S.R. ENGINEERING COLLEGE TECHNO TRENDS – 07 Paper on HVDC TRANSMISSION FREQUENCY, SYSTEM DAMPING ASSISTANCE AND STABILITY CONTROL THROUGH HVDC LINK Paper by N.SRINIVAS C.V.KRISHNA GOUR III Year EEE III Year EEE SREC SREC ABSTRACT: The HVDC transmission systems have led the way in providing assistance from a source other than a generator. This transmission made a modest beginning in 1954. But, since 1972 when the Nelson river transmission entered service in Canada. The D.C. System controls have been used to modulate the D.C. power to help stabilize the power frequency at either end of the link and dampen the power oscillations between weakly connected areas of the A.C. System. In India D.C. connections between separate electrical areas have been built to encourage energy trading. The systems they interconnect are relatively weak. This paper explains the effective use of HVDC over A.C., frequency and system damping assistance and stability control through HVDC links. KEY WORDS: HVDC Power Modulation Power System stability INTRODUCTION: The meshing level between transmission networks has become in the recent years more and more tight, due to the general trend of the utilities and system operators to exploit the networks as close as possible to their maximum transmission capacity and to the liberalization of the electrical industry, which is a well assessed or rapidly on going process in many countries. In particular the application of High Voltage D.C. links and compensated A.C. lines is becoming a frequent transmission design choice, especially in case of very long interconnections, often combined with the adoption of flexible A.C. transmission systems. In this growing interconnection scenario and in presence of high levels of power exchanges, some network operation problems may occur: increased active and reactive power losses, higher risks of voltage instability or collapse, stronger requirements for the load demands following and higher criticism associated with possible grid contingencies which typically results the most crucial and important. At this concern the simultaneous presence of A.C. and D.C. interconnections may create un expected and additional contingencies, with respect to the A.C. line faults or trips, due for instance to the D.C. line faults or communication failures at the inverter side, with following HVDC link temporary stops. In all the above depicted scenarios, power modulation controls through HVDC links is a well accepted technique to enhance A.C. systems performance. For a long time, such a technique has been used essentially to increase the power transfer capability of an hybrid A.C./D.C. power system and in particular to provide stability benefits to the A.C. system. Today the practical demands of power systems operation require the HVDC controls to be more able to regulate power flows, under adversity of operating conditions. At the same time HVDC technology (conventional or light) allows these systems to actively participate in the active/reactive power rescheduling. The D.C. current transmission offers asynchronous transfer of electrical poser. The transfer can be between independent systems or points with in a system. Though the treatment of these two cases will be different the essential contributions that comes with a D.C. link are: Power flow can be controlled precisely and very rapidly. In either direction, not only a wheel energy but also to help the control of either or both A.C. systems. By controlling its power transfer, the D.C. can help the system operator to dictate the power flows in the adjacent A.C. lines. By rapidly changing its power transfer, it can improve A.C. system stability. It can modulate its through put power to damp out past disturbance swings. Unlike asynchronous connection, D.C. transmits no fault current and buffers one network from faults in the other. As long as there is an A.C. Voltage, the D.C. terminal will operate. It can act as a voltage regulator by switching its reactive power banks or by adjusting control angles to absorb more or less reactive power. In this way it can exercise Mvar exchange with in specified limits. HVDC LINK SPECIAL DYNAMIC CONTROLS: HVDC links are typically operated according to their basic regulation modes, like individual phase rather than equidistant firing control, current control rather than voltage control mode, rectifier injection rather than inverter extinction angle control, power flow control etc. Some HVDC links are also provided with special dynamic controls, capable of responding to any deviation from the normal operating condition in the A.C. or D.C. systems, in such a way to not remain passive to special needs of the interconnected A.C. system. Some times these special dynamic controls are exclusively adopted for facing un- wanted dynamic interactions between the A.C. and D.C. systems, which may manifest themselves in a variety of voltage, harmonic and power instabilities, essentially related to the strengths of the A.C. and D.C. systems expressed by the short circuit capacity. Alternatively such A.C./D.C. interactions can be usefully exploited by using of the fast converters adoptability to achieve an over all more stable operation of the power system under control. More generally a considerable flexibility degree exists in respect to the levels of power that can be inter changed between A.C. systems interconnected through HVDC links. This property, already fully exploited in practical applications, is used to provide various types of D.C. power control, according to the operational needs briefly describe below: Network Frequency Control: The frequency of a small network interconnected to a larger one by an HVDC transmission link can be quickly controlled by means of an additional frequency feed back loop, typically acting on the D.C. link controls, such that the small network draws the required power change from the larger one. An example of frequency control is represented by the SACOI link where at the time of the original design, both the Sardinian and Corsican networks had the power rating and droop of the D.C. line comparable with or greater than the rating of the running generators in the A.C. system to which the line was connected: then the line terminal