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INDUSTRY I SAFETY Causes of Ground-Faults


Ground fault is the most common fault in PV and may be caused by the following reasons:  Insulation failure of cables, i.e. a rodent animal chewing through cable insulation and causing a ground fault;  Incidental short circuit between normal conductor and ground, i.e. a cable in a PV junction box contacting a grounded conductor incidentally;  Ground-faults within PV modules. A solar cell short circuiting to grounded module frames due to deteriorating encapsulation, impact damage, or water corrosion in the PV module.


Grounding


Using the US based NEC Article 690.41 as a guide to discussion, there are two types of groundings in PV arrays. The first one is system grounding: the PV system with system voltage over 50 volts should be solidly system-grounded. To achieve that, the negative conductor usually is grounded via the GFPD in the PV inverter at point G (see Fig. 1). The other one is the equipment grounding: the exposed non-current-carrying metal parts of PV module frames, electrical equipment, and conductor enclosures should be grounded.


When the PV array is working under normal conditions, each PV string is generating current. The current flowing out of the ith


string is Ii+, where i = 1 … n. If PV strings are all electrically identical and have the same environmental working condition,


then I1+=I2+=… =In+. The total current flowing out of the array is Ipv+= I1++I2++…+In+. Similarly, the current coming back to each string is I1-, I2- , … In- . Thus, the total current coming back to the array is Ipv- which should be equal to Ipv+.


Since no external ground point is involved, the current flowing


through the GFPD (Ig) should be zero. Notice that the PV array is supplying power, while the PV inverter absorbs the power and feeds it into the utility grid.


Kirchhoff’s Current Law (KCL) requires that at any node (or junction) in an electrical circuit, the sum of currents flowing into that node is equal to the sum of currents flowing out of that node, where a node is any spot where two or more wires are joined. From this point of view, a ground-fault point, positive/negative bus bar, or even the inverter can be viewed as a node (or junction) in PV systems. Therefore, the current relationships of the normally operating PV arrays are summarized in the following equations.


Figure 2: Schematic diagram of the PV system under a ground fault


 At the positive busbar: Ipv+ = I1++I2++…+In+  At the negative busbar: Ipv- = I1-+I2-+…+In-  At the system grounding point G: Ig = Ipv- - Ipv- = 0  At the inverter: Ipv+ = Ipv-


Ground Fault Analysis in PV Arrays As shown in Fig. 2, a ground fault occurs in String 1 of the PV array. The reason might be a short circuit between the conductor of String 1 and the grounded module frame. Consequently, the fault will cause electrical imbalance among the PV array, resulting in mismatched currents. Generally speaking, every module, string, and whole array, whether in normal or fault condition, has its own I-V characteristics and unique maximum power point (MPP). When PV modules are connected together, their performance is determined by the interactions among them. For this reason, PV modules perform together like a chain that is only as strong as the weakest link. In our case, the weakest link is the faulted String 1.


In our analysis, it is considered that the PV array is the only source of fault current. In other words, there is no overcurrent or overvoltage from any utility inverter, battery, lightning strikes or external sources. The reason is that most PV inverters are transformer-based that can provide galvanic isolation between the PV array and the utility grid. Also, the fault impedance is assumed to be zero.


The fault changes the configuration of the PV array and causes subsequent fault currents. After the fault, String 1 only has two modules left operating, since the rest of modules (Module 3 ~ Module n) are short circuited by two ground points F and G. As a result, String 1 is significantly mismatched with other normal strings. Meanwhile, the operating voltage of the PV array might be even larger than the open-circuit voltage of faulted String 1. Therefore, instead of supplying power, String 1 may be forced to work as a load in the 4th quadrant of its I-V curve (see Fig. 3). Now String 1 has a negative current backfeeding from other


normal strings. This current is often called backfed current (Iback, or reverse current). Iback will flow into the fault point F and become a part of Ig. The other part of Ig is I1-, which is the current coming from other (n-2) modules in String 1. Since Module 3 ~ Module n in String 1 are short-circuited by ground


Figure 1: Schematic diagram of a typical grid-connected PV system under normal conditions


points, I1-, will be equal to the short-circuit current of each PV module (Isc) under standard test conditions. Finally, the backfed current (Iback) and the current from other modules(I1-) will merge as the ground-fault current (Ig) at fault point F.


Issue VIII 2011 I www.solar-pv-management.com 23


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