PI or PID controller.

Although the voltage limit value U _max can be specified as a power-independent limit value, it has proven to be particularly advantageous if the voltage limit value U _max is a variable limit value that is dependent on the active power P fed in, ie the active power feed P, and is in particular lowered depending on the active power. In order to determine the size of this variable voltage limit value U _max , a voltage limit value-active power feed-in characteristic curve 5 is used, as shown, for example, in 3 is shown. This constant and monotonically falling voltage limit value active power feed-in characteristic curve 5 indicates the value of the voltage limit value U _max depending on the active power P fed in by the decentralized power generator PV4. As with the 2 the voltage limit value-active power feed-in characteristic curve 5 is described as a characteristic curve for regulating the decentralized power generator PV4, although the other power generators PV1 to PV3 of the distribution network 2 can also be controlled with the aid of the same voltage limit value-active power feed-in characteristic curve 5.

The voltage limit U _max is in 3 stated as a multiple of the nominal voltage of the distribution network 2, which corresponds to 230 V, for example. The active power P fed in is given as a multiple of the nominal power of the decentralized power generator PV4, whereby the nominal power is the active power P which the decentralized power generator PV4 can feed into the distribution network 2 at most under ideal conditions.

The voltage limit value active power feed characteristic curve 5 has a power threshold value P _rel , which divides the voltage limit value active power feed characteristic curve 5 into two areas with different dependencies of the voltage limit value U _max on the active power feed P. At the in 3 In the example shown, this power threshold P _rel is 60% of the nominal power of the decentralized power generator PV4.

If the fed-in active power P is below the power threshold P _rel , the variable voltage limit U _max has a constant value U _max,1 . This constant value U _max,1 is in the example 3 at 8th % of the nominal voltage of the distribution network 2. As long as the decentralized power generator PV4 feeds active power P below the power threshold P _rel into the distribution network 2, no additional reactive power Q is fed into the distribution network 2, provided that the local network voltage U is below this in comparison Compared to the known methods, the constant value U _max,1 is relatively high. Losses associated with the reactive power feed Q can be prevented in this way across this parameter range.

For active power feeds P above the power threshold value P _rel , the variable voltage limit value U _max falls linearly as the active power feed P increases until it assumes a minimum value U _max,2 at the active power feed P which corresponds to the nominal power (P=1) of the decentralized power generator PV4 . In the active power range between the power threshold value P _rel and the nominal power of the decentralized power generator PV4, the variable voltage limit value U _max drops from the value U _max,1 by the voltage offset U _offset to its minimum value U _max,2 .

In 4 the control behavior of the reactive power control with increasing active power feed P is an example of the in 1 Distribution network 2 shown with four decentralized power generators PV1 to PV4, each of the decentralized power generators PV1 to PV4 being regulated via its own control circuit 1 according to the reactive power control method described above. The local grid voltage U is plotted at the respective feed points 4 of the decentralized power generators PV1 to PV4. The local network voltage U is given as a multiple of the nominal voltage of the distribution network 2. The active power P fed in by the individual decentralized power generators PV1 to PV4 is as already in 3 shown as a factor of the nominal power of the respective decentralized power generators PV1 to PV4.

For a better understanding of the control behavior, the voltage limit value-active power feed-in characteristic curve 5 indicating the active power-dependent variable voltage limit value U _max is according to 3 as a dashed line in 4 drawn.

First, the local grid voltages U at the feed points 4 of the decentralized power generators PV1 to PV4 increase linearly as the active power feed P increases. As long as the active power feed P has not yet reached the power threshold value P _erl and the local network voltages U have not yet reached the voltage limit value active power feed characteristic curve 5, the curves are in a comparatively wide voltage dead band B, in which no reactive power Q is fed into the distribution network 2.

From the power threshold P _erl , the width of this voltage dead band B decreases as the active power P increases until the voltage dead band B reaches its smallest width at the nominal power of the decentralized power generators PV1 to PV4.

