DE2418468C2 - Process for controlling the converter stations of an HVDC system - Google Patents

Description

The invention relates to a method for controlling the converter stations of an HVDC system according to the preamble of claim 1.

Such a method is known from DE-OS 19 43 646.

In HVDC systems of the type in question, a Station half generally the brief shutdown of the disturbed station halves, so that with this load shedding a voltage increase occurs in the feeding three-phase network or on the converter transformers, which the converter stations connected to the converter transformers, here in particular the controllable converter valves, endangered.

In the method known from DE-OS 19 43 646 for avoiding the load shedding line-frequency voltage increase occurring in an HVDC system it is suggested that during the response time of a line protection a faulty one Station half the direct current of the undisturbed half of the station by reducing the voltage on the inverter is increased in such a way that the undisturbed half of the station reduces the power throughput between rectifier and inverter station increased by the reduced power of the disturbed half of the station compensate, at the same time the undisturbed half of the station to the three-phase network as a result of the increased Power throughput takes about the same reactive power as both before the occurrence of the disturbance Station halves together. As the undisturbed half of the station up to twice the power to maintain the energy supply at the consumer must be transmitted and it cannot be foreseen at which half of the station a fault will occur, the Thyristors of the rectifiers and inverters both

ίο station halves each for double normal Be designed power throughput, which means an expensive oversizing of the converter

The object of the invention is to provide a control method for reducing the increase in voltage on the network side Specify the shutdown of one half of the station at which the current in the half of the station that is not disturbed is low must rise above the mains current.

This object is achieved by the features characterized in claim 1.

The advantages that can be achieved with the invention are in particular that the current is not in the When the half of the station is switched off, it only rises a little above the nominal current and there is no costly oversizing the converter is necessary

An advantageous further development of the invention is characterized in the dependent claim

The invention is explained below with reference to the exemplary embodiments shown in the drawings It shows

jo F i g. 1 an HVDC system as the invention is based,

F i g. 2a, 2b show the course of the drawn direct current, direct voltage active power and reactive power during of shutdown and the following

Connection after the short interruption of the disturbed half of the station in the HVDC system according to FIG. 1,

F i g. 3 the time curve of the rms value of the three-phase voltage when switching off,
F i g. 4 the alternating current on the secondary side of the converter transformer as a function of the phase angle,

Fig. 5 shows the control angle and the related direct current / of the undisturbed station halves with 50% load shedding and constant three-phase voltage with two

■ 45 tax options.

In Fig. 1, a generator 20 is connected to a busbar 1. Although in FIG. 1, for the sake of simplicity, only a single-phase AC voltage network is shown, three-phase networks are used in normal application. Between generator 20 and busbar 1, the sub-transient generator reactance Xc, the machine transformer reactance Xtg and the reactance of the transmission line Xlc are shown, which together with the (not shown) bridge transformer reactance X _T b of bridge transformers 2,3 between the busbar 1 and rectifier bridges 4, 5 a Cause excessive voltage on busbar 1 due to load shedding.

The rectifier bridges 4, 5 form the two halves the one converter station, each of which has a direct current line 7 or 8 and a common ground 6 are assigned. The power passed through the rectifier bridges 4, 5 is transmitted via lines 9, 10,

b5 a control unit 13 and measuring elements 11, 12 in the Direct current lines 7, 8 regulated to a predeterminable setpoint value. Take over in normal operation both station halves, d. H. each of the two equal

richterbrücken 4,5 an approximately equal share of the Transmission power that is transmitted via inverters 17, 18 to a busbar 19 at the opposite station to the AC or three-phase network there is output. The output is there by means of a Control unit 14 and associated measuring elements 15, 16 held at a predeterminable setpoint, which with the Setpoint value of the rectifier bridges 4, 5 is matched. The control units 13 and 14 can be of protective devices are stimulated, for example by the Wandersrellen line protection, the line differential protection, the DC voltage-dependent current setpoint reduction, among other things. m.

Instead of a single bridge for each half of the HVDC system, several bridges can be used in be provided for the individual converter stations.

The following is an example to illustrate the occurring values of voltages, currents and Given inductances. For example, if you assign the following values to the reactances:

X _G = 14.5%

Xtg = Π%
X _w = 1.8%
X _TB = 16%
L = 0.75 H.

sub-transient generator reactance, machine transforming reactance,
Reactance of the three-phase line, bridge transformer reactance,
Smoothing choke inductance.

So is

dr

It was assumed that the DC voltage drops linearly after the line protection has responded by intervening in the grid control, reaches _{zero after f 0} = 5 ms and the negative end value after a further 5 ms, which is described by the following straight line equation:

(D

The direct current curve is then:

t- U - u « (_ _iL ₊ , V _M

IdN L ■ I _11n \ 21 ₀ )

(2)

Numerical values

/ ₀ = 5 ms
Uj _n = 266 kV
L = 0.75 H.
I _11n = 1800 A

The voltage and current curve of the disturbed half of the station is shown in Fig. 2a. The active and Reactive load curve with these current and voltage ratios results from the following equations:

JL

Ud = U _d0 (cos a - dx) « ü _d0 ■ cos φ,
cos φ = —f- (cos a _N - aW),

_Ö_

PdN

Jl.

IdN

Jk.

UdN

with

Q =

- [(cos ακ-

UdN

or with numerical values

q = 1.13 / νΊ- (0.866 «) ² ,

(3a)

whereby

as = 15 °

- 8%

p = u-i

Jda _

£ 4iv =

1.13
133 kV

The active and reactive power curve of the disturbed half of the station during the disturbance was Equations 3a and 4 calculated and is also in F i g. 2a applied.

