Inverter fuses - an overall introduction

With the rapid development of the LFP battery industry and home solar power system, high-power components are widely used, complex photovoltaic system design and diversified application scenarios have become trends, and inverter safety issues have become increasingly prominent. The inverter fuses play an important role in the safe use of the inverter. This article will give you a detailed introduction to inverter fuses.

1. What is an inverter fuse

Fuses are mainly used for short-circuit and overload protection of electrical equipment such as lines and power transformers, and are widely used in small-capacity electrical device with voltage levels of 60kV and below, and are often used to protect voltage transformers, for example, 2000w inverter or 3000w inverter. In 3 ~ 60kV systems, it is also often used with the load switch, recloser and circuit breaker to protect power lines, transformers and capacitor banks.

• Advantages: simple structure, small size, light weight, low price, easy maintenance, flexible use.
• Disadvantages: the protection performance is not very stable.

2. Structure of the inverter fuses

• Melts: When working normally, it plays the role of conducting the circuit, and in case of failure, the melt will be first peened, thus cutting off the circuit to realize the protection of other equipments. The Melts are expressed by two letters, such as "gg", "gM", "aM" and so on.
• Fuse link: Used to install and remove the fuse, often in the form of contacts.
• Base: Used to insulate and fix each conductive part.
• Melting tube: Used to place the melt, limit the burning range of the melt arc, and extinguish the arc.
• Filling: Generally solid quartz sand is used, to cool the arc.
• Fuse indicator: Used to reflect the status of the flame body, that is, indicating whether it’s intact or blown.

Classification of melts Melts can be divided into high melting point melts and low melting point melts. Low melting point materials such as lead, zinc, tin, etc., high melting point materials such as copper, silver, etc.

According to the range of breaking current, it can be divided into: “g" melt with full range breaking capacity and "a" melt with partial range breaking capacity. According to the use category, it is divided into: "G” type melt for general use and "M" type melt for motor protection.

The advantage of high melting point materials (silver, copper, aluminum) is low resistivity. At a certain resistance, the required cross-sectional area is smaller. There is less metal vapor during melting, which is beneficial to arc extinguishing and its breaking capacity can be improved.

It’s generally used for high current inverter fuses. However, due to its high melting point (960℃, 1083℃), it will lose protection against small overloads. To this end, the "metallurgical effect" is often used, that is, a small solder ball is soldered in the pin wire. When the melt temperature rises to the temperature of the small solder ball, the inverter fuses are blown, so that the protection for the lower overload can be achieved.

3. How inverter fuses work

The working process of inverter fuses is divided into the following four stages:

• The melts of the inverter fuses is heated to the melting temperature due to overload or short circuit;
• Melting and gasification of melt;
• The gap between contacts breaks down and generates arc;
• The arc is extinguished and the circuit is disconnected.

4. Protection characteristics of inverter fuses

The relationship between the melting time of the inverter fuses melt and the magnitude of the current is called the ampere-second characteristic of the inverter fuses, which is also called the protective characteristic of the inverter fuses. The protection characteristic of the inverter fuses is an inverse-time protection characteristic curve.

The rule is that the fusing time is inversely proportional to the square of the current. The protection characteristic curves of various types of inverter fuses are different and are related to the structural type of the inverter fuses.

0∞ is called the minimum melting current or critical current. The rated current IRN of the melt should be less than I∞. The ratio of I∞ to IRN is called the training coefficient, which is usually 1.5~2. This coefficient reflects the different protection characteristics of the fuse during overload.

5. Main technical parameters of inverter fuses

• Rated voltage: The normal operating voltage that the inverter fuses can withstand for a long time, that is, the rated voltage of the power grid where it is installed.
• Rated current: The long-term maximum operating current allowed to pass through the fuse housing part and the current-carrying part.
• Rated current of the melts: The maximum current that the melt is allowed to pass through for a long time without melting.
• Limit breaking current: the maximum short-circuit current that the inverter fuses can break.
• Selectivity: Inverter fuses are used as protective appliances in distribution trunk lines and branch lines. If the branch line is overloaded or short-circuited, the branch line fuse is required to melt. The main line fuse will not melt.
• Overcurrent selection ratio: The stipulated selection ratio is 1.6:1 to 2:1, that is, if b chooses 100A, choose 160A. If b chooses 100A, choose 200A and above.

