Adaptive protection method for impedance of parallel capacitors
An adaptive protection method for impedance of parallel capacitors comprises monitoring a terminal voltage and current of the capacitor; automatically calibrating the initial impedance of the capacitor when the capacitor is put into operation; calculating a real-time impedance of the capacitor during operation, dynamically updating the actual impedance of the capacitor periodically; comparing the real-time impedance of the capacitor with the actual impedance, updating the actual impedance if the relative value of the modulus of the real-time impedance change does not exceed the dynamic update limit threshold and the dynamic update reaches update period; generating an update failure alarm if the relative value of the modulus of the real-time impedance change exceeds the dynamic update limit threshold; generating a protection alarm if the relative value of the modulus of the real-time impedance change satisfies the protection alarm condition and the delay time is reached; and protecting the trip outlet if the relative value of the modulus of the real-time impedance change satisfies the protection trip condition and the delay time is reached. The protection method is applicable to various fault conditions of the capacitor and the capacitor bank, the set value is simple to calculate, the implementation is simple, and the sensitivity and reliability of the protection for capacitor are effectively improved.
1. An adaptive protection method for an impedance of parallel capacitors, characterized in that, the method comprising:
monitoring a terminal voltage and current of the capacitors;
automatically calibrating an initial impedance of the capacitors when the capacitors are put into operation;
calculating a real-time impedance of the capacitors during operation, dynamically updating an actual impedance of the capacitors periodically;
comparing the real-time impedance of the capacitors with the actual impedance, updating the actual impedance if a relative value of the modulus of the real-time impedance change does not exceed a dynamic update limit threshold and the dynamic update reaches an update period; generating an update failure alarm if the relative value of the modulus of the real-time impedance change exceeds the dynamic update limit threshold;
generating a protection alarm if the relative value of the modulus of the real-time impedance change satisfies the protection alarm condition and a delay time is reached; and
protecting a trip outlet if the relative value of the modulus of the real-time impedance change satisfies the protection trip condition and the delay time is reached.
2. The adaptive protection method for impedance of parallel capacitors according to claim 1 , characterized in that, the terminal voltage and current of the capacitors are monitored; the terminal voltage of the capacitors is a vector difference between a bus voltage of each phase and a neutral point voltage of the capacitors, or is a terminal voltage of a discharge potential transformer; the current is a current of each phase of the capacitors, or is a vector sum of the currents of each phase branch of the capacitors.
3. The adaptive protection method for impedance of parallel capacitors according to claim 1 , characterized in that, the protection trip condition of the impedance of the capacitors is calculated as follows:
ε dyn_(P) >ε set_trip
wherein, (P) refers to the three phases A, B, and C, ε dyn_(P) is a relative value of the modulus of the real-time impedance change per phase when the capacitors fail, and ε set_trip is a value of the impedance protection trip.
4. The adaptive protection method for impedance of parallel capacitors according to claim 1 , characterized in that, the protection alarm condition of the impedance of the capacitors is calculated as follows:
ε dyn_(P) >ε set_alm
wherein, (P) refers to the three phases A, B, and C, ε dyn_(P) is a relative value of the modulus of the real-time impedance change per phase when the capacitors fail, and ε set_alm is a value of the impedance protection alarm.
5. The adaptive protection method for impedance of parallel capacitors according to claim 3 , characterized in that, the relative value of the modulus of the real-time impedance change per phase of the capacitors is calculated as follows:
ɛ
dyn_
(
P
)
=
Z
dyn_
(
P
)
-
Z
real_
(
P
)
Z
real_
(
P
)
×
100
%
wherein, Z dyn_(P) is a real-time impedance per phase of the capacitors, and Z real_(P) is an actual impedance per phase of the capacitors.
6. The adaptive protection method for impedance of parallel capacitors according to claim 5 , characterized in that, the actual impedance per phase of the capacitors is calculated as follows:
Z
dyn_
(
P
)
=
U
.
dyn_
(
P
)
I
.
dyn_
(
P
)
wherein, {dot over (U)} dyn_(p) is a fundamental vector of the real-time terminal voltage per phase of the capacitors when put into operation, and İ dyn_(P) is a fundamental vector of the real-time current per phase of the capacitors when put into operation.
7. The adaptive protection method for impedance of parallel capacitors according to claim 5 , characterized in that, the actual impedance Z dyn_(P) of the capacitors is updated periodically with the real-time impedance Z real_(P) which satisfies requirements.
8. The adaptive protection method for impedance of parallel capacitors according to claim 7 , characterized in that, the real-time impedance update period is set by the value of dynamic update period of the actual impedance.
9. The adaptive protection method for impedance of parallel capacitors according to claim 7 , characterized in that, the dynamic period update of the actual impedance Z real_(P) of the capacitors satisfies the condition as follows:
ε dyn_(P) <ε set_alm_dyn
max(|ε dyn_A −ε dyn_B |,|ε dyn_B −ε dyn_C |,|ε dyn_C −ε dyn_A |)<ε set_ub_dyn
wherein, ε set_alm_dyn is a limit value of the relative value of the modulus of the dynamic update impedance change, and ε set_ub_dyn is a limit value of the inconsistency relative value of the modulus of the change of the three-phase dynamic update;
if the condition is satisfied, update the actual impedance; otherwise, the actual impedance is not updated and the dynamic update fails.
10. The adaptive protection method for impedance of parallel capacitors according to claim 5 , characterized in that, the initial value of dynamic period update of the actual impedance Z real_(P) of the capacitors is an actual impedance after automatic calibration when put into operation;
if the automatic calibration is accomplished, the actual impedance Z real_(P) is an initial impedance Z init_(p) of each phase after calibration of the capacitors when put into operation, otherwise the actual impedance Z real_(P) is a single-phase theoretical rated impedance
Z
ideal
=
U
.
n
S
.
n
of the capacitors; {dot over (U)} n is a rated line voltage, and {dot over (S)} n is a rated capacity.
11. The adaptive protection method for impedance of parallel capacitors according to claim 10 , characterized in that, the initial impedance of the capacitors is automatically calibrated when put into operation, wherein the calibration equation for the initial impedance is described as follows:
Z
init_
(
P
)
=
U
.
init_
(
P
)
I
.
init_
(
P
)
wherein, {dot over (U)} init_(P) is a fundamental vector of initial voltage per phase of the capacitors when put into operation, and İ init_(P) is a fundamental vector of initial current per phase of the capacitors when put into operation.
12. The adaptive protection method for impedance of parallel capacitors according to claim 10 , characterized in that, the initial impedance of the capacitors is automatically calibrated when put into operation, wherein the accomplishment of the automatic calibration is determined by the following:
ε inh_(P) <ε set_alm_inh
wherein, ε inh_(P) is a relative value of the modulus of inherent deviation of the initial actual impedance when put into operation, and ε set_alm_inh is a limit value of the relative value of the modulus of inherent deviation.
13. The adaptive protection method for impedance of parallel capacitors according to claim 12 , characterized in that, the relative value of the modulus of inherent deviation of the initial actual impedance when put into operation is calculated by the following equation:
ɛ
inh_
(
P
)
=
Z
init_
(
P
)
-
Z
ideal
Z
ideal
×
100
%