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Improvement of stability of HVDC Transmission System using Static Synchronous Series Compensator (SSSC).

INTRODUCTION

Nowadays, power Transmission systems applying HVCD lines are widely used as a supplement AC Systems and even in some cases they have been replaced ordinary lines. HVDC Transmission Systems are suitable for long distance overhead Transmission (more than 600 kilometers), underwater or underground transmission (more than 50 kilometers) (K. Meah and A.H.M. SadrulUla, 2009) and ties between two asynchronous power systems (connecting two AC Systems with different frequencies) (M. Szechtman, T. Wess, and C.V. Thio, 1991). HVDC Transmission Systems are economically more attractive than High Voltage Alternating Current (HVAC) Transmission systems for long distances and High Power Ratings. HVDC Transmission Systems have definite Technical and Environmental advantages over HVAC Systems for improve the Stability of the System. The operation and control of HVDC links pose a challenge for the designers to choose the proper control strategy under various operation conditions. The HVDC System traditionally uses PI controllers to control the DC Current thereby keeping the Current order at the required level. Recently, due to advancement of Power Electronics Technology, High Voltage Direct Current (HVDC) Transmission Technology has been utilized to Enhance power System Stability. The HVDC is very reliable, flexible and cost effective (Asplund. G., Eriksson. K, Svensson, 1997. Asplund. G, Eriksson. K, Druggle. B, 1997).

The Control of an HVDC Transmission System is greatly influenced by the AC/DC System Strength. The AC/DC System strength is defined by the relative term "Short Circuit Ratio (SCR)"(1995). The SCR can be expressed as:

SCR = Short Circuit MVA of AC System/DC Converter MW Rating

(i) Rectifier: SCR = 2-5 [angle] 84[degrees]

(ii) Inverter: SCR = 2-5 [angle] 75[degrees]

Lower SCR means more pronounced interaction between the HVDC Substation and the AC Network. AC Networks can be classified in the following categories according to strength:

(a) Strong systems with high SCR: SCR > 3.0

(b) Systems of low SCR: 3.0 > SCR > 2.0

(c) Weak systems with very low SCR: SCR < 2.0

In the case of high SCR HVDC Systems, changes in the active/reactive power from the HVDC Substation lead to small or moderate AC Voltage changes. Therefore the additional transient Voltage control at the busbar is not normally required. The reactive power balance between the AC Network and the HVDC Substation can be achieved by switched reactive power elements. In the case of low and very low SCR Systems, the changes in the AC Network or in the HVDC Transmission power could lead to Voltage oscillations and a need for special control strategies. Dynamic reactive power control at the AC bus at or near the HVDC Substation by some form of power Electronic reactive Power Controller such as a Static var Compensator (SVC) or Static Synchronous Compensator (STATCOM) may be necessary (L. Gyugyi, C. D. Schauder, and K. K. Sen, 1997). In earlier times, dynamic reactive power control was achieved with Synchronous Compensators. A Flexible Alternating Current Transmission System (FACTS) is a system composed of static equipment used for the AC Transmission of Electrical Energy. It is meant to Enhance Controllability and increase Power Transfer Capability of the network. It is generally a power electronics-based system. One direction of large AC System development is the Transmission of large amounts of power over long distances by high Voltage Transmission lines from remote power sources to load centers. Because of growing public impact on environmental policy, the building of new Transmission facilities, in general, lags behind the increased needs of Power Transmission. As a consequence, some Transmission lines are more loaded than was planned when they were built. With the increased loading of long Transmission lines, the problem of Stability after a major fault can become a Transmission power limiting factor. In some cases, this factor may be considerably lower compared to other limiting factors. Power electronic equipment, including appropriate control, offers effective solutions to this problem. Such equipment, including advanced control centers and communication links, is the basis of the so called Flexible AC Transmission System (FACTS). Some FACTS devices controlled by power thyristors such as, for example, Static Var Compensator or Controlled Series Compensation or their prototypes have been put into operation. The development of power semiconductor devices with turn-off capability opens up new perspectives in the development of FACTS equipment. The universal and most effective device is expected to be the Static Synchronous Series Compensator (SSSC). Static synchronous series compensator (SSSC), which consists of a voltage source inverter with DC capacitor, regulates the line reactance by injecting a controllable AC Voltage (M. Szechtman, T. Wess, and C.V. Thio, 1991). In this paper, the CIGRE model is extended in such a way that in addition to Voltage and Current, the frequencies of two AC systems (connected to each other by HVCD link) are considered as the parameters of the control system. In the proposed model, the frequency deviation is used (created during fault occurrence) as an additional control signal and the control system changes the DC Power in order to achieve frequency stabilization and to Improve DC Current and DC Voltage waveforms. To achieve that goal, a benchmark system for HVDC, known as the CIGRE Benchmark Model, was proposed in 1985 (1991). It provided a common reference system for HVDC System studies. Later in 1991, a comparison of four digital models has been carried out by the CIGRE Working Group (J. D. Ainsworth, 1985), (S. Rao, 1999), and a benchmark system for HVDC Control study was also proposed.

