Protecting off-grid PV systems

August 25th, 2014, Published in Articles: Energize

 

There are many studies on the short circuit behaviour of grid connected PV and renewable energy systems but not many on the protection of off-grid PV (OGPV) systems. Many national wiring codes are beginning to include protection of PV systems, but the subject is neither well nor widely understood. This article covers the protection requirements for such solar PV systems, including the DC and the AC sections of the installation.

Off grid systems comprise an electricity generation component and a reticulation and consumer component which are generally integrated into a single installation. Systems may be totally DC or a mixture of DC for the generation side and AC for the consumption side. Generally the protection requirements for generation and reticulation differ and must be treated separately.

Protection requirements

Protection is generally provided against electric shock and overcurrent. Other dangers exist in PV systems such as DC arc flash and long term low level over-currents, and special protection is required for these cases. Protection is required to ensure human safety and prevent damage to property. In any electrical installation the primary means of protection is a combination of earthing and overcurrent devices (generally fuses or thermomagnetic circuit breakers). Contact of a live conductor with an earthed component, or contact between live conductors results in an increase in current sufficient to operate the protective device. Residual current or earth leakage protection is a supplementary protection meant for AC socket outlets to cover the possibility of the earth wire becoming disconnected in the connecting cord. It is often applied to the whole installation and not only the socket outlets.

Protection devices operation

Two types of overcurrent protection device (OCPD) are typically used; circuit breakers and fuses.

Circuit breakers

A circuit breaker has two modes of operation, thermal and magnetic. Fig. 1 shows the operation curves of a typical DC rated circuit breaker. The thermal operation area is used for overcurrent protection and the time to operate is inversely proportional to the current, in this instance up to a current level of seven to ten times the rated current (In).

Fig. 1: Operating curves for thermomagnetic circuit breaker (Schneider Electric).

Fig. 1: Operating curves for thermomagnetic circuit breaker (Schneider Electric).

The magnetic curve is used for fault currents and gives instantaneous operation at a level ranging from seven to ten times the rated current. It is clear that a short circuit on a current limited device will not cause operation of the magnetic circuit, but will cause the thermal circuit to operate. The concern here is the operation time.

Fuses

Fuses operate on a thermal basis, with time to operate inversely proportional to the overcurrent. Fig. 2 shows the curve of a typical fuse designed for use with PV systems. From the curve it can be seen that instantaneous operation occurs at an overcurrent of approximately ten times the rated current. At an overcurrent of 120% the fuse will take 120 s to operate.

    Fig. 2: Fuse operation curves (Cooper Bussman [1]).

Fig. 2: Fuse operation curves (Cooper Bussman [1]).

DC protection

The DC side of a solar installation comprises the solar panel array, batteries, charge controllers, combiners and cabling. Where the complete system runs on DC, the consumer side comprises lighting and appliance circuits, which require different protection from the generation side .On the DC side of a solar system protection is required against:

  • Electric shock
  • Short circuits
  • Overcurrent
  • DC arc faults (arrays and inverters)

DC rated fuses and circuit breakers must be used. Zero crossing AC circuit breakers will not operate on a DC overcurrent as there is no zero crossing. Likewise AC rated fuses should not be used unless there is a DC rating for the same fuse.

With PV string voltages now in the range of hundreds of volts (600 to 1000 V), protection against electric shock is essential. Protection is provided by insulation and earthing. AC rated protection devices cannot be used on a DC circuit unless the DC rating is given.

Solar PV module protection

The photovoltaic panel is an inherent current limited device. I/V curves of a typical solar module (Fig. 3) show that the short circuit current (Isc) is of the order of 105% of the typical operating current at maximum power point. Problems however may occur when panels are paralleled, although the total current on short circuit of paralleled strings will also only rise to 105% of the normal current.

Fig. 3: PV panel I/V curves.

Fig. 3: PV panel I/V curves.

This poses a problem as normal protective devices are designed to operate instantaneously at current levels of seven to 10 times the rated current and will take long to operate at lower over-currents.

