Quantifying the Contribution of Dynamic Reactive Power Compensators on System Strength at LCC-HVdc Converter Terminals
Abstract : This paper presents improved indices to evaluate the contribution of reactive power compensators at HVdc converter terminals. The presence of such compensators impacts converter performance, such as the maximum available power (MAP) limit, susceptance to commutation failure, fault recovery time and temporary over voltage. Traditionally, the Effective Short Circuit Ratio (ESCR) has been used to indicate system strength. However, this index cannot be directly applied when power-electronics based converters are used to provide the reactive power support. The approach of this paper is to use an improved form of the Apparent increase in Short Circuit Ratio (AISCR) index to quantify the impact of the compensator on the HVdc system. This index is evaluated by comparing the performance with the reactive power device with a system without the device, but having the same performance. The paper shows that a single AISCR index cannot quantify all behaviors and suggest modifications to AISCR for each of several HVdc phenomena. In this paper two types of compensators are considered the Synchronous Compensator and the Static Synchronous Compensator (STATCOM).
? To reduce the amount of fixed capacitors for reactive power compensation an alternative is to use Static Var Compensators (SVC) as an alternative for AC transmission systems.
? However, experience from existing installations has shown that an SVC at the HVDC inverter terminal where the AC system has a very low SCR (SCR = 1.5) can cause an increased number of commutation failures during the recovery from single line to ground faults .
? HVDC transmission link still had a poor performance due to DC resonances frequencies in the system, which is also affected by the AC resonance existing between the AC system impedance and AC filters connected to the rectifier side of the HVDC transmission link.
? It is important to determine whether the frequency stability is a problem if a STATCOM is used as the reactive power compensator of the HVDC system.
? One adverse dynamic event in HVDC systems is commutation failure, which brings temporary interruption of HVDC power, and in some cases might induce more serious problems and longer power curtailment.
? It can, however, be noted that there is no STATCOM being used in a real HVDC system. In fact, the studies reported in those papers do not desccribe all problems of AC/DC interactions.
• A new control strategy for the VSC is proposed, which makes the converter less sensitive to resonances in the AC network. Its behavior is similar to that of an equivalent synchronous condenser.
• It should be noted that, from these indices, the low frequency values give an indication similar to that provided by the MPC curves proposed in (assuming that the systems are operating in the same control mode).
• The results confirm that the proposed Compensation Alternative 1, will improve the performance, but still problems are expected with the controller in some of the AC network configurations.
• Therefore the solution was found not robust for all studied combinations of the AC network.
? The transient performance of reactive power compensation options for HVDC Systems is studied by comparing their behavior during DC fault recovery, Temporary Overvoltage (TOV), and commutation failure.
? The purposes of the research discussed in this thesis are to investigate the performance of various reactive power compensation devices, to examine the mechanism of reactive power compensation for HVDC systems, and to develop guidelines for the design of reactive power compensation schemes for HVDC systems.
? Effectively designing and operating the reactive power compensation system is necessary in order to achieve and maintain good overall performance of HVDC systems.
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