def helm_coefficients_josep(Ybus, Yseries, V0, S0, Ysh0, pq, pv, sl, pqpv, tolerance=1e-6, max_coeff=30, verbose=False):
"""
Holomorphic Embedding LoadFlow Method as formulated by Josep Fanals Batllori in 2020
THis function just returns the coefficients for further usage in other routines
:param Yseries: Admittance matrix of the series elements
:param V0: vector of specified voltages
:param S0: vector of specified power
:param Ysh0: vector of shunt admittances (including the shunts of the branches)
:param pq: list of pq nodes
:param pv: list of pv nodes
:param sl: list of slack nodes
:param pqpv: sorted list of pq and pv nodes
:param tolerance: target error (or tolerance)
:param max_coeff: maximum number of coefficients
:param verbose: print intermediate information
:return: U, X, Q, iterations
"""
npqpv = len(pqpv)
npv = len(pv)
nsl = len(sl)
n = Yseries.shape[0]
# --------------------------- PREPARING IMPLEMENTATION -------------------------------------------------------------
U = np.zeros((max_coeff, npqpv), dtype=complex) # voltages
W = np.zeros((max_coeff, npqpv), dtype=complex) # compute X=1/conj(U)
Q = np.zeros((max_coeff, npqpv), dtype=complex) # unknown reactive powers
Vm0 = np.abs(V0)
Vm2 = Vm0 * Vm0
if n < 2:
return U, W, Q, 0
if verbose:
print('Yseries')
print(Yseries.toarray())
df = pd.DataFrame(data=np.c_[Ysh0.imag, S0.real, S0.imag, Vm0],
columns=['Ysh', 'P0', 'Q0', 'V0'])
print(df)
Yred = Yseries[np.ix_(pqpv, pqpv)] # admittance matrix without slack buses
Yslack = -Yseries[np.ix_(pqpv, sl)] # yes, it is the negative of this
Yslack_vec = Yslack.sum(axis=1).A1
G = np.real(Yred) # real parts of Yij
B = np.imag(Yred) # imaginary parts of Yij
P_red = S0.real[pqpv]
Q_red = S0.imag[pqpv]
Vslack = V0[sl]
Ysh_red = Ysh0[pqpv]
# indices 0 based in the internal scheme
nsl_counted = np.zeros(n, dtype=int)
compt = 0
for i in range(n):
if i in sl:
compt += 1
nsl_counted[i] = compt
pq_ = pq - nsl_counted[pq]
pv_ = pv - nsl_counted[pv]
# .......................CALCULATION OF TERMS [0] ------------------------------------------------------------------
U[0, :] = spsolve(Yred, Yslack_vec)
W[0, :] = 1 / np.conj(U[0, :])
# .......................CALCULATION OF TERMS [1] ------------------------------------------------------------------
valor = np.zeros(npqpv, dtype=complex)
# get the current injections that appear due to the slack buses reduction
I_inj_slack = Yslack * Vslack
valor[pq_] = I_inj_slack[pq_] - Yslack_vec[pq_] + (P_red[pq_] - Q_red[pq_] * 1j) * W[0, pq_] - U[0, pq_] * Ysh_red[pq_]
valor[pv_] = I_inj_slack[pv_] - Yslack_vec[pv_] + P_red[pv_] * W[0, pv_] - U[0, pv_] * Ysh_red[pv_]
# compose the right-hand side vector
RHS = np.r_[valor.real,
valor.imag,
Vm2[pv] - (U[0, pv_] * U[0, pv_]).real]
# Form the system matrix (MAT)
Upv = U[0, pv_]
Xpv = W[0, pv_]
