Inverters, Types, Ratings and Energy Supply for SS 1

Inverters

Performance Objectives

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

1. explain the meaning and function of inverters;
2. identify different inverters;
3. determine inverter ratings for different applications;
4. calculate the daily energy supply to an inverter.

Meaning and Function of Inverters

An inverter is an electronic device that converts direct current (DC) electricity into alternating current (AC) electricity.

Solar panels generate direct current (DC) when sunlight strikes their photovoltaic cells. Similarly, solar batteries store electrical energy in DC form. However, most household and industrial appliances, such as television sets, refrigerators, fans, and computers, are designed to operate on alternating current (AC). The inverter serves as the bridge between the DC power source and AC electrical appliances.

The primary functions of an inverter in a solar PV system include:

1. Power Conversion: Changing DC voltage from solar panels or batteries into standard AC voltage.
2. Voltage Regulation: Maintaining a stable AC output voltage despite fluctuations in the input DC voltage from the battery or solar array.
3. Frequency Control: Ensuring the output AC electricity matches the standard utility frequency, which is 50 Hertz (Hz) in Nigeria.
4. System Protection: Monitoring the system and shutting down automatically during electrical faults, overloads, under-voltage, or over-voltage conditions to protect both the inverter and connected appliances.

Types of Inverters

Inverters are classified based on their system configuration and the type of AC waveform they produce.

Classification by System Configuration

Based on how they connect to the solar PV system and the utility grid, inverters are grouped into three main categories:

1. Off-Grid (Stand-Alone) Inverters: These inverters operate independently of the public utility grid. They draw DC power from a battery bank charged by solar panels and convert it to AC power. They are commonly used in remote areas or locations with unreliable grid power.
2. Grid-Tied (On-Grid) Inverters: These inverters connect directly to the public utility grid. They convert DC power from solar panels into AC power and feed it directly into the home or grid. They do not require batteries, and they automatically shut down during power outages for safety.
3. Hybrid Inverters: These inverters combine the features of both off-grid and grid-tied systems. They can manage power from solar panels, batteries, and the utility grid simultaneously, directing energy where it is needed most and providing backup power during grid failures.

Classification by Output Waveform

Alternating current is characterised by a repeating wave pattern. Inverters produce different types of wave patterns, which affect how appliances perform:

1. Pure Sine Wave Inverters: These inverters produce an AC waveform that is identical to, or cleaner than, the utility grid power. This smooth, continuous wave is highly efficient and safe for all appliances, especially sensitive electronics like laptops, medical equipment, and microwave ovens.
2. Modified Sine Wave Inverters: These inverters produce a stepped waveform that approximates a sine wave. While cheaper to manufacture, they can cause some appliances, such as fans and motors, to hum, run hotter, or operate less efficiently.
3. Square Wave Inverters: These inverters produce a simple, blocky waveform. They are the cheapest type but are only suitable for simple resistive loads like incandescent light bulbs or basic heating elements. They can damage sensitive electronic devices.

Ratings of Inverters

Inverter ratings indicate the electrical limits and capabilities of the device. These specifications are printed on the manufacturer’s nameplate attached to the inverter housing.

The key ratings found on an inverter nameplate include:

1. Continuous Power Rating (Watts or Volt-Amperes): The maximum power the inverter can continuously deliver to connected loads under normal operating conditions. It is measured in Watts (W) or Volt-Amperes (VA).
2. Surge (Peak) Power Rating: The maximum power the inverter can deliver for a very short duration (usually a few seconds) to start motorized appliances like refrigerators, water pumps, or air conditioners, which require high initial current.
3. Input DC Voltage: The nominal DC voltage required from the battery bank or solar array. Common nominal DC input voltages are 12V, 24V, and 48V.
4. Output AC Voltage: The AC voltage delivered to the appliances. In Nigeria, the standard output voltage is typically 220V, 230V, or 240V.
5. Output Frequency: The frequency of the AC output, which must match the local grid standard. In Nigeria, this is 50 Hz.
6. Efficiency: The ratio of AC power output to DC power input, expressed as a percentage. High-quality inverters typically have efficiencies between 85% and 95%.

Determining Inverter Ratings for Different Applications

To select the correct inverter for a specific application, the total power demand of all appliances that will run simultaneously must be determined. The continuous power rating of the selected inverter should always be higher than the total connected load to prevent overloading and ensure system reliability.

A safety margin of at least 20% to 25% is standard practice when sizing an inverter. The formula for determining the minimum inverter power rating is:

$$\text{Minimum Inverter Rating (W)} = \text{Total Simultaneous Load (W)} \times 1.25$$

Worked Example: Sizing an Inverter

A household intends to run the following appliances simultaneously using a solar PV system:

1. 5 LED bulbs rated at 15W each
2. 1 Television set rated at 100W
3. 2 Standing fans rated at 60W each

Determine the appropriate continuous power rating for the inverter.

Step 1: Calculate the total simultaneous load.

$$\text{Power of Bulbs} = 5 \times 15\text{ W} = 75\text{ W}$$

$$\text{Power of Television} = 1 \times 100\text{ W} = 100\text{ W}$$

$$\text{Power of Fans} = 2 \times 60\text{ W} = 120\text{ W}$$

$$\text{Total Load} = 75\text{ W} + 100\text{ W} + 120\text{ W} = 295\text{ W}$$

Step 2: Apply the 25% safety margin.

$$\text{Minimum Inverter Rating} = 295\text{ W} \times 1.25 = 368.75\text{ W}$$

Based on this calculation, a standard inverter with a continuous rating of at least 400W or 500W should be selected.

Calculating Daily Energy Supply to an Inverter

Daily energy supply refers to the total amount of electrical energy that must be supplied to the inverter input from the DC source (battery bank or solar array) over a 24-hour period to meet the daily AC load demand. This calculation must account for energy losses within the inverter due to its efficiency rating.

