B4-212 OPERATING EXPERIENCES AND RESULTS OF ON-LINE EXTINCTION ANGLE CONTROL IN KII CHANNEL HVDC LINK

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1 21, rue d'artois, F Paris B4-212 Session 2004 CIGRÉ OPERATING EXPERIENCES AND RESULTS OF ON-LINE EXTINCTION ANGLE CONTROL IN KII CHANNEL HVDC LINK M. Takasaki * T. Sato, S. Hara H. Chishaki CRIEPI The Kansai Electric Power Co., Inc. TMT&D Corp. (Japan) 1. INTRODUCTION The Kii Channel HVDC Link has been in commercial operation since June 22, 2000 [1]. Figure 1 depicts the location of the Kii Channel HVDC link (±250kV, 1400MW). It transmits electric power from a coal-fired thermal power plant (Tachibana Bay P/S; 1050MW*2, 700MW) to a load center in the Kansai region via 48.9km submarine cables and 50.9km overhead lines. * takasaki@criepi.denken.or.jp Figure 1 Location of Kii Channel HVDC Link The HVDC link should fulfill important roles to ensure a stable power supply and to reinforce the power system interconnection in the western part of Japan. It requires the link to possess the high reliability of power supply as a trunk transmission route. In order to comply with the requirement for attaining high reliability, Kii Channel HVDC Link employs a continuous operation scheme with an innovative on-line extinction angle control (on-line AGR: Automatic Gamma Regulator) in addition to remarkable hardware technologies such as DC-GIS and large diameter LTTs. The continuous operation control makes HVDC system capable to maintain stable commutation even under ac faults close to a converter station followed by the fast restoration of dc power just after the ac

2 fault clearing [2]. This improves HVDC system capability in recovering dc power as fast as possible and in controlling the interconnected ac system stability with a power modulation control. Figure 2 illustrates the effect of the continuous operation scheme compared to a conventional stop-restart scheme in which dc power is interrupted for several hundred milliseconds. The minimization of the energy loss during an ac system fault brings the ac system transient stability improvement. The controllability of dc power during the disturbance can give additional effect in improving overall system stability. DC power On-line AGR VDCOL Power modulation Stop-restart AC fault Fault clearing Figure 2 The effect of continuous operation with the on-line AGR The on-line AGR installed in Kii Channel HVDC Link can prevent repetitive commutation failures of the inverter except for an immediate commutation failure that cannot be avoided by converter control strategies and it can adaptively determine the control angle so as to maintain the required minimum extinction angle during ac system fault disturbances. The control had worked as designed against inverter side ac system faults of about 30 times after the commissioning. Recorded results prove satisfactorily fast dc system recovery even in very severe cases with the ac voltage residue of less than 40% in the fault period. The on-line AGR which can adaptively identify the required control angle is expected to attain better recovery close to the optimal when compared to the recovery with the VDCOL (Voltage Dependent Current Order Limit). This paper first presents the salient features of a unique converter control applied to Kii Channel HVDC Link. The effect attained through the on-line AGR application is summarized based on the operating statistics after commissioning. The validity of the present design has been discussed with measured results and EMTP analyses. 2. HVDC SYSTEM CONFIGURATION Figure 3 illustrates a schematic single line diagram of Kii Channel HVDC Link. The outline specification is also included in the figure. Transmission capacity of the link is 1400MW (+/- 250kV, 2800A) and the HVDC system is designed to allow future expansion for doubling the transmission capacity. Some components such as smoothing reactors, DC-GIS and submarine cables have the rating for dc 500kV operation. The HVDC system has a bipolar configuration with metallic return. The designs of the return cables are the same as the main cables so as to transmit the rated power even when a main cable is out of service due to the permanent fault. For the thyristor valves, large capacity light triggered thyristors (LTTs) of 8kV-3500A are applied. This enabled a compact valve structure which gives excellent aseismic characteristics. The reliability of the thyristor valves is well improved as a result of applying LTTs. In addition to the above mentioned hardware features, the high reliability of power supply, which is required as an important transmission route, has been accomplished by installing the on-line AGR to optimize dc system recovery after the ac fault clearing. For applying the new converter control, the thermal duty of thyristor valves had been studied in system design stage of the project. It has found through the thermal analyses that the valve design to allow continuous operation is almost same as the conventional valve design in terms of the valve duty.

