Power Flow (PFC)

PFC uses the following network records documented in Network Modeling: BUS, LINE, SWITCH, TRANSFO, TRFO, NRTP.

The additional records specific to power flow computations are documented below.

Load and Shunt Data

Load and shunt data are specified in an extended version of the BUS record:

BUS NAME VNOM PLOAD QLOAD BSHUNT QSHUNT ;

Field	Description	Unit
NAME	Bus name (max 8 characters)	—
VNOM	Nominal voltage	kV
PLOAD	Total active power load (positive = consumed)	MW
QLOAD	Total reactive power load (positive = consumed)	Mvar
BSHUNT	Nominal reactive power of constant-susceptance shunt (positive = capacitor)	Mvar
QSHUNT	Reactive power of constant-power shunt (positive = capacitor)	Mvar

The total reactive power $Q$ produced by both shunt components:

$$Q = \text{BSHUNT} \cdot \left(\frac{V}{V_{nom}}\right)^2 + \text{QSHUNT}$$

Note

The PLOAD, QLOAD, BSHUNT, and QSHUNT fields are ignored by RAMSES.

Generator Data

GENER NAME BUS P Q VIMP SNOM QMIN QMAX BR ;

Field	Description	Unit
NAME	Generator name (max 20 characters)	—
BUS	Connection bus name	—
P	Active power produced	MW
Q	Reactive power produced (ignored if VIMP ≠ 0)	Mvar
VIMP	Imposed voltage (0 = PQ bus, nonzero = PV bus)	pu
SNOM	Nominal apparent power	MVA
QMIN	Lower reactive power limit	Mvar
QMAX	Upper reactive power limit	Mvar
BR	Breaker status (0 = open)	—

For PV buses, if reactive limits are exceeded, the bus switches to PQ type with the limit enforced. It switches back to PV if the voltage returns past the setpoint.

A variant with additional fields PMIN and PMAX exists but these are currently ignored by STEPSS.

Only one generator is allowed per bus.

Note

A generator producing constant active and reactive powers can be modeled as a negative load using the BUS record (negative PLOAD/QLOAD) without a GENER record.

Slack Bus

A slack bus is mandatory for power flow. A PV-type generator must be connected to it.

SLACK NAME ;

PFC can handle only one connected network. If the graph is disconnected, only the sub-network containing the slack bus is treated.

Static Var Compensators (SVC)

The SVC is modeled with a controllable susceptance $B$ at a controlled bus $i$, regulating the voltage at a monitored bus $j$:

$$I_{Qi} = G(V_j^o - V_j) V_i \quad \text{(voltage control)}$$ $$I_{Qi} = B_{max} V_i \quad \text{(upper limit)} \quad\quad I_{Qi} = B_{min} V_i \quad \text{(lower limit)}$$

Data Format

SVC NAME CON_BUS MON_BUS V0 Q0 SNOM BMAX BMIN G BR ;

Field	Description	Unit
NAME	SVC name	—
CON_BUS	Controlled bus	—
MON_BUS	Monitored bus	—
V0	Voltage setpoint (0 = constant power mode)	pu
Q0	Reactive power setpoint (ignored if V0 ≠ 0)	Mvar
SNOM	Nominal reactive power	Mvar
BMAX	Max nominal reactive power	Mvar
BMIN	Min nominal reactive power	Mvar
G	Gain	pu
BR	Breaker status	—

It is not allowed to connect both a generator and an SVC to the same bus.

Transformer Ratio Adjustment

PFC can adjust transformer ratios to bring a controlled voltage inside a deadband $[V_{des} - \epsilon,\; V_{des} + \epsilon]$.

Via TRFO Record

The ratio corresponding to tap position $p$ ($1 \le p \le \text{NBPOS}$):

$$n = \frac{\text{NFIRST}}{100} + \frac{p-1}{\text{NBPOS}-1} \cdot \frac{\text{NLAST} - \text{NFIRST}}{100}$$

Relevant fields in the TRFO record: CONBUS, NFIRST, NLAST, NBPOS, TOLV, VDES.

Via LTC-V Record

LTC-V NAME CON_BUS NFIRST NLAST NBPOS TOLV VDES ;

Phase-Shifting Transformer Adjustment (PSHIFT-P)

For phase-shifting transformers, PFC can adjust the phase angle to bring the active power flow in a monitored branch inside a deadband $[P_{des} - \epsilon,\; P_{des} + \epsilon]$.

