Grid-Tied Solar Set-Up – SS2 Solar Photovoltaic Installation & Maintenance Lesson Note (First Term)

A grid-tied solar set-up is a solar power system that stays connected to the public (street) power supply.

Behavioural objectives

By the end of the lesson, students should be able to:

1. Explain how a grid-tied set-up differs from a stand-alone (“island”) set-up.
2. Identify the components of a grid-tied system and state the use of each.
3. Describe the connection steps on the DC side.
4. Describe the connection steps on the AC side, including synchronisation and earthing.
5. Carry out and explain the shutdown test for anti-islanding protection.

How does a grid-tied solar set-up work?

A grid-tied set-up uses a grid-tied inverter to feed solar power into the house wiring and the street supply together.

• Grid reference: the inverter matches the grid’s frequency and voltage.
• House first: power from the panels goes to the house loads first.
• Extra to the street: any surplus power flows out to the grid.
• Shutdown: the inverter switches off when the grid goes off.
• No battery: the inverter does not need a battery to work.

What is the difference between a grid-tied and a stand-alone set-up?

A stand-alone (island) set-up is a solar system with no link to the street supply, so the house stands alone and usually needs batteries.

A grid-tied set-up is a solar system linked to the street transformer, so the house works as a “team player” with the grid.

What is the goal of the demonstration?

The demonstration shows how a grid-tied inverter works with the street supply and why it shuts down when that supply fails.

1. How the grid-tied inverter “listens” to the grid.
2. How power flows from the panels to the house first, and then to the street.
3. Why the system shuts down if the street power goes off (the “anti-islanding” rule).

Safety rules

• 230V AC: this practical uses mains voltage. Never touch a bare wire.
• Main Switch: make sure it is OFF before making any connection.
• Live panels: panels produce DC voltage whenever light falls on them, so treat the DC wires as live.

What components are used in a grid-tied set-up?

A grid-tied set-up has a PV array, a DC isolator, a grid-tied inverter, an AC breaker, a distribution board, a meter and an earth line.

• PV array: the solar panels, which are the DC source.
• DC isolator: a switch that cuts the panels off from the inverter during maintenance.
• Grid-tied inverter: changes DC to AC and needs no battery. It uses the grid as its “reference”.
• Dedicated AC breaker: protects the inverter’s AC output cable in the distribution board.
• AC distribution (main) board: the point where the street power enters the house.
• Bi-directional meter: measures power flowing in both directions, from the grid to the house and from the house to the grid.
• Earth line: carries fault current safely to the ground.
• Multimeter: checks polarity and voltage before connection.

How do we connect the DC side?

The DC side is the part of the system between the panels and the inverter’s DC input.

1. Mounting: fix the panels facing South at a tilt of about 15 degrees, which suits Nigeria’s location.
2. Stringing: connect the panels in series to add up their voltages.
3. Voltage check: make sure the string voltage is within the inverter’s DC input range. Many grid-tied inverters need a fairly high DC voltage to start.
4. Polarity check: test with a multimeter. Red is positive (+) and black is negative (-).
5. DC isolator: install it between the panels and the inverter, and leave it OFF.
6. Inverter input: connect the string to the inverter’s DC input through the isolator.

• Pro-tip: swapped polarity can damage the inverter, so always check before connecting.

How do we connect the AC side?

The AC side is the part of the system where the inverter joins the street supply.

1. Main Switch OFF: confirm the supply is off before touching any AC wire.
2. The connection: connect the inverter’s AC output to a dedicated breaker in the distribution board.
3. Earthing: connect the inverter’s metal casing to the house’s earth line.
4. Switch on: turn on the AC breaker, then the DC isolator.
5. Synchronisation: the inverter does not start at once. It “listens” to the grid, usually for about a minute, before it supplies power.

What does the inverter match during synchronisation?

• Frequency: 50Hz in Nigeria.
• Voltage: around 230V for single-phase supply.

What is anti-islanding protection?

Anti-islanding protection is the safety feature that makes a grid-tied inverter stop supplying power when the grid goes off.

• The danger: a technician may be repairing a fault on the street pole.
• The risk: if the solar system keeps sending power to the street, it could electrocute the technician.
• The result: the inverter stays “dead” when the grid is down, even in bright sun.

How do we test anti-islanding protection?

The test is a shutdown test done while the system is running in sunlight.

1. Confirm the sun is shining and the inverter is supplying power.
2. Flip the Main Grid Switch to “OFF”.
3. Observe the inverter. It must shut down very quickly, within seconds at most.
4. Note that it stops even though the sun is bright, because it no longer senses the grid.
5. Switch the grid back ON and watch the inverter wait, synchronise and restart.

• Pass: the inverter stops supplying power as soon as the grid is switched off.
• Fail: the inverter keeps running. Stop the work and report the fault.

Evaluation

1. What is the main difference between a grid-tied inverter and a normal inverter?
2. What does a bi-directional meter measure?
3. Why do we connect the panels in series, and why do we install a DC isolator?
4. What two things must the inverter match before it starts supplying power?
5. Describe the shutdown test and say what anti-islanding protection prevents.

Assignment

Write a short report on a grid-tied set-up with two parts:

1. Using the example of a technician climbing a pole, explain in your own words why a grid-tied inverter must shut down when the street power goes off.
2. List, in order, the connection steps for the DC side and the AC side.