What Are the Operating Modes of a PCS?

In-Depth Technical Analysis of the Four Major PCS Control Modes

PQ · VF · VSG · Grid-Following (GFL) · Grid-Forming (GFM)

1 Panoramic Quick View

1.1 Core Comparison

Mnemonic: Grid-following relies on phase-locking, grid-forming relies on virtual synthesis; when the grid is strong use grid-following, when the grid is weak use grid-forming.

2 Why Does a PCS Need "Multiple Identities"?

The awkward position of an energy-storage PCS: to whom does the "controllable" output ultimately listen?

• Listen to the grid → Worker (PQ)
• Listen to itself → Boss (VF)
• Dress up as the fighter-jet version of the Boss → VSG

2 PQ Mode (Worker)

"Boss (the grid) tells me how much power, I output exactly that much power"

2.1 Control Architecture

PQ mode adopts the classic "dual-loop + PLL" structure: outer power loop + inner current loop + phase-locked loop. The PLL is the key; without it the mode cannot operate.

2.2 PLL's Fatal Weakness

2.3 Applicable Scenarios

✅ Grid connection to a strong grid (SCR > 3), AGC/AVC dispatch response, peak shaving and valley filling ❌ Not suitable for weak grids or black-start

3 VF Mode (Boss)

"Grid is gone? No problem, I'll be the boss!"

3.1 Control Architecture

VF mode has no PLL; it directly uses an internal reference (U = 400 V, f = 50 Hz) as the voltage source. Voltage outer loop + current inner loop achieves constant voltage and constant frequency.

3.2 Multi-Unit Paralleling: Droop Control

Multiple PCSs require only communication to automatically share power. Principle: when output is higher, frequency/voltage droops slightly → automatic balance.

Tuning Principle: droop coefficient × rated capacity of every PCS must be equal → only then is power sharing achieved.

3.3 Hard Constraints of VF

• Load < PCS capacity (otherwise voltage collapses)
• Short-circuit current only 1.2–1.5× (no inertia)
• Fixed frequency (no governor)
• Poor resilience to impact loads (undamped second-order system)

3.4 Applicable Scenarios

✅ Microgrid islanded operation, black-start, UPS mode, master-slave switching of multiple PCSs ❌ Not suitable for scenarios requiring inertia support

4 VSG — Virtual Synchronous Generator (the Steady Boss)

"I pretend to be a real synchronous generator, with inertia!"

4.1 Core Idea

Inside the VSG there is a virtual flywheel that emulates the rotor motion of a synchronous generator, making the PCS behave like a real generator.

4.2 Complete Mathematical Model

(1) Rotor motion equation (core): J × angular acceleration = mechanical torque − electromagnetic torque − damping × frequency deviation

(2) Virtual impedance: EMF = terminal voltage + virtual impedance × current

(3) Excitation regulation: EMF = initial value + reactive-droop coefficient × (Q_ref − Q)

(4) Governor: mechanical torque = initial value + active-droop coefficient × (P_ref − P)

4.3 Influence of J / D / X_s Parameters

4.4 Frequency-Response Comparison

• 🔴 VF: on a sudden load change it drops instantly to 49.5 Hz, may trip
• 🟢 VSG: drops gradually to 49.8 Hz over 0.5 s, then recovers smoothly

The VSG's "dip slowly and recover" buys time for grid equipment to react — this is the value of inertia.

4.5 Golden Rules of Tuning

1. J must not exceed the system's equivalent inertia — otherwise frequency response is too slow
2. D must be greater than J × angular speed — otherwise underdamped oscillation ⚠️
3. X_s must match the grid impedance — otherwise power coupling
4. J/D of all GFM units must be coordinated — otherwise power circulating currents between parallel units
5. Frequency-droop coefficient must be uniform across the whole grid — otherwise power-grabbing

4.6 Applicable Scenarios

✅ Connection to weak grids (SCR < 2), high-renewable-penetration areas, black-start, islanded microgrids, frequency and peak regulation

5 Grid-Forming vs. Grid-Following

5.1 System Evolution Trend

In the renewable era, grid-following converters are proliferating, and the grid is becoming increasingly fragile. Grid-forming energy storage is the solution.

5.2 Core Comparison

5.3 Grid Strength (SCR) vs. Control Mode

6 Terminology Quick-Reference Table