2.5. Selection of Auxiliary Signal in STATCOM [7,11,12]

In HVDC control, as described in the well known CIGRE benchmark HVDC model [6], primary control parameter on rectifier side is DC current and secondary control parameter is alpha-minimum control. At the inverter side, primary control parameter is gamma-minimum control and secondary control parameter is DC current control.

In case of installing a STATCOM on the inverter side of HVDC system, a supplementary signal between HVDC and STATCOM must be considered in order to improve the coordinated performance between HVDC and STATCOM.

The following signals are candidates for suitable control coordination between HVDC and STATCOM: variations: in gamma (), DC current () and AC voltage (). Among these signals, is not a good transient state control variable at the inverter end since DC current can oscillate at fundamental frequency, and actually become a sinusoid during a prolonged commutation failure. However, in the rectifier side, DC current signal could be a good control variable. Also, AC voltage as a coordinated supplementary signal can be considered, but because this signal is directly proportional to AC voltage and reactive power, its use as a control signal in the case of the STATCOM is not preferred.

Since the trajectory of value between active power () and reactive power () in HVDC system moves according to cosine function (Figure 4). If gamma signal of HVDC is added to the controller of STATCOM, the gamma signal can be a useful signal for improving the performance of STATCOM in transient state.

However, there are two options for employing the gamma signal: either a mean value gamma signal or a minimum value gamma signal. The signal is more relevant for improving performance against a commutation failure, but the response time is inherently slower(when compared to the derivation of the) due to the computation algorithm. On the other hand, the is less relevant against commutation failure, but the response time is fast. Furthermore, since the is always higher than value. This comparison is shown in Figure 5(a) for the recovery of the system following a 3-phase fault at the inverter. Figure 5(b) shows the recovery of the 3-phase AC voltages, and shows the recovery of the corresponding DC current and DC voltage.

Due to the relative speed of response characteristics,

mean signal is selected as the coordinated supplementary signal of STATCOM. The supplementary signal of STATCOM is following as :

(11)

The signal which represents gamma variation, as shown in Equation (11), is added to STATCOM controller.

3. Impact of the STATCOM

The impact of the STATCOM is felt upon the following sysem aspects:

3.1. Stability Enhancement from Viewpoint of AC Network

From the viewpoint of reactive power, the relation equation of AC system is as following

(12)

where,

: Reactive power due to AC system impedance

: Reactive Power consumed by HVDC

: Reactive power by filter or capacitor

: STATCOM

From Equation (12), if the capacity of can covers an AC reactive power requirements, that is, can supply or absorbs the reactive power variations of AC system enough, it mean that Maximum Available Power (MAP) of HVDC can increases within the bounds of STATCOM control ability.

Using Equation (12) and Equations (1)-(5), MAP curve is shown in Figure 6, it shows that the capacity of STATCOM effects MAP capability as well as the voltage stability.

The case of Figure 6 is that STATCOM is not located in near HVDC station, but within AC system. Figure 7 shows that MAP is increased according to the location of STATCOM, which is related to the variation of AC impedance angle.

3.2. Counter-Acting Commutation Failures (CFs)

Commutation failure (CF) is one of the most frequent inverter failures in HVDC systems utilizing thyristor valves [13-16].

Before the valve can establish a forward voltage blocking capability, the internal stored charges in the valve, produced during the forward conduction interval, must be removed. Therefore, the valve requires a certain minimum negative voltage-time area, provided by the commutation margin angle, to re-establish its blocking capability.

Otherwise, the valve will immediately reconduct when it is again forward biased and that will result in an unwanted short circuit and a CF may ensue.

CFs can be detrimental to HVDC links. For instance, they can result in significant direct current increase and thus lead to overheating of converter valves which will shorten their lifespan. In addition, CFs can cause DC magnetic biasing in the converter transformers, and may even lead to the outage of the HVDC system. From this perspective, CFs can be considered as an index of nonavailability of the HVDC link.

A CF is a very complex phenomenon to analyze. Reference [9] proposed an empirical equation for defining CFs, as shown in Equations (13) and (14). In these equations, the possibility of CF is expressed by which is the onset voltage of CF. Equation (13) is for a 3-phase fault case and Equation (14) is for a 1-phase fault case

(13)

(14)

where, is larger DC current, is the impedance of converter transformer, is thyristor extinction angle, is thyristor operation angle, is AC voltages intersection deviation angle due to 1-phase fault. From Equation (13) and (14), larger DC current () can be re-arranged as shown in Equation (15)

(15)

where,

: fault clearing time

: AC voltage after fault

: AC voltage before fault

: an inductance of smoothing reactor

: an inductance of cable or overhead line

Equations (13) and (14) show that the primary reasons for CFs are current increase and the extinction angle deviations of the HVDC converter due to AC voltage reductions which can be caused by [17,18]:

• AC voltage faults/disturbances

• Transformer inrush current

• Capacitor inrush current

• Harmonic pollution or/and instability

• System induced resonances

From the above explanations, the obvious solution to reduce the risk of CFs is to maintain a constant margin angle and to maintain constant AC voltage. From Equation (14), the condition and equation for critical load variations (transformer energizing and capacitor switching etc.) to induce CFs can be shown as Equation (16).

(16)

where, is a short circuit capacity of equipment to connect to AC bus bar.

In this paper, several cases are simulated to show the impact of the STATCOM. The simulation conditions are as follows: SCR = 2.5 based on the CIGRE benchmark model, the capacity of STATCOM is 150 MVA and the operational set point condition is 0 MVAR (i.e. it is “floating”). And the simulation cases are with transformer energization and remote faults according to distance from the fault.

