在11KV回路中配置距離保護(hù).doc

Distance relaying of 11KV circuits to increase the installed capacity of distributed generationAbstract: The connection of generation to distribution networks may cause problems with existing protection schemes, while development of new protection schemes can facilitate an increase in generation the design and evaluation of a distribution network protection scheme based on distance relays is described. The scheme provides primary and back-up protection of 11 KV circuits. In the UK, voltage transformers and communication schemes are seldom are seldom installed on 11 KV circuits. Hence the distance relay restraint signal (voltage) is measured at the secondary winding of a distribution transformer (11/0.433 KV) to reduce costs. The development of the distance relay algorithm is discussed and a selection of simulation test results provided to illustrate the relay performance under fault conditions.Principal symbols=relay apparent (measured) impedance =relay short-circuit phase voltage and current =relay short-circuit zero sequence voltage and current =relay vectorial residual compensation factor=cable phase and sheath impedance=cable positive and zero sequence impedance =total zero sequence impedance appearing behind relay = total zero sequence impedance appearing behind relay= zero sequence impedance of 33/11 KV transformers =cable return path impedance =impedance of main body of earth =fault location (0 1 p.u.)1 Introduction Distribution substation typically consist of an 11 KV ring-main unit (RMU) connected locally to transformer (11/0.433 KV, Dyn). RMUs are now available with circuit breakers (CBs) for connection in the 11 KV primary circuit 1. They are also fitted with a microprocessor overcurrent and earth-fault relay fed form 11 KV line current transformers (CTs). The intention of this work is to incorporate distance relay in the RMU. The relay operate signal (current) will be measured at the 11 KV line CTs and the voltage measured at the secondary winding of the distribution transformer. This approach could improve both protection and network performance without significant extra cost. The main drawback is that the new type of RMU is not fitted throughout 11 KV networks. However, many utilities are presently upgrading distribution substation equipment to facilitate automation of supply restoration to 11 KV circuits. New types of plant, e.g. the new RMU, are integral to the automation of 11 KV networks.2 Advantages of using distance protectionExisting distribution network protection schemes usually consist of nondirectional overcurrent and earth-fault relays installed only at the primary 11 KV bus. Outgoing 11 KV circuits are usually radial and during fault conditions this results in a supply outage to all customers on the faulted circuit. These 11 KV radial circuits can usually operate as a ring via a normally-open point, however due to protection limitations this is only closed for short periods. Approximately 72% of customer interruptions are caused by faults on 11 KV circuits 2. The use of distance relays with directional elements, located at regular intervals along the 11 KV circuits would allow these circuits to be run as a ring. This would reduce the number of customer interruptions as only the faulted section would be disconnected for faults. Directional overcurrent relays can be graded to protect a ring if the ring is fed from a single utility source only. However, if the many relaying and CB points are included on the ring, the fault clearance times become excessive.Protection of 11 KV circuits becomes more onerous as the amount of generation connected directly at 11 KV or by step-up transformers increases. This is set to increase in the future. For weak utility sources the generator short-circuit capacity may approach that of the source and consequently large variations in fault level will exist when the generation is in or out of service. This