Voltage Stability Enhancement using VSWT with Direct Drive Synchronous Generators

Th is paper investigates about the enhancement in grid voltage stability while integrating the large-scale variable speed wind turbine (VSWT) with direct drive synchronous generators (DDSG). A dynamic modeling and simulat ion of a grid connected VSWT driven DDSG with controllable power inverter strategies suitable for the study was developed, tested and verified. Th is dynamic model with its control scheme can regulate real power, maintain reactive power and generate voltage at different wind speeds. For th is paper, studies were conducted on a standard IEEE 14 bus system augmented by a rad ially connected wind power p lant (WPP) which contains 100 wind turbine generators (WTG). The studies include examin ing the voltage stability (λ-V) curves, voltage magnitude, reactive power delivered, loading margin and voltage collapse of the system. These voltage stability studies are done for the normal state as well as for line contingencies. It is found that large scale VSWT with DDSG at the transmission level has the potential to improve the long-term voltage stability of the grid by injecting react ive power with the help of controllable power inverter strategy.


Introduction
Wind po wer is the most qu ickly g ro wing electricity generation source with a 20% annual growth rate for the past five years. Variable speed operation yields 20 to 30 percent more energy than the fixed speed operation, reduces power fluctuations and improves reactive power supply [1]. As wind energy is increasingly integrated into power systems, the stability o f exist ing po wer systems is beco ming a major concern fo r the power system planners and operato rs. A natural next step is now to utilize control of the active and reactive power of modern wind turbines to further enhance grid and wind energy installation interaction. The power elect ron ic interface isolates the generator characteristics fro m the rest o f the po wer system. On ly the cont ro lled converter characteristic is seen by the grid [2].There are many papers dedicated to dynamic model development of variab le speed wind turbine with DDSG [3,4]. Due to the continuous g ro wth in the d eman d fo r electricity with un match ed generat ion and t rans mission capacity expansion, vo ltage instability is emerging as a new challenge to power system planning and operation. .Contingencies such as unexpected line outages in stressed system may often result in voltage instab ility wh ich may lead to vo ltag e co llapse. A fter a voltage collapse, the system becomes dismantled owing to the wide spread operat ion of protective devices. Unavailab il ity of sufficient reactive power sources to maintain normal voltage profiles at heavily loaded buses are the prime reasons for the voltage collapse. Research efforts have been made in understanding the phenomenon associated with the voltage instability and suggesting the remed ial measures to protect the power system networks against such failures [5].
The voltage stability is the ability of a power system to maintain steady acceptable voltages at all buses in the system under normal operating conditions and after being subjected to a disturbance. Power system is voltage stable if voltages after a disturbance are close to voltages at normal operating condition. Vo ltage stability is also called load stability. The factors contributing to voltage stability are the generator reactive power limits, load characteristics, the characteristics of the reactive power compensation devices and the action of voltage control devices.
The objective of this paper is to investigate the grid reinforcing possibilities (voltage stability improvements) [6] that could be achieved by variab le speed wind turbine systems with power electronic converters. The wind turbine systems has been tested on IEEE 14-bus test system.Taking a IEEE five-machine, 14-bus system , we attach the WPP system radially through a transmission system and transformers at bus 1 in Figure 2.The equivalent WPP has a set of wind turbines connected in daisy-chain fashion. The DDSG is operated at variab le speed with capability to control the voltage at the regulated bus at constant power factor. The impact of wind-generation technology on power system voltage stability is also shown in [7].

Modeling
Figure1 presents a schematic diagram of the proposed VSWT with DDSG connected to the grid.

Wind Tur bine
The wind turbine is described by the following equations (1)(2) and (3).   (4) where β  is the blade p itch angle. For a fixed pitch type the value of β  is set to a constant value 4.5 0 .

Synchronous Generator
The DDSG is equipped with an exciter identical to IEEE type 1 model [8 ].The exciter plays a ro le of helping the dc lin k to meet the adequate level of inverter output voltage as given in (5) below where V AC_RMS is RMS line to neutral voltage of the inverter and D MAX is maximu m duty cycle. The exciter p lays a role of meeting the dc link voltage requirement.

