IS IT ADVISABLE TO COMBINE DC & AC EARTHING ?

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In Solar PV power plants, common question always rising in our mind that, "Whether is it advisable to combine DC & AC Earthing ?

Some of expertise people in the industry go ahead, to preferring to do combining of both the systems earthing, to arrive overall earthing grid resistance < = 5 Ohms / < = 1 Ohms. But technically speaking, this combining effect is not technically advisable. Why ? and How ?

Let's move on detailing. As we well aware, In DC systems, the flow of current in the circuit is based on the resistance ( R ) associated with that. Where as in AC systems, the flow of current in the circuit is based on the Impedance ( Z ) associated with that. From this preliminary concept, We understood clearly that, One Ampere of DC system current is not Equal to the One Ampere of AC system current.

From the above concept, we understood well that, Combining DC & AC earthing is not technically advisable. DC system working based on the Resistance (R) associated within, Whereas AC system working based on Impedance (Z) associated within. Normally in AC systems, the Short circuit level based on the Impedance. Presence of higher Impedance will resulted in higher Short circuit in the system.

The short circuit current of DC systems of PV plant based on the Pnom Ratio figure, which is detailed below :

From the above figure, it is clearly understood that, If DC loading increasing, then short circuit current level of the system will increase. Now a days, witnessing elevated technology on the inverter section, we have been getting single inverter unit rating like 2MW, 3.125MW, 3.75MW, 4MW & 5MW, in current PV market. By the way of adopting higher rating inverter & carrying with 60% DC loading, DC short circuit level maximum it can go nearing to 5 KA ( 5000A, rarely ) not more than that.

Sizing the earth conductor ( MMS Body earthing to earth pit ) is based on the maximum short circuit current carrying by the system + time to withstand the fault current. - This procedure not only for DC earthing, this is common procedure even for AC earthing too.

If designer carrying over general procedures to calculate proper sizing of earthing conductor for DC system, then he should have to check the Pnom ratio of plant along with Isc of each module & number of SCB's / SMB's connected to the each inverter.

Where as for the AC systems, the short circuit level derived based on two options widely by the industry experts.

Option - I : Short circuit apparent power based on IEC 60076 - PART - V.

Option - II : Plant connecting pooling substation short circuit level. ( The designer has to consider pooling substation short circuit level as the main data / figure, for his designing plant )

By the way of using the above available options, we can calculate easily required earth flat size connecting to the AC equipment's ( Transformer, LT / HT breaker, Isolator, CT, PT, Surge arrestor ) to the earthpits.

Example, The Short circuit current level is 25KA / 1 sec ( System has to withstand overall ), then required GI ( Galvanized Iron ) earth flat size as per IS 3043 - 1987 & International standards for earthing, is : 312.5 Sq.mm, so 50 x 8 mm is enough to carry the 25kA / 1 sec. For DC System 5KA short circuit current, required earth flat size is : 62.5 Sq.mm, So designer can proceed with detailing of 25 x 6 mm as the earthing flat between MMS to earth pits / SCB's to earthpit.

Conductor means it conducts / carry the current from one source to other source.It will allow to pass the current it's carrying. People generally prefer to combining both earthing systems, to arrive overall grid earth resistance is less than and equal to One ohms. Carrying over this concept strongly will not lead to failure free operation as overall. If we are willing to combining of DC & AC earthing through earthing conductors, then the chances for failure of whole system will be very high. The main reason behind is, AC systems have been carrying higher impedance and higher short circuit current compare than DC systems, specifically on Solar PV plants. In the combined earthing philosophy, AC system fault current may travel to reach even in DC systems ( MMS, SCB's ) and it will affect their operation life instantly. Sometime due to the combined effect, the higher fault current at HV level ( 11KV / 22KV / 33KV ) will take path through low resistance path /conductor and it will reach to Inverter DC portion and affect the whole inverter with severe damage. So combining both earthing system technically is not at all advisable one.

Now a days we have been using chemical bond Copper (Cu) earthing system for the whole PV power plants. Before using those materials / giving dispatch clearance to the site, either designer / QHSE officials should inspect those materials, whether those materials are certified by the Central / State ( India based ) body testing agencies like NABL, ERDA & CPRI or Not ? Whether vendors have been carrying valid proof of test report documents for his offering items or not ?

Example : Subjected to the CPRI test results available, 17.2 mm dia 3 meter long Cu.Rod can withstand fault current as 10kA / 1 Sec, where as 25mm dia 3 meter long Cu.Rod can withstand fault current as 25kA / 1 Sec. If the designer planning to use 17.2mm dia Cu.rod for the HV fault current, it will failure instantly / over a short span of time, once it experiences short circuit current of > 10kA continuously for more than one second.

From this post, the viewer understood that : (a) Should not combine DC & AC earthing, in solar PV power plants. (b) How to arrive the DC Short circuit of the PV Plant. ? and they will ensure proper earthing systems in the solar PV power plants.