Direct method of measuring gamma

2. Direct method of measuring gamma

In this method, the gamma measurement is derived from a

31.4.3.2 Gamma Control Loop

measurement of the actual valve voltage. Waveforms of the gamma measuring circuit are shown in the Fig. 31.14. An inter-

At the inverter end, there are two known methods for the nal timing waveform, consisting of a ramp function of fixed gamma control loop. The two variants are different only due slope, is generated after being initiated from the instant of zero to the method of determining the extinction angle: anode current. This value corresponds to a direct voltage pro-

• Predictive method for the indication of extinction angle portional to the last value of gamma. From the gamma values (gamma).

of all (either 6 or 12) valves, the smallest value is selected to

31 HVDC Transmission 837 circuit generates 12 gamma measurements; the minimum

μ γ gamma value is selected and then used to derive the control

firing pulse, valve 3

voltage for the firing pulse generator with symmetrical pulses.

anode current, valve 1

31.4.4 Hierarchy of DC Controls

Since HVDC controls are hierarchical in nature, they can be

anode-cathode voltage, valve 1

subdivided into blocks that follow the major modules of a converter station (Fig. 31.16). The main control blocks are:

gamma timing waveform

1. The bipole (master) controller is usually located at one end of the dc link, and receives its power order from

a centralized system dispatch center. The bipole con-

control voltage valve

troller derives a current order for the pole controller using a local measurement of either ac or dc volt-

FIGURE 31.14 Waveforms for the gamma measuring circuit [5]. age. Other inputs, i.e. frequency measurement, may also be used by the bipole controller for damping or modulation purposes. A communication to the remote

produce the indication of measured gamma for use with the terminal of the dc link is also necessary to coordinate feedback regulator.

the current references to the link. This value is compared to a gamma-ref value and a PI

2. The pole controller then derives an alpha order for the regulator defines the dynamic properties for the controller

next level. This alpha order is sent to both the positive (Fig. 31.15). Inherently, this method has an individual phase

and negative poles of the bipole. control characteristic. One version of this type of control

3. The valve group controllers generate the firing pulses implementation overcomes this problem by using a sym-

for the converter valves. Controls also receive measure- metrizer for generating equidistant firing pulses. The 12-pulse

ments of dc current, dc voltage, and ac current into

from current controller

V d γ ref +

K/(1

+ sT) Ring

MIN SELECT

MIN SELECT

12 valve voltages

FIGURE 31.15 Gamma feedback controller.

firing pulses

P ref

I o Value Group Pole

to remote bipole

FIGURE 31.16 Hierarchy of controllers.

838 V. K. Sood

I d Circuit

differential

Protection

Breaker DCPT

Pole

I ac I’

differential ac Protection

DCCT V ac Over Current

Protection I d

PT

Valve Group Protection

FIGURE 31.17 Monitoring points for the protection circuits.

the converter transformer. These measurements assist current extinction. Due to the phenomena of current chop- in the rapid alteration of firing angle for protection of ping of an inductive current, severe overvoltages may result. the valves during perturbations. A slow loop for con- The size of the smoothing reactor and the rectifier mini- trol of tap changer position as a function of alpha is mum current setting I min helps to minimize the occurrence also available at this level.

of CE.