Conventional Cigre Hvdc Benchmark System:

The conventional CIGRE HVDC benchmark system shown in Fig. 1 was proposed in (M.O. Faruque, Yuyan Zhang, and V. Dinavahi, 2006). The system is a monopolar 500 kV, 1000 MW HVDC link with 12 pulse converters on both Rectifier and Inverter sides, connected to weak AC Systems (short circuit ratio of 2.5 at a rated frequency of 50 Hz) that causes a considerable degree of difficulty for DC Controls. Damped filters and capacitive reactive compensation are also provided on both sides. The power circuit of the converter consists of the Following sub circuits.

(a) AC Side

The AC sides of the HVDC System consist of supply network, filters, and Transformers on both sides of the converter. The AC supply network is represented by a Thevenin equivalent Voltage source with equivalent source impedance. AC filters are added to absorb the harmonics generated by the converter as well as to supply reactive power to the converter

(b) DC Side

The DC side of the converter consists of smoothing reactors for both Rectifier and the Inverter side. The DC Transmission line is represented by an equivalent T network, which can be tuned to fundamental frequency to provide a difficult resonant condition for the modeled system.

(c) Converter

The converter stations are represented by 12-pulse configuration with two six pulse valves in series. In the actual converter, each valve is constructed with many thyristors in series. Each valve has a di/dt limiting inductor, and each thyristor has parallel RC snubbers. The control model mainly consists of Firing Angle ([alpha]) and Extinction Angle ([gamma]), measurements and generation of firing signals for both the Rectifier and Inverter. The used controllers in the control schemes are as follows:

* Extinction angle controller.

* DC Current Controller.

* Voltage dependent Current order limiter (VDCOL) (Narain G. Hingorani and Laszlo Gyugi).

(d) Rectifier control

The Rectifier Control System uses Constant Current Control (CCC) Technique. The reference for Current limit is obtained from the Inverter side. This is done to ensure the protection of the converter in situations when Inverter side does not have sufficient DC Voltage support (due to a fault) or does not have sufficient load requirement (load rejection). The reference Current used in Rectifier control depends on the DC Voltage available at the inverter side. DC Current on the Rectifier

Side is measured using proper Transducers and passed through necessary filters before they are compared to produce the error signal. The error signal is then passed through a PI Controller, which produces the necessary firing angle order (Narain G. Hingorani and Laszlo Gyugi).

It is desired to maintain the DC link Current constant by using a Current Controller at the Rectifier. This Controller can perform its function by generating a control Voltage which then controls the firing pulses, at some Delay Angle Alpha, to the Rectifier valves. The relationship between DC Current Id and delay angle a is obtained as follows:

The DC Voltage at the Rectifier is given by

[V.sub.dr] = [V.sub.dor] * cos [alpha] - [X.sub.cr] * [I.sub.d] .... (1)

Where [V.sub.dr] = DC line Voltage.

[V.sub.dor] = open circuit DC Voltage.

And [X.sub.cr] = Equivalent impedance of converter Transformer.

For constant Id, and small changes in alpha, we have

[DELTA][V.sub.d]/[DELTA][alpha] = -[V.sub.do] * sin [alpha] .... (2)

The relationship between DC Current Id and Alpha is given by

[I.sub.d] = [V.sub.dor] * cos [alpha] - [V.sub.doi] * cos [gamma]/[R.sub.cr] + [X.sub.cr] - Xci] .... (3)

Where Vdor = open circuit Rectifier DC Voltage.

