The continuous increase of installed wind power seen during recent years has forced the transmission system operators (TSO) to tighten their grid connection rules – also known as Grid Code - in order to limit the effects of power parks on network quality and stability. These new rules demand that power plants of any kind support the electricity network throughout their operation. Key issues are steady state and dynamic reactive power capability, continuously acting voltage control and fault ride through behavior. Some commonly used turbine designs have some limits in terms of achieving Grid Code compliance in several countries. For parks based on such turbines, additional equipment is needed. This paper presents the medium voltage STATCOM (Static Synchronous Compensator) technology which adds the missing functionality to wind parks in order to become Grid Code compliant. The STATCOM as a pure static device with no switched passive components provides outstanding performance for both steady state and dynamic operation. Especially, the fast dynamic voltage control and the behavior during balanced as well as unbalanced grid faults (fault ride-through) are highlighted. Based on medium-voltage converter platforms widely used for industrial applications, ZDDQ has successfully supplied STATCOMs to the wind power industry in order to integrate wind parks into grids with demanding connection requirements.

Harmonics The presence of harmonics in the waveform of the network voltage can be attributed to various causes such as rectifiers, variable speed drives, thyristors, saturated transformer, arc furnaces, etc. The main problems caused by harmonics are: • Interferences in telecommunications systems and equipment. • Distortion of the Electricity Supply Voltage • Erratic operation of control and protection relays • Failures in transformers and motors due to overheating caused by core losses. If the harmonic power is significant, i.e. THVD greater than 7 %, THID greater than 40 %, this may result in overvoltages and overloads, which may lead to the failure of the capacitors, circuit breakers, contactors etc.

Amplification of both voltage and current at the same time will occur if the resonant frequency is close or equal to one of the harmonic frequencies present in the distribution system. The power feeder (overhead line or underground cable) have an inductive impedance. By putting a capacitor in parallel with the load (for Power factor correction) it is possible for the combined system to have a resonance condition.

As the power factor of a three phase system decreases, the current rises. The heat dissipation in the system rises proportionately by a factor equivalent to the square of the current rise.

• Real Power is the power that actually powers the equipment and performs useful,productive work. • Reactive Power is required by some equipment (e.g. transformers, motors and relays) to produce a magnetic field for operation; however it does not perform any real work. • Apparent Power is the vector sum of Real and Reactive Power and corresponds to the total power required to produce the equivalent amount of real power for the load.

The time varying nature of electric arc furnace (EAF) gives rise to voltage fluctuations, which produces the effect known as flicker. Employing reactive power compensation devices such as static VAr compensator (SVC) is one of the main approaches to mitigate this phenomenon. By utilising prediction methods to forecast EAFs reactive power consumption for a half-cycle ahead, performance of SVC can be enhanced substantially. This study proposes a rolling Grey model and a Grey–Markov method to predict the actual reactive power of Mobarakeh Steel Company, Isfahan/Iran. To investigate the efficiency of the proposed methods the results are compared with the results of EAFs reactive power compensation when no prediction method is employed. Furthermore, autoregressive moving average (ARMA) models with updating coefficients, which are studied in the literature are used to predict EAF reactive power. Various methods for updating ARMA coefficients including normalised least mean square, recursive least square method and an online genetic algorithm are used. By comparing the indices which are defined using the concept of flicker frequency and power spectral density, the superiority of Grey–Markov and rolling Grey model over the aforementioned prediction methods is investigated.

The ability of static VAr compensators (SVC) to reduce the flicker caused by electric arc furnaces and other variable VAr loads depends on their speed, which is limited by delays in reactive power measurement and thyristor ignition. In this paper, a predictive method is proposed to compensate the delay time and, hence, to improve the SVC performance. Previous samples of the load reactive power are used to predict its future values. Predicted VAr is then applied as a reference for reactive power compensation

Loads on an electrical distribution system can be categorized as resistive, inductive and capacitive. Under normal operating conditions certain electrical loads (e.g. transformers, induction motors, welding equipment, arc furnaces and fluorescent lighting) draw not only active power (kW) from the supply, but also inductive reactive power (kVAr). All inductive loads require active power: kW to actually perform the work, and reactive power (kVAr) to maintain the electromagnetic field. This reactive power is necessary for the equipment to operate but it imposes an undesirable burden on the supply.