could share in the frequent regulation or even perform it unaided. Conversely, the GRITA link inter connecting the Southern Italian and Western Greek networks uses a combination of control modes, such that the control signal applied to the current controller is normally power flow, and it does so as long as the frequency remains with in predetermined limits. Out side these limits, frequency control takes over to assist the islanded network in emergency; in these operating conditions, if the minimum transmission power is approached, the frequency controller automatically starts a coordinated fast inversion maneuver. Small Signal Stabilization: In tightly connected systems dynamic instability is rare and electro mechanical swings are usually well damped for their characteristic frequencies; however, in case of long and relatively weak interconnections between large systems, low frequency swing modes may result. An Italian example of these modes has appeared in the frame work of the planning studies related to the new A.C. line interconnection between Sardinia and Corsica, in parallel to the existing D.C. link; another example in Italy could be represented by the A.C. cable interconnection between Sicily and Italy. In a worst case scenario, the response of the power system controls to the synchronizing swings associated with these low-frequency modes can produce sufficient negative damping to cancel the usual positive damping of the power system, so causing the occurrence of oscillations with increasing amplitudes. For damping these oscillations a control system to modulate the HVDC link power flow could be developed: a typical solution may be based on the small signal modulation of the D.C. power, in proportion to the frequency difference across the A.C. systems or to the power flow between themselves. Large Signal Stabilization: While the small signal modulation above discussed is suitable to maintain the state of equilibrium, it results often inadequate for the damping of very large disturbances: in this case large signal stability control is thus needed to regain the state following large disturbances, forcing for instance predefined higher steps in current order or angle set point. On this concern, with the increased power ratings of recent HVDC transmission schemes and with the introduction of VSC technology it is becoming more frequent to assess the effect of alternative converter control strategies on the transient stability levels of the interconnected A.C. systems. The thyristor valves used in HVDC transmission are in fact rate to with stand considerable over loads, without adverse effects to avoid unnecessary protective action, and this capability provides the basis for the first peak transient stability improvement. Each particular emergency strategy, i.e. current increase or decrease, temporary power reversal, etc., will use special purpose controllers, responding to power frequency and even absolute phase changes and giving the possibility to provide D.C. power burst to reduce first swing stability peaks. Further more in case of A.C. system interconnection through a tie line, a disturbance occurring on one system normally causes the trip of the line to prevent the disturbance itself affecting the other system, but at the same time the system in difficulty losses an essential in-feed. An HVDC link instead, even if equipped only with its basic controls, is capable to shield one system from disturbances on the other. In addition, although the specified power flow can continue, it is possible to vary the D.C. power setting to help the system in difficulty to the extent which the healthy system can allow, with out putting itself in difficulty, and subject to the rating of the link. An appropriate control is therefore capable to propagate a disturbance originated in one system with a pre-determined attenuation to the other system, without requiring any particular overload capability as to get through the first swing, exclusively by timely D.C. power set point reductions. Damping of Sub-Synchronous Resonance: The interaction between torsional oscillation modes of turbine generators and system oscillation modes of the electric power transmission system, by a synchronizing component with a relatively small negative damping, due to the resistance of the A.C. transmission line. In addition the damping windings and the mechanical effects resulting from stream flow, friction, etc. contribute to provide a relatively large positive damping to the torsional modes of vibration of the turbine generator shafts, which are therefore normally stable when connected to an A.C. transmission system. In presence of an HVDC system, its equivalent apparent loads changes slightly depending on the absence of control or more commonly in practice on the presence of the current control loop. The potential destabilization of torsional oscillations due to HVDC systems is similar to that caused by series compensated A.C. transmission lines and is commonly referred to as sub synchronous Resonance. The dynamic interaction with D.C. system can be solved relatively simply by providing D.C. power modulation control to cancel the negative damping impact of the basic constant power control loop. A. Indian example: In India there are five separate regions, with a disparity of resource and demand and a wide variation of operating frequency and voltage on a day-to-day basis. To synchronize any two was difficult for two reasons: the need to maintain stability and the organization necessary to schedule power exchange. The needs of resource allocation have however demanded an ability to transfer power between them. System studies were carried out to evaluate the viability of interconnecting two Indian regions using A.C. and D.C. Each is geographically large and with a substantial demand of approximately 19,000 Mw at peak times. Each of the regions is a relatively weak network even at 400 kv or 220 kv and consequently inter-regional synchronization is not readily achieved. The study primarily investigated the post fault transient stability of two alternative types of interconnection (A.C. and HVDC) between the regions, namely: A double circuit, 400 kv A.C. transmission line of nominal capacity 500 