VRE = coo_matrix((2 * Upv.real, (np.arange(npv), pv_)), shape=(npv, npqpv)).tocsc()
VIM = coo_matrix((2 * Upv.imag, (np.arange(npv), pv_)), shape=(npv, npqpv)).tocsc()
XIM = coo_matrix((-Xpv.imag, (pv_, np.arange(npv))), shape=(npqpv, npv)).tocsc()
XRE = coo_matrix((Xpv.real, (pv_, np.arange(npv))), shape=(npqpv, npv)).tocsc()
EMPTY = csc_matrix((npv, npv))
MAT = vs((hs((G, -B, XIM)),
hs((B, G, XRE)),
hs((VRE, VIM, EMPTY))), format='csc')
if verbose:
print('MAT')
print(MAT.toarray())
# factorize (only once)
MAT_LU = factorized(MAT.tocsc())
# solve
LHS = MAT_LU(RHS)
# update coefficients
U[1, :] = LHS[:npqpv] + 1j * LHS[npqpv:2 * npqpv]
Q[0, pv_] = LHS[2 * npqpv:]
W[1, :] = -W[0, :] * np.conj(U[1, :]) / np.conj(U[0, :])
# .......................CALCULATION OF TERMS [>=2] ----------------------------------------------------------------
iter_ = 1
range_pqpv = np.arange(npqpv, dtype=np.int64)
V = V0.copy()
c = 2
converged = False
norm_f = tolerance + 1.0 # any number that violates the convergence
while c < max_coeff and not converged: # c defines the current depth
valor[pq_] = (P_red[pq_] - Q_red[pq_] * 1j) * W[c - 1, pq_] - U[c - 1, pq_] * Ysh_red[pq_]
valor[pv_] = -1j * conv2(W, Q, c, pv_) - U[c - 1, pv_] * Ysh_red[pv_] + W[c - 1, pv_] * P_red[pv_]
RHS = np.r_[valor.real,
valor.imag,
-conv3(U, U, c, pv_).real]
LHS = MAT_LU(RHS)
# update voltage coefficients
U[c, :] = LHS[:npqpv] + 1j * LHS[npqpv:2 * npqpv]
# update reactive power
Q[c - 1, pv_] = LHS[2 * npqpv:]
# update voltage inverse coefficients
W[c, range_pqpv] = -conv1(U, W, c, range_pqpv) / np.conj(U[0, range_pqpv])
# compute power mismatch
if not np.mod(c, 2): # check the mismatch every 4 iterations
V[pqpv] = U.sum(axis=0)
Scalc = V * np.conj(Ybus * V)
dP = np.abs(S0[pqpv].real - Scalc[pqpv].real)
dQ = np.abs(S0[pq].imag - Scalc[pq].imag)
norm_f = np.linalg.norm(np.r_[dP, dQ], np.inf) # same as max(abs())
# check convergence
converged = norm_f < tolerance
print('mismatch check at c=', c)
c += 1
iter_ += 1
return U, W, Q, iter_, norm_f
def helm_josep(Ybus, Yseries, V0, S0, Ysh0, pq, pv, sl, pqpv, tolerance=1e-6, max_coeff=30, use_pade=True,
verbose=False, return_structs=False):
"""
Holomorphic Embedding LoadFlow Method as formulated by Josep Fanals Batllori in 2020
:param Ybus: Complete admittance matrix
:param Yseries: Admittance matrix of the series elements
:param V0: vector of specified voltages
:param S0: vector of specified power
:param Ysh0: vector of shunt admittances (including the shunts of the branches)
:param pq: list of pq nodes
:param pv: list of pv nodes
:param sl: list of slack nodes
:param pqpv: sorted list of pq and pv nodes
:param tolerance: target error (or tolerance)
:param max_coeff: maximum number of coefficients
:param use_pade: Use the Padè approximation? otherwise a simple summation is done
:param verbose: Print intermediate objects and calculations?
