Grid and PCC Subsystem Simulation Model

This document specifies the equivalent simulation model for the Grid and PCC (Point of Common Coupling) subsystem. It models the grid as a voltage source with a short-circuit impedance, computes the power flow at the PCC, simulates the metering and protection blocks, and defines the interfaces to the EMS and BESS systems.

(For automated code generation, refer to 05_grid_simulation_model_ai.md.)

Terms and Abbreviations

Term / Abbreviation	Definition
PCC	Point of Common Coupling — the physical point where the plant connects to the grid
CT/PT	Current Transformer / Potential Transformer — scales high-voltage sensing to meter-level inputs
IED	Intelligent Electronic Device — the protection relay controlling the grid breaker
OCV	Open Circuit Voltage (grid Thevenin source voltage, Vg)
Zg	Grid short-circuit impedance (Rg + jXg)
Vpcc	PCC voltage magnitude (kV)
Ig	Grid current injected at PCC (A, positive = flowing from grid to plant)
P_pcc	Active power at PCC (MW, positive = plant generating / exporting to grid)
Q_pcc	Reactive power at PCC (MVAr, positive = plant generating / exporting Q)
f	Grid frequency (Hz)
SIL	Short-circuit Impedance Level — characterises how "stiff" the grid is
FRT	Fault Ride-Through — plant must remain connected during brief grid voltage dips

System Overview

The Grid and PCC subsystem sits at the boundary between the plant and the external grid. The EMS reads measurements from this subsystem and adjusts plant output to track setpoints at the PCC. The BESS and other assets are connected to the Main AC Bus which is directly linked to the PCC.

Block Diagram

flowchart TD
    subgraph GRID_SYS ["Grid & PCC Subsystem"]
        GridSrc["AC Grid Voltage Source\n(Thevenin: Vg, Zg = Rg + jXg)"]
        GridLine["Grid Line / Feeder\n(series impedance)"]
        CTPT["CT/PT Sensors\n(scales V, I to meter level)"]
        PCCMeter["PCC Meter\n(computes P, Q, V, f, energy)"]
        BreakerIED["PCC Breaker + Protection IED\n(overcurrent, under-voltage, FRT)"]
        MainACBus["Main AC Bus / PCC\n(voltage node)"]
    end

    subgraph PLANT ["Plant Assets"]
        PVInv["PV Inverter\n(P_pv injected)"]
        WindConv["Wind Converter\n(P_wind injected)"]
        PCSInv["BESS PCS\n(P_bess injected)"]
    end

    subgraph EMS_SYS ["EMS"]
        EMSPCC["PCC Tracking Controller"]
        EMSALM["Alarm Manager"]
    end

    GridSrc -->|"Vg (source)"| GridLine
    GridLine -->|"Ig, Vg_drop"| MainACBus
    MainACBus -.->|"IF-CT01: V, I analog sensing"| CTPT
    CTPT -->|"IF-CT02: scaled V, I"| PCCMeter
    CTPT -->|"IF-CT03: scaled V, I"| BreakerIED

    PVInv -->|"I_pv"| MainACBus
    WindConv -->|"I_wind"| MainACBus
    PCSInv -->|"I_bess"| MainACBus

    PCCMeter -->|"IF-GM01: P_pcc, Q_pcc, V_pcc, f, energy"| EMSPCC
    PCCMeter -->|"IF-GM02: P_pcc, Q_pcc, V_pcc, f, energy"| EMSALM
    BreakerIED -->|"IF-BR01: breaker_status, trip, alarms"| EMSALM
    BreakerIED -->|"IF-BR02: trip command"| MainACBus

Interface Map (Grid ↔ EMS ↔ BESS)

This table shows all cross-system interfaces and ensures signal names are consistent across the three simulation models.

Interface ID	From	To	Signal Name	Type	Unit	Rate
IF-GM01	PCC Meter	EMS PCC Controller	`P_pcc`	float	MW	0.5–1 s
IF-GM01	PCC Meter	EMS PCC Controller	`Q_pcc`	float	MVAr	0.5–1 s
IF-GM01	PCC Meter	EMS PCC Controller	`V_pcc`	float	kV	0.5–1 s
IF-GM01	PCC Meter	EMS PCC Controller	`f`	float	Hz	0.5–1 s
IF-GM02	PCC Meter	EMS Alarm Manager	`P_pcc`, `Q_pcc`, `V_pcc`, `f`	float	MW, MVAr, kV, Hz	0.5–1 s
IF-BR01	PCC Breaker IED	EMS Alarm Manager	`breaker_status`	bool	OPEN/CLOSED	Event
IF-BR01	PCC Breaker IED	EMS Alarm Manager	`trip_alarm`	bool	True/False	Event
IF-BR02	EMS Alarm Manager	PCC Breaker IED	`breaker_open_cmd`	bool	True/False	Event
IF-BE01	BESS PCS (from BESS model)	Main AC Bus	`I_bess`	float	A	Per step
IF-BE02	EMS Coordinator (from EMS model)	BESS PCS	`P_bess_setpoint`	float	MW (+Chg/-Dis)	0.5–2 s
IF-PV01	PV Inverter	Main AC Bus	`I_pv`	float	A	Per step
IF-WD01	Wind Converter	Main AC Bus	`I_wind`	float	A	Per step

