Island network detection

Abstract

A circuit and a method are described for detecting islanding in grid-connected devices. The voltages on the input and output sides of the grid-connected device are compared, and the devices are safely disconnected from the input and output networks upon detection of islanding. The voltage comparison analysis takes into account both the phase shift between the voltages and deviations in amplitude, ensuring safe network isolation by the devices under any load conditions.

Description

Technical FieldThe invention relates to the island network identification in network coupling devices for separating the producer network and the consumer networkPrior ArtVDE AR 4105 requires all-pole shutdown for generating installations such as photovoltaic installations in the event that there is no connection to the supply grid. This state is referred to as an island network and can occur, for example, when a subnetwork is intentionally disconnected from the supply network for carrying out work. The all-pole shutdown of the generating installation ensures that operation can be safely carried out on the disconnected subnetwork.Monitoring voltage and frequency limits is the basis of the island grid detection methods. The reason for this is that maximum and minimum limits are already prescribed for safe network and installation operation. If these limits are exceeded, a separation of all generating systems must take place within 0.2 s.When separating network areas from the grid network, it is only rarely the case that generation and consumption are approximately the same. In order to achieve an admissible operating point in a potential island, frequency and voltage values must be set in a correspondingly stable manner. If the deviations are too great, the installations shut down directly.In this case, imbalance in the active power balance results in impermissible voltage values, imbalance in the reactive power balance results in impermissible frequency values, i.e., values that exceed or fall below the maximum limits for the deviation from the nominal value and thus result in shutdown, as was the case in the studyEffective methods for island network recognition in 0.4 kV networks by Markus Dietmannsberger, M. Sc. and Prof. Dr. Ing. habi. This is described in detail in Detlef Schulz.The methods of the prior art have the disadvantage that the shutdown takes place on the basis of deviations from predetermined limit values, but not on the basis of the actual measured deviation.SUMMARY OF THE INVENTIONThe invention is based on the object of making it possible to shut down on the basis of the comparison of actual measured values on the generator side and the consumer side.Line switching devices which connect a producer and a consumer network to one another constitute a production installation for the consumer network and must detect island network formation and reliably switch it off all-polarally.The reliable galvanic isolation of the generator and consumer network is effected by relays in phase and neutral conductors ("all-pole") both on the supplier side and on the consumer side. The two-sided disconnection is necessary because capacitors are located in the device, in which energy can be stored.The tests used to check whether the island grid detection and shutdown are functioning properly are defined in EN 62116.On the basis of the invention, unlike the prior art, island network recognition can be realized by measuring and comparing the voltages on the supplier side and the customer side. If these voltages are different, the devices are switched off.If the voltage on the supplier side or the customer side is zero, the measured voltages are obviously different and it is switched off.According to the standard EN 62116, however, the case must also be considered in which a load impedance is present on the consumer side which is so large that no current is drawn from the supply grid at this load impedance and the predefined, set output power of the devices.According to the invention, the output current is controlled in such a way that the phase shift between voltage and current becomes zero. This is usually achieved by adjusting the amplitude and phase position of the output voltage accordingly by a regulator, so that the phase shift zero between voltage and current results.In the case of loads which have a reactive component, this inevitably leads to a difference between the supplier-side and the consumer-side voltage being automatically established, which leads to disconnection.In the case of purely ohmic loads, the island network formation cannot be detected by the criteria voltage equal to zero or deviation between the voltages on the basis of the activity of the controller, since here the output voltage of the device is equal to the voltage of the network before the disconnection. For this case, an algorithm is implemented in the control of the devices, which briefly reduces the output power within the time interval which is defined for the detection of island network formation in the standard. With the lowered output power, a lower voltage drop results at the load impedance. By comparison with the supplier-side voltage, island network formation can also be detected in this case.FIG. 3 illustrates by way of example the measurements of the voltages required for island grid detection.The voltage measurement is carried out, for example, by ohmic voltage dividers at the phase L and the neutral conductor N, which relate to the ground potential of the evaluation circuit. The diodes ensure that the voltage dividers provide only the positive or negative half wave of the voltage for further evaluation. The voltage divider outputs "in_N_", "in_L_Lieferant", "in_N_Abnehmer" and "in_L_Abnehmer" can be evaluated with a microcontroller or an analog circuit by forming the differences of "in_N_Lieferant" minus "in_L_Lieferant" and "in_N_Abnehmer" minus "in_L_Abnehmer". In this way, the input voltage and the output voltage are reconstructed and can be used for island grid detection as described above.Since the voltage dividers bypass the relays which are required for the safe disconnection of the supplier network and the customer network, the resistors of the voltage dividers must be designed such that no hazardous currents can flow between the networks.The diodes D1,..., D4 are not connected directly in parallel with the resistors R3, R6, R9 and R12 but are connected to R2, R5, R8 and R11, respectively, since, for example, R2 and R3 form a voltage divider and the forward voltage of the diodes thus has only a negligible effect on the measurement.The advantages of this type of voltage measurement for island network detection are that no expensive components with galvanic isolation are required for voltage measurement and that the ground potential of the evaluation circuit can also be connected, for example, to the negative potential of the intermediate circuit of the network coupling device.The circuit shown can likewise be extended analogously for 3-phase circuits.Embodiment / Description of the Drawings:FIG. 1: Illustration 1:FIG. 1 shows the basic structure of a network coupling device (1) which represents a consumer for the supplier network and a generating installation for the consumer network. In the devices, the energy of the supplier network is rectified and transmitted to the supplier network through the intermediate circuit ( 3). When island network formation is detected, the intermediate circuit ( 3) is separated from both the producer network and the supplier network by the relays ( 2).FIG. 2 : Figure 2:FIG. 2 illustrates the operating mode of the island network identification of the network coupling device in the form of a state diagram. Starting from normal operation, there is on the one hand the transition condition that input voltage U_in and output voltage U_out are unequal. In this case, an immediate shutdown takes place. On the other hand, there is the transition condition that a time interval t_interesceeds. The transmission power is now reduced 'power down'. Starting from this, in the case of unequal input and output voltages U_in and U_out, the switch-off takes place or, if the voltages are the same, the time counter is reset for the time and a return to normal operation takes place.FIG. 3 : Figure 3:An embodiment of the circuit for island network detection is shown in the figure.List of reference characters1 Production plant also Pioneering power plant 2 Relay 3 Intermediate circuit with capacitors

