Solar power conversion system and method
A solar power conversion system includes a photovoltaic array having photovoltaic modules for generating direct current (DC) power. A power converter is provided in the system for converting the DC power to alternating current (AC) power. A transformer is coupled between the power converter and a power grid for transmitting the AC power to the power grid. The transformer is connected to the power grid at the point of common coupling (PCC) and to the power converter at output terminals. A reactance estimation module is provided in the system for estimating a short circuit reactance at PCC based on a small change in a measured voltage at output terminals with respect to a small change in a measured reactive power at the output terminals. Further, a maximum reactive power estimation module estimates a maximum reactive power based on the estimated reactance, the measured voltage at output terminals, and the measured reactive power at the output terminals. A controller in the system generates switching command signals for the power converter based on the measured voltage at output terminals and the estimated maximum reactive power.
1. A solar power conversion system comprising:
a photovoltaic array having photovoltaic modules for generating direct current (DC) power;
a power converter for converting the DC power to alternating current (AC) power;
a transformer coupled between the power converter and a power grid for transmitting the AC power to the power grid, wherein the transformer is connected to the power grid at the point of common coupling (PCC) and to the power converter at output terminals;
a reactance estimation module configured to estimate a short circuit reactance at PCC based on a small change in a measured voltage at output terminals with respect to a small change in a measured reactive power at the output terminals;
a maximum reactive power estimation module to estimate a maximum reactive power based on the estimated reactance, the measured voltage at output terminals, and the measured reactive power at the output terminals; and
a controller for generating switching command signals for the power converter based on the measured voltage at output terminals and the estimated maximum reactive power.
2. The system of claim 1 further comprising a DC to DC converter connected between the PV array and a DC link to maintain a constant DC voltage at the DC link.
3. The system of claim 2 comprising a DC to DC controller to control the operation of DC to DC converter.
4. The system of claim 1 further comprising filters connected between the power converter and the output terminal for removing harmonics from the system output power.
5. The system of claim 1 , wherein the power converter provides reactive power to the transformer to compensate for the PCC voltage variation.
6. The system of claim 1 , wherein the maximum reactive power estimation module estimates the maximum reactive power based on following equation:
Q
MAX
(
t
)
≈
Q
(
t
)
+
(
V
MAX
-
V
(
t
)
)
·
V
(
t
)
X
·
(
1
-
X
·
Q
(
t
)
V
2
(
t
)
)
where Q MAX (t) is maximum reactive power at time t, Q(t) is measured reactive power at time t, V MAX is the maximum allowable voltage by the power converter at time t, V(t) is the measured voltage V at time t and X is the short circuit reactance.
7. The system of claim 6 , wherein the reactance estimation module estimates the short circuit reactance based on following equation:
X
≈
V
·
d
V
d
Q
1
+
Q
V
·
d
V
d
Q
≈
V
d
Q
d
V
+
Q
V
V is the measured voltage at output terminals, Q is the measured reactive power at output terminals, dV is small change in measured voltage V and dQ is small change in measured reactive power Q.
8. The system of claim 7 , wherein V includes single phase line to neutral quantity or positive sequence quantity or three phase vector magnitude quantity.
9. The system of claim 1 further comprising a first controller to generate a reference reactive power magnitude for the power converter based on an error voltage.
10. The system of claim 9 comprising a first subtractor to generate the error voltage based on a reference voltage at output terminals and the measured voltage at output terminals.
11. The system of claim 9 , wherein the reference voltage at output terminals is determined by a system operator based on system conditions.
12. The system of claim 11 comprising a second controller to generate a q-domain reference current signal based on a reactive power error between a reference reactive power and the measured reactive power.
13. A method to transmit solar power to a power grid, the method comprising:
providing a photovoltaic array having photovoltaic modules for generating direct current (DC) power;
providing a power converter for converting the DC power to alternating current (AC) power;
coupling a transformer between the power converter and the power grid for transmitting the AC power to the power grid, wherein the transformer is connected to the power grid at the point of common coupling (PCC) and to the power converter at output terminals;
estimating a short circuit reactance at PCC based on a small change in a measured voltage at output terminals with respect to a small change in a measured reactive power at the output terminals;
estimating a maximum reactive power based on the estimated reactance, the measured voltage at output terminals, and the measured reactive power at the output terminals; and
generating switching command signals for the power converter based on the measured voltage at output terminals and the estimated maximum reactive power.
14. The method of claim 13 , further comprising connecting a DC to DC converter between the PV array and a DC link to maintain a constant DC voltage at the DC link.
15. The method of claim 13 comprising providing reactive power to the transformer from the power converter to compensate for the PCC voltage variation.
16. The method of claim 13 , wherein the maximum reactive power is estimated based on following equation:
Q
MAX
(
t
)
≈
Q
(
t
)
+
(
V
MAX
-
V
(
t
)
)
·
V
(
t
)
X
·
(
1
-
X
·
Q
(
t
)
V
2
(
t
)
)
where Q MAX (t) is maximum reactive power at time t, Q(t) is measured reactive power at time t, V MAX is the maximum allowable voltage by the power converter at time t, V(t) is the measured voltage V at time t and X is the short circuit reactance.
17. The method of claim 16 , wherein the short circuit reactance is estimated based on following equation
X
≈
V
·
d
V
d
Q
1
+
Q
V
·
d
V
d
Q
≈
V
d
Q
d
V
+
Q
V
V is the measured voltage at output terminals, Q is the measured reactive power at output terminals, dV is small change in measured voltage V and dQ is small change in measured reactive power Q.
18. The method of claim 17 , wherein V includes single phase line to neutral quantity or positive sequence quantity or three phase vector magnitude quantity.
19. The method of claim 13 further comprising generating a reference reactive power magnitude for the power converter based on an error voltage between a reference voltage at output terminals and the measured voltage at output terminals.
20. The method of claim 19 comprising generating a q-domain reference current signal based on a reactive power error between a reference reactive power and the measured reactive power.