DE4126274A1 - Increasing thyristor utilisation in rectifier - forming effective, permitted, valve-rated current in rectifier in dependence on actual DC voltage at input - Google Patents

Abstract

The utilisation and yield of thyristors is increased in rectifiers with superimposed d.c. voltage for compensation of reactive power. The thyristors (T1-6) have a presettable, max. valve OFF current. On exceeding of an effective, permitted valve-rated current the thyristors are switched-off, or controlled into a currentless state. The effective, permitted valve rated current of the thyristors in the rectifier (1) is formed in dependence on an actual d.c. voltage (Ud) at the rectifier d.c. voltage input (4, 5). Pref. the effective, permitted valve-rated current of the thyristors is proportional to a specified ratio, related to the idle d.c. voltage and the actual d.c. voltage at the input. USE/ADVANTAGE - GTO thyristors in rectifiers, with reactive power compensation and facility to load thyristors in reactive current operation up to thermal limit.

Description

The invention is based on a method for Increased utilization of thyristors in power converters with impressed DC voltage to compensate for Reactive power according to the preamble of patent claim 1.

STATE OF THE ART

With the preamble, the invention takes on a state of the Technology related, as he from H. Häusler, electrical engineering Basics of commutating on the DC side Power converter, electrical engineering magazine, edition A, 90 (1969) H. 15, pp. 363-367. There is in Connection with 2 asynchronous power grids use 6- and 12-pulse converter for receiving and dispensing Reactive power described. Because the DC polarity at the connections of the converter DC commutation independent of the Is the direction of energy flow, the transition from Rectifier operation for inverter operation and vice versa steadily and without reversing pole devices occur. Information about an effectively permissible valve Rated current, which is usually less than one from Component manufacturer specified, maximum permissible Valve shutdown current is missing.  

PRESENTATION OF THE INVENTION

The invention as defined in claim 1 solves the problem with a method of compensating for Reactive power to the effective permissible nominal valve current increase insofar as this is operational without restriction is possible.

An advantage of the invention is that the in Power converters used as controllable valves Thyristors in reactive current operation up to thermal Limit can be charged. Otherwise the same The converter can dimension the components control greater reactive power. If necessary, Save costs when designing the converter.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is illustrated below with reference to embodiments play explained. Show it:

Fig. 1 is a 3-phase inverter bridge circuit having an AC-side connected transformer,

FIG. 2a) -2f) Zündimpulsdiagramme for balanced operation of the power converter shown in FIG. 1,

Fig. 3a) -3c) current distributions on valves of the R phase of the power converter shown in FIG. 1 for a 90 ° lagging sinusoidal load current,

FIG. 4a) -4c) current distributions on valves of the R phase of the power converter shown in FIG. 1 for a 270 ° leading sinusoidal load current,

Fig. 5 mains voltage and load current in 6-pulse converter operation with inductive reactive load and

Fig. 6 Mains voltage and load current in 6-pulse converter operation with capacitive reactive load.

WAYS OF CARRYING OUT THE INVENTION

Fig. 1 shows a converter ( 1 ), the DC side to a positive pole ( 4 ) and a negative pole ( 5 ) of a DC voltage source, not shown, and the AC side via a choke or inductance or leakage inductance (L _R , L _S , L _T ) per AC phase (R, S, T) is connected to a mains transformer or transformer ( 2 ). The DC voltage source can be a rectifier, not shown. A capacitor (C) is connected between the negative pole ( 5 ) and the positive pole ( 4 ), on which a current DC voltage (U _d ) drops. The negative pole ( 5 ) can be grounded.

The converter ( 1 ) has 3 bridges (1 bridge / phase) with GTO thyristors or thyristors (T 1- T 6 ) which can be switched off and diodes (D 1- D 6 ) which are antiparallel to them in the bridge branches. (i _T1 , i _T2 ) denote thyristor currents through the thyristors (T 1 , T 2 ), (i _D1 , i _D2 ) diode currents through the diodes (D 1 , D 2 ).

