DC wiring (part 2)

All the necessary information for a correct wiring installation

Isolation switches

A battery isolation switch can be used to isolate the battery (or battery bank ) from the rest of the electrical circuit. Or to isolate a DC electrical source or consumer from an electrical circuit. Being able to isolate the battery or DC power consumers is useful if the system is not going to be used for a while or for maintenance.

When selecting an isolating switch, ensure that the switch’s current rating is adequate for the currents that can be expected in the system at full load.

Battery isolation guidelines and standards vary by country, but it is recommended that if isolation is required, only the positive battery cable be isolated.

It may not even be necessary to add an isolation switch. A DC system always has a main fuse. If the fuse is removed, the circuit will also be broken. So when maintenance work is required on the system, or if the battery needs to be replaced, simply removing the main fuse is sufficient to isolate the battery from the rest of the system.

Always use quality isolation switches. The isolation switch will add resistance to the system. A poor quality switch will have more resistance, which can increase voltage drop and cause system problems.

Types of isolation switches:

  • Battery isolation switch for mobile systems (normally 12 and 24 V).
  • DIN rail mounted circuit breakers for terrestrial systems for batteries and PV (typically 48V and above).
  • NH fuse holder switch for high current terrestrial systems for batteries and PV (typically 48V and above). 

Negative switching in multi-unit systems

In an inverter/charger system, the battery isolation switch is only located on the positive battery cable. But in some installations, it may be necessary to switch both positive and negative. This may be required by local regulations.

Switching both the positive and negative battery power cables is not a problem in a single Victron inverter/charger system. However, it can present problems in systems with multiple inverter/charger units connected in a parallel and/or 3-phase system.

In a parallel and/or three-phase system, each of the system units needs to communicate with the others through a communication cable that interconnects the VE. Bus connectors. Some of our inverter/charger models do not have galvanic isolation between the battery and the VE. Bus. The absence of insulation means that if the battery negative connection breaks in certain situations, a current would appear in the negative signal of the data cable, which can damage the inverter/charger communication chip.

In a system with more than one inverter/charger, if the negative has to be switched, follow this recommendation:

The battery negative connection of each unit must be connected to the negative connections of the other units. 

Only when the common negative is in place, the VE.Bus RJ45 cables can be connected to the units. 

When removing a unit from the system, all RJ45 cables must be disconnected before removing it.

Derivative

A shunt is added to the system to measure current flow. This is necessary to monitor the system or to calculate the state of charge of the battery.

A shunt is a resistive element. When current flows through it, there is a small voltage drop. If the current is small, the voltage will be low, but the voltage will be higher if the current is high. If the current flow is reversed,

the voltage drop will change the polarity. The shunt voltage is an indicator of the amount of current and its direction. This information can be used to find out how much current is passing through a system or to calculate the state of charge of the battery.

Shunts have a nominal current and voltage, for example, 500 A and 50 mV. This means that if a current of 500 A passes through the shunt, there will be a voltage drop of 50 mV (= 0.05 V) in it.

The shunt must have a nominal current that adjusts to the maximum DC current that will pass through all the electrical consumers in the system together.

DC wiring of a parallel and/or three-phase system

A large inverter/charger or 3-phase inverter/charger can be created by connecting multiple inverter/chargers. These units communicate with each other, and together, they make a great inverter/charger. All must be connected to the same battery bank. When wiring an installation like this, there are some important considerations regarding battery cables.

For proper operation, it is essential that each unit receives the same voltages. To ensure this, the DC path from the battery bank to each unit, or from the busbar to each unit, must be exactly the same.

If there is any difference in the thickness or length of the wires between the units, there will be a difference in the voltages of these units.

Different voltages imply different currents. The unit with the lowest voltage will have a greater current going through its electronics. The intensity of this current triggers the inverter/charger overload. So even though the power provided by each inverter is the same, the unit with the lowest voltage will have a higher current going through it and will go into overload before the other units. The system’s total inverter power will then be lower because when one unit goes into overload, the entire system stops working. The improperly wired unit will determine the performance of the entire system.

To achieve a balanced system, you will need to use the same type of power cable, with the same section and the same length, from the battery bank or from the connection bars to each unit. Also, make sure that the terminals of all cables are identical and that all connections are tightened with the same torque value. Consider using busbars or power terminals between the battery bank and the inverter/chargers.

When fusing the installation, consider using only one DC fuse per phase. If you cannot have a single large fuse, use one fuse per unit, but make sure they are all exactly the same.

To check if the wiring of a system is correct or to troubleshoot the wiring, do the following:

  • Charge the system up to the maximum charge. 
  • Attach the current clamp to the DC leads of each unit. 
  • Compare current readings, all units should have similar DC currents.

You can also measure the voltage on the busbar or battery bank and compare it to the voltages you have measured at each unit’s battery terminals. All of these voltage readings should be identical.

Large System Connection Bars

Large installations usually consist of several electrical consumers and several DC sources, such as several batteries, several inverter/chargers, and several MPPTs. They all connect to a central busbar. When making connections for these facilities, you need to consider several things.

