Method for preventing saturation of a transformer during fast voltage changes and a control system thereof
A control method for preventing saturation of a transformer during fast voltage changes by correction of magnetic flux of the transformer, the transformer having a primary winding with a power electronic converter and a secondary winding, the control method including estimating or measuring a primary signal including a primary voltage on the primary winding of the transformer and a primary current on the primary winding of the transformer, and injecting a correction signal including a correction voltage into the primary winding of the transformer so as to prevent saturation of the transformer.
1 . A method for preventing saturation of a transformer during fast voltage changes by correction of magnetic flux of the transformer, said transformer having a primary winding with a power electronic converter and a secondary winding, by estimating or measuring a primary signal, including a primary voltage on the primary winding of the transformer and a primary current on the primary winding of the transformer, and injecting a correction signal including a correction voltage, into the primary winding of the transformer so as to prevent saturation of the transformer, the method comprising:
acknowledgement of a commanded change of converter voltage, to be applied to the primary winding of the transformer, including initial values of voltage angle, voltage parameters and voltage change parameters on the primary winding of the transformer as the primary signal;
generating a reference signal including a reference voltage signal U* by a referencing unit;
inputting the reference voltage signal U* to a reference flux trajectory calculator that calculates a reference magnetic flux Φ* based on the reference voltage signal U*;
inputting a combination of the reference magnetic flux Φ* and an estimated magnetic flux Φ to a flux controller that generates a correction term ΔU* based thereon;
generating the corrected reference voltage signal, comprising a correction term, ΔU**, wherein U**=U*+ΔU*;
inputting the corrected reference voltage signal U** and a load of the secondary winding of the transformer and its change in time, to a flux estimator, which calculates an estimated magnetic flux Φ based thereon;
injecting the correction signal through the power electronic converter into the primary winding of the transformer;
wherein an auxiliary predictive control unit (ACU), which contains a reference mathematical model of the system, is configured to:
initialize the reference mathematical model of the system with the acquired initial values of voltage angle and voltage parameters, as well as the acquired or pre-configured voltage change parameters,
calculate an expected magnetic flux trajectory {circumflex over (Φ)}(t) using the reference mathematical model of the system;
classify the expected magnetic flux trajectory {circumflex over (Φ)}(t) so as to predict whether saturation of the transformer is expected to happen by establishing its mean value, its maximal value {circumflex over (Φ)} max , and its minimal value {circumflex over (Φ)} min , and compare them against a pre-determined magnetic core saturation limits ±Φ sat ;
evaluate a flux trajectory correction ΔΦ* (t) at which saturation is avoided or its occurrence is minimized; and
input the flux trajectory correction ΔΦ* (t) into the flux controller as an additional signal used by the flux controller to generate the correction term ΔU*;
wherein injecting the correction signal is initialized with the same initial conditions that are used for the initialization of the reference mathematical model of the system.
2 . The method of claim 1 , wherein the reference mathematical model of the system includes a detailed model of the flux control unit and mathematical models of the power electronic converter and of the transformer, and wherein the flux control unit comprises the reference flux trajectory calculator and the flux controller.
3 . The method of claim 1 , wherein during acknowledgement of the commanded change in converter voltage, the commanded voltage change is acquired from a pre-determined file, including expected real time change of physical load of the secondary winding of the transformer.
4 . The method of claim 1 , wherein during evaluation of the flux trajectory correction ΔΦ* (t), the flux trajectory correction ΔΦ*(t) is calculated according to:
ΔΦ
*
(
t
)
=
k
·
(
Φ
^
max
-
Φ
^
min
)
=
const
,
where k is a correction factor selected from between 0 to 1.
5 . The method of claim 1 , further comprising a deliberate predictive control procedure that requires an additional computational effort and results in a time delay between voltage change request acknowledgement and its reproduction by the converter.
6 . A control system for preventing saturation of a transformer during fast voltage changes by controlling a magnetic flux of the transformer, the transformer having a primary winding with a power electronic converter configured for receiving a correction signal U** and a secondary winding connected to a load, the control system comprising:
a device configured to measure or estimate a primary signal comprising information of a primary current and/or primary voltage and/or primary phase, on the primary winding of the transformer;
a referencing unit connected to the power electronic converter, the referencing unit configured for generating a reference voltage signal U;
a flux estimator connected to the referencing unit and configured for receiving the correction signal U** and also configured for receiving the primary signal;
wherein the flux estimator is configured for calculating an estimated magnetic flux Φ, a reference flux trajectory calculator connected to the referencing unit and configured for receiving the reference voltage signal U* and calculating a reference magnetic flux Φ*;
a flux controller connected to the reference flux trajectory calculator and the flux estimator;
wherein the flux controller is configured for receiving a combination of the reference magnetic flux Φ* from the reference flux trajectory calculator and the estimated magnetic flux Φ from the flux estimator;
wherein the flux controller is further configured for generating a correction term ΔU* based on said combination and outputting the correction term ΔU* as a component of the correction signal U**;
an additional control unit that contains a reference mathematical model of the system and that is connected to the referencing unit and to the flux controller;
wherein the additional control unit (ACU) is configured to:
calculate an expected magnetic flux trajectory {circumflex over (Φ)}(t) using the reference mathematical model of the system;
classify the expected magnetic flux trajectory {circumflex over (Φ)}(t) to predict whether saturation of the transformer is expected to happen by establishing its mean value, its maximal value {circumflex over (Φ)} max and its minimal value {circumflex over (Φ)} min and compare them against a pre-determined magnetic core saturation limits ±Φ sat ;
evaluate a flux trajectory correction ΔΦ* (t) at which saturation is avoided or its occurrence is minimized; and
input the flux trajectory correction ΔΦ* (t) into the referencing unit and input the flux trajectory correction ΔΦ* (t) into the flux controller as an additional signal used by the flux controller to generate the correction term ΔU*.
7 . The system according to claim 6 , wherein the reference mathematical model of the system includes a detailed model of the flux control unit and mathematical models of the power electronic converter and of the transformer; and wherein the flux control unit comprises the reference flux trajectory calculator and the flux controller.
8 . The system according to claim 6 , wherein the additional control unit is configured for calculating the flux trajectory correction signal ΔΦ* (t) according to:
ΔΦ*( t )= k ·({circumflex over (Φ)} max −{circumflex over (Φ)} min )=const,
where k is a correction factor between 0 and 1.