solar PV System Sizing: How much solar PV can contribute in a PV-DG Hybrid System?

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Tongatapu demand and diesel power output with

Tongatapu demand and diesel power output with


While we know that the solar resource is variable in nature and load is also variable in nature, it becomes critical to decide what level of solar penetration will be appropriate in a solar PV-DG hybrid system. National Renewable Energy Lab (NREL) has developed definitions for low, medium and high penetration hybrid systems based on its experience with wind and diesel hybrid systems. These definitions are detailed in following Table .

Penetration Class Operating Characteristics


Peak Instantaeous

Annual Average

  • Diesel(s) run full-time
  • Wind power reduces net load on diesel
  • All wind energy goes to primary load
  • No supervisory control system



  • Diesel(s) run full-time
  • At high wind power levels, secondary loads dispatched to ensure sufficient diesel loading or wind generation is curtailed
  • Requires relatively simple control system


20%- 50%

  • Diesel(s) may be shut down during high wind availability
  • Auxiliary components required to regulate voltage and frequency
  • Requires sophisticated control system

100%- 400%

50%- 150%

Recently IRENA (International renewable Energy Agency) has published a case study for  Tongatapu’s power plant consisted of seven diesel generators with a total maximum rated capacity of 11.28MW. The typical weekday demand curve for Tongatapu island shows that the  diesel generators are capable of easily matching their output to the grid’s demand for power.  This case study shows that how solar PV DG system can assist to meet the electricity demand of the proposed load curve in terms of low to medium to high levels of RE penetration.


Load Curve




Following figure shows that the PV generation will have a limited impact on the diesel generators, in low penetration about 1 MW of solar PV contribution is considered which will provide only around 4% of Tongatapu’s total electricity demand. The PV output is seen as a negative load by the generators, which continue to match their output to the changing demand profile and support power quality on the grid. The gap between the reduced diesel output and grid demand represents the fuel saved over one day of operation. Inverters on the PV system monitor the frequency and voltage on the grid and match the system’s frequency and voltage to these values. Even a 100% drop of the PV system output at the noon peak represents less than 10% of generator capacity and could be covered by diesel spinning reserves.



Tongatapu demand and diesel power output with 1MW PV


In order to achieve significant reduction in diesel fuel consumption it will be necessary for Tongatapu to install additional PV capacity. Following figure  shows the notable drop in diesel generator load that could be achieved with the deployment of a 5MW PV system. This would be classified as a medium penetration system because it will require that additional control systems to work effectively with the existing diesel generators. There are two key reasons why additional control equipment are required – protecting the diesel generators from low loading and limiting generator cycling.

As previously noted diesel generators have a lower load limit of approximately 30-40% of rated capacity. Below this limit the generator suffers from poor combustion that reduces efficiency, increases maintenance cost and can cause permanent damage that reduces the generator’s usable life span. In addition, below this lower loading limit the generator can no longer support power quality on the grid and there is a risk of damaging grid infrastructure and attached loads, and even of causing a black out.

Tongatapu demand and diesel power output with 5MW PV


Power limiting of 5MW PV system to prevent low generator loading

Power limiting of 5MW PV system to prevent low
generator loading



There are numerous strategies to prevent low generator loading, thus allowing higher levels of variable solar PV  onto the grid. One common and easy to implement approach is power limiting. In this situation control equipment monitors generator load and when high PV output risks reducing generator load below the lower loading limit the control equipment has the inverter on the PV system reduce the power output. This prevents the generator from dropping below the lower limit protecting it from poor combustion and loss of power quality. Figure 5 shows the same 5MW system and demand profile with power limiting systems installed.


While power limiting protects diesel generators from low loading it wastes the additional PV power available at the noon peak and reducing the fuel saving delivered by the PV system. Even with power limiting, the 5MW system still delivers significantly more fuel savings than the 1MW system. However the additional fuel offsets require additional control systems that increase the cost of the PV system.


Additional options exist to deal with low loading. Installation of dump loads can turn PV overproduction into useful energy (e.g. heat). The limited heating demand in the Pacific constrains the value of this option. Therefore load dumping for ice making for cooling demand is an option for dealing with low loading. Specialised low load diesel generators that are designed to operate efficiently and provide power quality down to 10% of rated capacity are an attractive option for Pacific islands with older generators but would represent a substantial cost for islands where the generators still have usable lifespan. The cost of each of these options will have to be weighed on a case-by-case basis against the value of offset fuel consumption.

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