WO2003071655A1

WO2003071655A1 - Method for monitoring decentralised power generation plants

Info

Publication number: WO2003071655A1
Application number: PCT/EP2003/001828
Authority: WO
Inventors: Kolm Hendrik
Original assignee: Retec Regenerative Energie Technik
Priority date: 2002-02-22
Filing date: 2003-02-22
Publication date: 2003-08-28
Also published as: AU2003214070A1; DE10207560A1

Abstract

Local power generation plants increasingly supply a high-voltage energy supply network (4) in a parallel manner via a local network and a low-voltage local network transformer (3). To achieve this, each power generation plant has on the load side a respective switch-isolator (2), which is used to isolate the respective power supply plant from the local network. To monitor the local power supply plants for overloads or network malfunctions, the amplitude and frequency of the phase-to-phase low voltages are measured on the 3-phase low-voltage side of the local network transformer (3). The measured values that have been determined for the amplitude and the frequency of the low voltages are transmitted in the form of digitally encoded data signals (8) to all power supply plants via the local network. The amplitude and frequency of the 3-phase output voltage are measured on the load side of each power generation plant. The data signals (8) that are transmitted via the local network are received by each power generation plant and are evaluated together with its own measured values for amplitude and frequency, in such a way that the switch isolator (7) of the relevant power generation plant is opened if no data signal (8) is received, or if the measured values of said installation lie outside a predefined tolerance range.

Description

METHOD FOR MONITORING DECENTRALIZED POWER GENERATION PLANTS

DESCRIPTION

The invention relates to a method for monitoring decentralized energy generation plants according to the preamble of the claim. Such a method is generally known.

In the case of decentralized energy generation plants on the low-voltage network of energy supply companies (EVUs), unwanted island operation must be reliably prevented to maintain safety. In the past, the RUs requested an activation point that was accessible to them at all times, which could be used to disconnect the feed-in system from the grid in the event of maintenance work or malfunctions.

However, with the increasing number of decentralized energy generation plants, especially also combined heat and power plants (CHP) and photovoltaic plants, this activation point becomes unprofitable. Imagine that every single-family home had its own production system installed, that would be it Therefore, the VDEW (Association of German Electricity Works) asked for automatic solutions. The three-phase network monitoring method was first used for this. As illustrated in Fig. 1, this is a purely passive method, since the network sizes are not influenced by the measuring device. The amplitude of the chained voltage of the 3 outer conductors and the frequency are monitored. With voltage _deviations outside the tolerance range 0.85 U _nam <U <1.10 U _neπ ,, and frequency deviations of +/- 0.2 Hz from the target value (50 Hz in Europe), the power generation system must be _disconnected from the grid within 0.2 s separate. This method is very easy to implement and is suitable for any type of energy generation system (for example synchronous or asynchronous generators or photovoltaic inverters). However, it has one crucial shortcoming. If a balance is struck between the sum of the power generated and the power taken off by local loads, this does not lead to changes in the voltage and frequency parameters to be monitored after the local network has been activated, which means that the system does not switch off and is therefore - to be avoided under all circumstances - Forms unwanted island operations that pose a significant health hazard. For example, a fuel cell delivers an electrical output of 4 kW _el . and a downstream inverter feeds into the low-voltage grid of the power supply company in parallel operation. At the same time, an ohmic load (heater) of 4 kW is connected to the supply area. Now for When maintenance work on the local network switch disconnector is open, the consumer absorbs the generated power from the inverter; due to the power balance, there is no switch-off criterion for the inverter, so that even after the local network has been activated, voltage is present, so-called unwanted island operation. There is another disadvantage: This method is not intrinsically safe, ie if the network monitoring is defective, the power generation system is not switched off, which is why repeat tests are required for the function.

In order to avoid these disadvantages, the impedance measurement has also been carried out. As shown in Fig. 2, this is an active method. The measuring device influences the network sizes and derives the network impedance from them. One possibility is the following:

In the area of the zero crossing of the AC mains voltage, the measuring device outputs a constant current value as a pulse to the network. The zero crossing now shifts in time until the network in turn compensates for this current to zero by building up the voltage of reversed polarity. This time shift is a measure of the network impedance and can be calculated.

