EPE Association: EPE 1991 - 25 - Session 3.3: HVDC AND REACTIVE POWER COMPENSATION

EPE 1991 - 25 - Session 3.3: HVDC AND REACTIVE POWER COMPENSATION
You are here: EPE Documents > 01 - EPE & EPE ECCE Conference Proceedings > EPE 1991 - Conference > EPE 1991 - 25 - Session 3.3: HVDC AND REACTIVE POWER COMPENSATION

   [return to parent folder]

   STATIC REACTIVE POWER COMPENSATOR WITH SELF-COMMUTATED INVERTER AT A LOW SWITCHING FREQUENCY
By D. Wuest [View]
[Download]

Abstract: The well-known voltage-source inverter can be used as a compensator for fundamental reactive power. Compared to conventional static compensators the reactive components of the circuit are much smaller, which means less cost and less room. To reach the lower limit of a reasonable power level for a static compensator, the inverter has to be equipped with large GTOs. Therefore a very low switching frequency of some 100Hz has to be accepted. Two specific problems which arise in this situation are discussed in this paper: an adequate PWM and a controller which is able to handle a system with tendency to oscillate. It is shown that an improved optimization of switching angles helps to keep harmonics of the line-currents at a low level. Further on, a state controller is discussed, which Iets the compensator appear as a fast controllable reactive current source. The simulation results have been verified with a laboratory model on reduced scale.

   STABILlTY OF PWM HVDC VOLTAGE REGULATOR BASED ON PROPORTIONAL-INTEGRAL FEEDBACK
By B. T. Ooi; Y. Guo; X. Wang; H.C. Lee; H. L Nakra; J. W. Dixon [View]
[Download]

Abstract: This paper shows that the Voltage Regulator, as in all feedback systems, has possibility of instability. The danger can be avoided by understanding its origins. One eigenmode is the Capacitor Charging Mode. This mode can be stabilized by proportional and integral feedback. Analytical insights for the design of this mode have been developed through simplified equations. Experimental results are given from 5 KVA laboratory converter driven by multi-DSP real time controller. There is a second eigenmode which becomes unstable when the line resistance is low. The stabilization of this second mode by pole-placement using an observer is the subject of a future paper.

   EXPERIENCE WITH A DIGITAL FULLY REDUNDANT CONTROL SYSTEM FOR HVDC PLANTS
By D. Knittler; L. Hügelschäfer [View]
[Download]

Abstract: The transport of electric power is constantly increasing in importance and this is accompanied by a corresponding increase in the importance of interconnections between differing grids, e.g. between the West European UCPTE Grid and neighbouring networks. In the case of two networks which are asynchronous, with slightly differing frequencies and the resulting phase shift, direct power exchange via an AC-link is not possible. HVDC (High-Voltage Direct Current) transmission is the only possibility for power interchange between two such networks. Digital systems (microcomputers) are used for the open-loop, closed-loop and protection systems of the HVDC link. This paper describes the digital closed-loop controls. A standard multiprocessor system which has proven itself in the field of drive controls is used. The closed-loop control has been designed as a two-channel redundant configuration to improve the availability and reliability of the HVDC link. While one channel actively controls the HVDC link, the second channel operates in hot-standby mode and can take over control at any time. Proper functioning of the closed-loop control system was proven by tests in a network analyser.

   PWM-CONTROLLED DRIVES REDUCE NETWORK HARMONIC DISTORTION
By P. Büchner; Th. Eulitz; S. Baumann [View]
[Download]

Abstract: Electric drives are more and more confronted with stronger national and international limitations of power quality, which is the key to guarantee the energetic electromagnetic compatibility and efficiency of distribution networks. Considering drives with a constant dc-voltage link, a mains-friendly drive can be designed using PWM inverters on both sides of the link. The primary PWM converter combines several different tasks: to convert threephase active power into dc-power of a constant voltage, to compensate reactive power dynamically at the point of coupling and to reduce network harmonic distortion of the coupling voltage up to a frequency less than half of the pulse frequency. The paper introduces a control strategy with several separate controllers in parallel for each lower harmonic acting in a synchronously rotating coordinate system. The behavior of the current component control and harmonics control depend on the varying network configuration and has to be optimized. Simulation results demonstrate how the proposed control strategy function and show some results of active filtering and reactive power compensation.

   COORDINATING CONVERTER CONTROLS AND SYNCHRONOUS CONDENSER CHARACTERISTICS IN HVDC BACK-TO-BACK LINKS
By B. Delfino; G. B. Denegri; M. Invernizzi; G. Garzi; S. Ruzza [View]
[Download]

Abstract: HVDC connection via back-to-back link is the usual way to allow power transmission between networks operating at different frequencies. Such installation permits power transfer in either directions regardless of the frequency and voltage in the AC network. If one of these networks is weakly connected to the converter station, isolated operating condition can frequently occur. In this case the control of the back-to-back tie has a considerable effect upon the design of all the components in order to meet operation requirements. The aim of this paper is to provide issues to justify the choice of project parameters and strategies concerning the HVDC converter control and the synchronous condenser placed at the commutating bus, in order to satisfy the maximum admissible steady-state frequency error and the frequency constraints under transient conditions.

   A UNIFIED STATISTICAL APPROACH TO VECTORIAL SUMMATION OF RANDOM HARMONIC COMPONENTS
By L. Pierrat [View]
[Download]

Abstract: The history and the state of the art concerning different methods available for the calculation of the resultant of random harmonic vectors are first presented in this paper. This is followed by the development of a unified analytical method that allows to find the statistical distribution of harmonic resultant. The proposed method is found to be able to assure the continuity between the perfect statistical compensation and the purely deterministic addition. Consequently, the calculation of the exceeding probability is included for all resultant values and for any number of harmonics. The limits of generalized sum method that is based on semi-empirical consideration are also discussed.