Power Electronics Control
This consists of an uncontrolled six-diode rect ifier and a six-IGBT voltage source converter (VSC), which is simple, cost-effective and widely used for industrial applications [9]. The VSC includes a LC harmon ic filter at its terminal to reduce harmonics it generates. The rectifier converts ac power generated by the wind generator into dc power and power control has to be imp lemented by the VSC. A current-controlled VSC can transfer the desired real and reactive power by generating an ac current with a desired reference waveform.

Voltage Instability
A system enters a state of voltage instability when a disturbance, increase in load demand, or change in system condition causes a progressive and uncontrollable drop in voltage. The heart of the problem is usually the voltage drop that occurs when active power and react ive power flow through inductive reactance associated with the transmission network. A power system becomes unstable when voltages uncontrollably decrease due to outage of equipment (line, generator etc.), decrement of production or weakening of voltage control. Vo ltage instability fro m the attempt of load dynamics to restore power consumption beyond the capability of the co mbined and generation system. The main factor causing voltage instability is the inability of the power system to meet the demands for reactive power in the heavily stressed system to keep desired voltages.

PV Curves for Voltage Stability Analysis
PV curves are used to analyze 'steady state' voltage stability which is the stability of the system in normal operation. The 'nose' of the PV curve defines the ma ximu m demand that can be served (the 'Power Limit') and the associated critical voltage. The upper part of the PV curve is considered to be stable whilst the lower part is considered to be unstable.

Loading Margin
The loading marg in is a measure to estimate power system voltage stability. The loading margin is the difference between operating point of the system and knee (critical loading) point of the system. The voltage co llapse points must be assessed in order to guarantee secure operation at the normal operation point.

Voltage Control Capability
Vo ltage control refers to the task of keeping the node voltages in the system within the required limits and of preventing any deviation fro m the no minal value to beco me larger than allowed.
In the DDSG, the reactive power exchange with the grid is not determined by the properties of the generator but by the characteristics of the grid side of the power electronic converter. The generator is fully decoupled fro m the grid. The power factor of the generator and the power factor of the grid side o f the converter can be contro lled independently[1 0].

Converter Rating
The converter rating in DDSG based wind turbine is large and thus more expensive. The turbines that are equipped with DDSG can control the terminal voltage. It was concluded that the terminal voltage variat ion is smoothest in the case of variable-speed wind turbines with voltage control. Furthermore, it was found that only wind turbines with voltage control can compensate a drop in grid voltage.

Simulation Results
The modeled controllable power inverter strategy of VSWT with DDSG is connected to IEEE 14-bus test system for voltage stability improvement under normal and contingency states. The VSWT with DDSG is connected at bus-1.Basic data used in the model is given in Tab le 1. The modeling of the VSWT with DDSG is imp le mented in MATLA B. The capacity of the VSWT with DDSG is chosen to be 1.66 M VA and real power 1.5 MW. The rated speed of the rotor is chosen to be 40 rp m. The rated wind speed is 15 m/s. the cut-in and cut-out speeds are 4 m/s and 23 m/s respectively. Here, the system is simulated switching frequency of the grid interface inverter is 1.040 kHz. The capacitor value of grid interface rectifier is 2500µF and d.c lin k voltage is 2.5 kV. The generated voltage of synchronous generator is 0.69KV. The transformer rating of grid connected side is 2kV/ 130kV. The p.u voltage magnitude of primary of the transformer is 0.99 p.u.. The grid voltage is 130kV.
For the variable speed operation of the WECS, a step change in wind speed is used in MATLA B, with a step size of 0.5, a wind speed of 8 m/sec. and 7.5 m/sec. is considered in this system is shown in Figure 2.

Contingencies Consi dered
To analyse the system under disturbance, several contingencies are considered.