Fuses are generally used for protection in small to medium to medium sized installations, although several manufacturers are offering circuits breakers specially designed for solar PV in the current range of off-grid systems. Special rapid melt fuses have been developed for solar PV applications, but these still take several seconds to operate. Fig. 2 shows the operate curve of a typical solar protection fuse. At a current of 150% of the rated current the fuse takes 20 s to operate. This will provide a short time disconnect, but will still allow the overcurrent to flow for an appreciable time.

PV panels are not only a current limited source but a variable current source. Under conditions of low radiation the short circuit current may be as low as 25% of the peak radiation short circuit current, which may be insufficient  to operate the OCPD. Designing protection must take into account the average radiation available at the site.

Single string earth or short circuit faults will result in a short circuit current equal to the Iscof the module. Many codes specify the rating of the OPCD at 125% of the Isc to allow for radiation levels higher than the standard values

An earth fault occurring within a string , as shown in Fig. 4 will result in back-current flow from other strings and the fault current can  be much higher than the short circuit current of a single module. The OPCD at the end of a string protects against this and current from other strings will ensure operation of the OPCD.

Protecting every module with an OCPD can be an expensive exercise and can even increase the risk of failure. The most common practice seems to be to protect each string at the point of highest current i.e. at combiner boxes’ inverter inputs.

Fig. 4: Back-current flow in strings connected in parallel.

Fig. 4: Back-current flow in strings connected in parallel.

Battery protection

The storage battery will form a major component of an off-grid system and will also be the main source of short circuit current. A battery does not stop operating when it is disconnected but remains a source of energy at its terminals. Multicell (or multiblock) batteries can be subject to earth faults on inter-cell connections, as well as external faults. The battery is capable of producing high currents on short circuit and of maintaining arcs for long periods, which could result in the ignition and maintenance of fires in the vicinity. Typical here is the experience of a telephone exchange in the US where the DC supply was located at the rear of the building. A DC fault resulted in an arc which caused a fire between the rear of the building and the entrance. Firefighters were able to switch off the AC supply but the DC supply was maintained by the batteries which kept feeding the arc and made it difficult to extinguish the fire.

On short circuit the battery current will rise to levels of the order of ten times its Ah rating [2]. For a battery with a 10 hr current rating of 10 A (100 Ah), the fault current could rise to 1000 A or higher.

Consider a 50 kW system operating at 500 V, with a normal current of 100 A. Ten hours of battery capacity would give an Ah rating of 1000 Ah and a potential short circuit current of 10 kA. 500 V at 10 kA is sufficient to sustain a healthy arc. For a solar installation on the roof of a building this could result in considerable damage. It is common to protect a multiblock battery string with a fuse in the centre of the string. This provides some protection against internal faults.

Batteries require special protection from overcurrents which could damage the battery. OCPDs which operate within a few seconds at overcurrents of three to four times the rated current are needed.

Arc faults

Arc faults have been experienced on both rooftop and ground based systems. In several cases the arc faults have resulted in fires, causing damage to both the PV array and the building . Arc faults are not limited to large installations. Two types of arc fault have been observed :

  • Series faults, where there is a break in the conductor string and an arc develops across the break. With voltages of 600 V and higher becoming standard, the potential for the development of arc faults increases.
  • Parallel or ground faults. This is where an arc develops between two conductors or a conductor and ground. There is a potential for higher currents on a parallel fault than on a series one and may be more difficult to extinguish.

Part of the problem is that, like batteries, PV panels do not shut off when the load is disconnected, but continue to produce power as long as the sun is shining on them. An arc developing within the wiring of an array may be very difficult to extinguish. Devices have been developed which detect the high frequency or noise current component of the arc rather than an increase in current [3]. Some manufacturers provide inverters with integrated arc fault detectors [4]. The American wiring code requires arc fault detection on all circuits operating above 80 V DC. Other national codes have similar requirements. The requirements for South Africa are uncertain at the time of writing.