1. Motor failure 2. Electrical or electronic equipment failure 3. Overheating of transformers, switchboards and cabling 4. Nuisance tripping of circuit breakers or fuses 5. Unstable equipment operation 6. High energy usage and costs

Static Synchronous Compensator (STATCOM), sometimes denoted as an 'Advanced Static VAr Compensator', is a Flexible Alternating Current Transmission System (FACTS) device connected in parallel with a power system that is capable to exchange reactive power with the system in both directions. The terms 'compensator' and 'synchronous' indicate that the device is equivalent to an ideal synchronous generator, which produces a set of three‐phase fundamental frequency sinusoidal voltages. The STATCOM device is mainly used for dynamic compensation in power systems, it can provide fast (dynamic) voltage support and might also be used for transient stability margin enhancement or oscillation damping improvement. The STATCOM consists of a voltage‐sourced converter (VSC), a magnetic circuit (MC), a shunt coupling transformer, a shunt breaker, and a control and protection unit. In a VSC, a number of square wave voltages are generated at fundamental frequency by operating the controllable semiconductor switches once per cycle of the fundamental frequency. STATCOM (Static Synchronous Compensator, also known as SVG). It is an important device for Flexible AC Transmission System (FACTS), which is the third generation of dynamic VAR compensation device after FC, MCR, and TCR type of SVC (Static VAR Compensator). Its appearance represents the application of most advanced technology for dynamic VAR compensation. It is also known as DSTATCOM when apply in power distribution. STATCOM is connected parallel in power grid and works as reactive current source. Its reactive current can be flexibly controlled and compensate reactive power for system automatically. It solves problem of harmonics interfere switching parallel capacitor banks. In another hand, it can restrain harmonics and improve power quality according to customers’ needs. STATCOM has superior performance in lots of aspect such as responding speed, stabilize voltage of power grid, reduce system power loss and harmonics, increase both transmission capacity and limit for transient voltage. It also has advantage of smaller in dimension.

Capacitors lower electrical costs two ways: In many areas, the electrical rate includes a penalty charge for low power factor. Installation of power capacitors on the electrical distribution system within a facility makes it unnecessary for the utility to supply the reactive power required by inductive electrical equipment. The savings the utility realizes in reduced generation, transmission, and distribution costs are passed on to the customer in the form of lower electrical bills. The second source of savings derived through the use of power factor correction capacitors is in the form of increased KVA capacity in the electrical distribution system. Installation of capacitors to furnish the non-productive current requirements of the facility makes it possible to increase the connected load by as much as 20 percent without a corresponding increase in the size of the transformers, conductors, and protective devices making up the distribution system which services the load.

Where open electricity markets have been introduced, the supply of electrical energy becomes competitive between the supply utilities. Although private distribution companies are obligated to run a profitable and successful business, they are also committed to maintain the quality of supply at a high level. Competition in an open electricity market creates new opportunities for even better quality of supply of electricity. One very important aspect of improving quality of supply is the control of power factor. Low power factor means poor electrical efficiency. The lower the power factor, the higher the apparent power drawn from the distribution network. This means that the supply company must install larger generation capacity, larger size transmission lines and cables, transformers and other distribution system devices, which otherwise would not be necessary. This results in a much higher capital expenditures and operating costs for the Electricity Supply Company, which in many cases is passed on to the consumer in the form of higher tariff rates. This is the main reason behind why the Electricity Supply Companies in modern economies demand reduction of the reactive load in their networks via improvement of the power factor. In most cases, special reactive current tariffs penalize consumers for poor power factors.