Power Conditioning & Control Equipment

System Controllers | Inverters | Battery Charge Controllers (Regulators) | Further Information | References |

The variable output from renewable energy devices means that power conditioning and control equipment is required to transform output into a form (voltage, current & frequency) that can be used by electrical appliances.

System Controllers

In systems with a number of power sources, careful design and matching of system components are required to ensure the system functions correctly. Most components have control functions inbuilt for the specific task for which they are designed. In some cases the inverter has inputs indicating the state of the system, and it can change the system's operation if necessary. The overall system control is automatically monitored by key components, such as the inverter (see Figure 1).
Typical functions performed by inverters and other controllers are:

• Disconnecting or reconnecting renewable energy sources
• Disconnecting or reconnecting loads
• Implementing a load management strategy
• Starting diesel generators if battery voltage is too low, or if the load becomes too high
• Synchronising AC power sources (e.g. inverters and diesel generators)
• Shutting systems down if overload conditions occur
• Monitoring and recording of key system parameters

 

Inverters

Renewable energy systems often provide low voltage, direct current (DC) from batteries, solar panels or wind generators. To use this DC power directly requires special non-standard appliances that may be available for camping and other portable or low power applications. Large DC, such as fridges, are relatively expensive. Electricity available from the main electricity grid is provided as alternating current (AC) at 240V in Australia, so most appliances are manufactured to suit this supply. The electrical energy used by the appliance is referred to as the load on the system.

An inverter is an electrical device that changes direct current (DC) into alternating current (AC). The inverter enables standard appliances designed for the main electricity grid to be used in SPS systems. Inverters often incorporate extra electronic circuits that control battery charging and load management. Generally, inverters used in most household systems now produce power of a similar quality to that in the main electricity grids. These are referred to as true sinewave inverters. Earlier model inverters produced lower quality power, which was adequate for most appliances. These are now only used in very small, inexpensive systems. They are often referred to as modified square wave inverters and sometimes as modified sinewave inverters.

Figure 1 Photo of an inverter

 

Battery Charge Controllers (Regulators)

A battery charge controller should always be used to protect the battery bank from over-charging and over-discharging. The simplest method used for charge control will turn off the energy source as the battery voltage reaches a maximum and will turn off the load when the battery voltage reaches a preset minimum. The battery charge controller for a system is more commonly referred to as a regulator. There are 3 main types of regulator; Shunt, Series and Pulse Width Modulated (PWM) regulators.

• Shunt regulators: when the batteries are fully charged, the power from the renewable source is dissipated across a dump load. These are commonly used with wind turbines.

• Series regulators: when the batteries are fully charged, the power from the renewable source will be disconnected from the cicuit in the simplest series regulator (see Figure 8). An improvement for this type is the proportional type, where the current is controlled by a variable component in series with the renewable source. These are used in PV systems.

• PWM Regulators: These regulators use a high frequency switching technique. The regulator switches the control device on and off quickly. When the batteries are discharged, the unit will be switched fully on. As the battery is reaching a fully charged state the unit will start switching the control device on and off in proportion to the level of charging required. When the battery is fully charged, no current will be allowed to flow to the battery. In solar systems the PWM technique is used in series with the solar modules. In wind turbine systems the PWM technique uses a shunt (dump) load to divert excess energy away from the batteries.

Figure 8 Plasmatronics series regulator

To stop the battery from being over discharged a load disconnector can be used. Quite often this can be through an auxiliary circuit in the battery charge controller or through a low voltage disconnect function built into the inverter.

Maximum Power Point Tracker

Another type of regulator is the Maximum Power Point Tracker (MPPT). This is a DC to DC converter that allows the array voltage to be different to the battery voltage used with photovoltaic modules to optimise the match between the panels and the battery bank. MPPT battery charge controllers measure the incoming power from the array and adjusts the arrays, so that the maximum power is being sent to the battery bank independent of the battery bank voltage.

Advancements in power conditioning technologies in recent decades is providing many new methods of providing quality power from sub optimal electricity supplies. The decreasing costs of many of these technologies, especially low power inverters is increasing access to power conditioning solutions to low voltage DC generating systems such as photovoltaics.

 

Further Information

Inverters – Solar Panel Info

RISE Information Portal Applications

 

References

None.