AC protection

On the AC side protection is required against:

  • Electric shock
  • Short circuits and earth faults
  • Overcurrent

Overcurrent, short circuits and earth faults

In a grid connected PV system, current to operate OCPDs is provided by the grid, and normal protection practices and devices can be used. In an off-grid system, the inverter has to supply the current to operate OCPDs. As inverters are current limited and current limiting devices, this necessitates special care in the design of a protection system, and can cause problems with the operation of protection devices. Operation of the OCPD will depend not only on the magnitude of the fault but on the loading of the inverter at the time of the fault, and under different load conditions, different conditions may result. Operation of the protection circuits will depend on the operation of the inverter under overcurrent and short circuit conditions.

Inverter internal protection

Inverters are generally equipped with internal short circuit and overcurrent circuits to protect the output electronics. This limits the overcurrent to 110 to 150% of the normal maximum current, depending on the make of inverter, and the inverter will shut down in a period ranging from one to ten output cycles (20 to 200 ms). In larger inverters the shut down time may be as short as 1 to 2 ms. There are two types of inverter protection: fast disconnection (i.e. in less than one cycle), and with continued operation for up to ten cycles [5]. The overcurrent trigger level can vary from 110 to 120% depending on the inverter.

The response of an inverter-fed protection circuit will depend on the loading of the inverter at the time of the fault. This can be understood by the following example:

Fig. 5: Inverter-fed AC installation.

Fig. 5: Inverter-fed AC installation.

Fig. 5 shows an off-grid system providing a single phase supply of 80A capacity: a normal domestic connection arrangement. The output of the inverter is fed via distribution board to several circuits, protected by OPCDs of different ratings. Consider circuit B protected by a 10 A rated device under two extreme conditions:

  • The circuit is the only one drawing current and the inverter is under very low load. An overcurrent condition of seven times the rating of the OPCD (70 A) will be sufficient to operate the device, but insufficient to cause the inverter protection to operate, and the protection will behave as normal.
  • The inverter is fully loaded at the time of the fault. An overcurrent condition of five times the rating of the OCPD (50 A) will cause the output current of the inverter to attempt to rise to 130 A, or >160% of the inverter capacity, which will cause the inverter protection to operate, possibly before the OPCD has time to operate. Lower fault currents will result in even slower operation of the OPCD, but with operation of the inverter protection. An overcurrent of three times the rating will raise the inverter output to 110 A, or 130% of the rating which will operate the inverter protection before the OPCD operates. This results in operation of the inverter protection but not of the OPCD, which could lead to an unsafe situation, as it is not possible to identify in which circuit the fault is located. Many safety codes require that a fault be identified before the supply is reconnected. An inexperienced or untrained person may be exposed to potential danger while trying to locate the fault using unsafe methods.

The characteristics of the inverter protection system need to be taken into account when selecting OCPDs for off- grid systems, and coordination of devices needs to be done carefully.

Electric shock

Normal residual current detection (RCD) devices do not require a large current source for operation and can be used. Alternatively, a number of manufacturers have incorporated RCD devices in their inverters.

References

[1] Cooper Bussman: “Photovoltaic system overcurrent protection”, 2009, www.cooperbussmann.com/pdf/9df1f7ec-8c62-4210-8cf8-9504927394f0.pdf

[2] RL Nailen: “Battery protection – where do we stand”, IEEE transactions on industry applications, Vol. 27 No 41, July 1991.

[3] S McCalmont: “Low cost arc fault detection and protection for PV systems”, NREL, 2013, www.nrel.gov/docs/fy14osti/60660.pdf

[4] SMA: “PV arc fault circuit interrupter”, www.sma-america.com/en_US/news-infos/resource-center/arc-fault-circuit-interrupter.html

[5] E Muljadi, et al: Dynamic model validation of PV inverters under short-circuit conditions”, IEEE Green Technologies Conference Denver, Colorado April, 2013.

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