Even after the power threshold value P _erl has been exceeded, the curves of the four power generators PV1 to PV4 are initially still below the voltage limit value active power feed-in characteristic curve 5, so that no reactive power is initially fed in. At point S1, the local grid voltage U at the feed-in point 4 of the decentralized power generator PV4 reaches the voltage limit value U _max applicable to this active power P, so that the reactive power feed Q to maintain the voltage, ie to maintain the voltage band of the distribution network 2, by the decentralized power generator PV4 begins becomes. Since the local grid voltages U of the remaining decentralized power generators PV1 to PV3 have not yet reached the voltage limit value U _max with this power feed P, the decentralized power generators PV1 to PV3 do not yet feed any reactive power Q into the distribution network 2.

As the active power feed P increases, the regulated decentralized power generator PV4 feeds increasingly more reactive power Q into the distribution network 2, which leads to a reduction in the local network voltages U of all decentralized power generators PV1 to PV4, without the predetermined voltage band of the distribution network 2 being violated.

The increase in the reactive power feed Q of the decentralized power generator PV4 continues with increasing active power feed P until at point S2 a maximum reactive power specified, in particular by the manufacturer of the inverter, is fed into the distribution network 2 by the decentralized power generator PV4. The decentralized power generator PV4 cannot feed any reactive power that exceeds this maximum reactive power into the distribution network 2. As the active power feed P continues to increase, the decentralized power generator PV4 continues to feed only the maximum reactive power into the distribution network 2, even if the voltage limit value U _max is exceeded.

Since the reactive power Q fed in by the decentralized power generator PV4 does not increase further from point S2, the local grid voltages U at the feed points 4 of the decentralized power generators PV1 to PV4 increase again as the active power feed P increases.

At point S3, the local grid voltage U at the feed-in point 4 of the decentralized power generator PV3 finally reaches the voltage limit value U _max that applies to this active power P, so that the decentralized power generator PV3 begins feeding in reactive power Q to maintain the voltage. As the power feed P increases, this voltage maintenance by the decentralized power generator PV3 also causes a decrease in the local grid voltages U until the decentralized power generator PV3 also feeds its maximum reactive power into the distribution network 2 at point S4. As the active power feed P continues to increase, the decentralized power generator PV3 only feeds its maximum reactive power into the distribution network 2, so that the local grid voltages U increase again as the active power feed P increases.

The increase in the local grid voltage U with increasing active power feed P continues until the local grid voltage U at feed point 4 of the decentralized power generator PV2 reaches the active power-dependent voltage limit value U _max at point S5. From point S5, the decentralized power generator PV2 begins to feed reactive power Q into the distribution network 2 until, with further increased active power feed P, this takes on the value of the maximum reactive power at point S6, which the decentralized power generator PV2 can maximally feed into the distribution network 2 . As the active power feed P continues to increase, the decentralized power generator PV2 continues to feed this maximum reactive power constantly into the distribution network 2 as reactive power Q.

Since the power generators PV1 to PV4 only feed inductive reactive power Q into the distribution network 2, the local grid voltage U drops during the phases in which the reactive power feed Q increases and does not increase, as would be the case if capacitive reactive power Q was fed in .

As in 4 As can be seen, the reactive power control method described can be used to prevent the local network voltage U at one of the feed points 4 of the decentralized power generators PV1 to PV4 from violating the predetermined voltage band of the distribution network 2, even when the power generator PV1 to PV4 can no longer increase the reactive power Q fed into the distribution network 2. Even the local network voltage U of the downstream decentralized power generator PV4 does not exceed the limit of 108% of the nominal voltage of the distribution network 2 with no active power P fed in and is permanently in a 10% voltage band around the nominal voltage of the distribution network 2.

With the help of the reactive power control method described above in the method for the corresponding operation of an inverter and such an inverter, it is possible to enable voltage maintenance at a feed point 4 of a decentralized power generator PV1 to PV4 and to reduce the system and network losses caused by the reactive power Q.

