Sep 23 2019

# Application of Series Compensation Capacitor in EHV Transmission System

## Introduction

A series compensation device is used on the transmission line to improve the stable transmission capacity of the system, improve the electrical parameters of the line, and realize the power of three lines to transport three lines, which not only improves the transmission power but also saves investment. This series compensation device installed in the 500kv Sanbao Substation of Xuzhou Power Supply Company has realized the stable transmission capacity of long lines and played an important role in the transmission network of “Nortel Transmission” of East China Power Grid.

## Catalog

 Introduction I Basic Information of Serial Compensation Device 1.1 Fundamental 1.2 Basic Wiring Form II Capacitor Bank Application 2.1 External Fuse Capacitor and Internal Fuse Capacitor 2.2 Effect of Fuse Blow on Capacitor Components 2.3 Advantages and Disadvantages of Two Types of Capacitors 2.4 Effect of Faulty Capacitor Components on Distributed Voltage III Metal Oxide Variable Resistor(MOV) 3.1 The Function of MOV 3.2 Working Condition of MOV in Case of System Failure 3.3 The Way to Start MOV IV Trigger Gap 4.1 Operation Process of Gap 4.2 Relation between Discharge Voltage and Gap Distance V Bypass Circuit Breaker VI Operation Control of Series Compensation Device Conclusion

## I Basic Information of Serial Compensation Device

1.1 Fundamental

The principle of increasing the stable Transmission capacity after installing the Series compensation device: the static and stable Transmission Power of High Voltage Transmission Line can be expressed by the following formula:

U1, U2 is the power supply voltage at both ends of the line; sinσ is the phase angle difference of the power supply voltage at both ends of the line; XL is the impedance of the line; U1U2/XL is the ultimate transmission power of the line (static stability limit). When series compensation capacitors are installed in the line, the stable transmission power of the line is:

Comparing the stable transmission power at both ends of the series capacitor circuit with the same phase angle difference sinσ: Ke=Xe/XL is the degree of compensation. In the 500kv ultra-high voltage transmission line project, if the compensation degree is set to 40%, the ratio of the stable transmission power to stable transmission power before installation is 1.67 times for each transmission line with series compensation capacitors. That is to say, two sets of series compensation devices are installed, which is equivalent to adding a transmission line.

1.2 Basic Wiring Form

The series capacitor compensation device consists of a capacitor bank, a metal oxide varistor (MOV), a discharge gap, a damping reactance, a bypass switch, an insulation platform, a protection and control system. The series compensation device adopts a fixed device, and the equipment for protecting the capacitor is an MOV, a shunt gap, and a bypass circuit breaker. The basic wiring of the series compensation device is shown in Figure 1.

Figure 1. Basic Wiring Diagram of Series Compensation Device

## II Capacitor Bank Application

The capacitor bank is the main equipment of the series compensation device. The main technical parameters and related performance are shown in Table 1.

Table 1. Main technical parameters and related performance

2.1 External fuse capacitor and internal fuse capacitor

There are two types of capacitors for series compensation: external fuse capacitors and internal fuse capacitors. The internal fuse capacitor is composed of 320 capacitor units per phase capacitor bank. The capacitor is an oil-immersed full-film capacitor with an actual designed electric field strength of 170V/um. The protection level of the capacitor bank is 2.3 pu and the protection voltage is 230.

The outer fuse capacitor is the fuse device mounted on the outside of the capacitor unit. The IEC standard stipulates that the fuse current of the external fuse should be more than 1.43 times of the rated current of the protected capacitor, and is generally 1.5 times.

As a capacitor for series compensation device, it is also necessary to consider that the fuse is not blown when the short circuit discharge at both ends of the capacitor bank occurs. Otherwise, when the system fails and the series capacitor bank is out of operation, the bypass capacitor or the shunt switch bypass capacitor bank will cause the external fuse of the capacitor bank to be blown.

Capacitors with external fuses, when the fault fuse is blown, the fuse tube will fall, and the patrol personnel are easier to find. However, there are also disadvantages. For example, a capacitor usually has a plurality of capacitor elements connected in series and in parallel according to a certain rule. When one of the components is broken down, the associated parallel group is short-circuited, and the capacitance of the capacitor unit is increased. At this time, the capacitor unit can still work; The operating current will flow through the fault point of the faulty capacitor component to expand the fault, and finally the entire capacitor unit will fail, the fuse action and the faulty capacitor unit will be taken out of operation. If the process is long, the fault point of the faulty component will continuously generate gas under the action of current, which may cause the capacitor drum to rupture or even the outer casing, so that the entire capacitor unit is taken out of operation. As a result, the capacitor bank loses a large capacity and the overvoltage on other healthy capacitor units is high.