variation in fault level makes overcurrent and earth-fault relay grading very difficult 3. This difficulty applies equally to radial circuits protected by nondirectional relays and ring circuits protected by directional relays. Furthermore, to maintain the stability of the distributed generators fault clearance times should be kept to minimum. Distance protection of 11 Kv networks can improve the operating performance, with or without generation connected.3. Power system modellingTo investigate how the proposed protection performed the power system studies were carried out using the simulation package PSCAD/EMTDC. Distribution network modelThe distribution network was modeled from 33 to 0.433 KV with the single-line diagram shown in Fig. 2. Neutral earthing of the 33/11 KV transformer was based on conventional UK arrangements of solid or resistance earthing. The neutral earhing resistor was rated at 6.35 . The outgoing 11 KV circuits consisted of a number of 3 km underground cable sections. These used three core consisting of the cable sheath and the main body of the earth. The generator was an unearthed 11 KV, 4.51 MVA,50HZ synchronous machine driven by a gas turbine 4. 4 New distance relay characteristics4.1 Distribution transformer compensation The relay voltage is measured at the secondary of the distribution transformer winding. This voltage (0.433 KV phase-to-phase) is fed directly into the relay or via a step-down auxiliary VT (0.11 KV phase-to-phase). Compensation for the presence of the distribution transformer has to be made to measure the correct distance to the fault.Distribution transformer load current:Figure 3 shows the equivalent circuit for a fault on (Fig.2), at fault location (3km), with closed and open. There is an error due to transformer load current . This manifests itself as a reduction in the positive-sequence voltage seen at the relay terminals. The current is not known so compensation is based on an estimate of transformer loading. Consider (1) and (2), (1) (2)The correct apparent impedance is , however transformer load current causes relay over-reach. If the short circuit current is large thenit can be seen that the distribution transformer load current will have a negligible effect,Distribution transformer loading is subject to seasonal variation. This can typically vary between a summer load of 35% () and a winter load of 100% (). The transformer load compensation factor (TLCF) was chosen to be the mean of these values. (3)The voltage dropis added to the positive-sequence voltage measured at the transformer 0.433 KV winding. The resultant impedance is, (4)From (4) the relay apparent impedance for the summer and winter loads are as follows: Winter load (5)Summer load (6)4.2 Distribution transformer vector groupThe yn vector group causes additional problems. The positive-and negative-sequence Voltages measured at the 0.433 KV winding are rotated through , respectively. This phase shift across the transformer due to load current is considered within the relay algorithm. The phase shift across the transformer due to load current is considered negligible. A more serious problem is the loss of the 11 KV zero-sequence voltage, since due to the Dyn connection it cannot be measured at the 0.433 KV winding.Full scheme distance protection has separate measuring elements for phase-to-phase and single-phase faults 6. The phase-to-phase measuring elements are designed to operate positive-and negative-sequence components of voltage and current to calculate the positive-sequence impedance (distance) between the relay and fault. For the protection of a symmetrical transposed line or a three core cable, knowledge of the zero-sequence components are not required for double-phase faults. Single-phase elements are designed to operate for single-phase faults only. They require zero-positive-and negative-sequence components of voltage and current to calculate the positive sequence