:return: V, converged, norm_f, Scalc, iter_, elapsed
"""
start_time = time.time()
npqpv = len(pqpv)
npv = len(pv)
nsl = len(sl)
n = Yseries.shape[0]
if n < 2:
# V, converged, norm_f, Scalc, iter_, elapsed
return V0, True, 0.0, S0, 0, 0.0
# --------------------------- PREPARING IMPLEMENTATION--------------------------------------------------------------
U = np.zeros((max_coeff, npqpv), dtype=complex) # voltages
U_re = np.zeros((max_coeff, npqpv), dtype=float) # real part of voltages
U_im = np.zeros((max_coeff, npqpv), dtype=float) # imaginary part of voltages
X = np.zeros((max_coeff, npqpv), dtype=complex) # compute X=1/conj(U)
X_re = np.zeros((max_coeff, npqpv), dtype=float) # real part of X
X_im = np.zeros((max_coeff, npqpv), dtype=float) # imaginary part of X
Q = np.zeros((max_coeff, npqpv), dtype=complex) # unknown reactive powers
Vm0 = np.abs(V0)
vec_W = Vm0 * Vm0
if verbose:
print('Yseries')
print(Yseries.toarray())
df = pd.DataFrame(data=np.c_[Ysh0.imag, S0.real, S0.imag, Vm0],
columns=['Ysh', 'P0', 'Q0', 'V0'])
print(df)
Yred = Yseries[np.ix_(pqpv, pqpv)] # admittance matrix without slack buses
Yslack = -Yseries[np.ix_(pqpv, sl)] # yes, it is the negative of this
G = np.real(Yred) # real parts of Yij
B = np.imag(Yred) # imaginary parts of Yij
vec_P = S0.real[pqpv]
vec_Q = S0.imag[pqpv]
Vslack = V0[sl]
# indices 0 based in the internal scheme
nsl_counted = np.zeros(n, dtype=int)
compt = 0
for i in range(n):
if i in sl:
compt += 1
nsl_counted[i] = compt
pq_ = pq - nsl_counted[pq]
pv_ = pv - nsl_counted[pv]
pqpv_ = np.sort(np.r_[pq_, pv_])
# .......................CALCULATION OF TERMS [0] ------------------------------------------------------------------
if nsl > 1:
U[0, :] = spsolve(Yred, Yslack.sum(axis=1))
else:
U[0, :] = spsolve(Yred, Yslack)
X[0, :] = 1 / np.conj(U[0, :])
U_re[0, :] = U[0, :].real
U_im[0, :] = U[0, :].imag
X_re[0, :] = X[0, :].real
X_im[0, :] = X[0, :].imag
# .......................CALCULATION OF TERMS [1] ------------------------------------------------------------------
valor = np.zeros(npqpv, dtype=complex)
# get the current injections that appear due to the slack buses reduction
I_inj_slack = Yslack[pqpv_, :] * Vslack
valor[pq_] = I_inj_slack[pq_] - Yslack[pq_].sum(axis=1).A1 + (vec_P[pq_] - vec_Q[pq_] * 1j) * X[0, pq_] + U[0, pq_] * Ysh0[pq_]
valor[pv_] = I_inj_slack[pv_] - Yslack[pv_].sum(axis=1).A1 + (vec_P[pv_]) * X[0, pv_] + U[0, pv_] * Ysh0[pv_]
# compose the right-hand side vector
RHS = np.r_[valor.real,
valor.imag,
vec_W[pv] - 1.0]
# Form the system matrix (MAT)
Upv = U[0, pv_]
Xpv = X[0, pv_]
VRE = coo_matrix((2 * Upv.real, (np.arange(npv), pv_)), shape=(npv, npqpv)).tocsc()
VIM = coo_matrix((2 * Upv.imag, (np.arange(npv), pv_)), shape=(npv, npqpv)).tocsc()
XIM = coo_matrix((-Xpv.imag, (pv_, np.arange(npv))), shape=(npqpv, npv)).tocsc()
XRE = coo_matrix((Xpv.real, (pv_, np.arange(npv))), shape=(npqpv, npv)).tocsc()
EMPTY = csc_matrix((npv, npv))
MAT = vs((hs((G, -B, XIM)),
hs((B, G, XRE)),
hs((VRE, VIM, EMPTY))), format='csc')
if verbose:
print('MAT')
print(MAT.toarray())
# factorize (only once)
MAT_LU = factorized(MAT.tocsc())
# solve
LHS = MAT_LU(RHS)
# update coefficients
Q[0, pv_] = LHS[2 * npqpv:]
U[1, :] = LHS[:npqpv] + 1j * LHS[npqpv:2 * npqpv]
X[1, :] = -X[0, :] * np.conj(U[1, :]) / np.conj(U[0, :])