(U _R , U _S , U _T ) denote AC line voltages of a 3- phase AC network ( 3 ) with an AC voltage of 360 V and a line frequency of 47 Hz, which are applied to the transformer ( 2 ) on the line side. (I _R ) denotes the fundamental oscillation of the current of phase (R) and (U _1R ) the converter alternating voltage of phase (R).

In FIGS. 2a) -2f) of the converter (1) are presented successively Zünddiagramme of the thyristors (T 1, T 6, T 3, T 2, T 5, T 4) for symmetrical operation, wherein the on the ordinates associated thyristor currents (i _T1 , i _T6 , i _T3 , i _T2 , i _T5 , i _T4 ) are plotted.

The Fig. 3a) and 4a) show the variation of the instantaneous value of the phase current (i) and the instantaneous value of the phase voltage (u) of the phase (R) for a φ a phase angle = 90 ° lagging or a leading to φ = 270 ° sinusoidal load current. The Fig. 3b) and 3c) and 4b) and 4c) show the current distribution on the valves (T 1, D 1) and (T 2, D 2) each for φ = 90 ° or 270 °. These current distributions show that half the reactive current is carried by the uncontrolled valves (D 1 , D 2 ) when the load is reactive.

FIGS. 5 and 6 show the variation of the instantaneous value of the phase current (i) and the instantaneous value of the phase voltage (u) of the phase (R) at 6pulsigem React.pow- power conversion legacy actuator, a constant AC line voltage (U _R) and a constant short-circuit reactance x = 0.2 of the three-phase network, from the connecting terminal of the converter ( 1 ) on the AC side, for ϕ = 90 °, ie inductive operation, or ϕ = 270 °, ie capacitive operation. It can be seen from this that the peak value of the current fundamental (zero crossing of the instantaneous value of the phase voltage (u)) increases the instantaneous value of the valve current (i) in capacitive operation with 270 = 270 ° and in inductive operation with ϕ = 90 °. With a purely inductive nominal current, the maximum occurring 6-pulse instantaneous value of the valve current (i) is approx. 50% lower than the maximum occurring instantaneous value of the valve current during capacitive operation.

In the case of purely 12-pulse operation, this is reduced Difference at the same short-circuit reactance x = 0.2 approx. 12% (not shown).

A current factor is calculated for a converter ( 1 ) with an impressed current DC voltage (U _d ) according to FIG. 1

k = I _TGQM / I _LNrms = k₀ · k₁ · k₂ · k₃ = 2.39

With:

I _TGQM = specifiable maximum valve _{shutdown current} ,
I _LNrms = effective permissible nominal valve current, k₀ = = 1.41 = peak value of the fundamental vibration,
k₁ = 1.4 = dynamic current change,
k₂ = 1.1 = factor for the intervention of a current limiter, not shown, and
k₃ = 1.1 = factor for the response of an emergency shutdown, not shown.

This factor (k) is applied regardless of the mode of operation of the converter ( 1 ). For a 3 kA GTO thyristor, the effective permissible nominal valve current I _LNrms = 1.255 kA. It is irrelevant whether the converter ( 1 ) is clocked at a basic frequency or at a higher frequency.

With k ₁ = 1.4, a stress on the valves is covered by a mains short circuit at the maximum current direct voltage (U _d ) of the capacitor (C). The rate of current rise in a switched-on GTO valve is given by the formula
di / dt = U _d / L
given, where t is the time and L is the sum of the effective inductances in the short-circuit path.

In stationary operation, the current DC voltage U _d results from:
U _d = U _d0 · (1-i _q · x)
With:
i _q = I _q / I _N (positive sign for ϕ = 90 °),
I _q = reactive current, I _N = load current at rated operation, U _d0 = open circuit _DC voltage at i _q = 0, short-circuit reactance
x = ωL · I _N / U _X ,
ω = angular frequency of the AC fundamental,
U _X = AC mains voltage, based on an earthed neutral point of the transformer ( 2 ), not shown, and X = one of the AC phases (R, S, T).

The factor k ₁ for the dynamic current change is calculated according to:

k₁ = [k _S · (1 - i _q · x)] / (1 + | i _q | · x) = k₁ ′ · U _d / U _d0

With:

k _S = 1.4 = predefinable safety factor,
k₁ ′ = k _S / (1 + | i _q | · x) = constant for x = constant and
ϕ = 90 °.

