EMC & Circuit Protection
Effective protection
The reliable protection of photovoltaic installations is vital but not always easy to achieve. Felix Corbett looks at one cost effective solution - thermally protected varistors and gas-filled surge arresters
P
hotovoltaic installations are being increasingly set up in exposed locations such as on roofs and in open spaces. As a result this makes them susceptible to high risks due to overvoltages, which are increased still further by the long leads between the solar modules and inverters at the DC level as well as the feed lines to the power network at the AC level. Figure 1 shows the structure of a typical solar inverter. It converts the DC voltage coming from the solar module to an AC voltage which can then be fed into the power network.
Comprehensive protection As a rule, metal oxide varistors with a rated voltage of 1000VDC are used for the DC input of the solar inverter. Depending on the line voltage to be handled, varistors with a voltage of 300V RMS, for example, are obvious candidates for the inverter output. In both cases, gas-filled surge arresters may also be used to provide additional protection. Figure 2 shows frequently used circuit variants for the DC input of solar inverters. The simplest version (Figure 2a) uses only a single varistor with a rated voltage of 1000V RMS and a disk diameter of 20mm, for example. In this case, the rated DC voltage is 1414VDC and the clamp voltages 2970V at 100A. The circuit in Figure 2b operates with two varistors connected in series. They should be designed for 550V RMS (745VDC) to provide the same protection. This variant offers the advantage that the clamp voltage is only 2710V at a current of 100A and thus further limits the overvoltage. In addition, the energy to be absorbed is distributed across both
24 September 2011
components, thus reducing the stress factor.
In the variant shown in Figure 2c, a gas- filled surge arrester is additionally connected between the varistors and ground. This variant still offers sufficient protection, especially in the event of failure or stress-caused aging of one or both varistors. The arrester must be selected so that it does not go over to a continuously conducting state if both varistors should fail.
In principle, the power line side offers the same circuit options. For grid voltages of up to 240 VRMS typical in Europe, varistors with rated voltages of 300 or 320 VRMS should be selected. Basically, these circuits are largely identical with the input circuits of standard power supplies designed for line operation. As solar inverters belong to a relatively young sector of technology, no long-term studies have yet been carried out about their ageing behaviour and how this affects the protection components used in them. However, it is known from power supplies and other equipment that degradation can occur in semiconductor- based components such as ceramic varistors. This is due to continuously repeated low amplitude pulse loads. Such degradation manifests in a rise of the leakage current. If the resulting higher power loss in the component cannot be dissipated by convection, then in extreme cases continuously increasing heating occurs which can lead to a short circuit and consequent destruction of the varistor.
Tougher requirements Various standardisation bodies such as UL and IEC have become alert to this safety
Components in Electronics Figure 1: A block diagram of a solar inverter
Left: Overvoltage protection (OVP) of the solar module and inverter input; right: line-side overvoltage protection inclusive of EMC filtering.
risk and insist that future applications assure thermal monitoring of the varistors as well as their disconnection in the event of a fault. However, the recently published IEC 62109-1 standard entitled “Safety of power converters for use in photovoltaic power systems – Part 1: General requirements” does not treat this set of problems explicitly. Other standards, such as the most
recent revision of IEC 60950-1, specify that exclusive use must be made of varistors that satisfy IEC 61051-2-2 as well as Annex Q (IEC 60950-1). A corresponding fuse must also be provided for the varistors. The requirements of insurance companies make the situation even more acute: they insist among other things that photovoltaic installations with an output of more than 50 kW must assure overvoltage protection to IEC 61643-11 Category II (coarse protection).
Technical data of the EPCOS ETVF series
Parameter/Type
Rated voltage [V AC] 130 to 420; 115 to 1000 (20 mm disk)
Surge current capability [kA] 6, 10 and 20 Response time [ns] <25
Max. energy absorption (2 ms) [J] 50 to 700
Operating temperature [°C] –40 to +85 To allow manufacturers of solar inverters
to satisfy the rigorous demands of the standards and insurance companies cost- effectively, TDK-EPC has developed the ETFV series (EPCOS ThermoFuse Varistor). These components are built up from a series circuit comprising a varistor and a thermal fuse. If the varistor degradation has advanced so far that the higher leakage current leads to overheating, the fuse blows and disconnects the varistor. A special feature of these components is the monitor lead: it extends the contact point between varistor and thermal fuse out so that it can drive an LED (Figure 3).
Varistors for bidirectional protection Varistors are voltage-dependent monolithic ceramic resistors. Their behaviour is defined by their rated voltage and their current capability. Below the response voltage, the component exhibits very high resistance:
only a negligible leakage current in the μA range flows. If the applied voltage exceeds the rated voltage of the component, the varistor becomes conductive, allowing the component to withstand currents in the range of several amperes, and of several kA for brief periods.
TTI Inc. |
www.ttiinc.com Felix Corbett is Director of Supplier Marketing, Passives and Discrete Products, Europe, at TTI
Figure 2: Circuit variants for overvoltage protection
Figure 3: The EPCOS ETFV has a monitor output. The LED signals the intact state of the varistor
www.cieonline.co.uk
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52