What is Grid Interconnection? The Fundamentals Explained

Grid interconnection refers to the linking of power generation systems to the utility grid. This article explains the fundamental mechanisms of interconnection and the necessary safety regulations for stable power distribution.

Did you know that until the 1980s, appliances in Japan often required a manual switch to adjust to the local power frequency? This was because the AC frequency differs between Eastern Japan (50 Hz) and Western Japan (60 Hz). While modern appliances switch frequencies automatically, this regional difference still affects the power grid. Since power cannot be directly transmitted between different frequency zones, frequency conversion is required.

To enable power interchange between the 50 Hz and 60 Hz regions, specific facilities like the Shin-Shinano and Sakuma frequency conversion stations, as well as the Higashi-Shimizu substation, connect these distinct power grids. This process of linking different power systems is a prime example of "grid interconnection."

Broadly speaking, grid interconnection refers to the parallel operation of multiple power systems--such as those with different voltages or AC/DC characteristics--to allow for the interchange of electricity. Because these systems operate on different transmission standards, connecting them requires precise conversion technology.

A familiar example is a residential photovoltaic (PV) system. During the day, households use the power generated by solar panels and charge their batteries, selling any surplus electricity back to the utility company. At night, they rely on battery storage or grid power. However, solar panels and batteries provide Direct Current (DC), while the utility grid uses Alternating Current (AC). Connecting these two requires a system to convert and synchronize the power.

This concept scales up for utility-scale PV plants (often called "Mega Solar" in Japan). In these large facilities, solar inverters convert the DC power generated by the panels into AC power. They adjust voltage levels and synchronize the output to match the transmission grid. This process requires both DC-to-AC conversion and voltage transformation--functions performed by advanced power supply technologies.

In grid interconnection, the core challenge is bridging the gap between DC sources (like solar panels and batteries) and the AC grid (typically 100 V or 200 V for residential use). The system must boost the DC voltage and convert it to AC. This is precisely the function of the bi-directional power supplies discussed in our previous article.

What are the key components of grid interconnection? Let's look at a residential solar power system as an example.

To connect to the utility grid, a "Solar Inverter" (or PV Inverter) is essential. This device draws DC power from the solar modules, converts it into AC, and synchronizes the phase and voltage with the grid. The system typically consists of the inverter unit and a grid protection device.

Modern inverters are equipped with Maximum Power Point Tracking (MPPT), harmonic suppression, and reverse power flow control. They convert DC power to match the grid's quality and supply it to home appliances. Any surplus power is sent (sold) back to the grid.

Grid protection devices are crucial for stability. Solar power generation fluctuates with the weather. Sudden changes in sunlight can cause voltage instability, which puts stress on the grid. To mitigate this, systems often incorporate automatic voltage regulators and battery storage to stabilize the output. Additionally, if a problem occurs on the utility grid side (such as a blackout), the system must detect it and perform "islanding prevention"--disconnecting from the grid to ensure safety. The solar inverter integrates these conversion and stabilization functions into a single system.

The importance of these control mechanisms is growing. For instance, in October 2018, Kyushu Electric Power had to curtail solar output because supply exceeded demand during favorable weather conditions. Both residential and utility-scale solar systems must be designed not just to generate power, but to contribute to a stable and reliable energy supply.

Stable voltage control is critical for grid interconnection. This stability is equally important for the DC power supplies used inside electronic devices. Most electronics, from home appliances to automotive systems, operate on DC voltages.

Inside these devices, AC grid power is converted to DC via transformers, rectifiers, and smoothing circuits. DC-DC converters then adjust this voltage to the specific levels required by internal circuits. If different voltage sources are connected improperly, or if the supply is unstable, it can lead to overcurrent, circuit damage, or device failure. Protection mechanisms, such as breakers, are used to disconnect the system during such anomalies.

Grid interconnection relies heavily on "AC/DC conversion" and "voltage transformation." Since the 1990s, the evolution of power semiconductors has revolutionized this field. While older systems used complex combinations of discrete parts, modern power semiconductors can handle high voltages and large currents with greater efficiency.

However, handling high power generates heat, which can lead to energy loss and component failure. To address this, the industry is adopting next-generation materials like Silicon Carbide (SiC) and Gallium Nitride (GaN). These materials significantly reduce power loss and improve heat dissipation.

In our next column, we will explore the four main functions of power semiconductors: converting DC to AC, converting AC to DC, frequency conversion, and DC voltage transformation.