Power Solution for High-voltage Static VAR Generator - Mornsun

Abstract:
Compared with traditional SVC such as modulator, capacitor reactor and thyristor controlled reactor (TCR), SVG is the best solution in the reactive power control field at present and has unparalleled advantages. In other words, SVG is currently the most advanced dynamic reactive power compensation device all over the world. MORNSUN PV45-29D-15 power supply for SVG's core power unit provides a highly reliable power solution to meet the customers’ demands.

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Key Words:

SVG, power unit, IGBT, high isolation, PV45

SVG stands for Static VAR (Volt-Ampere Reactive) Generator. It is also known as high-voltage dynamic reactive power compensation device and static synchronous compensator, which performs dynamic reactive power compensation by a self-commutated power semiconductor bridge inverter. 

 I. Applications of high-voltage SVG:

High-voltage SVG is suitable for many applications shown as below, such as PV solar, wind, railway, drilling platform, mill, hoist and electric arc furnace (EAF), etc. The market of high-voltage SVG (6KV/10KV/35KV) is promising and prospective.

    In power system, the reactive load not only increases the active energy loss of the power grid, but also seriously affects the quality of it. Real-time fast SVG is significant to improve the utilization ratio of power equipment and the stability of operation and ensure the power supply voltage within allowance. 

II. Basic structure of high-voltage SVG:

High-voltage SVG generally consists of control cabinets, power cabinets and starting cabinets. And power cabinets consist of multiple power units in which MORNSUN PV45-29D-15 used and play a key role in transmitting reactive power. Because power units are mainly composed of power supply (such as PV45-29D-15), power unit board, IGBT driver, film capacitor, surge absorption capacitor and heat dissipation device. 

III. SVG Application of PV45-29D-15:

    High-voltage SVG usually adopts the chain structure by using multiple H-bridges in series.   and then power power supply in power units. Therefore, the grid voltage actually determines the number of required power supplies. PV45-29D-15 directly gets power through the grid as diagram 4 below, removing from problems of unstable supplies’ voltages or troubles of adding external components. Therefore, it enhances the reliability and safety of the system. 

    For example, the fluctuating grid voltage in each H-bridge is 500V-V regarding 35KV bus bar. It’s recommended to use one PV45-29D-15 for every H-bridge to power two IGBT drivers respectively, totally four IGBT power tubes, due to its double 15V outputs.

VI. About PV45-29D-15

    PV45-29D-15 is specialized for high-voltage SVG and offers 10:1 ultra-wide input voltage range of 150VDC-VDC, VAC isolation and double outputs. It meets Industrial grade operating temperature of -40℃ to +85℃, m altitude requirements and ±2KV/4KV surge protection. And it has high voltage accuracy and excellent load regulation and cross regulation, solving the problem of load imbalance caused by double outputs. Moreover, its built-in multiple protections ensure the safety of load and the module itself under abnormal condition. 

Features:

● 10:1 ultra-wide input voltage range: 150 - VDC

Static var generators

Static var generators (SVG for short), also called active power factor compensators or correctors (APFC for short) or instantaneous reactive power compensators (IRPC for short), have been around since the s. Description of their topology and operating principle can be found as far back as . They were developed as a customised design of shunt active power filters (APF for short) to take care of the problems in the electric power system created by fast changing reactive power demand or by highly dynamic loads that conventional passive solutions like mechanically switched capacitor banks (MSC for short) and mechanically switched reactors (MSR for short) or conventional active solutions like thyristor switched capacitor banks (TSC for short) and thyristor switched reactors (TSR for short) could not handle.

SVGs can be applied to small, medium or large installations in a wide range of segments. They have many low and high voltage potential applications where their use offers many benefits including equipment or facilities where reactive power and/or power factor fluctuate rapidly or in big steps, welding machines, solar inverters, wind turbine generators, and loads with low power factor, to name a few.

Functions

SVGs deliver in real-time exactly the right amount of inductive and capacitive reactive current that the application demands, providing accurate power factor correction, mitigating flicker, reducing voltage variations and reducing unbalances in the system without the drawbacks of conventional solutions. 

Modern SVGs can take care of several power quality problems and support the development of clean energy by combining different control functions in a single device.

Connection

A SVG is a power electronics-based shunt compensation device connected in parallel with the equipment generating the power quality problems or that has issues to comply with grid code and energy efficiency requirements. The SVG behaves as a controlled current source providing any kind of current waveform (in terms of phase, amplitude and frequency) in real time (typical reaction time is under 50 microseconds and typical overall response time is under 100 microseconds).

SVGs can be connected to the electric power system as 3-wire or 4-wire devices:

• 3-wire SVGs are typically used for industrial and generation applications.
• 4-wire SVGs are typically used for applications in buildings.

The most common operating voltage range for SVGs is 200 V up to 690 V as they are built using low voltage IGBT switches. It is possible to connect them to higher voltages using a suitable step-up transformer.

SVGs for standby diesel generators

Electric power generators can be classified by their mode of operation in continuous, prime or standby. Continuous and prime power generators are very similar as they function as the main source of power and are designed to operate continuously or for extended periods of time. The major difference between the two is that continuous generators are designed to operate continually with a consistent load while prime generators are designed to operate for long durations at variable load. Standby power generators (also known as emergency power generators) are designed to be run only in a backup situation when there is an interruption in the source of power of a facility. 