2.2 Effect of fuse blow on capacitor components

Because of the high recovery voltage of the fuse in the capacitor unit, it is relatively difficult to manufacture the fuse. The fuse of the capacitor using an internal fuse is mounted inside the capacitor, and each capacitor element has a corresponding fuse. When a capacitor element fails, only the fuse of the capacitor element is broken and the capacitor element is removed. After the fault capacitor element is removed, the capacitor unit can still operate normally, and the lost capacitor capacity is small. Taking the capacitor bank design example, the capacitor unit loses only 1/52 of its capacity. Operating experience has shown that damage to single components in the internal fuse capacitor unit does not further amplify component failure. This is because the rated current of the component is small, the recovery voltage is low when the fuse is blown , the fuse speed is relatively fast, and there are not many by-products of the fuse, which will not cause harm to the operation of other components in the unit. The main shortcomings of using internal fuse capacitor bank: A. The inner fuse does not protect the fault between the terminal of the capacitor unit and its outer casing. If such a fault occurs, it is necessary to rely on the unbalanced protection of the capacitor bank to bypass the capacitor group. Practical experience shows that the probability of such failures occurring is very low. B. When a capacitor element or a capacitor unit fails, it cannot be observed directly, and it must be periodically measured by a dedicated instrument to be found. Since the failure of the components is randomly distributed in the respective capacitor units, the probability of failure of the capacitor elements is very low.

The DC test voltage of the capacitor bank is 1.9 * 1.414U according to IEC143 standard. For the capacitor bank, the DC test voltage is 437kV, and the inhomogeneity of the voltage distribution of the capacitance of the capacitor unit is no longer considered. This requires the accurate measurement of the capacitance of the capacitor unit when performing the grouping of the capacitor banks, so as to ensure that the error of the capacitance in each circuit after parallel connection is less than 1%.

In order to send out fault signal or bypass capacitor group in time when some capacitors fail, each phase capacitor unit is composed of H type, and four arms are respectively connected by 80 capacitor units in 4 series and 20 parallel.

As shown in figure 2. The rated parameters of each capacitor unit and component are shown in Table 1.

Figure 2. Composition of Each Phase Capacitor

2.4 Effect of faulty capacitor components on distributed voltage

The relationship between unbalanced current and overvoltage on capacitor components can be converted according to Table 2.

Table 2. Conversion Relationship Between Unbalanced Current and Overvoltage on Capacitor Elements

When some of the capacitor components fail out of operation, the voltage on the intact capacitor components will increase to a certain extent. When the overvoltage reaches 5%, the alarm signal should be sent; when reaches 10%, it should go through a certain delay permanent to permanently bypass capacitor group (see figure 1). In practical operation, it is very difficult to measure the overvoltage on each unit of the capacitor bank. Generally, the method of measuring unbalanced current is used to realize the overvoltage protection of the capacitor unit. The number of damaged components in Table 2 refers to the number of components in the same parallel group appearing in one capacitor unit, and no element damage is found in other capacitor units and other parallel groups in defective capacitor units. Obviously, the probability of multi-component damage in the same parallel group is very low. When the failed capacitor elements are interspersed in different capacitor units or in the same unit only in different parallel groups, the overvoltage on the unit or element is much lower. When the unbalanced current reaches 1.35A, the alarm signal is sent out, the bypass command is issued when the unbalanced current reaches 1.50A, and the damage of 13 components means that one unit is out of operation. Capacitor failure probability (empirical data): the total failure rate of capacitor components within 30 years is 2%.According to the number of capacitor units in two groups of series compensation devices, the number of damaged components in 30 years is 1996.8; On average, each unit is only 1.04, which shows that the failure rate is very low. Taking into account the random distribution of faulty components, the unbalanced protection of the capacitor's resistance does not work during practical operation. The capacitor bank unbalance protection is only possible when the casing of the capacitor unit is flashed.

