TEMPORARY OVERVOLTAGE LIMITER FOR HIGH VOLTAGE TRANSMISSION SYSTEMS

Abstract: The invention consists of a method to greatly enhance the Mvar absorption of a high voltage shunt reactor in response to excess Mvar suddenly appearing on a high voltage transmission system. A high voltage capacitor is connected in series with a shunt reactor in order to increase the reactor voltage, and thereby the Mvar absorption, as a function primarily of the capacitor value in combination with the dynamic impedance of the reactor in the saturated region of the reactor V/I characteristic. The new with the invention is that the Mvar absorption of the reactor is no longer dependent on the system voltage alone. The invention also includes a reactive device consisting of a multi phase shunt reactor with a capacitor connected in series with each phase.

Description: Temporary Overvoltage Limiter for High Voltage Transmission Systems The invention consists of a method and a device to immediately limit the magnitude of potentially high temporary power frequency overvoltages (- hereafter called temporary overvoltages) on high voltage electrical power systems, which could otherwise damage the equipment insulation, and lead to expensive repair and system outages. Of the three types of overvoltages broadly classified for power systems - lightning overvoltages, switching surge overvoltages, and temporary overvoltages, the first two are relatively easily neutralised using modern surge arresters. Temporary overvoltages however, can usually only be removed after the event, by switching out the primary cause, usually an element or elements of high capacitance producing appreciable reactive effect (Mvar) which the system cannot absorb. The excess Mvar production ΛO, in relation to the electrical strength of the system (- the short circuit level Sk) determines the temporary overvoltage magnitude V, by the approximate relationship : - V = - (1) Sk V is the temporary overvoltage in pu (per unit), ΔQ is in Mvar and Sk is in MV A. The situation is rapidly becoming critical in transmission systems being expanded using HVDC converter equipment which in operation requires about half as much reactive power (Mvar) from the electrical system, as the active DC power (MW) transmitted. The Mvar required are produced at the converter stations by the large capacitive filter banks, which are also used to remove higher harmonic currents produced by the HVDC rectification process. The sudden appearance on the system of the filter Mvar production during blocking of the converter valves after repeated commutation failure, or an HVDC side fault, results in high temporary overvoltages appearing on the electrical system. The effect is often enhanced, as blocking due to an AC power system fault, generally occurs simultaneously at all electrically close HVDC converter terminals ( - within about 100 km of each other). Thus the temporary overvoltage can exceed the capability of the circuit breaker equipment to disconnect the filters. As an illustrative scenario a section of a 400 kV three phase power system, having a short circuit level of 3000 MVA, with three 400 kV HVDC converters each operating at 600 MW requires a reactive power production of about 900 Mvar from the filters. The 400 kV system will experience a temporary overvoltage of up to 600 kV during blocking of the converter valves, when the Mvar production of the filters suddenly appears as excess Mvar on the system. This is well beyond the circuit breaker rating, and the power frequency withstand level of much of the equipment insulation. Methods to reduce temporary overvoltages include switching in large high voltage shunt reactors at a pre selected temporary overvoltage level, to absorb the excess Mvar from the filters, and also Thyristor Controlled Reactors (TCR). The method of direct switching has the disadvantage that because of the natural delay in the voltage measuring and switching system, many cycles of the high temporary overvoltage will be impressed on the power system before the reactors are in operation. Typically units of about 200 Mvar are available, thus many units may be required, together with the associated switching equipment. The installations cost typically about 1.5 - 2 M$ per unit. The method using power electronics to operate a Thyristor Controlled Reactor (TCR) already under voltage on the system is also slow, being delayed by the measuring electronics and the switching logic. A TCR of about 300 Mvar short term rating costs typically about 5 M$ With these methods the Mvar absorption of a shunt reactor is determined by the magnitude of the temporary overvoltage on the system as given by equation (1). The invention consists of a method to increase the Mvar absorption of an in service high voltage shunt reactor in response to excess Mvar suddenly appearing on the power system, without significantly increasing the system voltage. This is achieved by