Protection against both short and long-term power supply interruptions is essential for many critical applications. Static UPS systems and rotary UPS systems are not designed to deliver power indefinitely, because of that, standby generators are an ideal complement for UPS systems for applications that must remain operating continuously. UPS systems shield the protected equipment from brief anomalies, while also give standby generators time to start up and synchronise. Standby generators, once running, can continue operating for as long as necessary, subject to fuel availability.

Standby power is mandatory for many applications like data centers where mission critical processes require a reliable and uninterrupted supply of power. Standby generators typically used in data centers can be classified into three categories:

• Diesel generators (DG).
• Gas generators (GG): These generators can be further divided into natural gas generators (NGG) and propane gas generators.
• Dual fuel or bi-fuel generators.

Requirements

Background

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Data centers can contain hundreds or thousands of computers and data storage devices, and in order to be always available to the world, they must be always on. Additionally, these computers and data storage devices require climate control for reliable operation, which means the facilities in which they are housed, must have air conditioning and ventilation systems. Due to the value of the information being stored, emergency systems such as fire suppression, as well as communication and security systems are also required, and all these systems must be reliable and always on.

Diesel generators are commonly used as standby generators together with UPS systems to allow data centers to operate for extended periods during a power supply interruption or during maintenance activities. 

In this case the standby diesel generator trips under load conditions that appear to be well within the generator ratings. The situation where this occurs is when the loads become directly supplied by the standby generator.

The loads in this facility show a characteristic called “leading power factor”, which generators usually tolerate poorly. When the standby generator is forced to supply the leading power factor of the loads this produces a fault condition at the generator that causes it to shut down, often at the worst possible time, when it is needed most.

The generator can tolerate a small amount of leading power factor, so the facility may initially operate correctly but later develops a problem as the loads are changing over time, affecting the power factor of the installation. The problem is specifically caused by leading reactive current greater than a certain threshold value. When leading out-of-phase current passes a threshold value, the standby generator will lose control of its output voltage regulation and will trip off on overvoltage.

System description

Complex data centre power distribution systems, server racks and computers often produce non-unity capacitive load in normal operation. The capacitive loads of the data center are connected to a standby diesel generator at 415 V to provide emergency backup power if needed.

Solution

Analysis

To be able to dimension a solution it is necessary to collect power quality measurement data from the data center over a period of time by using a power quality analyser. The analyser is connected between the power supply and the capacitive loads of the data center.

The limit of capacitive reactive power for the standby diesel generator under full load condition is leading power factor 0.9, which means that if the leading power factor is lower than 0.9 when it works at full load , the standby generator could not connect to the electric power system.

The capacitive reactive power also affects the excitation system and the output voltage of the diesel generator, which affects its output capability When the system leading power factor is around 0.9, there is a considerable decrease in the output capability of the standby diesel generator.

Proposed solution

The two main problems of capacitive reactive power in the data center are the issues for starting and operating the diesel generator and the penalty because of low power factor in the installation. 

Generally, there are two options for correcting a situation of excessive leading out-of-phase current:

• Removing load from the standby diesel generator: This removes the sources of out-of-phase current.
• Installing a device in parallel with the standby diesel generator able to provide inductive reactive power.

Based on the data from measurements it is clear that a solution able to provide in real time both, capacitive and inductive (leading and lagging) power factor correction is required to fix both problems.

Installing a SVG rated 415 V 50 Hz +/-300 kvar in parallel with the standby diesel generator will solve the problems and comply with customer’s requirements. The SVG can provide in real time the exact amount of lagging out-of-phase current that can cancel the leading current of the loads. The SVG has the advantage that it can always adjust to the changing conditions of the loads or to installation’s topology.

Conventional solutions like reactor banks (even if equipped with thyristor switch modules) are often too slow to respond to the fast reactive power compensation requirements of these applications. Conventional solutions also do not avoid possible resonances that might occur in the system.

Based on the values monitored, the following functions are proposed for the SVG.

Conclusions

Data centers are critical facilities having remarkably high requirements for the reliability and availability of electric power supply. Any data losses due to power supply interruptions might lead to immense economic losses. Ensuring that power protection systems such as standby generators and UPS systems can adjust and react to situations of leading power factor is essential to guarantee proper operation of these facilities.

The inductive reactive power from SVGs can negate in real time the capacitive loading of the system to an adequate level, ensuring that the harmful effect of capacitive currents is avoided. SVGs can also respond to any short-term reactive power needs of the installation and balance the electric power system if necessary.

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About the author:

Pedro Esteban is a versatile, multicultural and highly accomplished marketing, communications, sales and business development leader who holds since a broad global experience in sustainable energy transition including renewable energy, energy efficiency and energy storage. Author of over a hundred technical publications, he delivers numerous presentations each year at major international trade shows and conferences. He has been a leading expert at several management positions at General Electric, Alstom Grid and Areva T&D, and he is currently working at Merus Power Plc.

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