After a pause that corresponds to the deionization time of the arc (approx. 200 ms), the DC voltage is brought back to the nominal value in approx. 30 ms using a ramp function. In accordance with the above equations, the curves of i, u, ρ and q have also been calculated for this process and plotted in FIG. 2b.

From the in F i g. 2a known curve of the related variables on the direct current side of the disturbed rectifier bridge 4 or 5 (FIG. 1) and the sum of the reactances on the alternating current side the time dependency shown in FIG. 3 results for the effective value of the mains AC voltage during the disconnection of the faulty converter station half

Assuming that the busbar voltage LL at the three-phase busbar 1 ( Fig. 1) is constant in the entire range of the power factor from cos φ = 0 to cosy = 1, FIG. 4 shows how the related alternating current / on the Secondary side of the current transformer depending on the power factor angle on the secondary side of the converter transformer 2 or 3 (Fig. 1) has to run if the alternating voltage LL is kept constant.

In contrast, in FIG. 4 shows with dashed lines how the alternating current / at 50% load shedding takes place if there is no intervention in the control of the half of the converter station that is not disturbed. (The numbers mean the times from the start of the disturbance, ie after the initiation of load shedding of the disturbed half of the station.) After passing through point A (ie approx. 13.3 ms after the start of load shedding), in which the two curves intersect the busbar voltage LL greater than the nominal value LLm To prevent this, there are two options for adjusting the control angle or the direct current of the converter station half that is not disturbed, as shown in FIG. 4 can be found.

First, the steering angle can not be disturbed

b5 rectifier station half are reduced, whereby the direct current would increase to double (Fig. 4, following the curve from A to C ). This option is feasible if the

Valves cannot be overloaded by the resulting high current.

The second possibility is to increase the control angle α of the rectifier station half that is not disturbed (FIG. 4, following the curve from A to B ).

By determining the course of active and reactive power as a function of the total transmission from the phase angle it is to be determined how α and / of the non-disturbed half of the converter station run, if the busbar voltage should be kept constant.

Here is

O = / Zx, ■ /, v, sin φ = reactive power,

P = - = - = active power.

The control angle can then be derived from the relationships

cos α ' - ^d

U _a

U _da · sin φ

determine.

The program sequence for the ignition angle adjustment in the control units 13 and 14 (FIG. 1) is shown in FIG. 5 for the second option described above. It shows the change in ignition angle from a point in time t _s (FIG. 3) after the occurrence of the fault, at which the busbar voltage U- is about to rise above the nominal AC voltage value U-n.

As can be seen, in the example shown, the control angle λ must go from 15 ° to approximately within 13.3 ms 42.5 ° and must have reached a final value of around 65 ° in a total of 15 ms. It's through the almost inertia-free intervention in the grid control of the undisturbed half of the station is possible.

The direct current must, as also from FIG is seen, briefly to about 1.4 times the nominal value and finally to a stationary value of about increase to 1.15 times the nominal value. The valves can, however, pass such a current flow without damage survive.

An increase in the current and the control angle is advantageously carried out simultaneously Intervention in the regulation of the rectifier station and inverter station.

If the control angle is «only at the rectifier station increased without the current setpoints of the rectifier and inverter being increased, see above

ίο the rectifier changes from current control to voltage control due to the given control characteristic, which means that after a certain time, which results from the voltage time area of the smoothing choke, the current setpoint is reduced from IdN to 0.9 IdN. Without increasing the DC current by influencing the inverter, however, the voltage cannot be kept exactly constant, instead a voltage increase of 1.5% occurs as opposed to a voltage increase of 6.5% without intervening in the grid control. In order to completely avoid an increase in voltage, the command to increase the current setpoint should advantageously be given jointly to one half of the rectifier station and inverter station that is not disturbed.

When the line protection responds, the setpoint adjustment cannot be performed in both stations at the same time begin, but at slightly different times, depending on the location of the fault location at most by simple travel times of the traveling waves on the line are apart. This is the case with a line length of 1500 km approx. 5 ms off. However, this small time difference is not important because the three-phase voltage rises above the nominal value only after a time of the order of magnitude of 10 ms. It therefore remains enough time to change the current setpoint accordingly.

Depending on the size of the reactive resistances in the short-circuit circuit (e.g. generator resp. Network reactance, reactance of the converter transformers, filter circuit capacities etc.) the control angle of the relevant converter station half can steadily increase can be brought to a value of approx. 60-70 °. The time interval in which this happens is also depending on the size of the above reactances and

is approx. 10 under normal network conditions and the usual design of a converter bridge circuit up to 25 ms.

In addition 5 sheets of drawings

Claims (2)

Patent claims: 1. Procedure for controlling the power converter stations an HVDC system, which is arranged in such a way that the converter of the energy (rectifier) and the converter of the energy-releasing (inverter) converter stations are divided into two halves connected to a common earth, where half of the converters are negatively charged and the other half are positively charged Transmission line is assigned and each half of the converter of a converter station is independent can be controlled by the other half of the converter of the same converter station, and at that in the case of the response of protective devices of one station half and the following Load shedding of this disturbed station half into the control of the not disturbed station halves in this way is intervened that the direct current is increased, characterized in that the control angle the rectifier station half that is not disturbed is increased in a time interval of which Order of magnitude of the oscillation period from the inductances and capacitances of the circuit resulting resonant circuit and that at the same time the current setpoints of the non-disturbed Rectifier and inverter station halves are increased together. 2. The method according to claim 1, characterized in that the increase in the control angle takes place according to a specifiable program of a programmer, the program in the It is designed in such a way that the control angle in the event of a fault depends on the value of the current value Operating control angle is continuously increased to 60 to 70 ° in a time interval of 10 to 25 ms.