# .......................CALCULATION OF TERMS [>=2] ----------------------------------------------------------------
iter_ = 1
range_pqpv = np.arange(npqpv, dtype=np.int64)
for c in range(2, max_coeff): # c defines the current depth
valor[pq_] = (vec_P[pq_] - vec_Q[pq_] * 1j) * X[c - 1, pq_] + U[c - 1, pq_] * Ysh0[pq_]
valor[pv_] = conv2(X, Q, c, pv_) * -1j + U[c - 1, pv_] * Ysh0[pv_] + X[c - 1, pv_] * vec_P[pv_]
RHS = np.r_[valor.real,
valor.imag,
-conv3(U, U, c, pv_).real]
LHS = MAT_LU(RHS)
# update voltage coefficients
U[c, :] = LHS[:npqpv] + 1j * LHS[npqpv:2 * npqpv]
# update reactive power
Q[c - 1, pv_] = LHS[2 * npqpv:]
# update voltage inverse coefficients
X[c, range_pqpv] = -conv1(U, X, c, range_pqpv) / np.conj(U[0, range_pqpv])
iter_ += 1
# --------------------------- RESULTS COMPOSITION ------------------------------------------------------------------
if verbose:
print('V coefficients')
print(U)
# compute the final voltage
V = np.zeros(n, dtype=complex)
if use_pade:
V[pqpv] = pade4all(max_coeff - 1, U, 1)
else:
V[pqpv] = U.sum(axis=0)
V[sl] = Vslack # copy the slack buses
# compute power mismatch
Scalc = V * np.conj(Ybus * V)
dP = np.abs(S0[pqpv].real - Scalc[pqpv].real)
dQ = np.abs(S0[pq].imag - Scalc[pq].imag)
norm_f = np.linalg.norm(np.r_[dP, dQ], np.inf) # same as max(abs())
# check convergence
converged = norm_f < tolerance
elapsed = time.time() - start_time
if return_structs:
return V, converged, norm_f, Scalc, iter_, elapsed, U, X
else:
return V, converged, norm_f, Scalc, iter_, elapsed
def helm_coefficients_josep(Yseries, V0, S0, Ysh0, pq, pv, sl, pqpv, tolerance=1e-6, max_coeff=30, verbose=False):
"""
Holomorphic Embedding LoadFlow Method as formulated by Josep Fanals Batllori in 2020
THis function just returns the coefficients for further usage in other routines
:param Yseries: Admittance matrix of the series elements
:param V0: vector of specified voltages
:param S0: vector of specified power
:param Ysh0: vector of shunt admittances (including the shunts of the branches)
:param pq: list of pq nodes
:param pv: list of pv nodes
:param sl: list of slack nodes
:param pqpv: sorted list of pq and pv nodes
:param tolerance: target error (or tolerance)
:param max_coeff: maximum number of coefficients
:param verbose: print intermediate information
:return: U, X, Q, iterations
"""
npqpv = len(pqpv)
npv = len(pv)
nsl = len(sl)
n = Yseries.shape[0]
# --------------------------- PREPARING IMPLEMENTATION -------------------------------------------------------------
U = np.zeros((max_coeff, npqpv), dtype=complex) # voltages
X = np.zeros((max_coeff, npqpv), dtype=complex) # compute X=1/conj(U)
Q = np.zeros((max_coeff, npqpv), dtype=complex) # unknown reactive powers
if n < 2:
return U, X, Q, 0
if verbose:
print('Yseries')
print(Yseries.toarray())
df = pd.DataFrame(data=np.c_[Ysh0.imag, S0.real, S0.imag, np.abs(V0)],
columns=['Ysh', 'P0', 'Q0', 'V0'])
print(df)
# build the reduced system
Yred = Yseries[np.ix_(pqpv, pqpv)] # admittance matrix without slack buses
Yslack = -Yseries[np.ix_(pqpv, sl)] # yes, it is the negative of this
G = Yred.real.copy() # real parts of Yij
B = Yred.imag.copy() # imaginary parts of Yij
vec_P = S0.real[pqpv]
vec_Q = S0.imag[pqpv]
Vslack = V0[sl]
Ysh = Ysh0[pqpv]
Vm0 = np.abs(V0[pqpv])
vec_W = Vm0 * Vm0
# indices 0 based in the internal scheme
nsl_counted = np.zeros(n, dtype=int)