Tables 2-4 show the occurrence of CFs according to transformer energization. The STATCOM operating point is varied from 0 MVA for Table 2, lagging 50 MVA for Table 3, and leading 50 MVA for Table 4.

From Tables 2-4, as the Capacity MVA is increased from 300 to 600 MVA, CFs appear earlier without the STATCOM, i.e., at 400 MVA level. Although it is difficult to quantify the actual significance of the STATCOM in reducing the occurrence of CFs, the beneficial trends are obvious from these tables.

Table 5 shows the result in case of a remote fault according to its distance from the commutating bus. The STATCOM is held in neutral mode (i.e. 0 MVA supply) for this case. With the STATCOM present, CFs are experienced at distances of 200 km or less. However, without the STATCOM, these distances have to be extended to 300 km or less. The beneficial impact of the STATCOM is evident.

Tables 6, 7 are the cases with or without the complementary control in STATCOM in operation. From Table 6, with the complementary control in STATCOM, CFs are experienced after 500 MVA capability of the transformer energization. However, without the complementary control in STATCOM, CFs are experienced at less than 450 MVA. Again the beneficial impact of the STATCOM is evident.

As shown in Table 7, the incidences of CFs with distance to a remote fault are investigated. With the STA-

TCOM complementary control in operation, the distance to the fault can be extended beyond 250 kms without causing CFs. However, without the STATCOM, this distance has to be increased to beyond 250 kms. Again the benefits of the STATCOM are evident.

3.3. Robustness against Harmonic Pollution

From Equations (13) and (14), the main causes of CFs are DC over-currents and AC voltage reduction. Also, AC harmonics are another major factor which can cause CFs in HVDC converters. Figure 6 conceptually shows how the extinction angle is reduced due to 3rd and 5th harmonic components in the AC voltage. The STATCOM is the designated equipment to supply reactive power to the AC network, functionally in the same manner as a synchronous condenser or a SVC, and structurally, in a similar manner to an active power filter or power conditioner. Fortunately, because a STATCOM is composed of self-commutating semiconductors, it has an inherent immunity against externally generated harmonics.

Table 8 show the reduced harmonic spectrums and the filtering capability against harmonics, in the cases that harmonics (2nd, 3rd, 5th and 7th harmonics) are arbitrarily injected in CIGRE model with and without STATCOM each other. In Table 8, 2nd harmonic is reduced to 60%, this result shows that STATCOM can contribute to stability enhance­ment reducing 2nd harmonic resonance condition which causes core saturation instability.

Also, two kinds of filters are normally used in HVDC systems, i.e. tuned and damped filters. Tuned filters are used for suppressing only characteristic harmonics.

On the other hand, damped filters are often employed for the suppression of both non-characteristic and characteristic harmonics. The capital cost of tuned filters is lower than for damped filters, despite the fact that a separate filter bank is required for each of the lower-order characteristic harmonics. This cost advantage is one of the reasons for the use of tuned filters.

However, the AC system has numerous other harmonic sources apart from the DC link. The AC harmonic filters play a major role in determining the waveform quality and amplitude of switching surge overvoltages on the converter bus bar. In HVDC systems, transients should be considered an integral part of the filter design for harmonic suppression. The effect on both the converter station and the existing AC system should be evaluated as part of the overall design process. Figure 7 shows fault recovery characteristics of the CIGRE HVDC model with either the damped filters or the tuned filters. The fault investigated is the recovery from a 3-phase to ground fault, which usually produces the highest overvoltages and harmonics, with the converters blocked. From Figure 7, AC network including HVDC with damped filters can have the good recovery characteristics after the fault as the same as the case with the tuned filter. Despite of the capital disadvantages, this characteristic is one of the reasons for the widespread use of tuned filters.

Consequently, if the STATCOM is connected to the

HVDC terminal, AC network with tuned filters can have the good recovery characteristics after the fault.

4. HVDC-STATCOM Simulations

4.1. Case Studies

Comparative studies of two system configurations of CIGRE model i.e. one with STATCOM and another without STATCOM were preformed. In these cases, the consideration was the converter transformer energization and its resulting saturation, which causes voltage and current distortion and repeated commutation failures at the inverter end. Results with two fault cases with either 1-phase or 3-phase faults at the inverter bus are presented below.

4.2. Results

In the results that follow, the following signals are depicted: AC Voltages (3-phase instantaneous and rms values at the inverter end (Upper trace), and DC voltage and current (lower trace).

Figure 8(a) shows the case of a 3-phase fault at the inverter end on the CIGRE Model without a STATCOM. During the recovery of the HVDC system, repeated commutation failures are seen. This should be compared with Figure 8(b) where the corresponding case with the STATCOM in place is depicted. The results show that the repeated commutation failures were not observed in Figure 8(b). In this case, the impact of the STATCOM on the recovery of the recovery of the system of the system is quite conclusive.

5. Conclusions

This paper deals with the use of a STATCOM at the inverter end of a conventional HVDC system for the dynamic reactive power support. This new topology has a number of benefits. First, with the aid of an auxiliary source, this topology permits a black start feature.

The proposed scheme may be a good solution for connecting island loads, offshore oil platforms, or offshore wind farms. The proposed solution can be used with both cables and overhead lines. Second, with the reactive power coordination between STATCOM and HVDC systems, it offers a robustness for recovery from a commutation failure, and other faults on the system. Third, with the ability of the STATCOM to act as an active filter, the design of the passive filters of the HVDC system can be eased somewhat; this will enable recouping some of the costs incurred in installation of the STACOM. In this paper, the coordinated control strategy

for the STATCOM-HVDC has been described.

6. REFERENCES