In addition to the partial capacitor unit being damaged and then running out, the sound capacitor unit will overvoltage, and when the current flowing through the capacitor bank exceeds the rated current, it will also cause the capacitor unit to overvoltage.

Whether the capacitor bank is overloaded is judged by whether the measured line current exceeds the rated current; when the line current exceeds the rated current, the timer of the capacitor group overload protection starts counting, and when the accumulated time of the timer exceeds the fixed value, the temporary bypass capacitor bank is started. If the capacitor group is frequently overloaded, the capacitor bank will be permanently bypassed.

## III Metal Oxide Variable Resistor(MOV)

When the line with series compensation device fails, the short circuit current of the system should flow through the series capacitor bank. When the current of steady-state short circuit is 20KV, the effective value of steady-state voltage on capacitor is as high as 600kV, so appropriate measures must be taken to limit the voltage on capacitor bank.

3.1 The Function of MOV

The function of MOV is to limit the overvoltage that appears on the capacitor bank and to protect the capacitor bank. When the out-of-area fault occurs, MOV will absorb all the energy and protect the capacitor bank; when the out-of-area fault disappears, the capacitor bank can be put into operation automatically. When a fault occurs in the area, the MOV before the discharge gap is broken will limit the voltage on the capacitor bank. After the discharge gap is broken through the 1ms, the MOV and the capacitor bank are bypassed, so that the MOV no longer absorbs energy, and the voltage at both ends of the capacitor bank is close to zero.

When the fault occurs in the area, the current in the fault is large, and the MOV absorbs energy very fast. The high current flowing through MOV will also make the voltage across the capacitor bank higher. The trigger gap is used to limit the overvoltage that appears on the capacitor bank, reducing the energy absorption capability of the desired MOV, and improving the ability of the system to dampen sub-synchronous oscillations.

3.2 Working condition of MOV in case of system failure

When an out-of-zone fault occurs in the system, the series-compensation device should still be in operation to improve the stable transmission capacity of the system. At this point, the MOV will flow through part of the fault current and absorb energy. The greater the fault current, the greater the energy absorbed by the MOV， and the longer the fault duration, the greater the energy absorbed by the MOV. When the fault occurs in the area, the fault current is large, and the current flowing through MOV is also large. If the duration of the fault (the time the MOV flows through the current) is the same as the time at which the fault occurs outside the zone, the energy absorbed by the MOV will be much greater than that absorbed when the fault occurs outside the zone. In order to reduce the absorbed energy of the MOV, the trigger gap should start the shunt capacitor bank and MOV in time, so that the fault current does not flow through the capacitor bank and the MOV. Therefore, the gap should be bypassed  MOV and capacitor bank within 1 ms after the fault occurs in the zone.

3.3 The Way to Start MOV

In order to avoid equipment damage caused by MOV overload, MOV is usually equipped with overload protection and short-circuit fault protection. MOV overload protection is used for fast bypass capacitor bank and MOV for internal faults in series-compensated lines. It has three starting modes: when the high current flows through MOV; When the speed of energy absorption by MOV exceeds a certain value; When the temperature exceeds a certain value.

Generally, when a short-circuit fault occurs outside the area of the series-compensated line, the current flowing through the MOV is small, the shunt gap and the bypass breaker do not operate, and the MOV should be able to withstand the absorbed energy.

The current to start the MOV overload protection should be greater than the current flowing through the MOV when a short-circuit fault occurs outside the area of the series-compensated line and leave a certain margin. The functions of MOV overload protection mainly include: (starting by large current) to realize three-phase bypass; (starting by large de/dt) to realize three-phase bypass; (starting from high temperature) to realize three-phase bypass.

## IV Trigger Gap

The trigger gap used in the series compensation device is non-self-extinguishing, and the gap itself does not have a strong arc extinguishing capability. The arc can only be extinguished after the bypass switch is closed or the line switch is tripped. Its main function is to be quickly breaked down under certain conditions for bypassing the capacitor bank and MOV, preventing MOV from overheating and damage, and protecting the capacitor bank from overvoltage. The analysis chart is shown in Figure 3.

