IMPORTANCE OF VSC IN HVDC

Transcription

1 IMPORTANCE OF VSC IN HVDC Snigdha Sharma (Electrical Department, SIT, Meerut) ABSTRACT The demand of electrical energy has been increasing day by day. To meet these high demands, reliable and stable transmission facilities are required. Due to certain disadvantages in AC transmission lines such as thermal limits, corona effect, skin effect, etc. HVDC came into existence. HVDC has proved to be more stable and reliable in case of long distance and under water transmission. HVDC lines are also used in asynchronous tie in two or more AC systems. To achieve more stability in HVDC systems, voltage source converters are used and thus termed as VSC- HVDC. In this paper we will study operation, topologies of VSC and also analyze the importance of VSC technology used in HVDC systems. 1. INTRODUCTION Previously mercury arc valves or thyristor valves were used in line commutated converters but with the advent of IGBT valves, Voltage source converter is seen to be more beneficial than line commutated converter. Voltage source converters are known as self commutated converters Because the devices are self-commutating they do not need a strong AC grid and can switch at high frequency (khz), eliminating low order harmonics and controlling the phase shift between output voltage and current on the AC side. This may eliminate the need for AC filters, DC filters, reactive power compensation and greatly reduces station footprint. 2. VOLTAGE SOURCE CONVERTER VSC is a six pulse converter consisting of six power semiconductor switching devices and anti parallel diodes. The antiparallel freewheeling diode is integrated in the same semiconductor package to ensure current capability in the reverse direction and to prevent the application of reverse voltage. From a direct voltage source, the VSC generates a set of controllable three phase output voltages at the frequency of the system voltage. PWM is used to control the firing of semiconductor switching devices, generating an average sine wave. PWM also helps in mitigate the amount of harmonics. Fig. Two level VSC 3. TOPOLOGIES OF VSC There are several VSC topologies currently in use in actual power system operation. Common aims of these topologies are:

2 To minimize switching losses of the semiconductor inside the VSC To produce high quality sinusoidal voltage waveform with minimum or no filtering requirements. 3.1 Two Level Converter It is the simplest type of three phase VSC and can be thought as a 6 pulse bridge in which the thyristors have been replaced by IGBT s with anti parallel diodes. Such converters derive their name from the fact that the voltage of the ac output of each phase is switched between two discrete voltage levels, corresponding to electrical potentials of the positive and negative dc terminals. Fig. Two level VSC The disadvantage of two level converter is that this would produce unacceptable levels of harmonic distortion so some form of PWM is always used to improve harmonic distortion of the converter. As a result of PWM, IGBT s are switched on and off approximately twenty times in each main cycle which results in high switching losses in IGBT s and reduce overall transmission efficiency. 3.2 Three Level Converter In an attempt to improve on poor harmonic performance of two level converters, three level converters came into existence. This can synthesize three discrete voltage levels at the ac terminal of each phase are (½)U d, 0, -(½)U d. A common type of three level converter is diode clamped or neutral point clamped converter where each phase contains 4 IGBT valves each rated at half of the dc line to line voltage along with two clamping diode values. The dc capacitor is split into two series connected branches, with the clamping diode values connected between the capacitor midpoint and one quarter and three quarter points on each phase. For (½)U d the top two IGBT valves are turned on, for -(½)U d bottom two IGBT valves are turned on, for 0 output middle two IGBT valves are turned on. It can compensate for unbalanced load currents in three phase four wire system. Fig. Three level VSC

3 The disadvantages are related to voltage balancing across the DC capacitors, uneven loss distribution between devices and excessive clamping diodes necessity in higher levels. 3.3 Modular Multi Level Converter It also consist of six valves each connecting one ac terminal to one dc terminal. However, where each value of two level converters is effectively a high voltage controlled switch consisting of large number of IGBT s connected in series, each valve of MMC is a separate controllable voltage source in its own right. Each MMC valve consist of number of independent converter sub modules each containing its storage capacitor. Each sub module contains two IGBT s connected in series across the capacitor with the midpoint connection and one of the two capacitor terminals brought out as external connections. Fig. MMC Depending on which of the two IGBT s in each sub module is turned on, the capacitor is either bypassed or connected into the circuit. Each sub module therefore act as an independent two level converter generating a voltage of either zero or U sm (U sm = sub module capacitor voltage). With suitable number of sub modules connected in series, the value can synthesize a stepped voltage waveform that approximates very closely to a sine wave and contains very low level of harmonic distortion 4. OPERATION OF VSC The basic configuration of a three phase converter is a bridge converter (Graetz bridge) which can be fed from transformer winding connected in star or in delta. There are six switches numbered in the order in which they are turned on. Each switch consists of a self commutated IGBT device and an anti parallel connected diode. The dc side is connected to the capacitor and the ac side is connected to the source voltage through inductors. In each leg of the converter, only one switch can be turned on at any given time to avoid short circuit across the capacitor and each switch conducts for Fig. VSC HVDC system