The draft standard of VDE 0126 provides this method as the most reliable way of preventing unwanted island operation, both in single-phase systems and in three-phase systems. The network impedance limits are defined as follows:

ZNetz <1, 25 Ω stable for 20 s before connection to the mains

ZNetwork <1.75 Ω during feed-in operation

Impedance jumps of> 0.5 Ω lead to a shutdown within 5 s.

This method has the advantage over three-phase voltage and frequency monitoring that it reliably prevents unwanted island operation and is intrinsically safe, which means that repeat tests are not necessary. However, the following problems arise:

, Since this method is an active one, this is in contradiction to the requirement of the power supply companies to feed 100% smelly electricity into the network if possible. With the inverter technology available today, this can also be achieved without any problems. It seems all the more traditional to distort the sine only for the purpose of network monitoring. For this reason, this method is not permitted in the USA and Great Britain, for example. Power generation plants that do not require an inverter, such as Combined heat and power plants with synchronous generators or small hydropower plants, especially for network impedance measurement, would have to be equipped with a measuring device that is capable of realizing this impedance measurement.

The network impedances of 1.75 Ω during operation cannot always be achieved in reality. Particularly in rural supply areas with larger distances from the entry point from the local network low-energy electricity transformer, significantly larger values can be expected. When purchasing power (i.e. not when supplying energy to the power supply company, but when accepting it from the power supply company), significantly higher line impedances did not lead to any noteworthy faults or other problems, which is why operators of feed-in systems now set up react with lack of understanding, if only for the 1 kWp photovoltaic system I don't understand why this impedance value is fixed regardless of the power of the power generation system, since the line losses increase with increasing feed-in power. On the other hand, the value of 1.75 Ω can be in a supply area already represent a limit value. In my view, there is reason to fear that it will become undesired during continuous operation Incorrect shutdowns due to high network impedance. A recent conversation with the regional energy supplier TEAG confirms this assumption. It is incomprehensible if an operator of an 800 Wp photovoltaic system threatens claims for damages due to lost feed-in tariffs because his inverter disconnects from the grid due to excessive grid impedance, but on the other hand he has had no problems for his business with a connected load of 50 for 18 years kW was supplied with electricity by TEAG

The problem of an unwanted incorrect shutdown due to the impermissible network impedance is exacerbated if the following circumstances are taken into account:

If several inverters are operated on one supply area according to the principle of impedance measurement (as provided in the draft standard of VDE 0126), these devices can influence each other. Since the impedance measurement is carried out by applying a current pulse from the measuring device to the inverter, all inverters would have to be synchronized with each other, because if e.g. If two inverters switch their current pulse to the grid simultaneously, the measured grid impedance is doubled. Synchronizing different brands of inverters is particularly problematic because their manufacturers have their Software protocols should be disclosed.

In contrast, the object of the invention is a method for monitoring decentralized ones

To create power generation plants, which does not require the application of Sfromimpulse and thus avoids distortion of the sinusoidal shape of the mains current.

This object is achieved by the characterizing features of claim 1.

The invention is explained below with reference to the drawings according to Figures 3 to 5. It shows:

Fig. 3 A block diagram of a local network with three decentralized power generation plants and a local network transformer, which connects the local network with a high-voltage power supply network;

Fig. 4 is a block diagram of a measuring transmitter arranged at the low voltage input of the local network transformer, and

Fig. 5 is a block diagram of a measuring receiver arranged on the load side of each power generation system.

As shown in Fig. 3, in terms of circuitry, a measuring device 1 is arranged in the building of the local network low-voltage transformer 3, which performs the following main function:

Measuring the concatenated 3 voltages of the phase conductors of the three-phase network R, S, T (low voltage) a) Measuring the frequency b) Digital transmission of the measured values cyclically (e.g. every second) through PWL communication to all power generation systems in the local network area 8

There is a measuring receiver 5 on the power generation systems, which is coupled to the allpoügen switch as a separating element (relay, contactor) 7 and performs the following tasks: a) Measuring the linked 3 voltages of the phase conductors of the three-phase network R, S, T (low voltage) 9 b ) Measuring the frequency 10 c) Receiving and decoding the digital 8 measurement signals from the transmitter in the transformer house (13, 12, 11) d) Switching off the power generation system if measurement signals 8 are not received by the measurement transmitter 1 or the self-measured values for voltage and frequency except Tolerance is 14 The principle of the network monitoring described above is based on a passive method, except for the modulation of the network voltage for the purpose of data transmission.