compt = 0
for i in range(n):
if i in sl:
compt += 1
nsl_counted[i] = compt
pq_ = pq - nsl_counted[pq]
pv_ = pv - nsl_counted[pv]
pqpv_ = np.sort(np.r_[pq_, pv_])
# .......................CALCULATION OF TERMS [0] ------------------------------------------------------------------
if nsl > 1:
U[0, :] = spsolve(Yred, Yslack.sum(axis=1))
else:
U[0, :] = spsolve(Yred, Yslack)
X[0, :] = 1 / np.conj(U[0, :])
# .......................CALCULATION OF TERMS [1] ------------------------------------------------------------------
valor = np.zeros(npqpv, dtype=complex)
# get the current injections that appear due to the slack buses reduction
I_inj_slack = Yslack[pqpv_, :] * Vslack
valor[pq_] = I_inj_slack[pq_] - Yslack[pq_].sum(axis=1).A1 + (vec_P[pq_] - vec_Q[pq_] * 1j) * X[0, pq_] - U[0, pq_] * Ysh[pq_]
valor[pv_] = I_inj_slack[pv_] - Yslack[pv_].sum(axis=1).A1 + (vec_P[pv_]) * X[0, pv_] - U[0, pv_] * Ysh[pv_]
# compose the right-hand side vector
RHS = np.r_[valor.real,
valor.imag,
vec_W[pv_] - (U[0, pv_] * U[0, pv_]).real # vec_W[pv_] - 1.0
]
# Form the system matrix (MAT)
Upv = U[0, pv_]
Xpv = X[0, pv_]
VRE = coo_matrix((2 * Upv.real, (np.arange(npv), pv_)), shape=(npv, npqpv)).tocsc()
VIM = coo_matrix((2 * Upv.imag, (np.arange(npv), pv_)), shape=(npv, npqpv)).tocsc()
XIM = coo_matrix((-Xpv.imag, (pv_, np.arange(npv))), shape=(npqpv, npv)).tocsc()
XRE = coo_matrix((Xpv.real, (pv_, np.arange(npv))), shape=(npqpv, npv)).tocsc()
EMPTY = csc_matrix((npv, npv))
MAT = vs((hs((G, -B, XIM)),
hs((B, G, XRE)),
hs((VRE, VIM, EMPTY))), format='csc')
if verbose:
print('MAT')
print(MAT.toarray())
# factorize (only once)
# MAT_LU = factorized(MAT.tocsc())
# solve
LHS = spsolve(MAT, RHS)
# update coefficients
U[1, :] = LHS[:npqpv] + 1j * LHS[npqpv:2 * npqpv]
Q[0, pv_] = LHS[2 * npqpv:]
X[1, :] = -X[0, :] * np.conj(U[1, :]) / np.conj(U[0, :])
# .......................CALCULATION OF TERMS [>=2] ----------------------------------------------------------------
iter_ = 1
for c in range(2, max_coeff): # c defines the current depth
valor[pq_] = (vec_P[pq_] - vec_Q[pq_] * 1j) * X[c - 1, pq_] - U[c - 1, pq_] * Ysh[pq_]
valor[pv_] = -1j * conv2(X, Q, c, pv_) - U[c - 1, pv_] * Ysh[pv_] + X[c - 1, pv_] * vec_P[pv_]
RHS = np.r_[valor.real,
valor.imag,
-conv3(U, U, c, pv_).real]
LHS = spsolve(MAT, RHS)
# update voltage coefficients
U[c, :] = LHS[:npqpv] + 1j * LHS[npqpv:2 * npqpv]
# update reactive power
Q[c - 1, pv_] = LHS[2 * npqpv:]
# update voltage inverse coefficients
X[c, :] = -conv1(U, X, c) / np.conj(U[0, :])
iter_ += 1
return U, X, Q, iter_
def helm_coefficients_andre(Yseries,
V0,
S0,
Ysh0,
pq,
pv,
sl,
pqpv,
tolerance=1e-6,
max_coeff=30,
verbose=False):
"""
Holomorphic Embedding LoadFlow Method as formulated by Josep Fanals Batllori in 2020
(----Reformulation to avoid Qk[n], Wk[n] calculation----)
This function just returns the coefficients for further usage in other routines
:param Yseries: Admittance matrix of the series elements
:param V0: vector of specified voltages (complex)
:param S0: vector of specified power (complex)
:param Ysh0: vector of shunt admittances (including the shunts of the branches)
:param pq: list of pq nodes
:param pv: list of pv nodes
:param sl: list of slack nodes
:param pqpv: sorted list of pq and pv nodes
:param tolerance: target error (or tolerance)