Vdoi = open circuit Inverter DC Voltage.

[alpha] = Rectifier Firing (delay) Angle.

[gamma] =Inverter Extinction Angle.

Rdc = DC line resistance.

Xcr = Equivalent resistance of Rectifier,

Xci = equivalent resistance of Inverter.

Eq. (3) is only valid under steady-state conditions due to assumptions made in its derivation, and therefore the plant dynamics are not indicated. If the Inverter DC Voltage is maintained constant by controlling gamma (CEA control) then the DC Current is a function of Cos [alpha] i.e. a non-linear relationship. This implies that the loop gain will be lowest for a = 90 degs. Resulting in non optimal dynamic properties for nominal values of [alpha] = 12 to 18 degs. Attempts to adapt the gain as a function of the operating angle have been used sparingly in the industry. This is partly due to the influence of other external parameters such as AC System strength and damping angle which are not considered in the equations above.

(e) Inverter Control

The Extinction Angle Control or ([gamma]) Control and Current Control have been implemented on the Inverter side. The CCC with Voltage Dependent Current Order Limiter (VDCOL) have been used here through PI Controllers. The reference limit for the Current Control is obtained through a comparison of the external reference (selected by the operator or load requirement) and VDCOL (implemented through lookup table) output. The measured Current is then subtracted from the reference limit to produce an error signal that is sent to the PI Controller to produce the required angle order. The (y) control uses another PI Controller to produce gamma angle order for the Inverter. The two angle orders are compared, and the minimum of the two is used to calculate the firing instant. Fig. 1 shows Rectifier and Inverter control system (Narain G. Hingorani and Laszlo Gyugi).

Flexible Ac Transmission System (Facts):

Devised by Hingorani (V.K. Sood, N. Kandil, R.V. Patel, and K. Khorasani, 1994), FACTS devices are based on Power Electronic Controllers that Enhance the Capacity of the Transmission lines. These controllers are fast and increase the Stability operating limits of the Transmission Systems when their controllers are properly tuned. These devices provide control of the power system through appropriate compensation of network parameters, such as line series impedance, line shunt impedance, current, Voltage, and real and reactive power. They help the operation of the power network closer to its thermal limits. The FACTS Technology Encompasses a combination of various controllers, each of which can be applied individually or in a coordination with other devices to control the interrelated parameters of the system as mentioned above. FACTS Controllers can be broadly divided into four categories, which include series controllers, shunt controllers, combined series-series controllers, and combined series-shunt controllers.

(a) Principles of the Series Controllers: A series controller may be regarded as variable reactive or capacitive impedance whose value is adjusted to damp various oscillations that can take place in the system. This is achieved by injecting an appropriate Voltage phasor in series with the line; this Voltage phasor can be viewed as the Voltage across impedance in series with the line. If the line Voltage is in phase quadrature with the line Current, the series controller absorbs or produces reactive power, while if it is not, the controllers absorbs or produces real and reactive power. Examples of such controllers are Static Synchronous Series Compensator (SSSC), Thyristor-Switched Series Capacitor (TSSC), Thyristor-Controlled Series Reactor (TCSR), to cite a few. They can be effectively used to control Current and Power flow in the system and to damp system's oscillations.

(b) Principles of the Shunt Controllers: Shunt Controllers are similar to the series controllers with the difference being that they inject current into the system at the point where they are connected. Variable shunt impedance connected to a line causes a variable current flow by injecting a Current into the system. If the injected current is in phase quadrature with the line Voltage, the Controller adjusts reactive power while if the Current is not in phase quadrature, the controller adjusts real power. Examples of such systems are Static Synchronous Generator (SSG), Static Var Compensator (SVC). They can be used as a good way to Control the Voltage in and around the point of connection by injecting active or reactive Current into the System.

(c) Principles of the Combined Series-Series Controllers: A combined Series-Series Controller may have two configurations. One configuration consists of series controllers operating in a coordinated manner in a Multiline Transmission System. The other configuration provides independent reactive Power control for each line of a multiline Transmission System and, at the same time, facilitates Real Power Transfer through the power link. An example of this type of controller is the Interline Power Flow Controller (IPFC), which helps in balancing both the Real and Reactive Power flows on the lines.