3 AC System Anan Thyristor Valves Pole P Pole 2 Yura DC Filters K ihoku AC Filters AC Filters DC GIS Smoothing Reactors Smoothing Reactors Fig. 2 AC System Configuration Ratings Thyristor valve Overload capacity Submarine cable Overhead line Schematic diagram of the Figure HVDC 3 Kii Channel HVDC Link Bipole, Metallic return 1400MW, +/-250kV 250kV, 2800A Quadruple-6tiered Air insulated, Water cooled 125%, 30 minutes 3000mm 2, 48.9km Main: 810mm 2 *4, 50.9km Neutral: 610mm 2 *2, 50.9km 3. ON-LINE EXTINCTION ANGLE CONTROL (ON-LINE AGR) Commutation failure is a frequent event mainly caused by the ac fault such as the transmission line grounding. It can be categorized into two types. The first type is an immediate commutation failure that occurs in the period of up to one cycle after the ac fault event. The second type occurs after the period of the first type. The latter can be prevented with the converter control strategy, while the former cannot be avoided with the control. Figure 4 illustrates a usual configuration of the converter control. Control angle of each converter is determined through the minimum value selection among ACR, AVR and AGR. In normal operating condition of the inverter, AVR is selected to maintain the dc voltage constant. If the ac bus voltage in a converter station drops due to the ac fault, AGR output decreases to maintain the extinction angle for continuing stable commutation. The conventional AGR, however, cannot maintain the extinction angle dynamically required. It is because the conventional AGR calculates its output using an ac voltage magnitude and does not take the phase shift or the harmonic distortion of ac voltage into account. The delay of PLL (Phase Locked Loop) also causes the situation that the conventional firing pulse control cannot output firing pulses with correct timing in the system transients. Therefore slow ramp recovery through an additional control was applied to obtain stable HVDC system recovery without repetitive commutation failures especially in the low SCR system. P ord + Pd + EFC PM + Vord Vd APR Iord ImCurrent margin ACR AVR L V G α Phase controller Firing pulse Id Vac Pord: Power order Iord: Current order Vord: Voltage order AGR APR: Automatic power regurator ACR: Automatic current regurator AVR: Automatic voltage regurator AGR: Automatic gamma regurator LVG: Low value gate Figure 4 HVDC converter control

4 The on-line AGR can adaptively determine the required minimum firing angle for sustaining stable commutations under unbalanced and distorted ac bus voltages. Salient features of a unique converter control strategy include capabilities; i) to prevent repetitive commutation failures during and following the ac system fault, and ii) to obtain a quasi-optimal dc system recovery according to the system condition such as SCR and the fault type. Figure 5 schematically shows the configuration of the on-line AGR installed in Kii Channel HVDC Link [3]. It has replaced the conventional AGR in Figure 4. The on-line AGR consists of three functional parts; i) extinction angle control by fundamental voltage, ii) auxiliary leading angle control by harmonics and iii) quick leading angle control after ac fault detection. Faul t Det ect i on Ti mer Det ect ed Vol t age Vac 6 phase AC Amplitude Det ect or Vect or Composi t i on k u f Fundamant al Vol t age Magni t ude and phase Har moni c Component Qui ck Leadi ng- Angl e Cont r ol up Base Angl e Cal cul at i on ƒó P uˆ Auxi l i ar y Leadi ng Angl e Cal cul at i on Fi gur e 2 Schematic Figure 5 diagramof Schematic on-line diagram extinction of the on-line angle AGR control { { Fi r i ng Angl e Out put ƒ ƒá LVG: Low Val ue Gat e Select the minimum value among the last 6 firing angle orders calculated within a cycle 1) Extinction angle control by fundamental voltage The conventional AGR decides the firing angle, which is needed to maintain the minimum extinction angle, using dc current, converter transformer reactance and ac voltage magnitude. It can output a correct firing angle to ensure a stable commutation in the normal operating condition. AC voltages during and following the ac fault, however, are unbalanced and distorted with harmonics. To calculate an adequate firing angle under the condition with these transient ac voltages, it is first required to detect the magnitudes and the phase angles of the fundamental components of every 6-phase commutation voltages. The magnitude V 1 and the phase angle Φ 1 are utilized in the Base Angle Calculation block to output the required firing angle order for the ac voltage fundamental component taking the phase shift angle into account. 2) Auxiliary leading angle control by harmonics In addition to the effect of the fundamental component, actual extinction angles are affected by harmonic components of the ac voltages. The Auxiliary Leading Angle Calculation block determines the required firing angle shift to maintain the minimum extinction angle under distorted commutation voltages by referring to a table installed in this control block. 3) Quick leading angle control after ac fault detection During the ac fault, inverter has to retrieve stable commutation as quickly as possible in order to minimize the loss of dc transmission power. As the base angle control and the auxiliary leading angle control do not have sufficient control speed due to filtering operation, the Quick Leading Angle Control, which outputs the firing angle order at the ac fault using the instantaneous voltage without filtering, is added to ensure fast retrieve of stable commutation.