connecting the reactor in series with a high voltage capacitor, the combination is hereafter called the reactive device. The reactive device forms a traditional rLC series circuit, in which the value of the inductance L depends on the degree of saturation of the shunt reactor according to the reactor's V/I characteristic. For all values of L throughout the operating range, the resonant frequency of the reactive device is by design well below the system frequency ω. By design ωL is considerably greater than 1/ωC, and thus a voltage amplification K occurs across the reactor which, neglecting r (« ωL), is given by: - K= ωL / ( ooL - 1/ωC), and K > 1 in terms of the system voltage. When an increase occurs in the system voltage, the voltage across the reactor increases correspondingly more by the factor K. The reactor rapidly enters saturation where L decreases and thus K increases. The Mvar absorption of the reactor is thus forced beyond that required by the system voltage alone, and the reactor then absorbs the excess Mvar appearing on the system with little increase in the system voltage. The V/I characteristic of the reactor is chosen such that the magnetic saturation point is about 10 % above the normal maximum continuous system operating voltage. Thus the reactive device is essentially passive during normal system operating conditions. The value of the capacitor is chosen in relation to the dynamic impedance of the reactor in the saturated region of the V/I characteristic, such that the reactive device first becomes strongly active in this region. In practice the upper limit of operation of the reactive device is determined by the short term withstand voltage level of the reactor, which must be compatible with the excess Mvar absorption demands. EP A 0.141.230 describes a combined AC filter and Mvar production device for use primarily with HVDC converter equipment. This device is entirely different from the present invention as excess Mvar appearing on the system is controlled by the conventional method of switching out banks of high voltage shunt capacitors using high voltage circuit breakers. The device also acts as a high pass filter with an adjustable resonant frequency at the 11 th harmonic and above. The present invention has a resonant frequency well below 50 Hz. SU A 158.40.31 describes a device consisting of a low voltage reactor in parallel with a capacitor, thereby forming a parallel LC circuit with a resonant frequency above 50 Hz. The device is connected in series with a Mvar producing high voltage capacitor bank and ground. The value of L is controlled by a separate DC magnetising signal in response to an increase in the system voltage. It is claimed that the impedance of the parallel branch is thus increased, thereby increasing the resulting capacitive impedance of the combined circuit, and reducing the Mvar production of the high voltage capacitor. This device is entirely unlike the present invention as it employs a parallel resonant circuit resonant above 50 Hz. In addition because of the separate source DC magnetising bias, the device-operating mode is asymmetric. The new with the invention is that by means of a traditional rLC series circuit tuned to a frequency well below the system frequency, it is possible to force an increase in the Mvar absorption of the shunt reactor (L), controlled primarily by the series capacitor (C), and not solely by the system voltage as with other methods. The reactive device operates both as a device to absorb excess Mvar appearing on the system, and as a temporary over voltage limiter. The operating method has the advantage that no power electronics or switching is involved, and thus the reactive device responds without delay with a greatly increased Mvar absorption when the system voltage rises above the saturation point on the reactor V/I characteristic. The saturation point on the V/I characteristic acts as a 'switch' to trigger the action of the reactive device, whereas below the trigger level the device operates essentially as a simple shunt reactor with a Mvar absorption corresponding to the continuous rated value. With a properly tuned reactive device, the system voltage does not rise significantly above the saturation voltage level of the reactor V/I characteristic, and the temporary system overvoltage is therefore largely independent of the excess Mvar suddenly appearing on the system. The reactive device is only required to withstand the increased current and voltage for at most several seconds, depending on the protective relay system used for disconnecting the capacitive element(s) causing the temporary overvoltage. A reactive device unit with a 100 Mvar continuous rating costs typically about 1-1.5 M$, thus with the much increased short time Mvar absorption capability the device is considerably less expensive than the previously discussed devices. The operation of the reactive device is illustrated below. Fig 