Converter solutions for future proof DC grids made by Querom GmbH

Designing a DC supply

The selection of suitable voltage levels for the supply network is of central importance. The selected voltage should not be too high so as not to excessively increase the demands on the connected electronics and protective devices. On the other hand, it should also not be too low in order to enable the transmission of high power with low currents and acceptable cable cross-sections.

The Open DC Alliance (ODCA by ZVEI) is also working on a standardized establishment of direct current technology and offers an important tool with its "DC Industry2 System Concept". The ODCA is committed to supporting DC technology and its increased use in the energy supply.

In a DC grid, the energy supply generally has different voltage levels (Figure 2). This diversity is due to the different applications and transmission distances as well as the easier integration of renewable energies. Specially designed DC/DC converters are required to link these different sectors.

Figure 2: Different voltage levels within a DC grid and their consumers (Image: Querom).

In summary, the transition from an AC supply to a DC grid has efficiency advantages above all, but these must be exploited through the appropriate design of the DC/DC converters involved. This brings us back to the question posed at the beginning: Which type of DC/DC converter is suitable at which point?

DC/DC converter topologies for coupling DC grids

The choice of a suitable DC/DC converter topology is crucial for the coupling of DC grids - and this depends on various criteria. Basically, DC/DC converters always consist of a combination of transistors, inductors and capacitors which, depending on the wiring and specific components, have different properties such as voltage and power range, degree of insulation and energy flow direction.

Regardless of the topology, protection in the form of a DC breaker between the DC/DC converter and the DC grid is absolutely essential. A DC breaker, whose function is based on switching elements such as semiconductors, possibly in combination with relays, disconnects individual sectors from the DC grid in the event of a fault. This ensures controlled operation of the unaffected sectors. A major challenge here is arc extinction and the control of inrush currents that can occur when voltage rails are connected.

Figure 3: DC/DC converters in the high-voltage range - with DC voltage from 60 volts - are subject to special requirements in terms of heat dissipation, EMC requirements and size (Image: Querom).

Connection of end devices

The basic planning and design of a DC industrial system is usually carried out by appropriately qualified engineering firms and will not be discussed further here. In addition to the aforementioned load distribution and dimensioning, they also take into account the integration of renewable energy sources and their storage, redundancy and reliability as well as grid integration and compliance with standards.

But one point that is essential for operation remains unanswered: How can I connect my existing or future consumers, previously supplied with AC, to the DC grid? From a purely technical point of view, this is basically possible by connecting the DC voltage between two phases of a power supply unit with a three-phase input. However, this operating mode often violates manufacturer specifications and is generally not covered by the certifications/testing carried out. It is also not possible to feed energy back into the DC grid.

For this reason, DC/DC converters are required, which convert the high-voltage voltage to a usable level either decentrally directly at the consumer or centrally in a control cabinet. Various concepts and topologies of corresponding converters are examined below.

Supplying loads with 24 or 48 V DC

When converting to 24 or 48 V DC, galvanic isolation is always necessary (see title image above). While a power factor correction (PFC) with a downstream LLC converter is generally used for isolation in existing systems with AC supply, this structure can be simplified for DC supply. For converters with higher outputs, the phase shifted full bridge (PSFB) is particularly suitable.

Six to eight circuit-breakers, a coil and a transformer are required here. The bidirectional operation of this topology is possible in principle, but makes the control of the power switches considerably more difficult - in unidirectional operation, all four transistors of the primary-side full bridge clock with a duty cycle of just under 50 percent, while the control of the output parameters is realized via an adjustable phase shift between the two bridge branches.

Despite the simple modulation scheme, ZVS and ZCS are possible over a relatively wide range. Zero Voltage Switching (ZVS) and Zero Current Switching (ZCS) are techniques that reduce switching losses in power electronics. While with ZVS the semiconductors are switched at a voltage of 0 V, with ZCS this is done at a current of 0 A in order to minimize switching losses. Both methods improve efficiency and reduce electromagnetic interference.

For the power range below around 2 kW, on the other hand, the two-transistor forward topology is often a more favorable choice. In addition to a storage choke and a transformer, only two power switches and four diodes are required. The control of the switches is extremely simple and is usually implemented as pulse width modulation, with both transistors receiving the same control signal. However, this simplicity also means that ZVS/ZCS operation cannot be easily implemented.

Power electronics for regenerative loads with 24 or 48 V

For regenerative loads, bidirectional operation is required in addition to galvanic isolation and the wide spread between input and output voltage. When using a unidirectional converter, the output voltage of the converter rises at the moment when the connected drive technology feeds back more energy than it consumes. The possible level of the voltage increase depends on the amount of energy fed back and the installed capacity of the output capacitors. In the worst case, it can lead to the destruction of the output circuit of the connected power supply unit if the correct countermeasures are not taken. In existing systems, brake choppers, which convert the energy into heat, are the method of choice.

While non-isolated topologies can be implemented bidirectionally very easily, isolated topologies generally require more effort on the control side - with the exception of the flyback converter with its quasi-symmetrical input and output side. All other common topologies such as forward half bridges, full bridge and PSFB (phase shifted full bridge) converters and all their common derivatives are asymmetrical, i.e. there is usually one side with and one side without a storage choke. This makes it difficult to change the direction of energy flow, as the modulation scheme of the semiconductors has to be changed at the same time.

In the PSFB converter, for example, the energy flow control from the HV side to the LV side is achieved by phase shifting between the two bridge branches. To turn the flow direction, it is not sufficient to modulate the LV-side full bridge with the same phase shifting scheme instead. Instead, all four switches of the LV-side full bridge must be switched on in the phase without energy transfer from the LV to the HV side.

Driven by the applied LV voltage, this causes the choke current to increase in the negative direction. In order to transfer the energy stored in the choke through the transformer to the HV side, two of the LV transistors must always be alternately deactivated crosswise. When the choke current has dropped far enough, it is increased again by a subsequent phase in which all four transistors are activated.

Implementing this transition between different types of modulation significantly increases the effort required on the control side for asymmetrical converters.

 

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