The technical parameters of the inverter fuses also include: rated breaking capacity, current type, rated frequency, breaking range, usage category and shell protection level, etc.

The technical parameters of the inverter fuses should be divided into the technical parameters of the fuse (base) and the technical parameters of the melt. Fuse bases of the same specification can be installed with melts of different specifications.

The rated current of the melt can be different from the rated current of the inverter fuses, but the rated current of the melt must not be greater than the rated current of the inverter fuses. The rated current is expressed in this form: rated current of the fuse base/rated current of the melt.

6. Difference between fuses, circuit breakers and isolating switches

① Working principle

• Inverter fuses:

The metal melt of the fuse is a conductor that is easy to melt. When an overload or short-circuit fault occurs in the circuit, the current through the melt increases, and the overload current or short-circuit current heats the melt.

Since the melt's own temperature exceeds the melting point, the melt melts before the temperature of the protected equipment reaches the point where it destroys its insulation. It will cut off the circuit to protect the electrical equipment in the line.

• Circuit breaker:

A mechanical switching appliance that can connect, carry and break current. It is composed of contact system, arc extinguishing system, operating mechanism, release, etc.

When a fault occurs, the fault circuit can be cut off for protection. In the situation of a short circuit, the magnetic field generated by the large current overcomes the reaction spring, and the release pulls the operating mechanism, causing the switch to trip instantly.

In case of an overload, the current becomes larger and the heat is intensified, and the bimetallic sheet is deformed to a certain extent to push the mechanism to move, causing the switch to trip.

The greater the current, the shorter the action time. The circuit breaker has strong arc-extinguishing ability, can break large short-circuit currents, and can protect multiple times. The overall protection characteristics are not affected by time and environment.  

• Isolating switch:

It mainly plays the role of isolation, so that there is an obvious disconnection point between the electrical equipment to be inspected and the power supply, ensuring the safety of the inspection work. In the past, the DC side of string inverters had manually operated manual isolation switches. As photovoltaic power plants become more intelligent, the manual shutdown method is upgraded to electric shutdown.

• Electric isolating switch:

Electric disconnection is realized through the inverter current transformer detection of string current + DSP signal control, and manual is upgraded to electric. Now 100-300 times of disconnections can be achieved at 1 time the current, and only 5 times of disconnections can be achieved at 4 times the current.

However, the upgrade of the manual isolating switch to the electric isolating switch is only an upgrade of the shutdown method and cannot be used as a protection device. The maximum heating current of the switch body is 50A.

When used within the rated current of 40A, it can exert its rapid breaking ability. It should be noted that in the case of multiple sinks (such as 5-way string connected to 1-way MPPT), the current will flow backward. If the temperature exceeds 50A, the internal heat of the switch is not easily dissipated, and problems such as deformation and jamming of the internal moving contact bracket are prone to occur.

In summary, inverter fuses and circuit breakers are both protective devices, which do not rely on external circuits and achieve reliable mechanical protection through physical effects; isolating switches are isolation devices, and whether electric isolation can be disconnected reliably depends on detection and control.

When the system is powered off or the control circuit fails, the performance of the isolation switch is limited and there is a risk that it cannot be disconnected.

② Protective effect

Above we compared the differences between fuses, circuit breakers and isolating switches. So what are the risks if an electric isolating switch is used alone without deploying fuses, circuit breakers and other protective devices? From the perspective of best solar inverter safety design solutions, different types of design solutions have different overcurrent consequences:

• One MPPT is connected to two strings of components: After a fault occurs, the maximum short-circuit current or backflow current in the system is only 1 time the component short-circuit current, and the isolation switch cannot be disconnected without affecting system safety. 
• One MPPT is connected to three strings or more components: Take one MPPT connected to five as an example. After a fault occurs, one circuit is short-circuited and the current of the remaining four strings will flow to the short-circuit point. The faulty circuit will withstand 4 times the current. If the isolating switch cannot be disconnected, related components will be damaged due to continued overcurrent until a fire occurs.

We can see that electronic isolating switches cannot be equated with circuit breakers. Electric isolating switches and circuit breakers have different principles, functions, and compliance standards, and cannot replace each other. The isolation switch can be used as a redundant device in combination with protection devices such as circuit breakers to improve system protection capabilities, but it cannot replace fuses or circuit breakers and be used alone as a protection device.

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