Improvement in Voltage Mag nitude
The results of voltage magnitude of bus-6 under normal and contingency states with only conventional synchronous generators (CSG) and when VSWT with DDSG is connected at bus-1is given in Table 2 and the voltage profile of Bus-6 is shown in Figure10.  Fro m Figure10, it is observed that VSWT with DDSG has much influence in providing the reactive power compensation. The voltage at bus 6 is within acceptable limits (Vbus5 > 0.95 pu).
The reactive power delivered by VSWT with DDSG at bus-1 to bus-6 and bus-5 are given in Tab le 3 and Table 5 respectively. ` Figure 11. rep resents the profile of.reactive power delivered to Bus-6 Figure 11. Profile of .reactive power delivered to Bus-6 The results of voltage magnitude of bus-5 under normal and contingency states are given in Table 4 and the voltage profile of Bus-5 is shown in Figure12.   Fro m the contingency analysis, line outages 1-5, 4-5 and 6-11 have found to be the most severe cases. These contingencies cause a large voltage drop at bus 5. Figure13 represents the profile of .react ive power delivered to Bus-5. The voltage at bus 5 is below the acceptable limit (Vbus5 < 0.95 p.u.). Th is could be observed from Tab le 4 and Figure  12. The reason for this lower voltage is that bus-5 is far-away fro m bus-1.A lso reactive power cannot travel over a very long distance. In both the above cases, the influence of VSWT with DDSG is more co mpared to the system without WTG. The controllable power inverter strategy of DDSG was applied to enhance the voltage stability under normal and contingency states.

Improvement in Loading Margin
The results of loading margin under normal and contingency states are given in Tab le 6. Figure14 represents the loading marg in curve with only CSG. Figure15 represents shows the loading margin curve when VSWT with DDSG is connected at bus-1 during line 4-5 outage. The profile of loading marg in is given in Figure 16.  With increasing reactive power injection fro m the wind turbine, the maximu m deliverable power to the load increases. So compared to CSG, VSWT with DDSG has much influence in imp roving the loading marg in.

λ-V Diagram of di fferent types of Wind Turbine Systems
Figure17 represents the Bus6 λ-V diagram with only CSG.  It is clear fro m Figure 18 that the maximu m deliverable power (Pmax) is increased by using the reactive power injection facility of the VSWT with DDSG. In other words, the voltage stability margin could be increased by reactive power in jection capability of the VSWT with DDSG.

Voltage Vs Ti me Curve after the Contingency
The result of high-voltage transmission-line disconnection is given here. The resulting voltage levels in the system are presented in Figures 19 and 20. After the line disconnection, the Bus-12 voltage drops due to the increasing reactive losses in the line, and to the reduced line charging. With only conventional synchronous generators, the transmission level voltage (Bus-12) drops to 0.825pu. The voltage at bus 12 is now below the acceptable limit ( Vbus12 < 0.95 p.u.),and can initiate a voltage collapse event . However, when VSWT with DDSG is connected, a possible voltage collapse event is avoided( Vbus12 > 0.95 p.u.). Injecting reactive power into the load bus is a well known method to imp rove the steady state power transmitted by the existing transmission line and also to improve the voltage stability limit [7]. In this case, the VSWT system utilizes its reactive-power in jection capability to maintain voltage on the transmission level (BUS12) within the allo wed limit( ±5% deviation) after the grid disturbance. Most of the load-side voltage (BUS12) is restored by this wind farm action. The above calculations show a grid stabilizing property of VSWT with DDSG. It is clear fro m the results that, the power electronic converter of the VSWT with DDSG can be utilized to increase the voltage stability limit of the nearby load bus. A dynamic model of a VSWT with DDSG and power electronic interface is imp lemented in MATLAB. The VSWT co mponent models and control scheme are built by using user-defined and built-in co mponents provided in the software. Contingency tests are carried out to study the voltage stability (λ-V )curves, voltage magnitude, reactive power delivered ,loading margin and voltage collapse of the system connected with VSWT under variable wind speed conditions.

Conclusions
It was found that VSWT with DDSG could assist the grid to delay or prevent a voltage collapse event. The improvement of voltage stability of a system when connected with VSWT and DDSG is large co mpared to the system with only conventional generators.
Further, it is found that the farms grid integrated on the distribution level and mixed with other loads may possibly increase the long-term voltage stability limit of the system when the control was modified. In the cases demonstrated in this paper, an instability is comp letely avoided. In addition, with the modified control, voltage dips could be mitigated by VSWT with DDSG.