Voltage bands are predefined voltage ranges in which the components of a DC network provide different functionality. The IEC technical report 63282 has defined 7 bands. Each band Bx is defined by an upper voltage limit Ux. In the nominal band from 600 V to 750 V (for controlled infeed, AIC), the components operate continuously.

 

The applications do not work in this band below voltage U1 = 400 V. Therefore, except for pre-charging, the DC grid is only in this range for a short time (e.g. in the event of short circuits). When the voltage drops below U1, the Smart Breakers open.

Voltages between 400 V and 485 V (for uncon-trolled rectifiers on 400 V AC) resp. between 400 V and 600 V (for AIC and for uncontrolled rectifiers on 480 V AC). 
This band is characterized by a large overload. It is imperative that the load is reduced so that the DC grid does not change to band B1. Devices are allowed to reduce their power. Due to the DC voltage being too low, AICs may only be operated here for a few milliseconds.

• 600 V to 750 V (AIC or uncontrolled rectifiers on 480 V AC)
• 485 V to 750 V (uncontrolled rectifiers on 400 V AC)

This is the normal operating range of the DC grid. It is characterized by a power balance between local generation (e.g. from PV systems), feed-in from the AC grid and consumption of the loads. All equipment in the grid is designed to operate continuously in this range. Limitations of the device performance can result from physical boundary conditions, such as the state of charge of storage devices, the level of the mains voltage for supply devices, insufficient motor voltage or the solar irradiation for photo voltaic.

Voltages between 750 V and 800 V.
In this band, measures must be taken to prevent the shutdown threshold of the devices (800 V) from being reached. This can be done by feeding back into the AC grid, charging of energy storage devices but also switching-on power resistors. This state should not last longer than 60 s and devices may reduce their power in this state. This band is not yet included in TR 63282.

Voltages between 800 V and 880 V.
Overvoltage can occur due to switching operations, faults and braking of drives. A temporary overvoltage of up to 880 V for one minute is not problematic for common 400 V electrolytic capacitors (two in series). In the case of commonly used 1200 V semiconductors, however, the limit of voltage resistance may be reached by internal switching actions; such devices may lose their function in this range in order to protect themselves. Overvoltage protection devices (SPDs) do not yet limit the voltage in this band. If the voltage is in this range for a longer time (> 5 s) the upstream Smart Breaker opens to protect all devices.

Voltages between 880 V and 1200 V.
In this area, the surge protection devices (SPDs) operate and try to protect devices from transient overvoltage caused by switching operations but also lightning strikes.

Voltages between 1200 V and 1500 V.
If the voltage is in this band, damage to the equipment is likely. Due to the damping properties of cables and input filters, short-term transient overvoltages do not usually cause damage to components in the devices.

Voltages higher than 1500 V.
In this voltage band, damage to the equipment (varistors, semiconductors, capacitors) is highly probable.

The basic safety standard IEC 61140 describes the protection measures up to DC 1500 V (and AC 1000 V). See here

An arc is a hot, conductive plasma, similar to a lightning bolt.   

There are two types of arcs: 

• Deliberately generated arcs, e.g. during electric welding or when opening switching contacts.
• Fault arcs caused by a fault, e.g. cable break, insulation fault, foreign parts in switch cabinets.

With DC, there are no current zero-crossings; therefore, additional measures must be imple-mented in the protective devices. In mechanical switches, these measures are, for example, magnetic fields and larger air gaps. Power electronics and hybrid switches are further options for preventing arcing in switchgear.

This cannot be answered in general – it depends on the specifications of the particular switch. Often AC switches are also approved for a certain DC voltage range according to the specific manufacturer's specifications and product standards.

The grounding reference is realized via the star point grounding at the transformer of the AC grid. The operational grounding takes place centrally at the transformer.
 

In the DC IT grid variant, the DC grid is galvanically isolated from the AC grid via an isolating transformer. The reason for this is not, as with a classic IT grid, that the DC IT grid should continue to operate after an initial ground fault, but rather to simplify the parallel switching capability of power supply units. 
In this DC IT grid, no further operation is permitted in the event of an earth fault; the fault location must be disconnected as quickly as possible by a circuit breaker (<10 s).

 

 

With this type of grounding, the midpoint of the DC voltage is earthed with low resistance. For this purpose, the supply unit can, for example, generate two DC voltages galvanically isolated from the AC mains, which are connected in series and whose midpoint is earthed with low resistance. In addition to the galvanic isolation, bipolar protection is required as with AC-side earthing.

 

 

Systems grounded on the AC side are recommended for systems installed in the vicinity of an AC hall (brownfield). IT systems are suitable for new installations as long as the insulation resistance is high (greenfield).
For extended installations with low insulation resistance, the center-point grounded DC-TN-S system is recommended.
The TT system with two different grounding points is not suitable because stray currents occur in the event of potential differences between the grounding points, which can lead to increased corrosion.

This is suitable for new plants and uses all the advantages of the DC systems such as energy efficiency and availability as described in question 1.

This can be easily and quickly integrated into existing plants for replacement or expansion. The new or replaced plant section is connected to the AC system via an active infeed converter (AIC).

Such devices are integrated in industrial DC grids as small consumers via suitable power supplies (DC/AC or DC/DC converters).

The new and existing system can be coupled with each other, with different voltages via DC-DC converters. The voltage bands from question 3 should be taken into account here.