Sign convention (consistent across BESS, EMS, and Grid models): - P_pcc > 0 → plant exports power to grid (generating) - P_pcc < 0 → plant imports power from grid (consuming/charging) - P_bess_setpoint > 0 → BESS is charging (consuming from AC bus) - P_bess_setpoint < 0 → BESS is discharging (injecting to AC bus)

1. AC Grid Voltage Source Model

The grid is modelled as a Thevenin equivalent: an ideal sinusoidal AC voltage source (Vg) behind a short-circuit impedance (Zg). For a simulation step size of 0.5–1 s, phasor-domain (steady-state) representation is sufficient.

1.1 Grid Thevenin Model

flowchart LR
    Vg["Vg\n(Grid source voltage)"]
    Zg["Zg = Rg + jXg\n(Grid impedance)"]
    PCC["PCC / Main AC Bus\n(Vpcc)"]

    Vg -->|"Ig flows through"| Zg --> PCC

Variables: - Vg: Grid open-circuit (Thevenin) source voltage (V RMS phase-to-phase). Default: 400 V - Ig: Current injected from the grid at PCC (A). Positive = grid supplying plant. - Vpcc: Resulting PCC voltage at the bus (V)

Parameters: - Rg: Grid resistance (Ohms). Default: 0.01 Ohms (stiff grid) - Xg: Grid reactance = 2 * pi * f * Lg (Ohms). Default: 0.05 Ohms - f_nom: Nominal grid frequency (Hz). Default: 50 Hz

1.2 PCC Voltage Calculation (phasor model)

The PCC voltage results from the grid source voltage minus the drop across the grid impedance:

Vpcc = Vg - Ig * (Rg + j*Xg)

For simplified scalar simulation (ignoring phase angle):

Vpcc ≈ Vg - Ig * Rg (resistive drop only, for slow EMS time scale)

PCC current from asset injections:

I_plant = I_pv + I_wind + I_bess (sum of all plant currents at the bus)

Ig = -I_plant (grid supplies what the plant does not)

1.3 Frequency Model

Grid frequency is treated as an independent disturbance input. In a simple simulation it is held constant or given as a time-series input signal:

f[k] = f_nom + f_disturbance[k]

For FRT testing: apply a frequency step or ramp (f_disturbance) to the input and verify that the EMS Alarm Manager responds correctly.

Default values:

Parameter	Description	Default
`Vg`	Grid source voltage (V, RMS line-to-line)	`400 V`
`Rg`	Grid resistance	`0.01 Ohms`
`Xg`	Grid reactance at 50 Hz	`0.05 Ohms`
`f_nom`	Nominal frequency	`50 Hz`
`f_disturbance`	Added frequency disturbance	`0.0 Hz`

2. CT/PT Sensor Model

The CT/PT sensors are hardware transformers that scale the high-voltage bus signals to a safe low-level range suitable for the PCC meter input. In simulation, they are modelled as ideal scaling blocks with an optional noise term for realism.

Variables: - V_bus: Actual PCC bus voltage (high-level, kV) - I_bus: Actual PCC current (high-level, A) - V_sec: Secondary voltage output to meter (low-level, V) - I_sec: Secondary current output to meter (low-level, A)

Equations:

V_sec = V_bus / PT_ratio + V_noise

I_sec = I_bus / CT_ratio + I_noise

Parameters:

Parameter	Description	Default
`PT_ratio`	Potential transformer ratio	`100` (e.g., 40 kV → 400 V)
`CT_ratio`	Current transformer ratio	`1000` (e.g., 1000 A → 1 A)
`V_noise_std`	Gaussian voltage noise std dev	`0.001 V`
`I_noise_std`	Gaussian current noise std dev	`0.001 A`

3. PCC Meter Model

The PCC meter receives the scaled secondary signals from the CT/PT and computes the power quality measurements. These are the primary feedback signals consumed by the EMS.

3.1 Computed Measurements

Active Power (MW):

P_pcc = (V_sec * I_sec * cos(phi)) * PT_ratio * CT_ratio / 1,000,000

Simplified scalar form for simulation:

P_pcc = P_pv + P_wind + P_bess

(Sum of all plant asset power injections, positive = export. This is the algebraic sum at the bus node.)

Reactive Power (MVAr):

Q_pcc = Q_pv + Q_wind + Q_bess + Q_grid

(Q from each asset; Q_grid is the reactive current drawn from or supplied to the grid.)