5 4. OPERATING EXPERIENCE OF THE ON-LINE AGR Table 1 shows an operation statistics of Kii Channel HVDC Link from the commissioning through the end of High energy utilization and availability have been achieved as a main power supply route from power plants in Shikoku to the load center in Kansai. Table 1 Operation statistics (commissioning on June 22, ) Items Operation Period (hours) Shikoku Kansai 5, , , Transferred Kansai Shikoku Energy (GWh) Total 5, , , Energy Utilization (%) Energy Availability (%) Forced Energy Unavailability (%) Scheduled Energy Unavailability (%) Table 2 summarizes the cases the on-line AGR in action due to the ac fault events in inverter side Kansai system during the same operating period. The cases are classified with the minimum ac bus voltage residue during the grounding fault. The voltage magnitude during fault principally depends on the distance between the converter station and the fault location. Most of the fault events are one line grounding faults of the ac transmission line. Table 2 shows that the on-line AGR can restrict the number of commutation failures to once for most of cases (24/27) including quite low ac bus voltage cases. The number of commutation failures is limited to twice a maximum. The HVDC system has been designed to reduce dc current when commutation failures continue longer than 60ms. Consecutive commutation failures over 200ms lead to the HVDC converter blocking. In all cases listed in Table 2, HVDC system successfully continues stable operation during and following the ac fault without the converter blocking. Table 2 Experienced on-line AGR action Minimum ac Number of Maximum Number Fault location bus voltage commutation failures restoration of times [pu] Close Remote time [ms] * Total * Necessary time to restore dc voltage and dc power to 90% of its pre-fault value after fault clearing The on-line AGR has enabled quick restoration of dc voltage and dc power immediately after the fault clearing. As shown in Table 2, Kii Channel HVDC Link can restore dc power to 90% of its pre-fault value within at least 150ms after the fault clearing. Figure 6 shows an example of the actual restoration characteristic which has selected among cases with longest restoration time. It takes about 133ms to 90% dc voltage, but the time restored to 80% is quite short of about 60ms.

6 AC bus voltage DC voltage DC current Firing angle Figure 6 An example of the HVDC system recovery with the on-line AGR 5. EVALUATION OF THE ON-LINE AGR DESIGN 5.1 Verification of EMTP model To analyze present design of the on-line AGR, an EMTP model of Kii Channel HVDC Link has been verified by comparing with actual measurement results. Some typical cases in which large ac voltage drops occurred due to adjacent transmission line faults have been selected for the comparison. In the EMTP model, HVDC system and adjacent ac systems are modeled in detail. AC networks beyond the adjacent substations are represented by reduced system models with equivalent impedances and constant voltage sources. Three generators in Tachibana Bay P/S are also modeled with constant voltage sources behind the transient reactance. Figure 7 compares the EMTP results with the actual measurement records for the case in which one line grounding fault occurred at nearby ac transmission line and converter bus voltage of the faulty phase decreased to 0.22pu. The anode-cathode voltages are shown for the 6-pulse bridge of the star-star winding side in a 12-phase converter. (X, Y, Z) indicate upper arms of a 6-pulse bridge and (U, V, W) indicate lower arms. The waveforms of anode-cathode voltages of actual measurement are biased because of the sensors for anode-cathode voltage measurement cannot detect the dc component correctly. The anode-cathode voltage in the conduction period should be at zero level similar to the EMTP analysis result. In the EMTP analysis, a grounding resistance and timing of fault initiation are adjusted to be consistent with the actual measurement. The inverter action immediately after the fault is a commutation from X-arm to Y-arm. AC bus voltage reduction causes a commutation failure at this commutation process and the arms of phase A (X, U) short-circuit the dc system. The commutation failure, however, restricted only once during the fault and the restoration period. The temporally increase of the reverse voltage can be observed at the recovery of ac voltages through the fault clearing. The peak value of the reverse voltage in EMTP analysis is about 2.4 times of its pre-fault value and that is well agreed with the measurement. Dynamic changes in the reverse voltage during the HVDC system recovery period are also well agreed between the EMTP analysis and the measurement. The same degree of agreement has been obtained for selected comparison cases. These results can prove that the EMTP model of Kii Channel HVDC Link has sufficient accuracy to design and evaluate the on-line AGR. 5.2 Validity of present design The validity of present on-line AGR design can be clarified through the analysis on HVDC system dynamic response. 1) Any excessive voltage or current which cause heavy stress on thyristor valves are not observed in the