Reference symbol:

1

control loop

2

Distribution network

3

strand

3.1

Start of strand

3.2

End of strand

4

Entry point

5

Voltage limit value-active power feed-in characteristic curve

6

Control device

7

Controlled system

8th

Actuating behavior

9

disruptive behavior

b

Voltage dead band

E

Deviation from the rule

P

Active power

Prel

Performance threshold

PV1-PV4

Power generator

Q

Reactive power

S1-S6

Point

U

local mains voltage

Umax

Voltage limit

Umax,1

constant value

Umax,2

Minimum value

Uoffset

Voltage offset

Z

Disturbance variable

100

central transformer

200

Medium voltage network

Claims (19)

Reactive power control method for the decentralized voltage maintenance of a distribution network (2), in particular a low-voltage network, with at least one decentralized power generator (PV1-PV4), wherein a local network voltage (U) of the distribution network (2) is detected at the feed point (4) of the power generator (PV1-PV4). , an active power (P) is fed into the distribution network (2) and a reactive power (Q) is fed in depending on the local network voltage (U), characterized in that the local network voltage (U) is set as a controlled variable to a voltage limit value (U _max ). Local mains voltage (U) is regulated and the reactive power (Q) is used as a manipulated variable. Reactive power control procedure Claim 1 , characterized in that the reactive power (Q) is only fed in when the voltage limit (U _max ) of the local grid voltage (U) is reached. Reactive power control method according to one of the Claims 1 or 2 , characterized in that the voltage limit value (U _max ) is a variable limit value dependent on the active power feed (P). Reactive power control procedure Claim 3 , characterized in that the variable voltage limit value (U _max ) is reduced depending on the active power. Reactive power control method according to one of the Claims 3 until 4 , characterized in that the variable voltage limit value (U _max ) is determined by means of a, in particular continuous and/or monotonically falling, voltage limit value-active power feed-in characteristic curve (5) depending on the active power feed-in (P). Reactive power control procedure Claim 5 , characterized in that the voltage limit value-active power feed-in characteristic curve (5) has a power threshold value (P _rel ), above which the dependence between the active power feed-in (P) and the voltage limit value (U _max ) changes. Reactive power control procedure Claim 6 , characterized in that when active power feeds (P) are below the power threshold value (P _rel ), the variable voltage limit value (U _max ) assumes a constant value (U _max,1 ). Reactive power control method according to one of the Claims 6 or 7 , characterized in that with active power feed-in (P) above the power threshold value (P _rel ), the variable voltage limit value (U _max ) drops with increasing active power feed-in (P), in particular linearly. Reactive power control method according to one of the Claims 3 until 8th , characterized in that the variable voltage limit value (U _max ) assumes a minimum value (U _max,2 ) when the active power feed (P) corresponds to the nominal power of the power generator (PV1-PV4). Reactive power control method according to one of the preceding claims, characterized in that a maximum reactive power is specified, up to which the reactive power (Q) to be fed in can be increased. Reactive power control procedure Claim 10 , characterized in that once the maximum reactive power is reached, this is fed in as reactive power (Q) if the voltage limit value (U _max ) is exceeded. Reactive power control method according to one of the preceding claims, characterized in that only inductive reactive power (Q) is fed in. Reactive power control method according to one of the preceding claims, characterized in that a control loop (1) is used without a stationary setpoint deviation. Reactive power control method according to one of the preceding claims, characterized in that the control takes place in discrete time steps. Method for operating an inverter, in particular a grid-following inverter, a decentralized power generator (PV1-PV4), in particular a photovoltaic system, in which a reactive power feed (Q) into a distribution network (2) for decentralized voltage maintenance Claim 1 is regulated. Computer program, comprising commands which, when the program is executed by a computer, cause it to follow the method Claim 1 to carry out. Computer-readable medium on which the computer program is written Claim 16 is stored. Inverter, in particular grid-following inverter, for a decentralized power generator (PV1-PV4), in particular a photovoltaic system, characterized in that this means, in particular a control device, for decentralized voltage maintenance Claim 15 has, in particular for executing the computer program Claim 16 . Distribution network (2), in particular low-voltage network, with several decentralized power generators (PV1-PV4), which have means, in particular control devices, for decentralized voltage maintenance Claim 1 have, in particular inverters Claim 18 .

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