:param max_coeff: maximum number of coefficients
:param verbose: print intermediate information
:return: U, "iterations"
"""
npqpv = len(pqpv) # number of non-slack buses
npq = len(pq) # number of pq buses
npv = len(pv) # number of pv buses
nsl = len(sl) # number of slack buses
n = Yseries.shape[
0] # number of buses; size(Yseries,n) = Yseries.shape[n-1]; size(Yseries) = Yseries.shape;
# --------------------------- PREPARING IMPLEMENTATION -------------------------------------------------------------
U = np.zeros((max_coeff, npqpv), dtype=complex,
order='C') # voltage series coefficients
conjU = np.zeros((max_coeff, npqpv), dtype=complex,
order='C') # conjugate voltage series coefficients
Vsp = np.abs(V0[pqpv]) # Specified voltage magnitude for PV-Buses
Vsp_2 = Vsp * Vsp # Specified voltage magnitude for PV-Buses Squared
if n < 2:
return U, 0
if verbose:
print('Yseries')
print(Yseries.toarray())
df = pd.DataFrame(data=np.c_[Ysh0.imag, S0.real, S0.imag,
np.abs(V0)],
columns=['Ysh', 'P0', 'Q0', 'V0'])
print(df)
Yrr = Yseries[np.ix_(pqpv, pqpv)] # Y_RED-RED; Yseries(pqpv,pqpv)
Yrs = Yseries[np.ix_(pqpv, sl)] # Y_RED-SLK
Vs = V0[sl] # V_SLK
Isl = Yrs * Vs # I_SLK
Ssp = S0[
pqpv] # Specified apparent power (Active for PQ and PV; Reactive for PQ)
Ysh = Ysh0[pqpv] # Y_RED
# indices 0 based in the internal scheme
nsl_counted = np.zeros(n, dtype=int)
compt = 0
for i in range(n):
if i in sl:
compt += 1
nsl_counted[i] = compt
pq_ = pq - nsl_counted[pq]
pv_ = pv - nsl_counted[pv]
pqpv_ = np.arange(0, npqpv, dtype=np.int64)
# .......................CALCULATION OF TERMS [0] ------------------------------------------------------------------
U[0, :] = spsolve(Yrr, -Isl)
conjU[0, :] = np.conj(U[0, :])
# .......................LINEAR SYSTEM MATRIX (MAT) ----------------------------------------------------------------
YV = Yrr.multiply(conjU[0, :].reshape(npqpv, 1))
YV = YV.tocsc()
# | Re{YxconjV} -Im{YxconjV} | * |LHS_ReU [PQ,PV]| = |RHS_P [PQ,PV]|
# | Im{YxconjV} Re{YxconjV} | |LHS_ImU [PQ,PV]| |RHS_Q [PQ] |
# | Re{V} Im{V} | |RHS_V [PV] |
ReYVp = YV.real
ImYVp = YV.imag
ReYVq = ReYVp[np.ix_(pq_, pqpv_)]
ImYVq = ImYVp[np.ix_(pq_, pqpv_)]
ReV = coo_matrix(
(U[0, pv_].real, (np.arange(0, npv, dtype=np.int64), pv_)),
shape=(npv, npqpv)).tocsc()
ImV = coo_matrix(
(U[0, pv_].imag, (np.arange(0, npv, dtype=np.int64), pv_)),
shape=(npv, npqpv)).tocsc()
# sparse matrix concatenation
MAT = vs((hs(
(ReYVp, -ImYVp), format='csr'), hs(
(ImYVq, ReYVq), format='csr'), hs((ReV, ImV), format='csr')),
format='csc')
if verbose:
print('MAT')
print(MAT.toarray())
# factorize (only once)
MAT_LU = factorized(MAT)
# .......................CALCULATION OF TERMS [>=1] ----------------------------------------------------------------
for c in range(1, max_coeff): # max_coeff_order
if c == 1:
UconjU = U[0, :] * conjU[0, :]
UconjU = UconjU.real
valor_v = 0.5 * (Vsp_2[pv_] - UconjU[pv_])
valor_pq = np.conj(Ssp) - Ysh * UconjU
else:
conjU[c - 1, :] = np.conj(U[c - 1, :])
valor_v = -conv4(U, conjU, c, pv_)
valor_pq = -Ysh * conv5(U, conjU, c, pqpv_) - conv6(
U, conjU, Yrr, npqpv, c)
# compose the right-hand side vector
RHS = np.r_[valor_pq.real, valor_pq[pq_].imag, valor_v]
# solve
LHS = MAT_LU(RHS)
# update voltage coefficients
U[c, :] = LHS[0:npqpv] + 1j * LHS[npqpv:2 * npqpv]
return U, max_coeff - 1