1500V solar DC system voltage: Is the time ripe?

5th Jun 18  

1500V solar DC system voltage: Is the time ripe?

A technology to sustain requires adequate support from the government. While this was the case with solar energy before, the market dynamics has changed a lot now. With the evolvement of cost competent market, optimizing every component of the solar power plant is important to ensure developers sustainability and profitability. System voltage is one such area where optimization has been at steady pace. The initial solar plants were designed at 600V DC system voltage. However, oncethe drive of cost reduction had begun designers/developers started looking for more novel ideas to keep their profit margins balanced. One such idea was to increase system voltage to 1000V (from the previous 600V systems) citing its various benefits. A similar idea has been in rounds for quite a few time now i.e. to further increase system voltage to 1500V(Figure 1) from the existing 1000V. With modernization of various components, this shift may seem easy in few countries only. For country like India lot of shortcomings needs adequate thinking and appropriate action before such shift is made. This blog aims to explain its readers on both advantages and shortcomings of 1500V system. Additionally it would also inform its readers on the way ahead for Indian market for such system.

Figure 1 : A typical arrangement of solar modules in 1500V plant (Source: PVTECH)

Before delving into the shortcomings of a 1500V system, let us first understand their advantages. The advantages of using higher voltage DC system (1500V here) are as follows:

  1. Reduced strings of solar modules: The first and foremost advantage of 1500V system is that we reduce number of strings. Let us take a typical example. The currently available typical solar modules havevoltage rating between 36 to 37 Volts. Thus in order to form a string of 1000V, we would need close to 27 modules in series (Figure 2) when compared to 42 modules for 1500V system (Figure 1). Thus for a power plant keeping its power output fixed, we get less number of strings for 1500V. Advantage of this reduction is that we need less DC combiner boxes and thus maintenance and hence the fault detection of such system becomes much easier. Additionally the cable length required for interconnection will decrease with decreasing number of strings.

    Figure 2 : A typical 1000V system (Source: Google images)

  2. Reduced DC cable losses : Energy losses while generation have always been a concern for solar power plants operators. Out of the possible losses (temperature loss, irradiance loss, etc.) in the power plant, only a few of them are controllable such as DC cable losses. DC cable losses are generally associated with the current flowing through the cable in form of heat. More the current flowing through the cable more is the heat lost and vice versa. With an increase of 50% in voltage (from 1000 to 1500V), the current flowing through the cable would decrease for given power which would lead to a significant decrease in DC losses. Additionally with decrease in array cable length (as mentioned above) the overall resistance of the cable decreases (see formulae below). This further reduced the DC power loss.
  3. Resistance = Resistivity x Length of wire /Area of wire &

    Power Loss due to heat = (Current)2 x (Resistance)

    • Reduced Capital cost: The third and most important advantage is that a fair reduction in capital cost can be achieved by using 1500V system. This can be attributed to multiple facts like:
      1. Reduction in number of strings which leads to reduction in number of Module Mounting Structure (MMS)
      2. Reduction in array cable length
      3. Reduction in number of DC combiner boxes/ junction boxes

With such advantages, 1500V system sure looks like an attractive technological shift. However there are still few concerns which are to be addressed before we shift to such systems. They are as below:

  1. Susceptibility to Potential Induced Degradation (PID) increases : Potential Induced Degradation (PID) as we explained you in our previous blog "What's and why's of PID!" is one of the cause of failure of PV modules. It has been observed that PID is primarily affected by system voltage. With increase in system voltage to 1500V, it has been estimated that the effect of PID on the module (connected at the negative end of the string) may increase. While there are materials for module (primarily EVA & cell) already in market which are anti-PID, using such higher voltage systems are expected to have a susceptible effect on solar modules. 

  2. Figure 3: Effect of PID on modules connected in a typical string Electroluminescence (EL) image (Source: Waaree Energies)

  3. Safety : One of the important concerns while implementing 1500V system is the requirement of improved protection devices. These additional up gradation are required in the following components:
    1. modules - as higher voltages may cause arcing when under maintenance leading to major safety hazard
    2. combiner boxes - due to increase in no. of modules per combiner box
    3. inverter - due to change in its input voltage (i.e. 1500V)
    4. transformer - due to change in output voltage of inverter (500 - 680V when compared to 1000V inverter 300 - 400V)

    Additionally, while handling such high voltages,it is also important that appropriate safety equipment and/or Personal Protective Equipment (PPE) is used to curb the risk of safety hazard.

1500V systems currently areonly beneficial at utility scale power plant systems and not on rooftop scales. However it is poised that with increasing awareness and further reduction of cost in conjunction with availability of material, 1500V systems shall experience meteoric rise. 1500V system would see an increased market penetration of around 95% in the next decade. Thus it is important for India that an early adaptability and the necessary infrastructure up gradation are done in order to be prepared for the next technical shift.

Figure 4 : Projected adaptability of 1500V system (Source: ITRPV)

Waaree Energies had understood this market trend and have already launched 1500V compatible modules and inverter. They are tested rigorously in order to ensure they perform optimally when under operation.

Let us all pledge to make solar energy the primary source of energy in the near future.