Voltage magnitude (kV):

V_pcc = Vpcc / 1000

Frequency (Hz):

f = f_nom + f_disturbance

3.2 Measurement Output with Data Quality

The meter applies a timestamp and quality flag to each published value:

MeterOutput = {
  "P_pcc":  P_pcc,          # MW
  "Q_pcc":  Q_pcc,          # MVAr
  "V_pcc":  V_pcc,          # kV
  "f":      f,              # Hz
  "energy": energy_acc,     # MWh (accumulated integral of P_pcc * dt)
  "quality": "GOOD",        # GOOD | SUSPECT | INVALID
  "timestamp": t_current    # ISO 8601 UTC
}

Energy accumulation (integrate per step):

energy[k] = energy[k-1] + (P_pcc[k] * delta_t / 3600) (converts Ws to Wh, /1e6 for MWh)

Parameters:

Parameter	Description	Default
`meter_update_rate`	How often meter publishes values (s)	`1 s`
`data_validity_window`	Age beyond which quality = SUSPECT (s)	`5 s`

4. PCC Breaker and Protection IED Model

The breaker/IED model controls the grid interconnection switch. In simulation it is a logical gate — the breaker is either CLOSED (operating) or OPEN (disconnected). The IED applies protection functions and can trip the breaker autonomously.

4.1 Breaker State Machine

stateDiagram-v2
    [*] --> OPEN : Startup
    OPEN --> CLOSED : close_cmd received AND enable_conditions_ok
    CLOSED --> OPEN : trip_cmd from IED, or open_cmd from EMS
    CLOSED --> OPEN : Protection trip condition
    OPEN --> OPEN : [default hold]

4.2 IED Protection Functions

The IED monitors conditions and trips the breaker automatically on fault:

Protection	Signal Checked	Trip Condition	Default Threshold
Overcurrent (OC)	`I_bus`	abs(I_bus) > `I_trip`	`I_trip = 1500 A`
Under-Voltage (UV)	`V_pcc`	V_pcc < `V_uv_trip` for > `t_uv`	`V_uv = 0.80 * Vnom`, `t_uv = 3 s`
Over-Voltage (OV)	`V_pcc`	V_pcc > `V_ov_trip` for > `t_ov`	`V_ov = 1.10 * Vnom`, `t_ov = 0.5 s`
Under-Frequency (UF)	`f`	f < `f_min` for > `t_uf`	`f_min = 47.5 Hz`, `t_uf = 4 s`
Over-Frequency (OF)	`f`	f > `f_max` for > `t_of`	`f_max = 51.5 Hz`, `t_of = 0.5 s`

4.3 IED Output Signals (sent to EMS via IF-BR01)

Signal Name	Type	Description
`breaker_status`	bool	TRUE = CLOSED, FALSE = OPEN
`trip_alarm`	bool	TRUE = IED triggered a protection trip
`trip_cause`	string	"OC", "UV", "OV", "UF", "OF", "Manual"
`ied_alarms`	list	Active protection alerts

Parameters:

Parameter	Description	Default
`I_trip`	Overcurrent trip threshold (A)	`1500 A`
`V_uv_trip`	Under-voltage trip level (fraction of Vnom)	`0.80`
`V_ov_trip`	Over-voltage trip level (fraction of Vnom)	`1.10`
`f_min_ied`	IED under-frequency trip (Hz)	`47.5 Hz`
`f_max_ied`	IED over-frequency trip (Hz)	`51.5 Hz`
`trip_delay_uv`	Under-voltage hold time before trip (s)	`3.0 s`
`trip_delay_ov`	Over-voltage hold time before trip (s)	`0.5 s`

5. Fault Ride-Through (FRT) Simulation

FRT tests verify the plant remains grid-connected during brief voltage dips. Implement as an input scenario applied to Vg and f_disturbance:

Test Scenario	`Vg` Change	Duration	Expected Result
Shallow dip	80% of Vnom	3 s	Plant stays connected, IED does not trip
Deep dip	5% of Vnom	0.5 s	Plant stays connected per EN 50549 / grid code
Frequency step	+/-2 Hz	10 s	EMS adjusts output, no IED trip
Frequency ramp	+0.5 Hz/s rise	5 s	EMS responds, Alarm Manager triggers

Apply the fault by setting Vg[k] = Vg_nom * dip_factor for the duration. Observe V_pcc, P_pcc, breaker_status, and EMS mode transitions.

6. Calibratable Parameters — Summary Table

Block	Parameter	Description	Default
Grid Source	`Vg`	Grid source voltage (V, line-to-line RMS)	`400 V`
Grid Source	`Rg`	Grid Thevenin resistance	`0.01 Ω`
Grid Source	`Xg`	Grid Thevenin reactance at 50 Hz	`0.05 Ω`
Grid Source	`f_nom`	Nominal frequency	`50 Hz`
CT/PT	`PT_ratio`	Potential transformer ratio	`100`
CT/PT	`CT_ratio`	Current transformer ratio	`1000`
PCC Meter	`meter_update_rate`	Publish rate (s)	`1 s`
PCC Meter	`data_validity_window`	SUSPECT age threshold (s)	`5 s`
Breaker IED	`I_trip`	Overcurrent trip (A)	`1500 A`
Breaker IED	`V_uv_trip`	Under-voltage trip (fraction)	`0.80`
Breaker IED	`V_ov_trip`	Over-voltage trip (fraction)	`1.10`
Breaker IED	`f_min_ied`	Under-frequency trip (Hz)	`47.5 Hz`
Breaker IED	`f_max_ied`	Over-frequency trip (Hz)	`51.5 Hz`