Power electronics devices

1. Charge controllers

Charge controllers are used in PV systems to control the charge and discharge process of batteries. Two main types of charge controllers are available in the market. One is the PWM-type and second is the MPPT-type.

(a) The PWM-type charge controller

The PWM-type employs a shunt or series switching topology to control the flow of current between the source and battery while the voltage is maintained at a constant level.

(b) The MPPT charge controller

The MPPT-type charge controller is a PWM controller with maximum power point tracking capability. The latter is achieved through impedance matching of the load through PWM such that maximum current is fed to the load at all times. In such a scenario, the voltage supplied by the controller is not constant.

Fig.1: IV-curve characteristic for PWM and MPPT controller

Mode of operation: Charge controllers have a series float charger which is a trickle charger with automatic on and off switch and this is why this charger type is mostly used in renewable energy systems. A float charger can have three stages:

  1. Bulk stage

  2. Absorption stage

  3. Float stage

The bulk stage is the stage during the charging process during which high current is allowed to flow from the source to the battery. Under the PWM mechanism with HIGH time representing the condition of a closed switch allows current flow is long enough to maintain a constant voltage increase.

The absorption stage is the stage during which the battery voltage reaches maximum level and current starts decreasing to maintain constant voltage. This is achieved by modulating ONs and OFFs of the switch; shorter time for OFFs.

The float stage is the stage during which current is pulsed to maintain a charged condition in the battery at a constant voltage. An additional stage known as the equalization stage may be also present in some charge controllers designed to be used in systems where more than one battery are used. Equalisation is the process whereby the battery voltage is overcharged at roughly one volt above the float voltage level to equalize charge in all cells in the battery bank.

Fig.2: Solar charge controller [1]

2. Inverters

Inverters convert dc voltage to an ac voltage. For PV systems, there are several inverter choices based on panel configuration and system size. String inverters are used in systems where individual solar panels are connected in series. Central inverters are used when the PV array consists of several strings of series-connected panels while micro-inverters are equipped with all modules in an array to optimize power output. Central and string-type inverters are the most commonly used inverters by PV installers/companies.


Table 1: Inverter type comparison
Details String Central Micro
Power rating <100 kW 5 kW – 2 MW <0.5 kW
Application Residential Residential/Commercial/Utility Module
Connection type Single/Three Phase Single/Three Phase Single Phase

Another power electronics module that may be used in PV systems is the power optimiser which is a dc-dc converter. It performs maximum power point tracking at module level before sending the optimized voltage to a central inverter for dc-ac conversion.


Table 2: String v/s central inverters
Inverter type Pros Cons
String inverters
  • Lower balance of system and maintenance costs.

  • Simpler design and modularity.

  • Ideal for limited inverter pad spaces.

  • Modularity of string inverters is better for systems with different array angles and orientations.

  • Fewer arrays are impacted with one inverter failure.

  • Higher dc watt unit cost.

  • More inverter connections.

  • Requires more distributed space to mount inverters.

  • Newer and less field-tested product.

Central inverters
  • Lower dc watt unit cost.

  • Fewer component connections.

  • Optimal for large systems where production is consistent across arrays.

  • Proven field reliability.

  • Higher installation cost.

  • Higher dc wiring and combiner costs.

  • Larger inverter pad footprint.

  • Less optimal for systems with different array angles or orientations since they default to highest producing strings within a range and block the production of lower producing strings outside of that range.



References:

[1] “Renewables Global Status Report - REN21.” https://www.ren21.net/reports/global-status-report/ (accessed Apr. 01, 2021).