Photovoltaic electronic system circuit protection design

The global demand for green energy has driven strong growth in the solar energy system market. Although a lot of development work is still focused on more efficient photovoltaic (PV) energy conversion, there is also an urgent need for more reliable, efficient and cost-effective solar energy transmission. The recent release of the "SunShot" action plan by the U.S. Department of Energy (DOE) highlights this need, which aims to reduce the overall cost of utility-scale photovoltaic energy systems by about 75%, making them more cost-effective than other power generation technologies.

Cutting-edge circuit design

The responsibility for practical photovoltaic electronic system development ultimately falls on the shoulders of circuit designers, including designers of companies that develop complete solar energy systems, designers of system integrators that provide "turnkey" systems to end users, and designers of various solar energy subsystems. Many of these designers are responsible for developing circuits that are primarily used to optimize the performance and cost of photovoltaic system installations. The circuits designed by these engineers are generally solar arrays, DC combiner boxes, or inverters.

Solar systems involve relatively new technology, so PV system designers often have extensive experience developing different types of electrical and electronic systems. For example, a company that now produces small solar inverters may have previously manufactured power conversion or UPS systems. In this new position, these new PV system designers may be asked to design a 1MW DC scale solar circuit connected to the grid. Designing circuits and specifying components for these high voltage solar applications is very different from the same tasks formulated when developing other DC power systems or even high power AC applications.

Basic circuit protection requirements

One area where designers can run into trouble is in selecting circuit protection devices for solar circuits. These circuits may be used in a wide variety of systems, ranging from residential applications to large industrial facilities and even grid-connected solar farms. In all of these systems, there are many locations where circuit protection devices are needed (Figure 1). Many application notes provide circuit protection device selection guidelines for AC power and digital communication systems used for monitoring and control. These areas are beyond the scope of this article. This article focuses on the design of the DC side of solar systems, where circuit designers are more likely to encounter unexpected problems.

Figure 1: Where circuit protection devices are needed in solar energy systems.

In a typical solar electronic system, individual solar panels or modules are connected in series to increase the output voltage and improve efficiency. Many battery strings need to be used in parallel to obtain the required output current and final power. Depending on the system size and design details, parallel battery strings can be connected in a battery string combiner box, which is connected in parallel in an array combiner box and then connected to the inverter (Figure 2).

Figure 2: Typical solar electronics system.

In most cases, multiple battery strings and arrays are connected together using combiner boxes at an operational location. These common connection points help simplify system assembly and maintenance. Regardless of where they are used, it is necessary to analyze the circuit, determine the possible fault current (i.e. short-circuit current) of the system, and compare it with the overcurrent capacity of the components, and then install appropriate circuit protection devices to prevent damage to photovoltaic modules, disconnectors, wiring and wiring equipment.

DC and AC circuit protection

Circuit breakers are often the preferred protection solution for the AC side of a solar system, and it may be tempting to use the same circuit breaker on the DC side. While the circuit breaker solution is generally convenient, it is not always the best approach. Designers must carefully determine whether the circuit protection devices used on the DC side of the solar system are designed according to the relevant photovoltaic standards and have been tested and certified to the standards by external organizations such as Underwriters Laboratories (UL) or VDE to ensure that the devices can operate normally in the event of a fault. It is much more difficult for circuit protection devices to interrupt DC voltage than to interrupt the equivalent RMS AC voltage. The fundamental reason for this is that the AC voltage reaches the zero voltage point twice in each voltage cycle, which is a key factor affecting the circuit protection device to safely interrupt the voltage and isolate the fault circuit.

Since solar PV panels generate DC power, the current and voltage are stable for a given amount of light energy received by the PV panels. Due to the presence of high voltage DC currents, typical circuit protection devices have difficulty reliably interrupting the circuit in the various operating conditions that may occur in solar systems. In the worst case, circuit protection devices that are not designed and certified for DC PV systems may suddenly fail and cause equipment damage, fire, or even personal injury. However, the most common problem is that the device does not operate fast enough under typical PV system overcurrent conditions.

For example, in a battery string, the short circuit current (ISC) may not be much greater than the normal current. A typical solar cell string might have an output current of 4.2A in normal operation, and its forward ISC is about 4.5A. When combined with other battery strings in a small 450VDC 10kW system, the short circuit current that a normally sized 10A overcurrent protection device (OCPD) is required to interrupt in the event of a battery string fault is about 20A. These high DC voltage, low overload conditions are a difficult challenge in designing cost-effective OCPDs, as they need to interrupt the circuit within the appropriate voltage, current and humidity ranges.

For these reasons, the most common first line of defense is an OCPD in the form of a fuse (Figure 3). Fuses, which are inherently passive devices, cost less than circuit breakers with the same performance characteristics. These PV system fuses and their testing are described in UL Standard 2579 (Low Voltage Fuses for Photovoltaic Systems) and IEC Standard 60269-6. These fuse standards are developed in conjunction with the PV panel standards UL 1703 and IEC60129 and the inverter standards UL1741 and IEC61727.

Figure 3: Block diagram of a typical battery string combiner box with fuses and other routing components.

