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Posted on 03 July 2019

Enhanced Over-Voltage Protection of Solar Installations

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Through-out its voltage / current characteristic, the Varistor is actually conducting

Varistors offer a cost effective solution for the protection over Solar Invertors against over-voltages. Thermally Protected Varistors from TDK-EPC can help to reduce down times and to optimize returns on investment.

By David Connett, Director IC Reference Design, EPCOS AG. A Group Company of TDK-EPC Corporation

 

With what seems to be sustainable growth rates in excess of 40% per annum, the expansion of solar energy systems remains phenomenal.

However, insufficient long term data is available to show whether the sensitive electronics in the solar inverter, are able to survive typical exposure to atmospheric influences and voltage fluctuations on both the DC input and the AC output stage.

This article will attempt to show the current protection concepts currently used, the typical aging effects of the individual devices and how these can be optimized to exceed their working lifetime.

Solar panel

Basic Inverter Block Diagram

On the left the DC Input from the Solar Module (Panel) enters the inverter. Overvoltage Protection (OVP) between the module terminals protects the Maximum Power Point Tracker (MPPT) and the Solar Panel itself. The DC/AC inverter needed to convert the DC voltage from the module into AC voltage for feeding into the mains supply and filtering to reduce electro-magnetic interference (EMI) are themselves protected against disturbances originating from the mains by the OVPs.

The simplified diagram does not show coarse protection devices that could be installed externally to the inverter. However, the coordination between fine and coarse protection should be considered in the design / installation stage.

Basic Inverter Block Diagram

Component Selection

Typically Metal Oxide Varistors (Varistor) with a DC rating upto 1000V are used for the DC input protection occasionally in combination for Gas Discharge Tubes (GDTs). The AC output protection is also Varistor based, however optimized for the network voltage (i.e. 300Vrms) again with a possibility of a combination with GDTs.

Metal Oxide Varistors

A Metal Oxide Varistor is a voltage dependant resistor. The clamp voltage of a varistor is defined by its voltage rating and its current handling capability. Through-out its voltage / current characteristic, the Varistor is actually conducting. In its normal, high resistance mode, a leakage current that can be measured in the µA range is always present. In the over-voltage, low resistance, mode in which the Varistor is conductive, currents, measured in amps or for short durations in kilo-amps, pass through the Varistor.

Metal Oxide Varistors

Gas Discharge Tubes

As its name suggests, the gas tube is tube filled with a gas. If a voltage exceeds the breakdown characteristics of the gas, the gas itself will ionize and will form a conductive path across which an arc is formed between the charged terminals. However, as the gas has a finite ionisation time, as the voltage rise time increases so does the breakdown voltage of the gas. For example, a typical Gas Tube with a DC sparkover voltage of 230V at 100V/s, its maximum firing voltage at 1.000V/μs could be closer to 600V. This firing voltage is commonly referred to as the impulse sparkover voltage or dynamic response.

Gas Discharge Tubes

Considerations

Varistors due to their high current handling capabilities and cost-performance ratio make ideal protection components. However, as with most semi-conductor based technologies, Varistors are subject to degradation (ageing) when exposed to repetitive pulse of low amplitude. The degradation takes the form of an increase in leakage current of the Varistor which can result in a phenomena called “thermal runaway”. In extreme cases, the thermal overload can result in a short circuit and rupture of the Varistor. This point has also been reviewed in a number of international standards (UL, IEC) and the net result is that thermal surveillance of varistors will need to be considered in the future.

Common Protection Concepts DC Input

For the DC Input Varistors, remain the most favoured primary protection. How they are deployed however is varied. A few examples follow – taking as an example an input upto 1000Vdc.

In Figure 5/1 a single Varistor is placed between the PV + and PV - terminals from the module. Rated at 1000Vrms, this 20mm varistor would have a maximum DC rated voltage of 1414Vdc and a clamp voltage of 2970V at 100A.

Figure 5/2 utilizes two varistors placed in series. Typically two 550Vrms (745Vdc) rated Varistors are deployed hereby achieving the same rating as Figure 5/1, with the advantage of a lower maximum clamp voltage of 2710V at 100A.

Figure 5/3 deploys the same concept as Figure 5/2 but with a separation to system earth provided by a GDT. This solution can assist with the compensation of ageing of the Varistors mentioned above in the normal operating conditions. However, the extinguish characteristics of the GDT must be taken seriously in account to avoid that the GDT remains in a conductive mode (arc or glow mode) after firing.

Common Protection Concepts

AC Output

Although, in this application, we are supplying AC power to the Grid, the connection provides a source of over-voltage and overcurrent, not to mentioned other disturbances such as EMI Noise. In this as such the protection of the Inverter can be compared to the input stage of a standard power supply.

The typical concepts are similar to those shown above. The main difference being the selection of Varistor ratings to suit the AC network voltage. Here as a general rule 300Vrms or 320Vrms rated Varistors are common for European line voltages of up to 240Vrms.

Specific Requirements

All of these solutions fulfil the intended requirements of providing over-voltage protection to the inverter, Figures 1 and 2 do not address the previously mentioned problems of ageing. The newly published IEC 62109-1 “Safety of power converters for use in photovoltaic power systems – Part 1: General requirements“, does not specifically address this point. However, to draw some analogies from other IEC standards, the latest revision of IEC 60950-1 (Information Technology Equipment) provides provision that only Varistors, qualified against IEC 62019-2-2 and which meet the requirements of Annex Q (IEC 60950-1), may be used as the primary protector. In addition, for protection of Varistors, overcurrent protection or a similar interrupting device / means is required to be supplied in series with the Varistors to ensure that in the case of thermal runaway that the Varistor itself does not become a safety issue.

Installation of External Overvoltage Protection

In general, the insurers of solar installations demand that Overvoltage Category 2 (coarse protection) SPDs are installed when the power of an insured installation exceeds 50kW. Below this rating, there are no clear guidelines and hence it would seem that the cost of protection versus the cost of replacement means does not justify additional external protection and that the fine protection inside the inverter should be sufficient.

Enhanced Protection in Inverters

As a result of the statements from insurance companies and the trend to extend warranty periods, the demand for improved protection which covers the previously mentioned problems of ageing coupled with a means of indicating failure will increase.

Thermally Protected Varistors

Awareness of the problems associated with ageing and thermal runaway have been addressed by the major producers of Varistors through so-called thermally protected varistors. These devices feature a thermal surveillance of the Varistor which can result in the disconnection of the Varistor to the supply when a threshold temperature is exceeded. Through these devices, e.g. the ETFV20K1000 from TDK-EPC, (ETFV – EPCOS Thermal Fuse Varistor) some of the harmful effects of aging of varistors have been eliminated while still providing a cost effective solution to Inverter designers. In addition, since these devices feature an external monitoring of the status of the protection through a LED. If the LED is no longer illuminated, then the user should be instructed to contact the service department for a prompt replacement of the thermally protected varistor otherwise the warranty is invalidated and so that the returns on investment can be optimised. In the case of replacement of the “opened” Varistor, the thermally protected ETFV Varistor could be hard-wired via screw terminals on the pcb and not soldered onto the pcb using conventional through-hole processes – this will leads to simple and effective replacement of the ETFV.

Thermal protected Varistor Block Diagram

Thermal protected Varistor Device

Summary

Varistors offer a cost effective solution for the protection over Solar Invertors against over-voltages but they themselves are the subject of ageing that can reduce their effective lifetime and make them a safety hazard. Thermally Protected Varistors from TDK-EPC can help to address this point and can be easily replaced following operation helping to reduce down times and to optimize returns on investment.

 

design-solutions@epcos.com

 

 

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