24-09-2013, 03:08 PM
HVDC Controls
HVDC Controls.pdf (Size: 1.29 MB / Downloads: 72)
INTRODUCTION
The historical background to the developments that took place in the evolu-
tion of HVDC controllers will be presented in this chapter. The basis and
formulations of modern controllers will be discussed. Because these con-
trollers have an important bearing upon the interconnected AC system, a
section will be devoted to inter-actions between the controllers and the sys-
tem with examples drawn from practical systems.
HISTORICAL BACKGROUND
To control the firing angle of a converter, it is necessary to synchronize the
firing pulses emanating from the trigger unit to the ac line commutation
voltage which has a frequency of (60 or) 50 Hz in steady state. In the early
1950s, when the first HVDC converter installations were implemented with
mercury arc valves, the relative size of the terminals was small compared to
the MVA capacity of the ac systems coupled to these converters. This essen-
tially meant that the grid firing system [1], which was synchronized directly
to the sinusoidal ac system waveform, could generate the firing pulses in a
relatively stable manner. However, since the three-phase sinusoidal ac
waveforms of the ac systems were used as the synchronizing elements, the
firing pulses were individually generated for each of the valves of the con-
verter.
FUNCTIONS OF HVDC CONTROLS
In a typical two-terminal dc link connecting two ac systems (Figure 4-1),
the primary functions of the dc controls are to:
Control power flow between the terminals,
Protect the equipment against the current/voltage stresses caused by
faults, and
Stabilize the attached ac systems against any operational mode of the dc
link.
CONTROL BASICS FOR A TWO-TERMINAL
DC LINK
A two terminal dc link is shown in Figure 4-2 with a rectifier and an
inverter. The dc system is represented by an inductance L and a line resis-
tance R; the value of the inductance L comprises the smoothing reactor(s),
dc line inductance whereas the value of R includes the resistances of the
smoothing reactor(s) and the resistance of the dc line etc.
CURRENT MARGIN CONTROL METHOD
The so called “Current Margin Method” of control for two terminal HVDC
system is the most widely accepted method in use at present. The method
relies on a defined zone of operation of the dc system, with clear functions
for both terminals. It also incorporates protection features to protect the dc
link [12].
Current error region:
A modification to the inverter characteristic (line PS’ in Figure 4-5) is often
made during the current margin period to avoid any instability due to multi-
ple operating points occurring with a weak ac system at the inverter.
This modification is illustrated in Figures 4-6a and b. Note that the VDCL
characteristic has been eliminated in these figures for reasons of simplicity.
INVERTER EXTINCTION ANGLE CONTROL
For the extinction angle control for the inverter, a technique similar to the
current controller at the rectifier is employed. However, the approach is
complicated due to the measurement of gamma. For the measurement of the
gamma, a direct method would be to measure the valve voltage VV, and the
gamma value would correspond to the period that the VV is negative. How-
ever, direct measurement of the VV is not always practically nor economi-
cally feasible, and alternative or indirect techniques to either measure or
predict gamma are used. Furthermore, since there are 6 (or 12) valves in a
converter, it is necessary to obtain the minimum value of the gamma of all
the valves. Different approaches for the measurement or prediction of
gamma have been reported in the literature [14,16,17,18].
Prediction of Gamma - Approach 2 [7]
In this method, a prediction of the remaining commutation voltage-time
area after commutation is made, and it is maintained to be larger than a
specified minimum necessary for successful commutation. The prediction is
approximate, but to increase its precision, a feedback loop is employed
which measures the error and feeds it back. The choice of the voltage-time
area is justified since commutation of a valve is a function of the remaining
commutation voltage-time area rather than just the remaining time period
alone.
The predictor continuously calculates (by a triangular approximation) the
total remaining voltage-time area if firing would occur at that instant. Since
the predictor is common to all the valves in one 6-pulse converter, it oper-
ates for a period of 60 degrees per valve. Figure 4-11 shows this function for
one commutation voltage
for valve 3 of the converter bridge.
HIERARCHY OF CONTROLS
The terminal at one end of the DC transmission system is shown in the Fig-
ure 4-13. The terminal can be divided into sub-sections i.e. a Bipole which
comprises of the positive and negative poles. Each pole can be further sub-
divided into the star valve group and the delta valve group depending on the
transformer configuration used. Each valve group comprises a 6-pulse con-
verter.