Mw, assuming that matching of the two system frequencies and voltage levels is achievable. A double circuit 400 KV AC transmission line of nominal capacity 500 Mw, with a 500 Mw HVDC back-to-back link connected at one end. Operation of the system was simulated under peak load flow conditions with transfer through the inter-regional tie. With the regions interconnected, three phase faults of different intensities were applied at representative locations around the networks. The variations with time of generator rotor angles and terminal voltages were examined together with the power flows in the interconnection. B. Canadian Example: To give an example, over 60% of the load of the province of Manitoba in Canada is supplied by the Nelson river HVDC project which brings hydro-generated power 900 km to the load center at Winnipeg. The project consists of two bipoles. It supplies four A.C. systems which, though synchronous, tend to oscillate at a natural frequency of 0.5 to 1 Hz. If the tie line to one of the four systems opens, a power imbalance will result. The HVDC controls respond to other the DC power flow by the same amount as the tie line was previously carrying, so that the disturbance is cancelled out. A further control of tie-line power oscillations is necessary. It responds to changes in the AC bus bar voltage angle, a convenient measure of the change in tie- line flows within the critical frequency of oscillation, and alters DC power flow, bringing substantial damping improvement to the operation of the network. Alternatively the modulation of power is possible in response to the frequency of one or both. A major benefit of HVDC transmission compared to AC is the inherent robustness of the inter connection in the face of difficult A.C. system conditions and its ability to isolate an A.C. system from the worst effects of a transient disturbance in an adjoining network. An HVDC link will continue to regulate power even under the conditions of varying A.C. voltage, frequency and phase angle. The ability of the HVDC to cope with the varied AC conditions can even permit the interconnection to be made with DC when it would be impossible with A.C. Results Fig. 5, 6 and 7 show results of a three phase fault applied to one circuit of an AC transmission line in region B. If there is no AC or DC interconnection, the system on its own can recover from this fault. In contrast, Fig.5 shows that, when the regions are connected with an AC tie, the tie will be lost with continuously increasing oscillation of both terminal voltage and line power flow in Region B. The AC line power is measured in one of the circuits that supplies the link. However, the results for the same study with an HVDC back-to-back link in series with the AC interconnection can be seen, in Fig. 6, to be stable with the voltages and power flow returning to new stable values with the voltages and power flows returning to new stable valves with decreasing oscillations. With the AC interconnection the effect of faults was also transferred to region A resulting in similar unstable oscillation in region A as shown in Fig.5. However, when the interconnection include back-to-back DC link there is little disturbance in region A as shown in Fig.7. The DC link has buffered the disturbance in Region B from region A and ensured stability of the whole system. These studies results match the performance of the real systems in which it has been found that even when conditions have permitted the interconnection using an AC time, the system will become unstable, but with the HVDC tie, operation is satisfactory. Additional Control benefits of HVDC: Power flows can be controlled through an HVDC link in response to control inputs and these can be programmed to act for the overall benefit of the AC system or systems. In addition to power oscillation damping or power modulation already described, it is possible to apply rapid changes in the level of power transfer in response to changed system conditions. Such applications are specific to the system conditions and must be evaluated for each case. One strategy that has been successfully applied is to reduce the power through the DC link in response to dropping AC voltage in the supplying system. The principle is to relieve a system under stress by reducing some of its load, a strategy less disruptive than load shedding, since no customer is interrupted. There will however be a reduction of power supplied to the receiving system, which must then draw on its reserves during the disturbance. Finally the power exchange can be adjusted over a slightly longer time scale to provide frequency control, provided that the change in power flow that will assist one system can readily be supplied by the other. Dynamic over voltages can arise on one side of a DC link when power flow is interrupted by a voltage collapse on the other side. An HVDC link can be utilized to control voltage through its inherently variable power factor. The valve firing process is maintained in effect developing a DC by-pass on the faulted side, the current through the by-pass being controlled to regulate the voltage on the healthy side. As a regular feature, steady state assistance can be provided to either system because there is reactive power available at the converter. By advancing the firing angle the converter will absorb more reactive, after which filter switching may be appropriate. Conclusion: Power electronics have proved a powerful force in contributing to power system stability. Damping signals can modulate the power transferred by a HCDC link to stabilize oscillations where methods based on generators control reach. By avoiding the synchronization of two systems, a DC link transfers power but offers a barrier to the transfer of faults. The paper has demonstrated that instability occurring when two regional systems are synchronized can be solved by the insertion of a back-to-back DC link. A substantial array of control options becomes available when such a link is introduced. Both the power passed through can be adjusted to relieve a system under stress and the reactive power can be made sensitive to the system’s transient requirements of voltage control.
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