(d) Principles of Combined Series-Shunt Controllers: A combined Series-Shunt Controller may have two configurations, one being two separate series and shunt controllers that operate in a coordinated manner and the other one being an interconnected Series and Shunt component. In each configuration, the Shunt component injects a Current into the system while the series component injects a series Voltage. When these two elements are unified, a Real power can be exchanged between them via the power link. Examples of such controllers are UPFC and Thyristor-Controlled Phase-Shifting Transformer (TCPST). These make use of the advantages of both Series and Shunt Controllers and, hence, facilitate effective and independent power/current flow and line Voltage Control.

Static Synchronous Series Compensater:

Static Synchronous Series Compensator (SSSC) is one of the important series FACTS devices. SSSC is a solid-state Voltage source inverter, injects an almost sinusoidal Voltage, of variable magnitude in series with the transmission line. The Injected Voltage is almost in quadrature with the line Current. A small part of the injected voltage, which is in phase with the Line Current, provides the losses in the inverter. Most of the injected voltage, which is in quadrature with the line Current, emulates an inductive or a capacitive reactance in series with the Transmission line. This emulated variable reactance, inserted by the injected Voltage source, influences the electric power flow through the Transmission line. A SSSC operated without an external Electric Energy source as a Series compensator whose output Voltage is in quadrature with, and controllable independently of, the line current for the purpose of increasing or decreasing the overall reactive Voltage drop across the line and thereby controlling the transmitted active power. The SSSC may include transiently rated energy storage or energy absorbing devices to enhance the dynamic behavior of the power system by additional temporary real power compensation, to increase or decrease momentarily, the overall resistive voltage drop across the line.

The SSSC injects the compensating Voltage in series with the line irrespective of line Current. The Transmitted power Pq versus the Transmission Angle [delta] relationship therefore becomes a parametric function of the injected Voltage, Vq, and it can be expressed for a two machine system as follows:

[P.sub.q] = [V.sup.2] sin [delta]/[X.sub.L] + [V/[X.sub.L]][V.sub.q]cos[[delta]/2] .... (4)

Fig. 2 shows that the SSSC increases or decreases the Transmitted power by a fixed fraction of the maximum power Transmittable by the uncompensated line, independently of [delta] in the important angle range of 0 [less than or equal to] [delta] [less than or equal to] 90[degrees] fig.3 and Fig.4 show the SSSC with PI Controller and SSSC with PI Controller design respectively.

Pi Controller:

The HVDC Transmission system traditionally uses the PI controller to control the converter systems. In a two terminal system, the Current margin rule is used, where the Rectifier is equipped with a Current Controller (CC) and the Inverter side is equipped with a constant extinction angle (CEA) controller. The Inverter system also has a Current Controller in parallel with a CEA. An error signal, Ie which is a difference between the reference current, Iref and measured Current, Id from the system is fed to the PI-controller. The error output of the controller is acted upon by the PI gains to provide the required alpha order for the HVDC Converter. The optimal gain calculation of the PI controller is difficult due to the fact that the HVDC Transmission system is uncertain and nonlinear. Consequently, combined field tests and simulation studies are conducted to fine tune these gains (K.G. Narendra, V.K. Sood, K. Khorasani, R.V. Patel, 1997). Fig 5 shows the structure of PI Controller and Fig. 6 HVDC system with PI Controller.

Simulation Results:

The rectifier and the inverter are 12-pulse converters which are two Universal Bridge blocks connected in series. The converters are interconnected through a 850-km line and 0.597H smoothing reactors as shown in Fig. 7 The converter transformers (Wye grounded/Wye/Delta) are modeled with Three-Phase Transformer (Three-Winding) blocks. The transformer tap changers are not simulated. The tap position is rather at a fixed position determined by a multiplication factor applied to the primary nominal voltage of the converter Transformers (1.01 on the Rectifier side, 0.989 on the Inverter side).