Claims

Device for detecting an island network, characterized in that the voltages on the generator side and on the load side are measured with respect to their amplitude and are compared with one another, and in that, in the event of a deviation of the voltage values from one another, a signal for switching off the generator network takes place by means of one or more isolating switches.Device according to claim 1, characterised in that the voltages on the generator side and on the consumer side are measured with respect to their amplitude and phase position and are compared with one another in order to be able to adjust the phase difference to the value zero.Device according to one of the preceding claims, characterized in that, in the case of purely ohmic loads, i.e. in the case of a pure active power load on the load side, the transmitted power is lowered briefly on the generator side.Device according to one of the preceding claims, characterized in that if a load with a reactive power component is connected on the load side, this is detected in that a deviation in amplitude and phase position occurs between the generator-side and load-side voltages.Device according to one of the preceding claims, characterized in that, for detection, the connection of a purely ohmic load on the consumer side, which load has no reactive power components, the setpoint value of the consumer-side output current is briefly reduced, with the result that the consumer-side output voltage has to be adapted accordingly and a deviation occurs as a result between the generator-side and consumer-side voltage, which can then be detected.Device according to one of the preceding claims, characterized in that, in order to identify an island network at regular time intervals, the active power which is fed into the consumer-side network is lowered sufficiently high so that, in the case of consumer-side loads having any active or reactive component, deviations between generator-side and consumer-side voltages or currents are identified, the lowering of the setpoint value having to be so great that it is greater than the measurement tolerance.Device according to one of the preceding claims, characterized in that the comparison of the voltages or currents measured on the generator side and on the load side is converted with an analog-to-digital converter and then compared with the aid of a programmable integrated circuit, preferably a microcontroller, and a control signal is generated therefrom.Device according to one of the preceding claims, characterized in that the comparison of the analog voltages or currents measured on the generator side and on the load side is compared with a comparator and a control signal or control signal is generated therefrom.

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