Depending on the specific application and system design, the DC battery string voltage is generally in the range of 300V to 1000V, but it may be as high as 1500VDC in systems connected to the grid. Therefore, it is necessary to select appropriate fuses, disconnectors, routing devices, etc. for the combiner box. In addition, UL and IEC standards have special performance requirements for OCPDs used in these applications.

When the OCPD is a fuse, it must be able to protect the PV source circuit operating at its short circuit current rating and also protect the PV source circuit in the event of a circuit fault. The NEC article defines fault current as 125% of the PV current ISC plus any reverse or feedback current in the opposite direction of the normal current.

Generally speaking, the source of reverse current during a fault may come from the backfeed current (IBACKFEED) of other battery strings in the affected array, see Figure 4. It can be approximated by the formula Isc x (n-1), where n is equal to the number of battery strings in the affected array. UL1703 and IEC60129 define PV cell test specifications to ensure that the panel will not experience dangerous overheating conditions when the backfeed current is equal to or less than 135% of Istring fuse x for 2 hours. The UL PV fuse standard later defined the PV fuse open circuit characteristic as a current not exceeding 135% of Istring fuse x for 1 hour. This ensures proper coordination when using UL or IEC panels with UL fuses.

Figure 4: Feedback current due to a fault.

Learn more about low current interrupting and UL and IEC fuse standards#e#

Learn more about low current interrupting and UL and IEC fuse standards

Most circuit designers naturally equate the label rating of a circuit protection device with the load current value that will cause the device to open. However, this is not always the case in circuit protection devices designed to interrupt DC voltages, high AC voltages, or provide extreme short-circuit current limits. Because interrupting high-energy faults is particularly challenging, circuit protection designers often have to sacrifice low overload protection capabilities. In most applications that require this type of circuit protection, such as UPS systems or variable frequency drives (VFDs), this is an acceptable compromise because the UPS or VFD system uses microprocessor or solid-state type control circuits to detect and interrupt these low overload currents. It is very cost-prohibitive to use this type of low overload protection circuit on tens, hundreds, or thousands of nodes in a solar photovoltaic system, so designers use individual OCPDs.

As long as the OCPD is designed and certified as a full range fuse, this is a perfectly acceptable design. A full range fuse is defined as any fuse designed to interrupt currents between 110% of the UL marked rating and 113% of the IEC and marked maximum interrupting rating. For UL this includes all listed fuses and some recognized fuses. Care must be taken when using a recognized fuse to ensure that it is also a full range fuse. For IEC this includes all fuses with a characteristic name beginning with "g" (such as gPV and gR). For DC photovoltaic circuit protection, fuses with a characteristic name beginning with "a" are unacceptable and should not be used.

Other photovoltaic system circuit protection issues

In addition to the important coordination and full range protection requirements of solar panel and string protection devices, UL and IEC standards also address other unique electrical characteristics of solar PV systems, such as different environmental conditions and high-level current cycles.

Solar energy systems often operate in harsh outdoor environments, where temperature conditions may cause thermal shock. Temperature cycle tests such as those mandated by UL2579 and IEC60269-6 help ensure that no significant temperature drift (aging characteristics or other performance drift) associated with fuse operation occurs. UL and IEC requirements for temperature cycle testing will further limit the fuses that can be used in solar energy systems.

Solar systems use high voltages to efficiently transmit energy, and the design requirements are fundamentally different from 120V or 240V designs. When designing protection circuits and other components of solar systems, robust, long-life performance requirements need to be kept in mind. Although a five-year life is acceptable for DC power supplies in consumer electronic devices, it is completely unacceptable for solar systems, where the expected life is often up to 25 years. Remember that electronic devices work outdoors, are exposed to high and low temperatures, and may also withstand ESD surges caused by nearby lightning strikes. Therefore, it is important to select components that are robust—from circuit board traces to bus bars to mechanical components. The enclosure should be both rugged and waterproof. Surge suppression devices should be installed in appropriate circuit locations.

Engineers are often under pressure to provide cost-effective designs, but taking shortcuts in PV system development can easily lead to problems. For example, it may seem attractive to use a circuit breaker to combine the disconnect and OCPD functions within the PV system combiner box, but using properly certified components may result in a non-optimal cost structure, while using cheaper components may lead to safety and reliability issues. In most cases, the cost of fuses, fuse clamps, and stand-alone disconnects will have a lower initial cost and lower maintenance costs when needed. To ensure proper operation and avoid damaging the manufacturer's reputation, many circuit breaker manufacturers require annual testing and recalibration of their products. The circuit breaker must be removed from service, cooled, and then recalibrated according to manufacturer instructions. This annual maintenance requirement adds considerable expense, difficulty, and safety hazards.

This article has focused on the protection circuits required on the DC side of a PV system. However, as shown in Figure 1, other circuit protection devices are required in many other locations. System designers also need to protect other components in the system from transient overvoltages, ESD, and AC overcurrent. Fortunately, the use of devices such as MOVs, TVS diodes, and AC fuses to prevent these threats has been typical of other systems that are properly designed and protected for decades. These applications are generally no different in PV energy systems, but safe and careful design work is still of great value.