1. Introduction
Line commutated HVDC converters inherently consume large amounts of reactive power; typically, the reactive power demands of the converter are 50% - 60% of the DC power being transferred. For the design and safe operation of HVDC thyristor converters, there are special concerns when connecting to weak AC systems such as high temporary over voltages (TOVs), low frequency resonances, risk of voltage instability, harmonic instability, long fault recovery times and increased risk of commutation failure. Many of these concerns are closely related to the AC voltage regulation at the converter bus. Some possible means of voltage regulation were the synchronous compensator (SC) (now virtually obsolete), the SVC (Static Var Compensator) and now the latest method, employing a STATCOM (Static Synchronous Compensator). The STATCOM option is likely to be employed with variable speed wind generator systems which will use voltage source converter(VSC) technology to connect to the grid [1-3].
Until now, HVDC systems and their associated reactive compensators were mostly operated and controlled independently. And the interactions between HVDC system filters and reactive power compensator were largely considered under steady state conditions only. If the control between HVDC system and its reactive power compensator can be actively coordinated, the performance of the HVDC will be improved in transient state as well as resulting in an improved dynamic performance. With the industry increasingly leaning or being forced towards the integration of HVDC systems at weaker AC networks, the transient performance of such systems is of vital importance.
Earlier research [4] has indicated that the combination of SVC with SC provided much faster system response than SC or SVC alone. And [2] proposed a hybrid HVDC system coordinated with a STATCOM. A more modern topology, which considers the characteristics of the line-commutated HVDC with a STATCOM at the inverter end, is proposed in this paper. The proposed system comprises a black start function and a HVDCSTATCOM coordination control scheme. Furthermore, this paper investigates the advantages of the new STATCOM based system from the point of view of the cost reduction of the HVDC link filter design, recovery from commutation failures and overvoltage control as well as the dynamics of recovery from various system disturbances including undervoltage events. The main objectives of the proposed topology are 1) to dynamically control the AC voltage at the inverter end of the HVDC link, and 2) to do the coordination control with HVDC system.
The paper is structured as follows: first, the combined HVDC and STATCOM test system, along with the control strategies employed and the choice of the coordinateing signal, are described in Section 2; second, the impact studies of the STATCOM are presented in Section 3; next, a number of dynamic simulation studies performed with EMTDC/PSCAD are discussed. Finally, some concluding remarks are made.
2. HVDC-STATCOM Systems
2.1. Overall Test System
The diagram of the proposed HVDC-STATCOM system is shown in Figure 1. The DC capacitor(Cdc) of the STATCOM is also powered by an auxiliary supply consisting of a rectifier “B” that derives its energy from a diesel engine (“C”). The capacity of the system to provide reactive power support is determined by the STATCOM’s MVA rating while its capacity to provide active power support depends mainly on the energy storage on the DC capacitor. In Figure 1, a diesel engine and a rectifier are for the “black start” of the HVDC system which may be required to recover from a complete system shutdown. Before restarting the system, it will be necessary to disconnect the load from the HVDC inverter.
The STATCOM is pre-charged to supply the power to HVDC system through the small diesel generator and a rectifier. The DC capacitor continues to be fed by the auxiliary power supply until the HVDC converter starts. When the DC capacitor is fully charged, the STATCOM output voltage is ramped up (giving smooth energization of the transformer) and then the HVDC converter can be deblocked to commence transmitting active power.
After HVDC system has recovered, the disconnector switch is opened to isolate the auxiliary power supply to the DC capacitor of the STATCOM.
2.2. HVDC Test System
As shown in Figure 1, the AC network parts of the HVDC study system and its DC controls are identical to those in the CIGRE benchmark model except for the STATCOM that is added to the AC bus bar at the in-
Figure 1. Configuration of proposed STATCOM system.
verter end [5-7].
The study system models a 1000 MW, 500 kV, 12 pulse, DC link with a low SCR receiving AC system. The STATCOM provides about 150 MVar at steadystate to fully compensate the inverter reactive power requirement. The simulations are conducted using the EMTDC transients simulation program.
With the STATCOM placed into the CIGRE benchmark model, the AC filter and fixed capacitor bank ratings have to be modified in order to keep the reactive power demand of the inverter. Table 1 shows the parameters of the CIGRE model used on the simulation. For the purposes of this study, the STATCOM is modeled as a two-level Voltage Source Converter, switched at 1050 Hz, and rated at ±150 Mvar.
2.3. Relation between HVDC System and STATCOM
Based on the HVDC system model of Figure 2, the mathematical model of the system is following as [4];
HVDC Modeling,
(1)
(2)
(3)
(4)
AC system Modeling,
(5)
(6)
where, the variables are:
: Line to line voltage at the ac busbar
: Commutation impedance
: Dc current
: Transformer turns ratio
: Inverter firing angle
: Inverter extinction angle
: Power factor angle of inverter ac current
: Inverter dc voltage
: Inverter dc Power
: Ac filter impedance, 
Table 1. Parameters of CIGRE model.
Figure 2. Inverter connected to Ac system.
: Ac system source voltage magnitude
: Phase angle between 
and 
: Ac system impedance, 
2.4. STATCOM Modeling (Figure 3)
The terminal voltage and current of the STATCOM, at the point of connection, can be modeled by a vector representation [8-10]. This vector representation is extended by a d-q model which leads to the definitions of instantaneous reactive current and active current. The voltage equations in the stationary a-b-c frame are: 
, (7)
Also, the voltage equations in the rotating d-q frame and the input voltages in the rotating d-q frame are shown on Equation (8).
, (8)
Since the active power (P) supplied from the input is directly proportional to the d-axis current 
, the d-axis reference current 
is generated from the proportional and integral (PI)-type voltage controller for the DC-link voltage regulation. And the reactive power (Q) is directly proportional to the q-axis current 
, therefore, the reactive power equation and active power equation is shown in Equation (9) respectively.
, (9)
The voltage equations shown in Equation (7) are transformed from the stationary a-b-c frame to the rotating d-q frame as follows:
, (10)
Figure 3 shows the STATCOM control block described in this section.