7 anode-cathode voltages or the valve currents during the fault and the HVDC system recovery. 2) Change in the reverse voltage during the HVDC system recovery shows the on-line AGR can output the required minimum firing angle with the delay of about 4 cycles. The recovery characteristic achieves a good compromise between the secure recovery without commutation failure and the fastest dc power recovery. AC bus voltage va vb DC voltage DC current Firing angle Vd vc Id AC bus voltage V d I d α Anode-cathode voltage Anode-cathode voltage 200ms (a) Measured result (b) EMTP result 200ms Figure 7 - Comparison between the measured and the EMTP results 5.3 Comparison with VDCOL VDCOL reduces dc current order depend on ac or dc voltage decrease at the ac fault. This action limits reactive power consumption of HVDC converter in the transient period and achieves a quick HVDC system recovery [4]. Fast retrieve of stable commutation during the fault in both schemes are accomplished with an additional firing angle control such as the Quick Leading Angle Control. The difference between the on-line AGR and VDCOL mainly appears in the recovery characteristic. 1) VDCOL should be designed to have fixed slow recovery characteristic to ensure stable HVDC system recovery considering the worst system condition with the minimum SCR. The on-line AGR can adaptively attain the best recovery characteristic according to the system conditions. 2) The on-line AGR can identify the required firing angle to prevent commutation failure at every moment. This enables HVDC system to respond adequately even if an unexpected ac voltage

8 distortion appears in the recovery period. 3) In the VDCOL design, many cases of system analyses are needed to determine the fixed recovery time and the setting of control parameters differs from system to system. In the on-line AGR design, control parameters can be applied commonly to the other HVDC system. HVDC system with the on-line AGR can recover dc transmission power as fast as possible after the ac fault clearing. The new control expands HVDC system applicability as a trunk transmission system. Thermal analyses at the equipment design stage show that the valve duty when compared with VDCOL installation case has not changed so much. These results explain the sufficient reason to discuss the on-line AGR installation particularly into a large capacity HVDC system having a role of main power supply. 6. CONCLUSIONS 1) The on-line AGR installed in Kii Channel HVDC Link can adaptively determine a precise minimum firing angle taking all factors which affect an actual extinction angle into account. The factors include magnitude unbalance and phase shift of the ac voltage fundamental component, and harmonic components as well. In addition to these control functions, a quick firing angle control to retrieve stable commutation as fast as possible even during the nearby ac fault. 2) In all the 27 activated cases, the new control attained quick and stable HVDC system recovery without commutation failures. The new control made an important contribution to operate Kii Channel HVDC Link as a main power supply to a load center. 3) An EMTP model developed has been verified to have sufficient accuracy in discussing the on-line AGR design and its effect. This has been done through the comparison between the EMTP simulations and the actual measurements. 4) The analyses of the recovery characteristics with the EMTP and the measurement results proved that present on-line AGR design achieves a good compromise between the secure recovery without commutation failure and the fastest dc power recovery. 5) The practical advantage over VDCOL is that the new control can adaptively attain the best recovery of HVDC system according to the system and operating conditions such as SCR and pre-fault dc power. 7. REFERENCES [1] T. Shimato, T. Hashimoto & M. Sampei, The Kii Channel HVDC Link in Japan, CIGRE 2002 Paper No , Paris, August [2] M. Takasaki, T. Hayashi, K. Uyeda, Stabilization of HVDC System Operation under the Unbalanced AC System Voltage, CIGRE Symposium on AC/DC Transmission Interactions and Comparisons, No , Boston, September [3] S. Tamai, H. Naitoh, F. Ishiguro, M. Sato, K. Yamaji & N. Honjo, Fast and Predictive HVDC Extinction Angle Control, IEEE Transactions on Power Systems, Vol. 12, No. 3, pp , August [4] J. Arrillaga, High Voltage Direct Current Transmission 2 nd Edition, IEE Power and Energy Series 29, pp , 1998.