The measuring transmitter 1 in the transformer house measures the three external conductor voltages in the local network and the frequency directly on the secondary winding of the three-phase transformer 3. After analog / digital conversion 11, the measured values are digitally transmitted to the measuring receivers 5 in the local network area 8. The frequency band for the data transmission is in the range from 95 kHz to 150 kHz. This is internationally approved for data transmission for free use and is currently used to operate home automation devices. The operation of devices on the low-voltage network for the purpose of data transmission is regulated in the harmonized standard EN 500 65 - 1 CENELEC, so that reference is made to this. This data 8 can be received and detected at all points in the local network. Beyond the three-phase transformer 3 in the direction of the medium-voltage level 4, the transformer 3 acts as a bandstop, which is why the data flow in this direction is interrupted. The low-voltage network thus forms a communication network that is open to every user for as long as EN 500 65 - 1 CENELEC is complied with.

The measuring receivers 5 measure the voltages of the outer conductors 9 and the frequency 10 directly on the power generation system (photo voltaic inverter, hydropower plant, combined heat and power plant, etc.); In addition, they receive the data 8 sent at cyclic intervals from the measuring transmitter 1 in the transformer house. The measuring receiver 5 acts on the switching element (relay, contactor) 7, which couples the energy generation system to the network.

The following criteria ensure that the power generation system is switched off in measuring receiver 5:

1. Voltage out of tolerance (0.85 Uneπn <U <1.1 Unenn)

2. Frequency out of tolerance (49.8 Hz <f <50.2 Hz in Europe)

(59.7 Hz <f <60.3 Hz in USA)

3. Failure of data stream 8 from measuring transmitter 1 - (for example, by opening the switch disconnector in the transformer house)

4. Voltage drop between the feed node and transmitter 1 in the transformer house> 10 V (proposal)

Criteria 1) and 2) correspond to the three-phase voltage monitoring as required by the task.

Criterion 3) occurs when either the measuring transmitter 1 is defective, the local network switch disconnector 2 has been opened, for example for maintenance work by the utility company, in the event of a line break, for example due to earthworks, or if there is no medium voltage on the primary side of the local network transformer 4, with the result that whereby the power supply of the measuring transmitter 1 is interrupted. This makes the entire system intrinsically safe. Functional tests can be omitted.

Criterion 4) makes it possible to indirectly monitor the line impedance or line losses as a function of the power of the power generation system and to trigger appropriate switching operations. For this purpose, the actual value of the current fed in per phase is made available to the measuring receiver 5.

The power generation system knows your fed-in current Ie at all times, furthermore it knows its measured voltage at the feed-in node, as well as the voltage at the transformer, which is transmitted by the measuring transmitter. According to the calculation basis.

RNetz = Uτr-Ue RNetz - ohmic part of the network impedance Ie UTΓ - voltage at the transformer

Ue - voltage at the feed node Ie - fed current The line impedance can be calculated in the event that all fed power is delivered to the transformer 3. However, their absolute value alone is not a switch-off criterion, because if the power fed in is reduced again directly by local loads 6 at the feed-in point, the absolute impedance between transformer 3 and feed-in node only plays a subordinate role.

This variant takes into account the fact that large feed-in capacities require lower line impedances than smaller ones.

For example, if the line impedance is 2 Ω, which would lead to a shutdown for a long time according to VDE 0126 - draft, a 1 kWp system can be safely operated in one phase without a shutdown, since 4.35 A x 2 Ω = 8.7 V below the Cut-off voltage. The switch-off would only take place at 5 A feed current and the resulting power of 1150 Wp.

So far, the data flow within my description was considered as a one-way street from measuring transmitter 1 (transformer house) to measuring receiver 5, with which the main function, the avoidance of undesired island operation, can be safely accomplished. It is also conceivable to equip the measuring transmitter 1 with a radio clock and the Real-time signals also to be sent to the measurement receiver 5, as a result of which network events can be logged in real time.