PSHIFT-P CONTRFO MONBRANCH PHAFIRST PHALAST NBPOS SIGN PDES TOLP ;

Field	Description	Unit
CONTRFO	Name of the controlled phase-shifting transformer (max 20 characters)	—
MONBRANCH	Name of the monitored branch (max 20 characters)	—
PHAFIRST	Phase angle at first tap position	degrees
PHALAST	Phase angle at last tap position	degrees
NBPOS	Number of tap positions	—
SIGN	+1 or -1 — determines direction of power flow increase with phase angle increase	—
PDES	Desired active power on monitored branch	MW
TOLP	Tolerance on active power	MW

The phase angle $\phi$ at tap position $p$ ($1 \le p \le \text{NBPOS}$) is:

$$\phi = \text{PHAFIRST} + \frac{p-1}{\text{NBPOS}-1}(\text{PHALAST} - \text{PHAFIRST})$$

Note

PFC performs a sensitivity analysis to determine whether the phase angle should be increased or decreased. If this analysis indicates a direction opposite to SIGN, a warning is issued and SIGN is ignored.

Note

A transformer cannot be controlled by both an LTC-V and a PSHIFT-P record.

Bus Voltages: Initial Values and Results (LFRESV)

On output, PFC produces a file with the computed bus voltage magnitudes and phase angles. These are stored in LFRESV records. The syntax is:

LFRESV BUS MODV PHASV ;

Field	Description	Unit
BUS	Bus name (max 8 characters)	—
MODV	Voltage magnitude	pu
PHASV	Voltage phase angle, referenced to slack bus	radians

Note

If LFRESV records are provided as input, they are used as initial voltages for Newton iterations.

Note

Default initialization: PQ buses start at 1 pu magnitude and 0 angle; PV buses start at the generator-specified voltage and 0 angle.

Note

The output LFRESV records from PFC can be fed back as input — this results in zero Newton iterations (round-trip property).

Note

LFRESV is the output of PFC that initializes RAMSES dynamic simulation.

PFC Computation Control Parameters

PFC uses Newton–Raphson iterations to solve the power flow equations. Convergence is achieved when both the active and reactive power mismatches fall below specified thresholds, all transformer ratio and phase-shift controls are satisfied, and all generators and SVCs are within their reactive limits.

The following records control the computation. Each record starts with $ and has a single numeric field.

Parameter	Default	Unit	Description
$SBASE	100	MVA	System base power (on which pu values are expressed)
$TOLAC	0.1	MW	Convergence tolerance on active power mismatch ($\epsilon_P$)
$TOLREAC	0.1	Mvar	Convergence tolerance on reactive power mismatch ($\epsilon_Q$)
$NBITMA	20	—	Maximum number of Newton iterations
$MISQLIM	20	MVA	Apparent power mismatch threshold below which generator/SVC reactive limits are checked and enforced (set to 0 to skip)
$MISBLOC	10	MVA	Apparent power mismatch threshold below which the Jacobian is kept constant
$MISADJ	10	MVA	Apparent power mismatch threshold below which transformer ratios and phase shifts are adjusted (set to 0 to skip)
$DIVDET	0	—	Set to 1 to activate divergence detection; 0 to skip

Note

Divergence is detected when $\varphi(k) > 1.1\,\varphi(k-1)$, where $\varphi$ is a norm of the power mismatches. The test is temporarily suspended following generator/SVC limit adjustments or transformer ratio/phase-shift adjustments.

Record Sharing Between PFC and RAMSES

The following table summarises which records are used by PFC and RAMSES respectively.

Record	PFC	RAMSES
BUS	All 6 fields	First 2 fields (NAME, VNOM)
LINE	All fields	All fields
SWITCH	All fields	All fields
NRTP	All fields	All fields
TRANSFO	All fields	All fields
TRFO	All fields	Fields 1–9 and 15 only
SHUNT	Ignored	All fields
GENER	All fields	Ignored
SVC	All fields	Ignored
SLACK	Used	Used
LFRESV	Input: initial values; Output: solution	Input: initial values for RAMSES
LTC-V	Used	Ignored
PSHIFT-P	Used	Ignored