In these systems, you will have to use bus bars, but it is still important how each piece of equipment is connected to the bus bar and in what order. It is important to connect the inverter/chargers and the MPPTs to the busbar alternately. This will reduce the current through the busbars. Simply put, current entering the busbar from an MPPT can go a shorter way directly to the inverter or the battery. This current does not need to pass through the entire busbar. This keeps local “traffic” low.

When making connections, make sure all inverter/chargers have the same cable length. Also, MPPTs must have cables of approximately equal lengths. And the same for batteries.

If your system has only one battery bank, you should connect the battery bank in the middle of the busbars. But if there are multiple battery banks in parallel or smart batteries, they also need to be evenly distributed along the busbars.

Voltage detection and compensation

Voltage detection is a feature of the battery charger. It works by measuring the difference between the unit voltage and the voltage at the battery terminals. As soon as a difference is detected, the load voltage is increased to compensate for cable losses during loading. This ensures that the batteries are always charged to the correct voltage. This feature will normally only compensate for voltage drops of up to 1 V. If the system losses are greater than 1 V (for example, 1 V on the positive connection and 1 V on the negative), the battery charger, the controller MPPT solar charging or the inverter/charger will reduce their charging voltage such that the voltage drop remains limited to 1 V. The reason this is so is that if the losses are greater than 1 V.

Voltage sensing can also be used to compensate for voltage drops when using diode separators. A diode separator has a voltage drop of 0.3 V across the diode.

Suppose the product has a voltage sense terminal (V-sense). In that case, two sense leads can be directly connected from the V-sense terminal to the positive pole of the battery and the negative terminal of the distribution. Use a wire with a section of 0.75 mm².

If the inverter/charger is equipped with the VE. Bus smart dongle, the voltage sense cables are not required. The Smart dongle takes care of voltage detection. If it is an MPPT charger, connect a Smart Battery Sense sensor to the battery and pair it with an MPPT.

Voltage sensing in a solar DC energy storage system (ESS)

In an ESS system with an MPPT, the inverter charger/charger is disabled. This is because the MPPT charges the battery, and the excess solar energy is returned to the grid. The CCGX controls this process. For this to work, the CCGX will set the MPPT to a higher DC voltage than the inverter/charger.

When the battery is almost full, the voltage across the battery will be slightly higher than the inverter/charger’s DC voltage. This is the “signal” that the inverter/charger uses to reduce this “surge.” To do this, it returns

energy to the grid. In a 48 V system, this overvoltage is fixed at 0.4 V, and in a 24 V system at 0.2 V.

For this process to work properly, it is essential that the battery receives the correct voltage from the MPPT. Special attention must be paid to the design and placement of DC wiring, fuses, and connections, as they could cause a voltage drop in the system.

A voltage drop can reduce the “surge” that the inverter/charger needs before returning power to the grid.

The total resistance is 1.78 mΩ, and the voltage drop at 100 A is 178 mV.

The solution is to use an MPPT with automatic voltage drop compensation. As a result, the MPPT output voltage will rise slightly with increasing current. But if the MPPT does not have voltage sensing, it is better to connect the MPPT directly to the MultiPlus.

Solar panels

The solar panels can not be connected directly to a battery. A solar charger needs to be placed between the solar panels and the batteries. The solar charger converts the solar panel voltage, which is higher, into a suitable voltage for charging batteries. If a solar panel is connected directly to a battery, the battery will be damaged.

In most cases, the solar panel has special waterproof connectors, usually MC4 connectors, to connect solar panels to a solar charger. There are two types of these connectors: male and female.

The male connector connects to the positive cable coming from the solar panel and the female connector connects to the negative cable.

If the solar panel cables are not long enough, it will be necessary to use an extension power cord. The extension cord usually has MC4 connectors already mounted. Solar cables have a male connector on one end and a female connector on the other.

MC4 connectors can be attached to a 4mm² or 6mm² solar cables.

A solar cable is a special cable. It is a very resistant cable designed for use outdoors in solar panel installations. It is resistant to dust, aging, and UV radiation and has tinned copper wires.

Solar cables for small PV arrays, such as automotive or marine applications, are typically dual-core. In these applications, the cables also have to be resistant to UV radiation and have tinned copper wires.

The thickness of the cable required will depend on the size of the solar array and its tension. This will determine the current and the thickness of the wire. More information on this can be found in the section on selecting the appropriate wire.

Two models of MPPT solar chargers are sold, with MC4 connectors or with screw connectors on the PV face. This is how they connect to a solar panel viewed from the back of the solar panel.

Sometimes the solar panel does not have cables. So you will have to put them yourself. To do this, open the junction box on the back of the panel and connect the cables there. You can use solar cables with or without MC4 connectors. If you are connecting the solar panel directly to the MPPT.

In many solar installations, a single solar panel is not enough. In this case, it is necessary to mount a solar or photovoltaic (PV) set. A solar array is made up of several solar panels connected to each other.

If the solar panels are connected in series, the voltage increases, and if they are connected in parallel, it decreases. The same happens when building a battery bank with independent batteries.

To facilitate parallel connections, MC4 connectors can be used.

To determine the total energy of the solar array, you only have to add the energy of each module regardless of whether they are connected in series or in parallel.

When designing a solar array, ensure that the array’s open-circuit voltage (Voc) does not exceed the rated voltage of the MPPT.

Previous in part 1.

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