The HVDC Transmission link uses 12-pulse thyristor converters. Two sets of 6-pulse converters are needed for the implementation stage. AC filters and DC filters are also required to minimize harmonics. The firing-angle control system is configured based on two 6-pulse converters in series, one of which is operated as a modified HVDC bridge. The HVDC power converters with thyristor valves will be assembled in a converter bridge of twelve pulse configuration. This is accomplished by star-star connection and star-delta connection. Reduction of harmonic effects is another factor of investigation. Here, MATLAB/SIMULINK program is used as the Simulation tool two 6-pulse Graetz bridges are connected in series to form a 12 pulse converter. The two 6-pulse bridges are 345Kv, 50 Hz totally identical except there is an in phase shift of 58.4[degrees] for the AC supply Voltages. Some of the harmonic effects are cancelled out with the presence of 60[degrees] phase shift. The harmonic reduction can be done with the help of filters. The firing angles are always maintained at almost constant or as low as possible so that the Voltage control can be carried out. Six or eight of equal rating bridges are the best way to control the DC Voltage. More than these numbers of series bridges are not preferable due to the increase in harmonic content. The control of power can be achieved by two ways i.e., by controlling the Current or by controlling the Voltage. It is crucial to maintain the Voltage in the DC link constant and only adjust the current to minimize the power loss. The Rectifier station is responsible for Current Control and Inverter station is used to regulate the DC Voltage. Firing angle at Rectifier station and Extinction Angle at Inverter station are varied to examine the system performance and the characteristics of the HVDC System. Both AC and DC filters act as large capacitors at fundamental frequency. Besides, the filters provide reactive power compensation for the Rectifier consumption because of the firing angle.

Without Fault:

Fig 8 and Fig. 9 shows the system with no fault in Voltage and Current waveforms at Rectifier and Inverter sides using PI Controller and Static Synchronous Series Compensator (SSSC). From the simulation results, it is observed that DC Voltage and Current reaches the reference value of 1.0Pu at 0.5 sec, i.e. about 0.1 sec later after starting HVDC System. It is clear that for no fault, the PI Controller performs well and gives a better transient performance. Further, with the addition of Static Synchronous Series Compensator (SSSC) at time t=0.5 sec the system damps oscillation and improves Stability of HVDC System. The complete HVDC System reaches stable state after 0.5 sec.

With Faults:

In Fig. 10 and fig. 11, it is observed that DC fault occurs at Rectifier and Inverter sides of HVDC system. The PI Controller activates and clears the fault. Fig. 10 and fig. 11 shows the waveforms after 0.6sec DC fault occurrence at the Rectifier and Inverter sides. A large number of oscillations have been observed in DC link Current and Voltage magnitudes in case of a conventional controller. From the Rectifier and Inverter DC Voltage plots it is clear that in case of conventional controller. Further, with the addition of Static Synchronous Series Compensator (SSSC) at time t=1.5sec the System damps oscillation and improves stability of HVDC System. Once the fault is cleared, at t=1.5 sec the system comes back to its normal operation.

A step change of 20% is applied to the reference Current and Voltage at Rectifier and Inverter sides. From the simulated results (Figure 12 and fig. 13), it is clear that for a 20% step change, the PI Controller performs well and gives a better transient performance. Further, with the addition of Static Synchronous Series Compensator (SSSC) at time t=1.5 sec the system damps oscillation and improves Stability of HVDC System.

In Fig. 14 and fig 15, it is observed that a line-to-ground fault occurs on the rectifier and Inverter sides of HVDC System. The PI Controller activates and clears the fault. The rectifier and Inverter DC Current suffers from prolonged oscillations and consequently more commutation failures occur in the case of fixed-gain PI controller. The fixed-gain PI controller takes longer time to recover after fault is cleared due to the narrow range of optimum controller gain parameters. Further, with the addition of Static Synchronous Series Compensator (SSSC) at time t=1.5 sec. damps the oscillation and improves stability of HVDC System. Once the fault is cleared, at t=1.5 sec the system comes back to its normal operation.

