Transient Performance Analysis of Reactive Power Compensators at LCC-HVDC Station Feeding Weak AC System

This paper focuses on analyzing the dynamic performance of various reactive power compensators at weak LCC-HVDC terminals. Four scenarios based on combination of active and passive reactive power compensation schemes are presented. The comparative performance analysis of various compensators in different types of AC and DC system faults are evaluated using electromagnetic transient simulation program PSCAD/EMTDC. The analysis shows that fixed capacitor has poor performance and results in reducing effective short circuit ratio of system, while STATCOM on the other hand, increases AC system strength and has significant performance in areas like immunity to commutation failure, suppression of temporary overvoltage, AC bus voltage regulation and fault recovery time.


INTRODUCTION
Line Commutated Converter (LCC) based High Voltage Direct Current Transmission (HVDC) is widely implied throughout world for transmission of power from distant parts to local AC grid stations [1,2]. Due to rapid expansion of LCC-HVDC links, the receiving end AC system becomes weak. Weak AC system having high impedances and low inertia, results in various operating issues like voltage instability, limitations on power transfer, temporary overvoltages, fault recovery problems, commutation failures, harmonic resonances and frequency deviations [3]. To overcome these operating issues, various compensating equipment like Fixed Capacitor (FC), Synchronous Compensator (SC), Static Var Unfortunately, few publications are available regarding design and analysis of dynamic reactive power compensators utilized for LCC-HVDC station attached into weak AC networks. Reference [3] compared different compensator schemes (Fixed Capacitors, SVC, SC, combination of SVC and SC) for LCC HVDC station attached into AC systems with SCR value of 1.5 and frequency of 60 Hz. An RL load is considered in study system to control damping and oscillations. It is identified that weak LCC-HVDC system gives best performance in all disturbances, when combination of SVC and SC is utilized as reactive power compensating source. Reference [4] design advanced static VAR compensator STATCON incorporating nine level Gate Turn-Off Thyristor (GTO) and its operating characteristics are analyzed for various faults. STATCON is then compared with SVC, SC and FC. It is assessed that STATCON has better performance than mentioned compensators, in areas like fault recovery time, voltage regulation dc fault recovery, while operating with AC system having low SCR value. Reference [5] compared the transient performance of STATCOM and SC for LCC-HVDC system with SCR of 1.5. An RL load is considered in study system for analysis. Both STATCOM and SC almost show equal performance during various transients. Actually when RL load is connected in parallel with AC bus, it results in decreasing equivalent impedance, which increases SCR value and the system becomes strong. This paper investigates in detail the dynamic performances analysis of different reactive power compensators at weak LCC-HVDC system without connecting RL load to AC bus. Various scenarios based on passive and active shunt compensators are considered for analysis of different studies like temporary overvoltage, commutation failure and fault recovery time for various disturbances. The paper is organized in following approach: Section 2 illustrates system strength. Section 3 gives description about study system. Section 4 includes analysis of reactive power compensators, focusing mainly on operation principle and control strategy of STATCOM. Section 5 illustrates results of study system. The paper is then finally concluded in Section 6.

SYSTEM'S STRENGTH
AC-DC system's strength is generally determined by index called Short Circuit Ratio (SCR). SCR is the ratio of short circuit capacity (S) of AC network to DC network power rating (Pd), as given in (1). An AC system associated with SCR value of 2 is considered as weak AC system, while strong AC system has usually SCR ≥ 3 [7]. SCR includes only impedance of AC source (Zac) in calculation of Short circuit capacity of AC network. SCR becomes Effective Short Circuit Ratio (ESCR), if impedances of capacitors (Zc) and filters (Zf) are also included in calculation of short circuit capacity, as given in (2) [4].

SET UP OF STUDY SYSTEM
LCC-HVDC system based on CIGRE benchmark is used for analysis [7]. In original benchmark model, the inverter side AC system with SCR of 2.5 employs fixed capacitor and filters for providing reactive power. The six-pulse STATCOM connected to inverter side of HVDC system via converter transformer is shown in Fig.1 Four scenarios are presented in this paper to analyze behavior of LCC-HVDC system connected to weak AC system. These four scenarios comprise of different reactive power compensation schemes utilized for voltage regulation at inverter bus. In all scenarios, size of filters is kept constant. The features of four scenarios are:

Scenario 1 (Filters +Fixed Capacitor System):
System is analyzed while connecting filters and passive shunt capacitors to inverter bus for supplying reactive power. At steady state, filters and fixed capacitor provide about 500 Mvar and 125 Mvar respectively.

