AC side: cable sizing, phase, connecting at the meter, anti-islanding

The DC side gets most of the attention, but many service calls come from the AC side: inverters tripping on high grid voltage, wrong phase connections, and DISCOM inspectors refusing a connection because the solar feed was tapped in the wrong place. This lesson follows the power from the inverter's AC terminals to the DISCOM's line.

Sizing the AC cable

Like DC cable, AC cable must pass two checks: current rating (as installed, after derating) and voltage drop. Many specifications cap the AC drop at 2% from inverter to the point of connection. On the AC side there is a third reason to keep the drop low: voltage rise, which we will come to.

Formulas

• Single phase: Vd = 2 × L × I × ρ ÷ A, drop % on 230 V
• Three phase: Vd = 1.732 × L × I × ρ ÷ A, drop % on 415 V (line voltage)

Use ρ = 0.0172 Ω·mm²/m for copper and 0.0283 for aluminium. For the short runs and small sizes on homes, reactance can be ignored.

Worked example: 5 kW single phase, 30 m

I = 5,000 ÷ 230 = 21.7 A.

4 mm² copper: Vd = 2 × 30 × 21.7 × 0.0172 ÷ 4 = 5.60 V, which is 2.43%. Too high.

6 mm² copper: Vd = 3.73 V, which is 1.62%. Acceptable, and it also carries the 32 A MCB rating in most installation conditions.

Worked example: 10 kW three phase, 40 m

I = 10,000 ÷ (1.732 × 415) = 13.9 A.

4 mm² copper: Vd = 1.732 × 40 × 13.9 × 0.0172 ÷ 4 = 4.14 V, which is 1.0%. Good.

Single phase or three phase?

The choice is set by the building's supply and the DISCOM's rules.

• Single-phase supply: single-phase inverter. Each state's supply code caps the load on a single-phase connection (commonly in the 5–10 kW range). Check the local limit and the sanctioned load.
• Three-phase supply: above about 5 kW, use a three-phase inverter. It shares the output equally across all three phases, which keeps the building's phases balanced.
• Small single-phase inverter on a three-phase home: allowed in many places for small systems, but connect it to the phase that carries the most daytime load. All the export will flow on that one phase, which can raise its voltage.

On a three-phase connection, ask the DISCOM how their net meter registers export. Most modern static meters add up energy across the three phases, but the customer should not be surprised later.

Connecting at the meter

The solar AC must join the building's wiring on the consumer side of the DISCOM's meter, usually in the main distribution board, after the main switch.

• Net meter: replaces the old import meter. It is bidirectional and records import and export separately. The DISCOM supplies or approves it (often tested in their lab) and seals it.
• Generation (solar) meter: many states and subsidy schemes ask for a separate meter at the inverter output to record total solar generation. Check the state rule.
• Never connect to the meter terminals, the service cable or the DISCOM's cut-out. These are DISCOM property, and tampering with sealed equipment is an offence.
• Phase and neutral: a reversed phase and neutral on the inverter can cause odd faults and, in some meters, mis-registration. Check with a tester before you energise.

Anti-islanding

An "island" is when part of the grid is cut off from the main supply but stays energised by local generators. Picture a lineman who has opened the feeder to repair a broken conductor. He believes the line is dead. If a rooftop inverter keeps feeding the line, the conductor in his hands is live at 230 V, and through the distribution transformer it can even appear at 11 kV on the high-voltage side.

Grid-tied inverters must therefore stop exporting within about 2 seconds of losing the grid. They detect it from voltage, frequency and other changes. You can test this at commissioning: switch off the AC MCB or the main switch and time how long the inverter takes to stop.

Diesel generators. Many Indian homes and shops have a DG set on a changeover. When the DG runs, an on-grid inverter may see its output as "grid" and try to export into it. A small DG cannot absorb power, its voltage rises and it can be damaged. Solutions: a DG synchroniser or zero-export controller that limits the inverter when the DG is on, or wiring the inverter so it is on the grid side of the changeover and goes off with the grid. Never let an on-grid inverter run in parallel with a DG without one of these.

Voltage rise: why inverters trip at noon

When the inverter exports, current flows from it to the grid, so the voltage at the inverter is higher than at the meter by the cable's voltage drop. If the local transformer is lightly loaded at noon and the line voltage is already high, say 246 V, the extra rise can push the inverter past its over-voltage trip.

Example: a 5 kW inverter on 30 m of 4 mm² cable adds 5.6 V. With 246 V at the meter, the inverter sees about 252 V. On a rural feeder that reads 250 V at noon, the inverter sees almost 256 V and starts tripping. Upsizing to 10 mm² cuts the rise to about 2.2 V.

Fixes, in order: shorter and thicker AC cable, asking the DISCOM to check the transformer tap, and using the inverter's grid-support functions (reactive power or volt-watt) where the DISCOM allows them. Never raise the inverter's trip limits beyond the settings required by the DISCOM.

Key points

• AC cable must pass current-rating and voltage-drop checks; aim for well under 2% drop.
• Single-phase drop uses 2 × L; three-phase drop uses 1.732 × L on 415 V.
• Connect on the consumer side of the net meter, through the ACDB and (where required) a generation meter; never tap DISCOM equipment.
• Anti-islanding stops export within about 2 seconds of grid loss to protect linemen; test it at commissioning.
• On-grid inverters must not run in parallel with a DG set without a synchroniser or zero-export control.
• Cable voltage rise causes many noon-time over-voltage trips; thicker, shorter cable is the first fix.

Check yourself

1. A 3 kW single-phase inverter (13 A) is 20 m from the main DB on 4 mm² copper. What is the drop %?
2. Why must the solar connection be on the consumer side of the net meter?
3. A customer's inverter trips every sunny noon with "grid over-voltage". Name two things you would check before blaming the inverter.