Example 1: x = 0.2

In inductive operation with ϕ = 90 °, the factor k₁ is: k₁ = (1.4 x 0.8) / 1.2 = 0.933.

For capacitive operation with ϕ = 270 °, the factor k₁ is: k₁ = (1.4 * 1.2) / 1.2 = 1.4.

Example 2: x = 0.3

In inductive operation with ϕ = 90 °, the factor k₁ is: k₁ = (1.4 x 0.7) / 1.3 = 0.754.

For capacitive operation with ϕ = 270 °, the factor k₁ is: k₁ = (1.4 x 1.3) / 1.3 = 1.4.

The usability of a GTO thyristor is therefore theoretically 50% higher in inductive operation than in capacitive operation. The control range of a reactive converter can be increased by 25% if the thermal load capacity of the thyristor allows this. The choice made in this way of the dynamic overcurrent factor (k ₁ ) is still conservative, in particular for converters ( 1 ) with fundamental oscillation, as can be seen from FIG. 5. The higher the current DC voltage (U _d ) and thus the converter voltage, the greater the risk of an excessive current. It is therefore proposed according to the invention to make the dynamic current measurement factor (k ₁ ), which represents a reserve for the breaking capacity, dependent on the current DC voltage (U _d ).

It goes without saying that the safety factors k _S , k ₂ and k ₃ can be chosen to be approximately 50% larger or smaller than indicated above.

Claims (9)

1. A method for increasing the utilization of thyristors in converters with impressed DC voltage to compensate for reactive power,

• a) which thyristors (T 1- T 6 ) have a predefinable maximum valve _{shutdown current} (I _TGQM ) and
• b) are switched off or are de-energized when an effectively permissible nominal valve current (I _LNrms ) is exceeded,

characterized,

• c) that the effectively permissible nominal valve current (I _LNrms ) of the thyristors (T 1- T 6 ) in the converter ( 1 ) is formed as a function of a current DC voltage (U _d ) at the DC voltage input ( 4 , 5 ) of the converter ( 1 ).

2. The method according to claim 1, characterized in that the effectively permissible nominal valve current (I _LNrms ) of the thyristors (T 1- T 6 ) of the converter ( 1 ) is formed in proportion to a ratio U _d0 / U _d , wherein U _{d0 is} the open circuit _DC voltage and U _d mean the current DC voltage at the DC voltage input of the converter ( 1 ). 3. The method according to claim 1 or 2, characterized in that the effectively permissible nominal valve current (I _LNrms ) is formed as a function of a short-circuit _reactance (x) of an AC network ( 3 ), calculated from an AC-side terminal of the converter ( 1 ). 4. The method according to claim 3, characterized in that the effectively permissible nominal valve current (I _LNrms ) is formed inversely proportional to a term (1 + x). 5. The method according to any one of claims 1 to 4, characterized in that the effectively permissible nominal valve current (I _LNrms )

• a) depending on the peak value of the fundamental (k ₀ ) of the respective phase current (I _R ),
• b) in particular that it is formed inversely proportional to.

6. The method according to any one of claims 1 to 5, characterized in that the effectively permissible nominal valve current (I _LNrms )

• a) depending on a safety factor (k ₂ ) for the intervention of a current limiter,
• b) in particular that it is formed inversely proportional to a safety factor k ₂ = 1.1.

7. The method according to any one of claims 1 to 6, characterized in that the effectively permissible nominal valve current (I _LNrms )

• a) depending on a safety factor (k ₃ ) for the response of an emergency shutdown,
• b) in particular that it is formed inversely proportional to a safety factor k ₃ = 1.1.

8. The method according to any one of claims 1 to 7, characterized in that the effectively permissible nominal valve current (I _LNrms ) is formed inversely proportional to a current factor k = k ₀ · k ₁ · k ₂ · k ₃ , with: k ₀ =, k ₁ = factor for the dynamic current change, k ₂ = safety factor for the intervention of a current limiter, k ₃ = safety factor for the response of an emergency shutdown.