impedance between the relay and fault.5.Protection of 11 KV radial circuitsNew relay settingsAn additional parameter needs to be entered into this relay compared with those required for the classical distance relay: this is the zero-sequence impedance appearing behind the relay In classical distance relays the vectorial residual compensation factor is used to calculate the faults 7. It compensates for the additional impedance of the earth-return path, which would otherwise cause relay under-reach. Considering (7)-(9), (7) (8) (9)The cable zero-sequence impedance , with an earth-return path consisting of the cable sheath and the main body of the earth is . Substituting this into (8) and (9) results in residual compensation settings of and . These values were based on an earth resistivity of 6.Referring to Fig.2, an 18 km radial circuit is formed when is closed and is opened. Following the approach adopted by Ziegler 8, the short-circuit loop for a phase-to-earth fault on is shown in Fig.4. Relay 5 (R5), at , protects the faulted cable . At R5 the zero-sequence voltage is equal to the product of the zero-sequence current and the zero-sequence impedance appearing behind R5, 9 (10)At R5 the zero-sequence current is measured by the 11 KV-line. The zero-sequence impedance appearing behind R5 is calculated using (11) (11)Where is the zero-sequence impedance of the 33/11 KV transformer/s and is the cable zero-sequence impedance behind R5. This can be calculated from (12). (12)Where is equal to the cable positive-sequence impedance behind R5 and is the cable earth-return impedance behind R5 which consists of the cable sheath and the main body of the earth in parallel.The cable zero-sequence impedance appearing behind the relay and the residual compensation settings are shown in Fig. 5. The relay settings are based on an earth render these settings inaccurate and cause errors in the apparent impedance. Studies were carried out over a range of earth resistivity of 100m. A variation in earth resistivity will render these settings inaccurate and cause errors in the apparent impedance. Studies were carried out over a range of earth resistivity between 1 and 1000m.在11KV回路中配置距離保護(hù)提高配電的裝機(jī)容量摘要:雖然新的保護(hù)方案的開發(fā)可以促進(jìn)發(fā)電容量的增加和設(shè)計(jì), 但是評(píng)價(jià)一個(gè)依距離保護(hù)為基礎(chǔ)的新的保護(hù)方案的配置在現(xiàn)有的保護(hù)制度下,從發(fā)電網(wǎng)到配電網(wǎng)的連接仍會(huì)引起一系列的問(wèn)題.本文所提到的方案為11KV電網(wǎng)提供了主保護(hù)和后備保護(hù).在英國(guó),11KV電網(wǎng)中很少安裝電壓互感器和通訊系統(tǒng).因此,距離保護(hù)限制在配電變壓器的二次側(cè)繞組側(cè)得的電壓信號(hào)以降低成本.人們還在討論距離繼電保護(hù)繼電器的算法,并已經(jīng)提供了一系列的模擬測(cè)試結(jié)果來(lái)解釋繼電器在故障情況下的工作原理.主要符號(hào): =繼電器的視在阻抗(測(cè)量值)=繼電器的短路相電壓和電流=繼電器的短路零序電壓和電流 =繼電器的方向剩余功率補(bǔ)償系數(shù)=電纜外部阻抗=電纜正序和零序阻抗 =RS后備保護(hù)總的零序阻抗 = CB后備保護(hù)總的零序阻抗= 33/11KV變壓器的零序阻抗 =線路阻抗=接地阻抗 =故障點(diǎn)(0 1 p.u.)1. 概述:典型的配電變電站由一個(gè)11KV的環(huán)型主接線單元(RMU)連接到變壓器低壓側(cè)(11/0.433KV,DYN)上的,在11KV主網(wǎng)中,環(huán)型主接線是通過(guò)許多回路斷路器連接而成的.這些環(huán)型主接線單元也安裝了微機(jī)過(guò)流保護(hù)和由11KV線路電流互感器供電的接地故障保護(hù).這樣做的目的為了把距離保護(hù)包含在(RMU)里面保護(hù)的動(dòng)作信號(hào)(電流信號(hào))范圍內(nèi).由11KV線路的電流互感器處側(cè)得,電壓信號(hào)由配電變壓器的二次繞組獲得. 這種做法在不增加的基礎(chǔ)上既可以提高保護(hù)效率又可以提高網(wǎng)絡(luò)的作用.新型RMU的主要缺點(diǎn)是無(wú)法在整個(gè)11KV網(wǎng)絡(luò)中安裝.然而,目前人們還在研制許多類型的設(shè)備來(lái)提高配電變電站的設(shè)備和11KV電網(wǎng)的自動(dòng)化程度.新型設(shè)備比如說(shuō)RMU是可以和11KV電網(wǎng)兼容的.2. 使用距離保護(hù)的優(yōu)勢(shì): 現(xiàn)有的配電網(wǎng)保護(hù)方案是由非定向過(guò)流保護(hù)和安裝在11KV母線上的接地保護(hù)組成的,11KV網(wǎng)的出線往往是輻射狀的,所以故障情況下,這種接線方式會(huì)導(dǎo)致故障回路所有用戶的供電中斷.這些11KV輻射狀線路通常通過(guò)一個(gè)繼電器運(yùn)行,但是由于保護(hù)的局限性,這個(gè)繼電器也必須短時(shí)閉合。11KV回路的故障會(huì)導(dǎo)致大約72%的用戶供點(diǎn)中斷.裝設(shè)定向距離保護(hù)可以使這些回路以環(huán)網(wǎng)形式運(yùn)行.這些方向性元件通常安裝在11KV回路的繼電器處,只要故障部分能被及時(shí)切除,就會(huì)減少用戶供電中斷次數(shù).如果環(huán)網(wǎng)是由單電源供電那么定向過(guò)流保護(hù)通常被分得布置,然而如果許多繼電器和電流互感器都包括在保護(hù)范圍內(nèi),那么保護(hù)動(dòng)作時(shí)間會(huì)變得很長(zhǎng).隨著11KV發(fā)電廠和高壓變電站的增加,11KV回路的保護(hù)變得越來(lái)越艱巨了.對(duì)弱的設(shè)備來(lái)說(shuō)發(fā)電機(jī)的短路容量會(huì)使其燒毀.因此當(dāng)發(fā)電機(jī)運(yùn)行或退