AC Circuits, Waveforms, and the Role of the Inverter

AC vs. DC: Two Kinds of Electrical Flow

Every PV cell produces direct current (DC) — electrons flow in one direction only, from the negative terminal to the positive terminal. If you plotted DC voltage on a graph over time, you would see a flat, horizontal line. That steady, unchanging characteristic makes DC ideal for charging batteries and for the internal workings of electronics, but the power grid runs on something different.

Alternating current (AC) reverses direction periodically. In the United States, the grid operates at 60 Hz, meaning the current completes 60 full cycles every second. On a graph, AC voltage traces a smooth sine wave — it rises to a positive peak, falls back through zero, drops to a negative peak, then returns to zero. One full trip from zero up, across, and back to zero is called one cycle. The time it takes to complete one cycle is the period (T), measured in seconds. Period and frequency are inversely related: T = 1 ÷ f. At 60 Hz, the period is approximately 16.7 milliseconds.

RMS Voltage: The Effective Value

Because AC voltage swings continuously between positive and negative peaks, quoting the peak value alone is not very useful for calculating real power. Engineers use root mean square (RMS) voltage, sometimes called the effective voltage, because it represents the equivalent DC voltage that would deliver the same power to a resistive load.

For a pure sine wave, the relationship between peak voltage and RMS voltage is:

> V_RMS = V_peak ÷ √2 ≈ V_peak × 0.707

US residential service is commonly described as 120 V AC. That figure is an RMS value. The actual peak voltage is approximately 170 V, and the swing from negative peak to positive peak (peak-to-peak) is about 340 V. When you work with inverter datasheets and utility interconnection requirements, RMS values are always the reference unless a document specifically states otherwise.

Frequency

Frequency is how many complete cycles occur per second, measured in hertz (Hz). North American grids are standardized at 60 Hz. Many other countries use 50 Hz. An inverter connected to the US grid must output AC at exactly 60 Hz. If the frequency drifts outside the grid's tolerance, protective relays and the inverter's own anti-islanding circuitry will disconnect the system — intentionally and quickly — to protect utility workers and equipment.

Power Factor

Power factor (PF) describes how effectively current is being used to do real work. It is the ratio of real power (watts) to apparent power (volt-amperes). A power factor of 1.0 means all the current is doing useful work. Inductive loads such as motors cause the current waveform to lag behind the voltage waveform, reducing power factor below 1.0. Grid-tied inverters are designed to operate at or very near unity (1.0) power factor, and some advanced inverters can actively supply reactive power to help support grid voltage. For most PV installation work, you will see power factor listed on inverter spec sheets as a value between 0.99 and 1.0 at rated output.

The String Inverter: Gateway Between PV and the Grid

A string inverter is the most common inverter type for residential and small commercial PV systems. It receives high-voltage DC from a series string of PV modules and converts that DC into grid-compatible AC. The main stages inside a string inverter are:

1. DC input filtering and protection — capacitors smooth the incoming DC; fuses or DC disconnect switches protect against overcurrent and ground faults. A ground fault protection device (GFPD) monitors for unintended current paths to ground and shuts the inverter down if one is detected.

2. DC-to-AC conversion — power transistors (typically IGBTs) switch the DC on and off at very high speed in a pattern that synthesizes a sine wave.

3. Output filtering — inductors and capacitors clean up the synthesized waveform so it closely matches the smooth utility sine wave.

4. Grid synchronization — the inverter constantly samples the utility voltage to match its output frequency and phase before allowing current to flow.

5. Display and communications — a screen or indicator lights show operating status, and many inverters include a data port or wireless interface for monitoring.

String inverters have clearly labeled DC input terminals (usually marked + and −, with the acceptable voltage range stamped nearby) and AC output terminals (usually marked L1, L2/N, and Ground for split-phase residential service). Never reverse these connections.

Why Grid Matching Matters

The utility grid is an enormous shared resource. For a PV system to feed power into it safely, the inverter's AC output must match the grid in three ways: voltage level (e.g., 240 V RMS for US split-phase residential), frequency (60 Hz), and phase angle. If any of these are mismatched, the inverter cannot synchronize and will not export power. More importantly, a mis-synchronized inverter could damage loads and create safety hazards for utility workers. This is why inverters include anti-islanding protection — they shut down automatically when the grid is de-energized, preventing the inverter from energizing a line that a lineworker believes is dead.

Safety Reminder

When examining any inverter — even a demonstration or cutaway unit — verify that lockout/tagout (LOTO) procedures have been completed and confirmed before touching internal components. DC input terminals on a string inverter may carry voltages exceeding 600 V from the PV array even when the inverter AC disconnect is open, because the array generates voltage whenever light is present. Wear appropriate PPE and treat all conductors as energized until proven otherwise with a calibrated meter.