HVDC Control Schemes
HVDC Control Schemes
HVDC Control Schemes
Let at any instant the rectifier is operating with firing angle α1 and the inverter with extinction
angle γ1 as shown in Fig.1. Both the converter characteristics should intersect at a point, as the dc
link voltage Vd of the rectifier and that of the inverter basically represents the same value. In
particular, Vd is the voltage of a node in the HVDC link. Position of this node in a HVDC link
has been represented in the equivalent circuit of a HVDC system as shown inFig.2
Vd (per unit)
CIA:α1
A CEA : γ1
1.0
Id (per unit)
1.0
Rcr Rl Rci
Id Vd
Vacr Vdor Cosα
Vdoi Cosγ
Vaci
Vdor Vdoi
Line
Rectifier Inverter
(a) The line resistance is very small causing low damping effect in the line. Thus, it will be
difficult for the controller to arrest the power swings.
(b) The dc power is given by Pd Vd I d . Thus, to maintain a constant power in the dc link
either dc link voltage Vd or dc link current I d or in situations both needed to be
controlled. Therefore control algorithm becomes very complex as depending on
characteristic of the fault it needs to decide which parameter of dc link Vd , Id , or both
should be brought under control action.
(c) In case of any fault in the dc link, if Vd decreases then to maintain constant power in the
dc link current will rise causing damage of the dc link as well as the valves
The control algorithm in a HVDC link can be discussed under following headings. The
control scheme for the rectifier illustrating play of CIA and Constant Current Control (CC) of
rectifier is shown below in Fig.3.
Idref Id Vc2
Id Controller
Firing Angle
SELECTOR
αref Controller α
α-
α Controller Vc1
From the relation between dc link voltage Vd and dc link current I d as stated above,
surely with increase of rectifier end ac side voltage Vdor , current in the dc link will
increase. The same has been illustrated in Fig.4, which shows that due to increase of Vdor
rectifier characteristic has raised from grey colored dashed line to orange colored line and
therefore dc link current I d has increased due to shifting of operating point from ‘A’ to
‘B’. Thus, to bring down I d the term cos needs to be decreased. In particular, ignition
angle of rectifier side valves should be increased from its operating value of 1 (grey
colored dashed line to 2 (red colored line) to lower the characteristic from the new
position of orange colored line ( operating point B) to previous position (indicated by red
colored line) as shown in Fig.4. To be noted that, situation may not be brought under
control by firing angle control, as considering the constraint of poor line power factor
caused by rise of ignition angle, the maximum limit of is (10° -20°). In such cases, on
load tap changer of the rectifier side should be operated and even if satisfactory result is
not achieved on load tap changer of the inverter side is controlled.
Vd (per unit)
Shift of rectifier characteristic for increase of
Vdor ,α=α1
B CEA : γ1
1.0
A
(Red)CIA:α2
Grey CIA : α1
Id (per unit)
1.0
Fig.4: Rectifier –Inverter characteristic of a HVDC system
Thus, fromFig.3 it can be understood that, increase of valve ignition angle from 1 to
2 shifts the previous operating point from ’A’ to ‘B’.
Fig.4: Rectifier –Inverter Characteristic of inverter
From the relation between dc link voltage Vd and dc link current I d as stated above,
surely with decrease of rectifier end ac side voltage Vdor , current in the dc link will
decrease. The same can also be analyzed from Fig. 5, which shows that due to
decrease of Vdor rectifier characteristic has lowered from grey colored dashed line to
orange colored line and therefore dc link current I d has decreased due to shifting of
operating point from ‘A’ to ‘B’.
Vd (per unit)
Shift of rectifier characteristic for decrease of Vdor ,α=α1
A’
1.0 CEA : γ1
B A (Red)CIA:α2
Grey CIA : α1
Id (per unit)
1.0
Fig.5: Rectifier –Inverter characteristic of a HVDC system
So, current controller of the rectifier end will initiate control action and to maintain
constant dc link current, the control action is to raise cos term by reducing ignition
From the relation between dc link voltage Vd and dc link current I d as stated above,
surely with increase of inverter end ac side voltage current in the dc link will
decrease. The same can also be analyzed from Fig. 6, which shows that due to
increase of Vdoi inverter characteristic has raised from red colored line to black
colored line and therefore dc link current I d has decreased due to shifting of
operating point from ‘A’ to ‘B’.
Orange CIA : α2
Shift of inverter characteristic for increase of Vdoi ,γ = γ1
Vd (per unit)
A’ CEA : γ1
B
A CEA : γ1
1.0
Grey CIA : α1
1.0 Id (per unit)
Fig.6: Rectifier –Inverter characteristic of a HVDC system
So, current controller of the rectifier end will initiate control action and to maintain
constant dc link current, the control action is to raise cos term by reducing ignition
From the relation between dc link voltage Vd and dc link current I d as stated above,
surely with decrease of inverter end ac side voltage current in the dc link will
increase. The same can also be analyzed from Fig. 7, which shows that due to
decrease of Vdoi inverter characteristic has lowered from red colored line to black
colored line and therefore dc link current I d has increased due to shifting of operating
point from ‘A’ to ‘B’.
Orange CIA : α2
Shift of inverter characteristic for increase of Vdoi ,γ = γ1
Vd (per unit)
A CEA : γ1
1.0
A’
B
Grey CIA : α1
CEA : γ1
1.0 Id (per unit)
In the event of severe fall of rectifier end ac side voltage Vdor the rectifier
characteristic may be lowered to such an extent that the CIA characteristic of rectifier
fails to make an intersection with the CEA characteristic of inverter as shown in
Fig.7.
A CEA : γ1
1.0
Grey CIA : α1
CEA : γ1
1.0 Id (per unit)
The same can also be analyzed from Fig. 8, which shows that due to decrease of Vdor
rectifier characteristic has lowered from grey colored dashed line to orange colored
line and therefore dc link current I d is zero as there is no intersection between the
orange colored CIA and red colored CEA. In such cases the HVDC link practices
VDCOL, i.e, voltage dependent current order limit. With such reduced voltage it
becomes difficult for both the converters to maintain current which is equal to Idref
and so current order of both rectifier and inverter is reduced and reduced power is
sent over the dc link. To be noted in VDCOL the dc link current is maintained not by
CC of rectifier, but by CC of inverter as shown in Fig. 8.
Vd (per unit)
Blue: CIA: α1
A CEA : γ1
1.0
CC of
inverter CC of
rectifier
Id (per unit)
Blue: CIA: α1
A
1.0
B CEA : γ1
CC of
inverter
CC of
rectifier
Id (per unit)
In such case, to avoid hunting of dc link for exact value of operating point the inverter
is shifted from constant ignition angle control to constant β control as shown in
Fig.10.
Vd (per unit)
Blue: CIA: α1
Β - control
A
1.0
B CEA : γ1
CC of
CC of rectifier
inverter
Id (per unit)