The mathematical steps to establish the daily energy supply are:

1. Calculate the daily energy consumption of each AC appliance in Watt-hours (Wh) by multiplying its power rating by its daily operating hours.
2. Sum the energy consumption of all appliances to find the total daily AC energy demand.
3. Divide the total daily AC energy demand by the inverter’s efficiency (expressed as a decimal) to find the required daily DC energy supply to the inverter.

The formulae used are:

$$\text{Daily AC Energy Demand (Wh)} = \text{Power (W)} \times \text{Operating Hours (h)}$$

$$\text{Daily DC Energy Supply to Inverter (Wh)} = \frac{\text{Total Daily AC Energy Demand (Wh)}}{\text{Inverter Efficiency (decimal)}}$$

Worked Example 1: Calculating Daily Energy Supply

A school computer laboratory runs the following appliances daily:

1. 4 Desktop computers rated at 150W each, operating for 5 hours per day.
2. 6 LED tubes rated at 20W each, operating for 8 hours per day.
3. 2 Wall fans rated at 80W each, operating for 6 hours per day.

If the system uses an inverter with an efficiency rating of 88% (0.88), calculate the daily energy supply required from the DC source to the inverter.

Step 1: Calculate the daily AC energy demand for each appliance type.

$$\text{Energy for Computers} = 4 \times 150\text{ W} \times 5\text{ h} = 3000\text{ Wh}$$

$$\text{Energy for LED Tubes} = 6 \times 20\text{ W} \times 8\text{ h} = 960\text{ Wh}$$

$$\text{Energy for Wall Fans} = 2 \times 80\text{ W} \times 6\text{ h} = 960\text{ Wh}$$

Step 2: Calculate the total daily AC energy demand.

$$\text{Total Daily AC Energy Demand} = 3000\text{ Wh} + 960\text{ Wh} + 960\text{ Wh} = 4920\text{ Wh}$$

Step 3: Calculate the daily DC energy supply to the inverter.

$$\text{Daily DC Energy Supply} = \frac{4920\text{ Wh}}{0.88} \approx 5590.91\text{ Wh}$$

Therefore, the DC power source must supply approximately 5590.91 Watt-hours (or 5.59 kWh) of energy to the inverter daily to satisfy the school’s AC load requirements.

Worked Example 2: Calculating Daily Energy Supply for a Small Shop

A barber shop operates the following loads daily:

1. 2 Hair clippers rated at 15W each, operating for 8 hours per day.
2. 1 Sound system rated at 50W, operating for 10 hours per day.
3. 1 Television set rated at 80W, operating for 6 hours per day.

Determine the daily energy supply to an inverter with an efficiency of 90% (0.90).

Step 1: Calculate the daily AC energy demand for each appliance.

$$\text{Energy for Clippers} = 2 \times 15\text{ W} \times 8\text{ h} = 240\text{ Wh}$$

$$\text{Energy for Sound System} = 1 \times 50\text{ W} \times 10\text{ h} = 500\text{ Wh}$$

$$\text{Energy for Television} = 1 \times 80\text{ W} \times 6\text{ h} = 480\text{ Wh}$$

Step 2: Calculate the total daily AC energy demand.

$$\text{Total Daily AC Energy Demand} = 240\text{ Wh} + 500\text{ Wh} + 480\text{ Wh} = 1220\text{ Wh}$$

Step 3: Calculate the daily DC energy supply to the inverter.

$$\text{Daily DC Energy Supply} = \frac{1220\text{ Wh}}{0.90} \approx 1355.56\text{ Wh}$$

The DC source must supply 1355.56 Watt-hours (or 1.36 kWh) of energy to the inverter daily.

How to Read Inverter Nameplates and Verify Ratings

This practical procedure guides learners on how to safely inspect an inverter, read its nameplate specifications, and verify its operating parameters using a multimeter.

The required materials and tools are:

1. An offline/disconnected solar inverter
2. A digital multimeter with test probes
3. An inverter nameplate or technical datasheet
4. A calculator
5. Insulated hand tools

The procedure involves the following steps:

1. Ensure the inverter is completely disconnected from both the DC battery source and the AC utility grid or load distribution board.
2. Locate the manufacturer’s nameplate on the back or side panel of the inverter housing.
3. Read and record the following parameters from the nameplate: Model Number, Continuous Power Rating (W/VA), Nominal DC Input Voltage (V), AC Output Voltage (V), Output Frequency (Hz), and Efficiency (%).
4. Set the digital multimeter to the DC Voltage mode.
5. Measure the voltage across the battery bank terminals connected to the inverter input to verify if it matches the nominal DC input voltage specified on the inverter nameplate. Record this value.
6. With the system safely turned on and running under supervision, switch the multimeter to AC Voltage mode and carefully measure the AC output voltage at the inverter’s output terminals. Verify if it matches the output AC voltage rating (e.g., 220V) on the nameplate.
7. Turn off all equipment and return the tools to their proper storage locations.

Summary of Inverter Parameters

The table below summarizes typical inverter parameters for common residential installations in Nigeria:

Review Questions

Answer the following questions to assess understanding of the unit:

1. Define an inverter and state its primary function in a solar PV system.
2. List and explain three types of inverters based on their system configuration.
3. Why is a pure sine wave inverter preferred over a square wave inverter for sensitive electronic appliances?
4. Identify four key technical specifications commonly found on an inverter nameplate.
5. A clinic uses 4 LED lamps (10W each) for 12 hours daily, and 1 vaccine refrigerator (120W) for 24 hours daily. If the inverter efficiency is 85%, calculate the daily DC energy supply required by the inverter.