AGC vs. AVC System: What is the Difference in Power Grids?
Document Type: Technical Articles
Document Published: 2026-07-18
Last Updated: 2026-08-03
Core Summary
AGC regulates active power to stabilize grid frequency; AVC regulates reactive power to stabilize nodal voltage. The two systems control two independent electrical quantities of the power grid and coordinate to guarantee safe and high-quality grid operation.
1. Basic Definition & Controlled Objects
AGC (Automatic Generation Control)
- Controlled quantity: Active Power (P) (kW/MW, real power transmitted for electricity supply)
- Core objectives: Maintain system nominal frequency at 50 Hz, balance tie-line interchange power among control areas, implement economic load dispatch
- Fundamental principle: The total active power generation must match total grid load; any imbalance directly causes frequency deviation
AVC (Automatic Voltage Control)
- Controlled quantity: Reactive Power (Q) (MVar, used to establish electromagnetic fields and sustain voltage levels)
- Core objectives: Keep bus voltages of the entire grid within qualified ranges, realize hierarchical & zonal local reactive power balance, reduce network losses
- Fundamental principle: Reactive power flow triggers line voltage drop; reactive power imbalance directly results in overvoltage or undervoltage
2. Comparative Table of Key Differences
| Comparison Item | AGC System | AVC System |
|---|---|---|
| Regulated Electrical Parameter | Active Power P, controls total power output | Reactive Power Q, controls voltage magnitude |
| Grid Stability Target | Frequency stability (50 Hz rated) | Nodal voltage stability |
| Regulating Equipment | Thermal power CCS & turbine governor, hydro turbines, active power control loops of wind/PV/energy storage inverters | Generator excitation systems, SVG, capacitor/reactor banks, on-load tap changers (OLTC) of main transformers, reactive power control loops of renewable inverters |
| Dispatching Control Hierarchy | Global unified control via provincial EMS, cross-area coordination | Hierarchical & zonal control (provincial dispatching + local dispatching + substations), local reactive power balance priority |
| Regulation Response Cycle | Real-time second-level frequency regulation, continuous dynamic tracking of load fluctuation | Minute-level optimized voltage regulation with slow tuning; fast reactive power support under fault conditions |
| Core Control Algorithm | ACE (Area Control Error), economic load dispatch | Nine-zone diagram, secondary voltage optimization, reactive power optimal power flow |
| Applicable Power Stations | Thermal, hydro, wind, PV and energy storage (all adjustable active power sources) | Power plants, substations, renewable energy stations (all sites equipped with reactive power regulation resources) |
| Major Operation Risks | Frequency collapse, grid splitting, unit overspeed / low-frequency vibration | Voltage instability, insulation breakdown, equipment failure due to undervoltage, soaring network losses |
3. Operating Principle & Regulation Logic Difference
AGC Operation Flow
- The dispatching center collects system frequency and tie-line power data to calculate ACE (Area Control Error);
- Distributes active power regulation setpoints to power plants;
- Thermal units adjust turbine steam intake, hydro units adjust wicket gates, renewable stations modify inverter active power output;
- Closed-loop feedback eliminates frequency and tie-line power deviations to realize secondary frequency regulation.
AVC Operation Flow
- The dispatching center collects bus voltage and reactive power flow data across the grid;
- Calculates optimal reactive power demand by zone and issues target voltage/reactive power setpoints;
- Power plants adjust excitation to increase/decrease generator reactive power; substations switch capacitor/reactor banks and adjust transformer tap positions; renewable inverters generate capacitive or inductive reactive power;
- Follow the principle of local reactive power balance to minimize long-distance reactive power transmission and reduce line losses.
4. Coupling Relationship Between Active & Reactive Power (Interaction of AGC and AVC)
- Adjusting AGC active power P narrows the generator’s reactive power regulation range, indirectly restricting AVC tuning capacity;
- Massive switching of reactive power Q via AVC redistributes grid power flow, slightly impacting system active power balance and frequency;
- Engineering specification requirement: Logical interlock shall be configured between AGC and AVC slave stations to avoid conflicting bidirectional regulation, with coordinated control strategies deployed.
5. Popular Analogy
- AGC = vehicle accelerator: It governs longitudinal driving force (active power) to stabilize traveling speed, corresponding to maintaining the grid rated frequency of 50 Hz.
- AVC = vehicle steering and ESP stability control system: It regulates lateral balance (reactive power) to stabilize vehicle posture, analogous to sustaining qualified nodal voltage distribution across the power grid.
6. Mandatory Grid Connection Requirements
- Conventional thermal and hydropower units must be equipped with both AGC and AVC regulation functions;
- Wind farms, PV power stations and energy storage systems are mandatorily required to deploy AGC (active power tracking) and AVC (voltage/reactive power regulation) per grid connection codes; grid connection approval is withheld for non-compliant facilities;
- Dispatching assessment metrics: AGC is evaluated by regulation rate and response accuracy; AVC is assessed by the voltage compliance rate and availability rate of reactive power compensation equipment.