Scenario 2 (Filters +Fixed Capacitor + STATCOM 150):
In this scenario, STATCOM along with fixed capacitor is connected to inverter bus system. Reactive power is provided by filters and shunt capacitor in steady state while in transient state STATCOM operate to make the system stable.

REACTIVE POWER COMPENSATIONS OF LCC CONVERTER STATIONS
The reactive power requirement of LCC converter station is fulfilled by filters and shunt capacitors, when AC system is strong. However, for weak AC system, dynamic reactive power compensators like Static Var Compensator (SVC), Synchronous Condenser and STATCOM are used to regulate bus voltage. In the following section, operation principle and control mode strategy of STATCOM is presented.

STATCOM Fundamentals
It is a Flexible Alternating Current Transmission System (FACTS) device, shunt connected to AC transmission and distribution network for voltage regulation. Although, it is power electronics based (voltage source converter or current source converter) regulating device, its behavior is similar to Synchronous condenser. STATCOM being an active device is capable of absorbing or generating reactive power without any need of shunt reactors and capacitor banks. It can also supply active power, when energy source like battery, flywheel is connected to DC side capacitor. The schematic diagram of STATCOM is shown in Fig.2.
Step Up Transformer with Leakage Reactance (X t )  (4). Moreover, the reactive power may be controlled by varying the output voltages of VSC converter as given in equation (5). If the output voltage of converter is greater than bus voltage, reactive power will be generated due to reactive current flow from converters into AC system. If output voltages of converter is less than bus voltages, reactive power will be absorbed due to reactive current flow from AC system to converter. If both voltages have same magnitude, no reactive current will be exchanged through reactor and STATCOM will be in floating state.

Control Strategy of STATCOM
d-q decoupled control method [9] is used for controlling of STATCOM as shown in Fig. 3. It includes two close loop PI controllers. The outer loop controllers generate d-axis and q-axis current references (idref & iqref) using dc voltages and AC voltages respectively as controlled parameters. The inner loop controllers are used to adjust d-axis and qaxis currents to set with reference currents. The voltage orders (Vdo and Vqo) are produced by combining output of inner loop PI controllers, measured voltages and decoupling terms. These voltage orders are utilized to produce switching signals for converter using PWM technique. The reactive power is controlled by Id. The STATCOM parameters are summarized in Table I [2]. The STATCOM capacity is limited by maximum current (Imax) given by equation (5).
where St is the transformer capacity in MVA and Vti is bus voltage.  Fig.3: d-q decoupled controlled method of STATCOM [9] is most severe when AC system is weak. TOV greatly depends on AC system's impedance, their angle and reactive power compensating devices connected to AC busbar.
The temporary overvoltage for all scenarios given in Table 2, is calculated using Power flow equation. These theoretical results are then validated using electromagnetic transient simulation program. Fig.4 shows the simulation results of temporary overvoltage

Computation Failure
Commutation failure is a worst dynamic event during inverter operation. It occurs when converter off going valve continues to conduct without transferring current to ongoing valve [10]. Commutation failure is mainly caused by low AC voltage condition or with increase in DC current [11]. It results in interruption of power flow to AC system. The interruption will be significant if AC system is weak. Moreover, the recovery process becomes difficult due to implication of large inverter firing angles and requirement of large amount of reactive power [2].
Commutation Failure Immunity Index (CFII) is used to evaluate the vulnerability of LCC converter to commutation failure. CFII is the ratio of worst critical fault MVA to rated DC power of converter [10].
The above equation shows that greater the fault MVA, higher will be the CFII and less vulnerable is the LCC to Commutation Failure. CFII considers HVDC controls and AC system strength and their effect on commutation process. CFII uses electromagnetic transient simulation program to determine the susceptibility of LCC converters to commutation failure. Three-phase inductance fault with increasing severity is occurred at inverter bus, while continuously changing fault point on wave. Repeated simulations are conducted for different point on waves and critical fault levels at commutation failure occurs is then utilized to evaluate CFII.

DC Power Recovery Time
Weak LCC-HVDC station has usually longer fault recovery times. This fault recovery time could be minimized in order to mitigate energy losses [12,13]. In this work, DC power recovery time is utilized as fundamental metric to assess quantitatively the system recovery performance.  good results, but the capacity of STATCOM is not as high that it can better support bus system in fault states. Scenario 4, the optimum option shows very promising results. This is due to the reason that surplus reactive power capacity of STATCOM helps in regulating system bus voltage during transient state.