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Introduction

DiVER: Direct Current low Voltage Electrical Grid for Renewable energies

• Any voltage over about 60VDC is not safe to the touch
• Any voltage over 60VDC needs a special license to operate in Germany, e.g. you may not build and connect a grid over 60V, unless a proper Electrician checks everything (VDE)
• Small permanent magnet alternators used in a generator at a wind turbine or a water turbine usually operate at below 100V
• It is very expensive to generate a true sine wave alternating current out of low voltage DC from the renewable energies
• Solar panels output low voltage DC
• TEG (thermo electric generators) output low voltage DC

Only a few systems really do need the high voltage AC of 230V or 110V.

• Desktop Computers (although terribly inefficient compared to laptops and smartphones) do need 12 VDC, 5 VDC and 3,3 VDC.
• Laptops need about 12-20 VDC
• TFT Flatscreens run on DC, especially if they have an LED backlight.
• Mobile phones, smartphones, tablet PCs etc are all charged and run with 5VDC

Difficulties and negative points about a low voltage DC grid:

• Grid size is smaller with a lower voltage or:
• available Power is lower at the same AWG / cable dimensions
• special connectors have to be chosen

A hybrid 230 VAC + DiVER infrastructure for OSEG/FeFG: 230VAC for high power appliances like some machines, DiVER for any consumer electronics (wherever possible) and efficient lighting.

Household circuit breakers may be re-used for a DiVER grid, e.g. anything from 8-63A per phase.

System voltage:

• N: 0V, GND
• L1: 12VDC, 10-15VDC (one lead acid battery)
• L2: 24VDC, or higher, to be determined. e.g. 24-48VDC (three lead acid batteries)

A cheap NYM-J 2x2.5mm² (1 Phase) or 4x2.5mm² (2 Phase) cabling may be used for low power branches of the grid.

E.g. for L2 with nominal voltage 3x12V = 36V: equals three lead acid batteries connected in series. If fully charged, grid voltage at the source would be 3*14V = 42V, at empty batteries about 32V.

The L2 grid voltage (only the voltage!) may be compatible with PoE, Power over Ethernet 802.3af (802.3at Type 1) and 802.3at Type 2, if the grid voltage is above 40 V.

At 36V and 63A, there are 2268W available in the grid, if we assume proper cables and connectors. It depends on the application, but I would have switched over to 230 VAC already at this power rating.

One could make the 12V phase on/off grid redundant with one efficient ATX power supply+two Schottky diodes or with active switching. It is much easier to switch DC synchronous vs AC synchronous, because one does not have to establish a phase lock to get the waveform in sync.

Smart DiVER

The start would be to equip an energy monitoring system, like DiVER.Wilssen. Communication could take place via

• Wireless connections (even to non-grid-tied appliances. NRF24L01+, RFM, xBee)
• Wired LAN connection (expensive and uses additional cabling)
• PLC (Powerline Communication) with an Open Source protocol and hardware design.

Research: What about RS485 between two phases, would that be possible? Is a choke / low-pass needed at the low impedance energy sources and appliances? What frequency is used best for transmitting data via a modulated power line? Coupling via passive RC-highpass? There might be an integrated circuit for low voltage DC PLC communication?

DiVER.Wilssen

The Wireless Logging System for Sourcing ENergy - Controller is monitoring some or all grid parameters and is connected with the BMS (battery monitoring/managment system) or may even contain BMS functions. Wilssen is the brain of a DiVER grid, Wilssen is recommended but not necessary for operation.

• battery voltage of each battery (multiplexed) (2s1p, 3s1p, 3s2p, 3s4p etc)
• battery bank voltage
• l1 phase current (has got to be signed for bidirectional measurement)
• l2 phase current (has got to be signed for bidirectional measurement)

• battery bank temperature sensor (OneWire preferred)
• over current protection
• under voltage protection
• Uninterruptable Power Source function.
• SSR usage, no conventional electromechanical Relays.