Figure 3. Trigger Gap Schematic for A Typical Series Compensation Device

4.1 Operation process of gap

According to the setting conditions, the control unit of the trigger gap on the platform receives the control signal, and the control unit sends a high voltage pulse to the ignition ball gap of the triple ball gap device, so that the ignition ball gap can be broken and turned on; A small arc produced by the breakdown of the ignition ball gap, which causes the conduction of the ignition trigger gap under the action of 1 ≤ 4 voltage of the voltage on the capacitor divided by the voltage divider; The voltage of the ignition trigger gap increases, resulting in the breakdown of the exact trigger gap; After the ignition and the precise trigger gap are turned on, the voltage of the series capacitor bank is applied to the upper main gap to cause it to be turned on. After the upper gap of the main gap is turned on, the voltage of the series capacitor bank is turned on and applied to the lower portion of the main gap due to the action of the current limiting resistor connected in series in the trigger gap circuit, thereby completing the entire gap being broken and conducting. In order to ensure that the three-ball gap device can be broken through reliably, two sets of trigger signal loops should be adopted.

4.2 Relation between discharge voltage and gap distance

When any type of fault occurs in a transmission system equipped with a series-compensation device, the ignition triple-ball gap should not be actuated as long as there is no trigger signal. To meet this requirement, the discharge voltage of the ball gap must be higher than the voltage divided by the ignition three-ball gap at the highest voltage that may occur in the capacitor bank.

According to the relationship between the discharge voltage and the gap distance of the ignition three-ball gap provided by the SIEMNENS manufacturer (see Figure 4), the ignition voltage is usually set with the actual situation of the system. Generally, the ignition three-ball gap discharge voltage should be 10~15% higher than the voltage divided by the ignition three-ball gap at the highest voltage. The trigger gap should be able to withstand 10 times of 40KA short-circuit current for 100ms, and once for 40kA of short-circuit current of 500ms or discharge for 25 times without maintenance. And the trigger gap can withstand 100kA dynamic steady current and the discharge current of the container group without deformation or damage.

Figure 4. Relationship Between Discharge Voltage and Gap Distance of Ignition Three-ball Gap

## V Bypass Circuit Breaker

Bypass circuit breakers are used to enter or exit series capacitor banks. Another function is: when the system has a fault in the area, in order to protect the MOV from damage because of overload, the bypass gap will be broken down in a short time. Because the bypass gap is a non-self-extinguishing gap without arc extinguishing ability, only the short circuit gap of bypass circuit breaker can be used to extinguish arc. The bypass circuit breaker is only used to input or exit the series capacitor bank, and does not need to open the short-circuit current, so the circuit breaker is not required to have a large breaking capacity. Its maximum breaking current is the load current of the transmission line.

## VI Operation Control of Series Compensation Device

6.1 Precautions for operation of the device

1) Serial compensation operation and monitoring microcomputer (WINCC) shall not withdraw from operation without authorization;

2) The WINCC microcomputer should run normally under the “string compensation operation and monitoring system”, and the system should not be replaced at will. During the operation, the operator should regularly inspect the power supply and deal with the problem in time. If the power failure cannot be repaired in time, it should be reported immediately;

3) If the WINCC microcomputer is found to be faulty, the attendant should report to the work area in time;

4) It is strictly forbidden to use floppy disk and CD-ROM on the serial-compensation WINCC operation microcomputer, and it is not allowed to copy the system software of WINCC microcomputer;

5) It is forbidden to perform all operations unrelated to the series compensation operation on the serial compensation operation/WINCC microcomputer;

6) The operation of series compensation shall be strictly in accordance with the sequence operation logic, and the operation logic shall not be changed or unlocked without authorization. If you find that there is a problem with the logic locking circuit, you should report it in time;

7) When the operator operates on WINCC, the switching operation system should be strictly implemented;

8) When the series compensation device is overhauled and someone is working with, the operating power of the string compensation knife and the knife gate must be pulled;

9) Before the series-compensation net door is closed, it should be checked that there is no staff on the platform and the net door in each phase. The platform small door should be closed, and the three-phase ladder has been put down and locked.

## Conclusion

Through the operation practice of installing the series compensation device in 500KV, the stable transmission capacity of the long line is achieved, the load distribution between the parallel lines is improved, the line loss is reduced, and the voltage quality is effectively improved. The series compensation device is operated and controlled effectively, and its use has obvious economic and social benefits.

However, because there are few series compensation devices used in ultra-high voltage lines, operation experience and maintenance experience are not mature, so it is more beneficial to eliminate oscillations after system failure if partial controllable series compensation mode is selected in the device.

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