It would also be conceivable to query data 8 of the power generation system, such as type (CHP, PV, hydropower), maximum power, power currently fed in, or to operate load management from the measuring transmitter 1, for which purpose the grain communication via the network takes place bidirectionally, every transmitter is also a receiver and vice versa.

By disclosing the data protocol (similar to PTB Braunschweig's Maiflingen time signal transmitter), every device developer is able to participate in this data traffic, thereby achieving independence from manufacturer and country regulations, as well as different network systems (voltage, frequency). This is currently handled differently in each country. The VDE's wish to incorporate draft standard VDE 0126 into a harmonized international standard is currently not foreseeable. Even if the proposed principle is not reflected in a regulation, it offers the highest level of security and reliability, the expenditure on equipment is relatively low. With energy generation systems without inverters, this method makes it possible to avoid unwanted island operation in the first place. In the following, the example of a measuring transmitter 1 will be described with reference to Fig. 4.

The 3 phases of the secondary winding of the three-phase transformer 3 RS and T and the neutral conductor are fed to the measuring transmitter 1. The measuring transmitter 1 draws its supply voltage from the phases via its internal power supply 13. The power supply unit 13 is equipped with phase couplers, so that the data 8 to be transmitted are transmitted on all 3 phases at the same time as the N as reference potential. The BG 9 prepares the amplitude of the 3 outer conductor voltages for the microprocessor 11, so that after its A / D conversion, the network amplitude of the 3 phases is available in digital form. The frequency monitor 10 prepares the zero crossings of the mains voltage so that they can be converted into a digital measurement signal by the microprocessor 11. The microprocessor 11 cyclically sends a data frame in bit-serial fashion via its T x D line to the powerline modem 12, which, depending on the mode of operation, generates a frequency- or amplitude-modulated signal and modulates it to the mains voltage. This procedure is internationally approved and is bindingly regulated in EN 500 65 - 1 CENELEC. If the software-defined limits for the voltage amplitude (0.85 Unenn <U <1.1 Uneπn) or for the frequency (49.8 Hz <f <50.2 Hz in Europe) are not met, the microprocessor 11 interrupts the data stream 8. The data stream 8 is also interrupted when the power supply 13 of the Measuring transmitter 1 is not guaranteed or because of a defect. If the load disconnector 2 in the transformer house is opened for the purpose of maintenance work or if there is a line break between it and a measuring receiver 5, the measuring transmitter 1 continues to send valid data, but it does not arrive at the measuring receiver 5, which is detected as an error by the latter. whereupon this separates the power generation system from the network via the coupling 7. This procedure is intrinsically safe and does not require any additional retests.

An example of a measuring receiver 5 is described below with the aid of Fig. 5.

The modules 9-13 are functionally identical to those of the measuring transmitter 1 in Fig. 4. In addition, however, the measuring receiver 5 has a power amplifier 15 in order to control the coupling element (relay or contactor) with which the energy generation system (generator) is separated from the mains becomes.

The module 14 for current measurement is optional in receivers that are not part of an inverter. In any case, the phase currents are measured in the inverter, but for external devices, an impedance calculation can be carried out by calculating the current phase currents. For the basic function of the ENS is the height the phase currents are not relevant, however, this impedance calculation is implemented in inverters integrated in the inverters, since no additional hardware expenditure is required for the current measurement.

Claims

P A T E N T A N S P R U C H

Method for monitoring local power generation systems, which feed paraUel via a local network and a low-voltage local network transformer (3) into a high-voltage power supply network (4), each power generation system on its load side each having a load break switch (2), which occurs when Overloads or network disturbances disconnect the respective power supply system from the local network, characterized in that the amplitudes and frequency of the linked low voltages are measured on the three-phase low-voltage side of the local network transformer (3), that the measured values determined for the amplitudes and the frequency of the low voltages in Form of digitally coded data signals (8) are transmitted over the local network to all power generation plants, that the amplitude and frequency of the 3-phase output voltage is measured on the load side of each power generation plant, and that each power generation plant age the data signals (8) transmitted via the local network are received and evaluated together with the own measured values for amplitude and frequency in such a way that the load-break switch (7) of the power generation system in question is opened if no data signal (8) is received or the own measured values outside one specified tolerance range.