INDUSTRY I SAFETY
According to KCL, the current relationships of the ground-fault PV array are summarized as below.
At the positive busbar: Ipv+ = -Iback+I2++…+In+ At the negative busbar: Ipv- = I1-+I2-+…+In- At the ground-fault point F: Ig=Iback + I1-, where I1- = Isc At system grounding point G: Ipv+ + Ig = -
Analysis under Ground-Fault Condition In the worst case of a ground fault, the ground fault current
could reach as high as n Isc. In this scenario, the other (n-1) normal strings have approximate total current (n-1) Isc backfeeding into String 1. Meanwhile, the modules below the
ground fault point F in String 1 are flowing Isc into F.
The fault scenario could be explained with the help of I-V characteristics analysis in Fig. 3, in which Voc is the open-circuit voltage of one PV module. Before the fault, the PV array is
working at MPP with operating voltage Vmpp and operating current Impp. At the moment of the ground fault, the PV array’s configuration and I-V characteristics are changed suddenly.
Meanwhile, the operating voltage at Vmpp is dropping to Vf immediately. Since Vf is the open-circuit voltage of the faulted array, the array becomes open-circuit and there is no current feeding into the PV inverter.
However, the current of each string inside the PV array is not zero. For (n-1) normal strings, their operating point is at ( Vf, Iother). Notice that the operating voltage Vf is close to the knee of their I-V characteristics.
Therefore, each string is providing the current close to Isc. In other words, Iother is close to (n-1) Isc. Iother has no path to go but to backfeed into String 1. As a result, Iback at String 1 becomes ( n-1) Isc, flowing into the ground-fault point F.
In summary, the worst-case current relationships of the PV array under a ground-fault can be re-written in the following equations.
At the positive busbar: Ipv+ = -Iback + ( n-1) Isc = 0 At the negative busbar: Ipv - = nIsc At the ground-fault point F: Ig = nIsc
Ground-Fault Current and Beckfed Current Assume that in our case, for instance, the rated current of the GFPD (fuse) is 1A, there are 5 parallel strings in the PV array
(n=5), and Isc of each module is 5A. Simple calculation by the equations above will give us Ig =25A in the worst case. According to the melting time vs. current diagram in Fig. 4, the
1A fuse takes less than 0.01s to clear the ground fault. In this instance, the fault path is detected by the fuse and the fault is interrupted successfully. If a grounded conductor is opened to interrupt the ground-fault current path, the grid-connected PV inverter fed by the faulted PV array shall automatically cease to supply power to the grid. Meanwhile, an indication of the fault should be provided. After the shutdown of the PV inverter, the whole PV array goes into the open-circuit condition, waiting for maintenance personnel to fix the problem.
Need specific solutions To meet safety needs it is required that ground-fault overcurrent protection be installed and that if a ground-fault is detected, the inverter must be removed from operation in order to prevent electrical fire. Mersen and other companies offers a variety of fuse technologies designed to protect solar PV systems from ground-fault incidents. Fuses can be found for ground-fault protection only and for internal to the inverter cabinet the better option is a touch-safe DIN rail mountable fuse holder but for door-mount or panel-mount options select the touch-safe GPM series fuse holder. For fuses that require ground-fault protection and remote indication the need is to protect the ground based circuits that require mechanical indication or signaling
Conclusion
Improperly detected and interrupted ground-faults can present the risk of fire hazards in solar PV arrays. Excess current flow, to faulted strings can overload conductors and/or connectors leading to overheating and risk of fire. Faulted strings can also negatively impact maximum power point tracking, ultimately affecting the efficiency and power output of the PV array. To properly protect PV arrays from damage, National Electric Code Article 690.5(A) specifies that the GFPD or system must be capable of detecting a ground-fault current, interrupting the flow of fault current and providing an indication of the fault.
Figure 3: I-V characteristics of the PV array during a ground fault 24
www.solar-pv-management.com I Issue VIII 2011
© 2011 Angel Business Communications. Permission required.
Figure 4: Melting time vs. fault current of
protection fuses
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