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Hybrid systems

Hybrid systems, singlephase, threephase

The main feature of a hybrid system is the use of two or more different electricity sources.

Alongside solar energy, photovoltaic hybrid systems generally employ a diesel generator, a wind turbine or the public grid as a further electricity source. The inverters used in hybrid systems, which have integrated battery chargers, supply the connected AC loads according to demand from the battery bank of solar energy or the second electricity source. These devices also allow the batteries to be recharged from the extra energy source.

Photovoltaic hybrid systems offer the advantage that the solar generator does not have to be significantly oversized for periods of low sunlight. This avoids substantial costs. When selecting its energy source, the system always gives priority to the energy provided by the module. In combination with a controllable second source, the energy supply remains reliable and available 24 hours a day, all year round.

Single-phase DC hybrid systems

The central, intelligent element within the system is the Steca Tarom or Power Tarom solar charge controller (B): it controls the energy flow and protects the battery against critical states. Steca Tarom /Power Tarom is directly connected to the battery, just as the DC bus is. Using a shunt, the Steca PA HS200 (E), which is situated on the minus cable attached to the battery, the battery current is measured and this information is passed on to the Steca Tarom / Power Tarom (B). Further components, such as an inverter or the Steca PA 15 remote control, are directly connected to the DC bus. In order to automatically start a diesel generator (G) if the battery’s state of charge (SOC) falls below an adjustable threshold, the output of the Steca PA 15 is connected to a relay. The normally open contact of the relay switches the diesel generator on, and subsequently switches it off again.

The Steca Tarom / Power Tarom controls the DC hybrid system. The Steca PA HS200 current sensor (E) transfers all infor mation on the charge and discharge currents at the DC bus to the Steca Tarom / Power Tarom. With the aid of this data, the controller is able to calculate the current state of charge of the battery. This information is transferred via the DC cabling (powerline modulation) to all connected Steca PA 15. Every Steca PA 15 can be independently configured to a certain switch-on and switch-off threshold of the state of charge.

If, in the above example, the inverter is discharging the battery, then this information is transferred to the Steca Tarom / Power Tarom, which calculates the state of charge. As soon as the state of charge falls below the appointed threshold value of the connected Steca
PA 15 (e.g. 30 %), the controller switches on the diesel generator via a relay. The load is now being supplied from the generator (G), and at the same time the battery is being recharged. After the state of charge has reached the Steca PA 15’s appointed upper value (e.g. 90 %), the diesel generator is switched off again.

In order to create an automatic energy management system, the AC output of the diesel generator is connected to the AC input of the inverter (with integrated battery charger). The load is always connected to the output of the inverter. If the diesel generator is running, and this current flows to the inverter, then the inverter automatically switches to transfer mode. The loads are supplied from the diesel generator whilst the battery recharges via the inverter. If the AC output voltage of the diesel generator falls under a certain voltage level, which can be adjusted on the inverter, then battery operation is automatically switched back on.

This system allows for automatic energy management which gets optimum use from the available solar energy, maintains the batteries reliably, and ensures electricity supply around the clock.

Three-phase DC hybrid systems

The control concept is similar to that of the single-phase system. If more than one Steca Tarom / Power Tarom is employed, one of the devices must be desig­nated as the master Tarom. All other charge controllers are then automatically designated as slave Taroms. The master Tarom / Power Tarom is directly connected to the battery and all slaves are connec­ted to the DC bus. Only the master Tarom / Power Tarom shows the correct state of charge on its display and controls the energy flow around the system. Slave Taroms / Power Taroms perform the function of controlling the charging from the connected PV modules.

In order to assemble a three-phase energy supply, three inverters are connected to the DC bus. Various three-phase generators can be connected to the three inverters for controlled recharging of the battery via a Steca PA 15 and a relay. These may be wind, water, or diesel generators or the public grid. Suitable inverters with integrated battery chargers in three-phase mode are the Steca Xtender devices (XTS, XTM, XTH). In total, a maximum of 72 kW can be supplied.

Both single-phase and three-phase hybrid system concepts are based on the same principles of energy management. With the help of the Steca PA HS200 current sensor, the charge and discharge currents of the components, such as slave Taroms / Power Taroms, inverters etc., are determined and communicated to the master Tarom / Power Tarom. Based on the calcu­lated state of charge of the battery, the Steca PA 15 switches the extra generator on or off. The three single-phase inverters switch off if the voltage falls below a given threshold in order to protect the battery from deep discharge.

Single-phase and three-phase AC hybrid systems

With very large load requirements, AC-coupled hybrid systems can provide a sensible alternative to the very effective and cheap to implement DC hybrid systems. This topology is beneficial if the largest part of the loading is required on the AC side (L) during the day. Steca AC hybrid systems can be implemented using the Steca grid and sine wave inverters (B and C).

Various generators (A and F) are coupled to the AC bus. In addition, bi-directional sine wave inverters (C) are deployed, which are used for charging the batteries and can also be used for supplying the load if the AC generators (A and F) supply insufficient power. In addition, it is also possible to couple solar generators via a Steca solar charge controller (D) directly to the batteries (H) on the DC side.

If not enough energy should be available in the system in order to supply the load, a diesel generator (G) can be started automatically. When the diesel generator is running, it must be ensured that all grid inverters (B) have been disconnected from the grid. This is necessary in order to prevent the inverters (B) from feeding back into the diesel generator and destroying it when the battery is full. As soon as the diesel generator has been switched off, the grid inverters (B) can again be automatically connected to the grid. The loads are then again supplied by the PV generators (A) via the grid inverters (B).

The Steca Xtender battery inverters (C) here create the grid into which the grid inverters (B) feed, and from which the loads (L) are supplied. If the PV generators (A) produce a higher output than the loads (L) take up, the battery inverters (C) charge the batteries (H) with the excess power difference.

Steca droop mode
When the batteries (H) have reached the cut-off voltage, they can no longer fully take up this power difference. There is then more output available in the system than can be used. The battery inverters (C) then activate the Steca droop mode.

The coolcept grid inverters with the droop mode are specially designed to meet the demands of AC-coupled hybrid systems and interact perfectly with the Steca Xtender battery inverters (C). These increase the frequency of the AC grid in a linear fashion, depending on the excess output of the grid inverters. The more excess output available, the higher the grid frequency. The grid inverters then restrict the feed output to precisely the feed output which fully supplies the loads (L) and maintains the batteries (H) at the cut-off voltage. In this way, they create a balanced output level in the hybrid system. If the level of the load changes, the grid inverters immediately adjust their feed output and continuously offset the output balance so that the batteries (H) can be fully loaded in an optimum manner. As soon as the excess output from the grid inverters decreases, the battery inverter (C) again reduces the grid frequency until the standard grid frequency with a balanced output level has been reached. If not enough output is provided by the grid inverters (B) to supply the loads (L), the necessary difference comes from the battery inverters (C) in the batteries.

With very large outputs, this kind of Steca AC hybrid system can also be designed as a three-phase system in order to supply corresponding loads directly. Here the StecaGrid grid inverters (B) provide direct three-phase feeding on the AC side.

The required bi-directional Steca Sinus inverters Steca Xtender (C) can be used in both single-phase and three-phase cases. Up to three devices can be connected in parallel per phase. This means that a total of 24 kW per phase is available, with a maximum of 72 kW in three-phase operation.

Diesel generators (G) can be used to produce approx. 100 kW, while grid inverters (B) are used for up to 70 kW. Thus, loads of up to 70 kW can be supplied. The power per phase of the grid inverters must not exceed the rated power of the Steca Xtender(s) in a phase.

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