In Fig. 16 and fig 17, it is observed that line-to-line fault occurs on the rectifier and Inverter sides of HVDC System. The PI Controller activates and clears the fault. The Rectifier and Inverter Current suffer from prolonged oscillations and consequently more commutation failures occur in the case of fixed-gain PI controller. The fixed-gain PI controller takes longer time to recover after fault is cleared due to the narrow range of optimum controller gain parameters. Further, with the addition of Static Synchronous Series Compensator (SSSC) at time t=1.5 sec. damps the oscillation and improves stability of HVDC System. Once the fault is cleared, at t=1.5 sec the system comes back to its normal operation

In Fig. 18 and fig. 19, it is observed that a Three-phase fault occurs on rectifier and Inverter sides of HVDC System. The PI Controller activates and clears the fault. The Rectifier and Inverter DC Current suffers from prolonged oscillations and consequently more commutation failures occur in the case of fixed-gain PI controller. The fixed-gain PI controller takes longer time to recover after fault is cleared due to the narrow range of optimum controller gain parameters. Further, with the addition of Static Synchronous Series Compensator (SSSC) at time t=1.5 sec. damps the oscillation and improves stability of HVDC System. Once the fault is cleared, at t= 1.5 sec the system comes back to its normal operation.

Fig. 20, fig. 21 and fig. 22 shows the change process of the active power of HVDC System after a line-to-ground fault, line to line fault and three phase faults are occurred with PI and the Static Synchronous Series Compensator (SSSC). It is clear that for a line-to-ground fault, the PI controller performs well and gives a better transient performance. Further, with the addition of Static Synchronous Series Compensator (SSSC) at time t=1.5 sec the system damps oscillation and improves Stability of HVDC System.

Conclusion:

In this paper, the CIGRE model is developed for improving the performance of the System. For HVDC links where very large transient conditions are involved in the plant operation, it is more convenient to improve the PI Control Strategy rather than to work out complicated dynamic models which require Sophisticated Control Strategies. In the paper, it is proposed a PI Control Scheme, intended to improve the performance of the HVDC System. Besides, the system controlled by PI Controller also has overshoot and oscillations when the system is in the process from system fault time to steady time. The operation of the SSSC is examined and tested under abnormal operating conditions occurring in the Transmission line. SSSC provides Capacitive and Inductive Compensation at the Transmission level with minimum harmonic Distortion introduced into the HVDC System. The SSSC is able to successfully switch from capacitive operation to inductive operation in a few milliseconds. The SSSC enhances the overall Stability of the HVDC System to the changes take place in the power demand. When the HVDC System is incorporated with SSSC, the system is not losing its synchronism and is stable. But without SSSC, the system is losing its synchronism and is unstable. This shows the Capability of SSSC in improving the Stability margin of the HVDC System. The simulation results prove the effectiveness of the SSSC to maintain operation even when faults occur during different operating modes of the compensator. After clearing the fault the system restores back to its pre disturbed state. From the simulation results, it is shows that SSSC is very effective in controlling the power flow, and the response time is very fast.

ARTICLE INFO

Article history:

Received 29 July 2014

Received in revised form 14 November 2014

Accepted 04 December 2014

Appendix 'A':

Following are the parameters of the HVDC System chosen for the simulation studies: CIGRE HVDC Benchmark System Data

Parameters            Rectifier            Inverter

AC Voltage Base       345kv                230kv
Base MVA              100MVA               100MVA
Transf.tap(HV side)   1.01pu               0.989pu
Voltage source        1.088 [angle]        0.935 [angle]
                      [22.18.sup.0]        [-23.14.sup.0]
Nominal DC Voltage    500kv                500kv
Nominal DC Current    2KA                  2KA


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(1) M. Ramesh and (2) Dr. A. Jaya Laxmi

(1) Associate Professor & HOD, EEE, Dept, Medak College of Engineering and Technology, Kondapak Medak Dist, Research Scholar, EEE Dept., JNTU, Anantapur-515002,Andhra Pradesh, India,

(2) Professor, Dept. of EEE & Coordinator Centre for Energy Studies, Jawaharlal Nehru Technological University, College of Engineering, Kukatpally, Hyderabad-500085, Andhra Pradesh, India

Corresponding Author: M. Ramesh Associate Professor & HOD, EEE, Dept, Medak College of Engineering and Technology, Kondapak Medak, Dist, Research Scholar, EEE Dept., JNTU, Anantapur-515002, Andhra Pradesh, India.

E-mail:marpuramesh223@gmail.com, Mob: +91-7729992705
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Publication:Advances in Natural and Applied Sciences
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Date:Nov 15, 2014
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