Along with the increase of installed capacity of wind-driven power, wind power grid-connected generating system is increasing on the impact of electrical network, from keeping the angle of power system stability, requires the wind-powered electricity generation unit can keep not off-grid operation when line voltage falls.At present, GB/T9963-2011 " wind energy turbine set access power system technology regulation " has proposed the requirement of wind energy turbine set low voltage crossing (LVRT) ability, requires wind electric converter to possess the LVRT ability.
For the full power convertor based on synchronous generator, when line voltage falls fault, for keeping power output constant, net side converter output current increases, when reaching maximum constraints, full power convertor motor side input power can be greater than the grid side power output, and DC bus-bar voltage raises.For the double-feed current transformer based on double fed induction generators, because instant of failure double-fed asynchronous generator magnetic linkage can not suddenly change, stator current increases, because the close coupling of stator, rotor relation, so that rotor produces huge energy impact, and by rotor-side converter inflow dc bus, because line voltage falls the power delivery ability that has limited the net side converter, cause DC bus-bar voltage to raise rapidly.Raise so need to take certain measure to suppress DC bus-bar voltage, guarantee that current transformer runs without interruption.A kind of method commonly used at dc bus relief arrangement in parallel.Discharging circuit is made of device for power switching and electric power generation unloading resistance usually, controls the input of electric power generation unloading resistance and cuts out inhibition dc bus overvoltage by power device.
Discharging circuit control mode at present commonly used is only with the decision condition of DC bus-bar voltage as the off-load switching, do not consider the electric power generation unloading resistance off-load ability upper limit under the actual condition, line voltage easily causes electric power generation unloading resistance to burn because of frequent operation when falling the faults frequent generation.CN201774451U " direct-current unloading circuit of double-fed wind power converter " has provided the implementation of hardware discharging circuit, but the control method of discharging circuit and the guard method of electric power generation unloading resistance are not provided.CN101136582A " a kind of control method of DC side-discharging circuit of full power convertor " proposes a kind of by Gather and input active power, output reactive power and dc voltage; ask for the deviation of power as input variable; regulate the scheme that the control off-load drops into duty ratio by PI; this kind mode has been improved the impact of off-load action on busbar voltage; but response speed is slow, and does not provide necessary protection to electric power generation unloading resistance equally.
Summary of the invention
Electric power generation unloading resistance can't effectively be protected, the problem that causes the electric power generation unloading resistance lost of life and even burn in order to overcome existing current transformer DC side-discharging circuit control method.The present invention proposes a kind of control method of current transformer DC side-discharging circuit, realizes the protection of electric power generation unloading resistance.The present invention can be applicable to based on the full power convertor of synchronous generator with based on the double-feed current transformer of double fed induction generators.
When electrical network generation voltage fell fault and causes busbar voltage to rise, the DC bus-bar voltage outer shroud controller action of net side converter by increasing the active current of net side direction electrical network output, suppressed busbar voltage and raises.But slow because of busbar voltage outer shroud response speed, or after busbar voltage outer shroud adjuster was saturated, busbar voltage can rise uncontrollably.Just need this moment DC side-discharging circuit to participate in, by the part energy on the electric power generation unloading resistance release bus, to suppress busbar voltage.
The present invention uses the off-load controller that the discharging circuit that is made of off-load switch and electric power generation unloading resistance is controlled.The off-load controller gathers d-c bus voltage value, the d-c bus voltage value that gathers is at first passed through the high frequency ripple composition in the low pass filter filters out DC bus-bar voltage, then input voltage hysteresis comparator, the output valve of voltage hysteresis comparator is as off-load on off state controlled condition one.Simultaneously, the off-load controller reads current off-load on off state, and calculate the energy accumulation value of electric power generation unloading resistance according to current off-load on off state, and the result of calculation of energy accumulation value is as the input of energy hysteresis comparator, and the output valve of energy hysteresis comparator is as off-load on off state controlled condition two.When satisfying off-load on off state controlled condition one and condition two simultaneously, by controller output control signal, drop into electric power generation unloading resistance, otherwise cut out electric power generation unloading resistance.
Described low pass filter can be the analog filter of shared-converter controller, also can be digital filter.Described low pass filter is the fertile hereby low pass filter of Bart, by the selection of frequency take can the filtering grid side and the switching frequency of motor side current transformer near ripple as target.
The stagnant ring scope of described voltage hysteresis comparator can be adjusted according to actual conditions, the ring upper limit that stagnates maximum is limited by bus capacitor and grid side motor side current transformer power device withstand voltage, need leave enough surpluses, the stagnant ring bus voltage value of minimum value must normally move greater than current transformer time that roll off the production line will keep surplus equally.Generally, stagnant ring should be narrow as far as possible, the fluctuation range of busbar voltage when moving to reduce discharging circuit.