1 shows a reactor of XI Ω alone with an applied voltage of Vab. The Mvar Ql absorbed by the reactor is given by : - Vab 2 Qi = XI Fig 2 shows a reactor in series with a capacitor ofXc Ω (where numerically Xc <Xl). The Mvar Qab absorbed by the combination of the reactor and the capacitor, is given by : - Oak Vah 2 XI - Xc Thus the Mvar absorbed by the reactive device is greater than that of the reactor alone for the same applied voltage. Figs 3, 4, 5 and 6 illustrate for simplicity the steady state operation of a single phase version of the reactive device. Voltage and current rms values are used for the simplified reactor V/I characteristic of fig 3, which has the magnetic saturation point at 1 pu voltage ( ~ 400kV). As shown the value of NZ is in the range 1600> X > 400 Ω depending on the reactor voltage VI This corresponds to a single phase 400 kV reactor with a rating of 100 Mvar. Xc has the parameter values of 100, 150, — 350 Ω. The calculated results showNZ, VI, and Qab, withNc as parameter and Vab maintained at 1.15 pu voltage (~ 460 kV) such that the reactor operates into the saturated region of the V/I characteristic. Fig 6 shows that the reactive device absorbs about 500 Mvar with Xc = 275 Ω ( ~11.5 μF ). The reactor acting alone without the capacitor will absorb about 200 Mvar for the same system voltage conditions of 460 kV. The temporary overvoltage stressing the reactor (fig 5) is approx 2 pu, and for the series capacitor approx 1 pu. In a power system the duration of these voltages is determined by the actual protective scheme, which in all circumstances will be capable of disconnecting the elements producing the excess Mvar within a few seconds. For a three phase reactor on a 400 kV system the voltage on each phase stressing the reactor is approx. 460 kV. The reactor therefore requires a somewhat higher basic insulation level than normal such that the short time withstand level, - say the standard 1 min. 50 Hz test value, corresponds to the maximum temporary overvoltage across the reactor. Fig 7 shows a practical 400 kV three phase transmission system set up used in simulations of the performance of the reactive device. A reactive Mvar load is suddenly disconnected from the system -simulating for example blocking of the valves in an HVDC convertor station. The Mvar production of the high voltage filter capacitors C/is suddenly thrown on to the system and the system is subjected to a potentially large temporary overvoltage. Fig 8 illustrates the performance of the reactive device on the practical high voltage transmission system of fig 7, for the following circuit conditions : - - the V/I characteristic of a typical 100 Mvar 400 kV three phase reactor, series capacitors of 15 μF, - the system short circuit level SA; in the range 1000 - 6000 MVA, - the excess Mvar from the capacitance Cf suddenly thrown on to the system, in the range 300 - 900 Mvar. For the above practical equipment under normal operating conditions, the resonant frequency/ of the reactive device is 21 Hz, the series resistance is 3 Ω corresponding to the losses of the reactive device. The quality factor Q is high, and the bandwidth is narrow thus steady state ferroresonance is avoided. The simulations were carried out as a transient voltage analysis using the EMTDC software package. For the extremely wide range of system conditions covered, the device, while using standard component values, is capable of limiting the system temporary overvoltage to less than about 1.13 pu (< 450 kV). Harmonic currents generated when the reactive device operates briefly in the saturated region of the reactors V/I characteristic are less onerous than those produced by the inrush current of a similar rated power transformer brought into saturation during energising, as the transformers V/I characteristic is much more non-linear than that of a shunt reactor.

Claims: Patent Claim. 1. A method to greatly increase the Mvar absorption of a high voltage shunt reactor in response to excess Mvar suddenly appearing on a high voltage transmission system characterised by a capacitor connected in series with the shunt reactor in order to increase the reactor voltage, and thereby the Mvar absorption, as a function primarily of the capacitor value in combination with the shunt reactors dynamic impedance in the saturated region of the reactor V/I characteristic, and not the system voltage alone. 2. A reactive device to avoid high temporary overvoltages on a high voltage power system due to excess Mvar suddenly appearing on the system according to patent claim 1 characterised by a high voltage capacitor connected in series with each phase of a multi phase shunt reactor, the reactive device being normally connected to ground (fig 7).

Inventors: BOYD HUGH ALLISON, BOYD, HUGH, ALLISON
Applicants: BOYD HUGH ALLISON, NESA A S, BOYD, HUGH, ALLISON, NESA A/S
Application year: 2000
Main classification: H02H9
IPC codes: H02J3