At a future revision, Wilssen may also:

• switch on chargers or grid-tied SMPS (switch mode power supplies) if source impedance gets to high/grid voltage too low.

optional: digital ZVD (zero voltage diode) function via comparator+ISR at ADC or interrupt attached to a plain digital port pin. SSR could then be used as OC protection, UV protection and ZVD.

SSR module may be replaced with a MOSFET at low power applications.

Wilssen Hardware

• Voltage sensing should be through passive voltage dividers with appropriate headroom and a high impedance connection to the ADC, we don't need the galvanic isolation at these low voltages.

• Current sensing should not be shunt based, but rather with a hall effect sensor or inductive. At DC, inductive sensing is not possible I guess, so we have to stick to hall sensors.

The integrated current sensor packages from Allegro are quite expensive. The following will be suitable for a <200A grid. Bidirectional integrated hall effect current sensor: ACS759 ±50A to 200A ACS756 ±50A to 100A

• SSR example:

100V, 100A type: http://www.mercateo.com/p/139A-1779776/SSR_100A_100V_SIP_Typ_D1D100.html?showSimplePage=NO&ViewName=live~showGrossColumn&utm_source=product-search&utm_medium=web&utm_campaign=Halbleiterrelais#crydom-ssr-100a-100v-sip-typ-d1d100-crydom

• 2row LCD display for easy overview at the controller, showing the momentary power consumption and accumulated energy, grid voltage, ...

Physical Layout: Cabling, Sockets, Fuses

If the infrastructure is build from scratch and completely new, then it's best to start with a hybrid AC/DC DiVER grid:

cabling:

• open or closed ring topology with branches, e.g. 3p AC household EIS: 1 x NYM-J 5G2.5, laid in parallel with 1x NYM-J 4x2.5 (up to 4x10)

Important: How to determine the cables other than by their inner topology? DiVER cables should be marked differently.

Connectors, sockets, plugs, terminals

Suggestions for sockets and plugs for DiVER:

• large screw terminals (which is somewhat legal, because the DIVER grid is safe to the touch )
• PowerCon sockets (expensive, proprietary, e.g. by Neutrik. How many poles?)
• SpeakOn sockets (moderately expensive, well suited because they carry 4 poles and are aimed for moderately high currents)
• XLR sockets (cheap, but can't carry much current. maybe for small appliances like phone chargers)
• Open Source screw terminals with M6 screws. 3D printed or milled.

Any other recommendations?

• CEE sockets are way too expensive, overkill and not meant for DIVER voltages. They may also be confused with the mains grid.

• DiVER Fuses, circuit breakers, Wilssen, BMS, PDUs and so on are encased in a separate enclosure and are nowhere near the mains.

XLR specifications from Neutrik NC3-FX

Capacitance between contacts ≤ 4 pF
Contact resistance ≤ 3 mΩ (inner)
Dielectric strength 1,5 kVdc
Insulation resistance > 2 GΩ (initial)
Rated current per contact 16 A
Rated voltage 50 V

Separated or safety extra-low voltage (SELV)

IEC defines a SELV system as "an electrical system in which the voltage cannot exceed ELV under normal conditions, and under single-fault conditions, including earth faults in other circuits".

There exists some confusion regarding the origin of the acronym: "SELV" stands for "separated extra-low voltage" in installation standards (e.g., BS 7671) and for "safety extra-low voltage" in appliance standards (e.g., BS EN 60335).

A SELV circuit must have:

• protective-separation (i.e., double insulation, reinforced insulation or protective screening) from all circuits other than SELV and PELV (i.e., all circuits that might carry higher voltages)
• simple separation from other SELV systems, from PELV systems and from earth (ground).

The safety of a SELV circuit is provided by

• the extra-low voltage
• the low risk of accidental contact with a higher voltage;
• the lack of a return path through earth (ground) that electric current could take in case of contact with a human body.

The electrical connectors of SELV circuits should be designed such that they do not mate with connectors commonly used for non-SELV circuits.