The computational methods of described energy accumulation value are as follows:
At first calculate the energy value that electric power generation unloading resistance adds up or reduces in the control cycle T.When off-load is in the input state, busbar voltage V
DcBe added in electric power generation unloading resistance R two ends by the off-load switch, because it is very large to unload charged current, unload the energy that energy that charged current produces at electric power generation unloading resistance R discharges much larger than the electric power generation unloading resistance natural heat dissipation, therefore cumulative energy value Q of off-load monocycle
Off-load is in when cutting out state, and electric power generation unloading resistance is by radiation, conduction and force the type of cooling to be dispelled the heat.The ceiling capacity that can bear when known electric power generation unloading resistance impacts Q
MaxReturn to the required time T of initial temperature after impacting with ceiling capacity
Interval, can calculate the approximation Q of off-load monocycle change energy
Suppose that the electric power generation unloading resistance energy is Q
Sum, and current transformer to power on when initial be 0, then difference form is described as:
In the formula, Q
Sum(n) the current control cycle energy value of expression, Q
Sum(n-1) the last control cycle energy value of expression, Δ Q represents the changing value of monocycle energy.When this cycle off-load switch is in conducting state, Q is arranged
In, when this cycle off-load switch is in cut-off state, Q is arranged
Out, at last to electric power generation unloading resistance energy value amplitude limit 0≤Q
The stagnant ring of energy hysteresis comparator upper limit maximum is limited by the electric power generation unloading resistance energy limit, needs less than Q
Max, minimum value guarantees according to actual needs to pass through electrical network in the most abominable situation and falls fault and get final product.The stagnant ring minimum value that rolls off the production line is 0.
The conditional combination arranged side by side of the controlled condition one of off-load on off state and condition two uses with gate logic and realizes.
Control method of the present invention can be controlled the switching of DC side-discharging circuit effectively and accurately, prevents the dc bus overvoltage, prolongs the useful life of dc bus capacitor and DC side-discharging resistance.Particularly for powerful wind-powered electricity generation unit, especially full power convertor, in the low voltage crossing process during DC side unbalanced power, mainly absorb unnecessary power by discharging circuit, power such as simple increase current transformer DC side-discharging resistance, must cause the discharging circuit increase that takes up space, cost rises simultaneously.This control method can all be realized by digitial controller, need not additionally to increase hardware, and is safe and reliable.By adjusting the stagnant ring scope of two hysteresis comparators, can be easy to realize the adjusting of bus ripple effect and the variation of various power grades simultaneously.
Further specify the present invention below in conjunction with the drawings and specific embodiments.
The overall control scheme is controlled by the discharging circuit 101 of 102 pairs of current transformers of off-load controller as shown in Figure 1.
The ceiling capacity that the electric power generation unloading resistance that at first provides according to electric power generation unloading resistance manufacturer can bear impacts Q
MaxReturn to the required time T of initial temperature after impacting with ceiling capacity
IntervalCan obtain the energy approximation value Q that electric power generation unloading resistance reduces among the single control cycle T
For cumulative energy value Q of off-load monocycle
InApproximation is according to current busbar voltage V
DcAnd electric power generation unloading resistance resistance R draws:
Off-load controller algorithm block diagram as shown in Figure 2.The off-load controller gathers the original busbar voltage U of direct current
Dc, the high frequency ripple composition by in the low pass filter 201 filtering DC bus-bar voltage obtains filtered DC bus-bar voltage V
Dc, difference input voltage hysteresis comparator 202 and energy accumulation computing module 204.The logic of voltage hysteresis comparator 202 is:
v(n) the current control cycle output valve of expression, SS
v(n-1) the last control cycle output valve of expression, V
CutinExpression stagnates and encircles lower voltage limit value, V
CutoffExpression stagnates and encircles the upper voltage limit value.The output valve conduct of voltage hysteresis comparator 202 and first input value of door 206.Simultaneously, the off-load controller obtains the off-load on off state of previous control cycle from reading current off-load on off state with door 206 outputs through delay link 203, input to energy accumulation computing module 204.Energy accumulation computing module 204 is respectively on off state 0 and state 1 counting, when on off state is 1, and electric power generation unloading resistance energy value Q then
SumAccumulate once Q
InValue when on off state is 0, then makes Q
SumAccumulate once Q
MaxAmplitude limit input to energy hysteresis comparator 205, its logic is:
q(n) the current control cycle output valve of expression, SS
q(n-1) the last control cycle output valve of expression, Q
EnExpression stagnates and encircles lower energy value, Q
DisExpression stagnates and encircles the upper energy value.Energy hysteresis comparator 205 output valves as with second input value of door 206.Finally by driving the off-load switch motion with door 206 output valves, drop into or cut out electric power generation unloading resistance.With door 206 output valves be 1 o'clock, the off-load switch conduction is the cut-off of 0 o'clock off-load switch.