Voltage Rise Calculations

Standards for solar electrical projects

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Introduction

Voltage rise can occur in solar PV systems on the AC system between the power inverters and the network connection point and is with respect to the network system voltage.  Voltage rise above the power system network voltage occurs when power flows into the network from the inverter.

Maximum limits for voltage rise are required in order to avoid excessive voltages within the consumers installation and to help reduce the occurence of overvoltage protection trips on the inverters.

The limits set by the Standards for solar PV voltage rise (AS/NZS 4777) are lower than for voltage drop because the normal power system voltage operates above its nominal value.  For example the two common nominal voltage levels in Australia are 230 V (-6 %, +10%) and 240 V (+/- 6 %) however the average system voltage lies between 240 V and 250 V.

Limits set by the Standards

The limit for voltage rise is 2 % per Standard AS/NZS 4777.1:2016 from the inverter to connection point.

Equations

Voltage rise is really no different to voltage drop calculations.

According to AS/NZS 4777.1:2016, the voltage drop from the inverter to the point of supply must not exceed 2%.  Therefore, after selecting the proper size of cables, the voltage drop calculations should ensure that the voltage drop is within the limits.  Otherwise, the cable size needs to be increased to fit within the limits.

The current rating of the consumer mains cables needs to exceed the total rated current of the inverters.  The Standard AS/NZS 3008.1 can be used to determine the current rating and voltage drop for LV cables.

Cable Pro Web software can be used to easily and accurately perform voltage rise calculations.
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Example

This example from Standard AS/NZS 4777.1 is for an installation with a 400 V three-phase supply, a 30 metre consumer mains length, a 60-metre final sub-circuit length to the three-phase inverter and only the main switchboard for connections.  It is necessary to select suitable cable sizes for the complete installation based on requirements for current-carrying capacity and voltage rise.  For this example, two segments are considered: the consumer mains (existing installation) and the final sub-circuit to the inverter (new installation).

The required minimum current-carrying capacity for cables in the installation is determined based on the rated current of the inverters.  The current rating for the 3 phase, 30 kVA inverter equates to a current rating of 43.35 A per phase.

The overall voltage drop combining the consumer mains and the final sub-circuit to the inverter must not exceed 2 % per the Standard.  For the three phase 400 V system this equates to an overall voltage rise (or voltage drop) of 8 V.

A 6 mm2 conductor size satisfies the minimum size requirement of the electricity distributor’s service rules.  A 6 mm2 conductor size has a current-carrying capacity of 45 A.  This rating is based on three single-core thermoplastic copper cables enclosed within an underground wiring enclosure, as per AS/NZS 3008.1.1:2017, Table 7, Column 24.

However, the 6 mm2 conductor size does not meet the voltage drop requirements.

Cable Pro Web software was used to easily determine the minimum conductor sizes while meeting the maximum voltage rise requirements.  All cables had XLPE (90 deg.C.) insulation, copper conductors and were buried below ground in separate conduits.

For the consumer mains cables a voltage drop of 1/3 the overall 2 % (0.67 %) was allowed since it was a 30 m run of the overall 90 m.  The minimum conductor size for consumer mains was determined to be 16 mm2.  See the software result screen image below.

For the final subcircuit cables a voltage drop of 2/3 the overall 2 % (1.33 %) was allowed since it was a 60 m run of the overall 90 m.  The minimum conductor size for final subcircuit was also determined to be 16 mm2.  Software result not shown.

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