4 A basic VSC HVDC, as shown in Figure consists of two converter stations which use series-connected fast-switching IGBTs to transform three-phase AC voltage to DC and vice versa at each end of the DC link. With the SPWM (Sinusoidal Pulse-Width-Modulation) controlled VSC, it is possible to deliver virtually any phase angle and voltage amplitude to the AC grid, by changing the PWM modulation depth and relative phase displacement respectively [13]. The reactors are used for controlling the active and reactive power fow by regulating the current through them and for reducing the high frequency harmonic content of the ac line current caused by the switching of the VSCs. Tuned shunt filters are needed to reduce the high frequency switching ripple on the ac voltage and current. The transformers reduce the ac system voltage to a value suitable for the converter. The dc capacitors provide an energy buffer to keep the power balance during transients and reduce the voltage ripple on the dc side [10]. 5. CONTROL SCHEMES VSC HVDC control scheme is divided into four parts: Fig. Schematic representation of control system hierarchy 5.1 Firing Control It is said that valve control is at the lowest level. The desired input is taken by it and then it determines that pulses need to be generated or not. At appropriate instances, pulses are generated by communication of IGBTs with firing logic. These firing instances are synchronized using phase locked loop (PLL). The pattern of pulses depends upon the topology of the bridge and switching methods Individual Phase Control: The generation of firing pulse in each valve is independent of each other in this scheme and these firing pulses are synchronized with commutation voltages. Two ways in IPC are: Constant α control: From converter AC bus and voltage transformers six timing voltages are derived six pulses are generated at the same time delays corresponding to respective voltage crossings. The delay circuit produces delays which are controlled by control voltage Vc derived from current/extinction angle controllers.

5 Inverse cosine control: In each phase six timing voltages are phase shifted by 90 0 and added separately to a common control voltage Vc. To initiate the firing pulse for particular valve the zero crossing of the sum of the two voltages is considered. The delay angle α is nominally proportional to the inverse cosine of control voltage. It is advantageous as the average DC voltage across the bridge varies linearly with the control voltage Vc Equidistant Pulse Control (EPC): By using ring counter, the firing pulses are generated at equal intervals of 1/pf. In this scheme, firing pulses are generated by using phase locked oscillator. Three variations of this scheme are: Pulse frequency Control (PFC): In this scheme, VCO is used which consists of an integrator, comparator and a pulse generator. The frequency of voltage controlled oscillator is derived by control voltage which gives output pulses of generator to activate ring counter and reset the integrator. Fig. Pulse frequency Control K 1 (V c +V 1 )dt = V 3 K 1 V 1 (t n -t n-1 ) = V 3 As t n -t n-1 = 1/pf 0 Therefore, in steady state, gain K 1 = pf 0 V 3 / V 1 Pulse period Control: This is similar to PFC except the way in which control voltage is handled. Here, K 1 V 1 dt = V 3 + V c K 1 V 1 (t n -t n-1 ) = V 3 + V c Pulse phase Control (PPC): The firing pulses are generated according to the equation: K 1 V 1 dt = V 3 + V cn - V c(n-1) Constant Extinction Angle Control (CEA): Since current reference in the inverter is reduced by a amount called current margin, the direct current determined by the rectifier in the other end of the line will be more than the current reference at the inverter. This gives constant error which forces CC to go to saturation at the inverter and leave the inverter with CEA control. CEA is controlled through close loop. The actual value of gamma is evaluated between the time interval between the valve current extinguishing point to zero crossover point of communicating voltage. The controller must act fast in the direction of enlarging gamma.

6 Fig. Constant Extinction Angle Control 5.2 Current/Inner Control The current is controlled by phase reactors. Here, we use decoupled control i.e. voltage and currents are decomposed in d and q components which are controlled independently. Inner control gives the output as desired control voltage. The design of inner control is such that it is faster than outer controller. The inner current controller generates the PWM pattern for the actual VSC. The voltage is such that currents through the phase reactor correspond to the reference values of the controller [4]. The implemented scheme has at its base level a fast inner current control loop controlling the dq components of the AC currents. An extended Park s transformation is used to transform the electrical variables from the abc reference frame into a synchronous rotating dq reference frame. The advantage of this transformation is that the controllable electrical variables are now DC values. This feature is useful for design, analysis, and for decoupled control of the two AC dq current components. Fig. Current Control 5.3 Outer Control DC voltage controller:

7 Fig. DC voltage controller If the direct-voltage controller were to operate directly on the error, the closed-loop dynamics would be dependent on the operating point. This inconvenience is avoided by selecting the direct-voltage controller (DVC) as a PI controller operating instead on the error Active power control: Fig. Active power control In a two-terminal VSC HVDC system, one converter sets the active power, while the other converter controls the DC voltage. In general, an n converter VSC HVDC system has n 1 converters controlling active power, and one controlling the DC voltage Reactive power and AC voltage controller: Fig. Reactive Power control Fig. AC voltage control In a VSC HVDC system, every converter can independently control its reactive power injection in the power system. When the system voltage is aligned with the q-axis, the reactive power Q can be calculated as : Q REF = I prref d U ctrl q

8 AC voltage is mainly dependent on Q, When PI block is used, the equation of the AC voltage controller in the Laplace domain is as follow: 6. CONCLUSION Voltage source converter used in HVDC has been proved a sreat booster in transmission system. The additional controllability in VSC gives many advantages, notably the ability to switch the IGBTs on and off many times per cycle in order to improve the harmonic performance. Being self-commutated, the converter no longer relies on synchronous machines in the AC system for its operation. A voltage sourced converter can therefore feed power to an AC network consisting only of passive loads, something which is impossible with LCC HVDC. REFERENCES [1] Analysis of the Control Algorithms of Voltage-Source Converter HVDC ; Cuiqing Du, Ambra Sannino, Member, IEEE, and Math H.J. Bollen, Fellow, IEEE [2] Analysis of response of VSC-based HVDC to unbalanced faults with different control systems ;Cuiqing Du, Ambra Sannino, Member, IEEE, and Math H.J. Bollen, Fellow, IEEE [3] Multiterminal Voltage-Sourced Converter-Based HVDC Models for Power Flow Analysis; Xiao-Ping Zhang, Member, IEEE [4] HVDC power transmission system; KR Padiyar [5] Voltage sourced converter (VSC) valves for high-voltage direct current (HVDC) power transmission Electrical testing, IEC 62501:2009, Annex A.