EPE 1991 - 25 - Session 3.3: HVDC AND REACTIVE POWER COMPENSATION
You are here: EPE Documents > 01 - EPE & EPE ECCE Conference Proceedings > EPE 1991 - Conference > EPE 1991 - 25 - Session 3.3: HVDC AND REACTIVE POWER COMPENSATION
	[return to parent folder]

	STATIC REACTIVE POWER COMPENSATOR WITH SELF-COMMUTATED INVERTER AT A LOW SWITCHING FREQUENCY By D. Wuest	[View] [Download]
Abstract: The well-known voltage-source inverter can be used as a compensator for fundamental reactive power. Compared to conventional static compensators the reactive components of the circuit are much smaller, which means less cost and less room. To reach the lower limit of a reasonable power level for a static compensator, the inverter has to be equipped with large GTOs. Therefore a very low switching frequency of some 100Hz has to be accepted. Two specific problems which arise in this situation are discussed in this paper: an adequate PWM and a controller which is able to handle a system with tendency to oscillate. It is shown that an improved optimization of switching angles helps to keep harmonics of the line-currents at a low level. Further on, a state controller is discussed, which Iets the compensator appear as a fast controllable reactive current source. The simulation results have been verified with a laboratory model on reduced scale.

	STABILlTY OF PWM HVDC VOLTAGE REGULATOR BASED ON PROPORTIONAL-INTEGRAL FEEDBACK By B. T. Ooi; Y. Guo; X. Wang; H.C. Lee; H. L Nakra; J. W. Dixon	[View] [Download]
Abstract: This paper shows that the Voltage Regulator, as in all feedback systems, has possibility of instability. The danger can be avoided by understanding its origins. One eigenmode is the Capacitor Charging Mode. This mode can be stabilized by proportional and integral feedback. Analytical insights for the design of this mode have been developed through simplified equations. Experimental results are given from 5 KVA laboratory converter driven by multi-DSP real time controller. There is a second eigenmode which becomes unstable when the line resistance is low. The stabilization of this second mode by pole-placement using an observer is the subject of a future paper.

	EXPERIENCE WITH A DIGITAL FULLY REDUNDANT CONTROL SYSTEM FOR HVDC PLANTS By D. Knittler; L. Hügelschäfer	[View] [Download]
Abstract: The transport of electric power is constantly increasing in importance and this is accompanied by a corresponding increase in the importance of interconnections between differing grids, e.g. between the West European UCPTE Grid and neighbouring networks. In the case of two networks which are asynchronous, with slightly differing frequencies and the resulting phase shift, direct power exchange via an AC-link is not possible. HVDC (High-Voltage Direct Current) transmission is the only possibility for power interchange between two such networks. Digital systems (microcomputers) are used for the open-loop, closed-loop and protection systems of the HVDC link. This paper describes the digital closed-loop controls. A standard multiprocessor system which has proven itself in the field of drive controls is used. The closed-loop control has been designed as a two-channel redundant configuration to improve the availability and reliability of the HVDC link. While one channel actively controls the HVDC link, the second channel operates in hot-standby mode and can take over control at any time. Proper functioning of the closed-loop control system was proven by tests in a network analyser.

	PWM-CONTROLLED DRIVES REDUCE NETWORK HARMONIC DISTORTION By P. Büchner; Th. Eulitz; S. Baumann	[View] [Download]
Abstract: Electric drives are more and more confronted with stronger national and international limitations of power quality, which is the key to guarantee the energetic electromagnetic compatibility and efficiency of distribution networks. Considering drives with a constant dc-voltage link, a mains-friendly drive can be designed using PWM inverters on both sides of the link. The primary PWM converter combines several different tasks: to convert threephase active power into dc-power of a constant voltage, to compensate reactive power dynamically at the point of coupling and to reduce network harmonic distortion of the coupling voltage up to a frequency less than half of the pulse frequency. The paper introduces a control strategy with several separate controllers in parallel for each lower harmonic acting in a synchronously rotating coordinate system. The behavior of the current component control and harmonics control depend on the varying network configuration and has to be optimized. Simulation results demonstrate how the proposed control strategy function and show some results of active filtering and reactive power compensation.

	COORDINATING CONVERTER CONTROLS AND SYNCHRONOUS CONDENSER CHARACTERISTICS IN HVDC BACK-TO-BACK LINKS By B. Delfino; G. B. Denegri; M. Invernizzi; G. Garzi; S. Ruzza	[View] [Download]
Abstract: HVDC connection via back-to-back link is the usual way to allow power transmission between networks operating at different frequencies. Such installation permits power transfer in either directions regardless of the frequency and voltage in the AC network. If one of these networks is weakly connected to the converter station, isolated operating condition can frequently occur. In this case the control of the back-to-back tie has a considerable effect upon the design of all the components in order to meet operation requirements. The aim of this paper is to provide issues to justify the choice of project parameters and strategies concerning the HVDC converter control and the synchronous condenser placed at the commutating bus, in order to satisfy the maximum admissible steady-state frequency error and the frequency constraints under transient conditions.

	A UNIFIED STATISTICAL APPROACH TO VECTORIAL SUMMATION OF RANDOM HARMONIC COMPONENTS By L. Pierrat	[View] [Download]
Abstract: The history and the state of the art concerning different methods available for the calculation of the resultant of random harmonic vectors are first presented in this paper. This is followed by the development of a unified analytical method that allows to find the statistical distribution of harmonic resultant. The proposed method is found to be able to assure the continuity between the perfect statistical compensation and the purely deterministic addition. Consequently, the calculation of the exceeding probability is included for all resultant values and for any number of harmonics. The limits of generalized sum method that is based on semi-empirical consideration are also discussed.