The post How to estimate electrical network stress during large motor starting in pre-design phase appeared first on Schneider Electric Blog.

]]>In a previous blog post, we learned that simplified calculations can be used to evaluate voltage drop caused by large motor starting and determine potential solutions in the design project stage. Those calculation simplifications are valid if cables can be omitted. The calculations are using only reactances.

Reduced voltage soft starters (RVSS) and variable speed drives (VSD), are commonly used electronic starters for large motors. In this blog post, we will provide guidelines to establish an order of magnitude to determine how RVSS and VSD impact voltage drops compared to using direct online (DOL), which is the most frequently used motor starting solution.

Let’s consider a basic situation with one motor to start. We can express the voltage at motor terminals, i.e. starting voltage as:

With,

Using a direct online starter results in a full starting current, therefore k=1. When applicable, RVSS can reduce the starting current up to 50% (k=1/2). A variable speed drive starter can reduce the starting current to less than a quarter (k=1/4).

The following figure illustrates the relationship between effective starting current and maximum current for the three starting modes discussed above:

This relation can be observed in the following graphical representation and allows the 3 steps for voltage drop estimation to be defined:

Let’s consider the following example:

Pn = 2.5MW centrifugal pump motor with 6 x In starting current

Ssc =100MVA short-circuit power at motor busbar level.

In step 1, the ratio between short-circuit power and motor rated power has a value of 40 (100MVA/2.5MW), shown as point (A) on the figure.

In step 2, the starting modes and respective prospective starting currents are analyzed. In direct online starting, the motor will start at 6 In (B), RVSS with 3 x In (D) and VSD at 1.5 x In (F).

In step 3, the respective voltage drop for each starting mode is determined by reading the corresponding value from the voltage drop scale. For DOL, the voltage drop corresponding to 6 x In starting current is 15% (C), using RVSS it will be 8% (E) and using VSD it will be less than a 5% voltage drop (G).

However, it is important not to confuse this short-circuit current *I _{SC}* with the busbar short-circuit current rating. Such approximation will lead to an overestimation of short-circuit power and an optimistic calculation of voltage drop, inducing changes in commissioning phase and additional costs.

Using the proposed method, it is easy to discriminate starting methods in early project phase where data is limited and inexact.

Interested to learning more about motor management? Let our expertise be your guide in this free brochure:

Motor Management for LV and MV high-power motor applications

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]]>The post Avoid electrical network stress during large motor starting using simplified calculations at design stage appeared first on Schneider Electric Blog.

]]>Voltage drop during load energization is a major concern in heavy industries. Furnaces, transformers, and large motors are examples of loads with high inrush currents, generating voltage drop. In the same time motors exposed to voltage drop will have their torque reduced, experiencing deceleration and even stalling. Sensitive electronic devices can fail when voltage drop occurs. To cope with these issues, voltage drop protection is used to disconnect loads at a defined threshold. If voltage drops are not addressed in the design phase, they may later be responsible for process downtime and production losses.

For large motors, it is best to address the issue as early as possible in the early design stage of a project. However, at this stage fewer inputs are available, so this can be challenging.

That is why, quick simplified calculations can be relevant, compared to time-consuming simulations based on multiple assumptions.

A simple method of calculation is to focus only on predominant reactances, neglecting resistances, and consider the electrical network as an impedant circuit. This method is valid if cables can be omitted.

Consider the following electrical network with one starting motor:

Where,

Based on the figure above, the starting voltage and voltage drop can be expressed as:

Several parameters can help to maintain a high starting voltage and reduce the voltage drop during starting.

These parameters are shown in the next table:

Effectively, limiting the voltage drop, Vdrop, is possible through:

Increasing equivalent starting reactance with:

- A load current (1) decrease by disconnecting or reducing loads before starting a large motor. This method is limited to specific processes (oil and gas, mines, etc.) and exceptional starting (one start/year) of very large motors (>10MW). This is a process automation-based solution.
- A motor starting current (2) decrease using starting equipment. This method brings the additional benefit of mechanical stress reduction. Starting current (2) can also be reduced with a specific low inrush current motor, which can be a more economical solution than using a starter.

Reducing transformer reactance with:

- Higher transformer power (3) and/or lower short-circuit voltage (4)

Reducing upstream utility or generator reactance with:

- Verify utility short-circuit power (5) with effective inputs. Underestimating the value could have economic consequences due to pessimistic evaluation of starting voltage drop.
- An increase of generator power (6) or design of the generator with lower sub-transient impedance (7).

**Case study:**

Let’s consider the example of 4 x 315kW 0.4kV 550A motors supplied through a 1.5MVA 22/0.42kV transformer with a 6% short-circuit voltage and 750MVA upstream short-circuit power.

For one motor starting at 6 x In with 3 similar motors that are fully loaded and running in parallel, the variables can be calculated as:

Then the voltage during start can be calculated as:

Coming next in our blog series: How to estimate voltage drop with large motor starting.

Interested to learning more about motor management? Let our expertise be your guide in this free brochure:

Motor Management for LV and MV high-power motor applications

The post Avoid electrical network stress during large motor starting using simplified calculations at design stage appeared first on Schneider Electric Blog.

]]>The post 3 safety measures for motors with individual power factor correction appeared first on Schneider Electric Blog.

]]>Whether direct on line (DOL) or with a motor starter (auto-transformer, soft-starter, variable speed drive), a motor can be individually paired with dedicated capacitor unit to fulfill installation power factor objectives and ensure power quality.

Usually not well understood, capacitor protection is often underestimated. Several phenomena have to be considered in order to design an optimum protection and guarantee the safety of people and goods:

- capacitor internal fault and failure mode
- inrush current,
- auto-excitation with motor
- harmonics

**Internal faults** can occur in a capacitor. Protective devices shall be used to isolate promptly the faulty capacitor before the unit case rupture. Properly rated, High Rated Current (HRC) fuses is the most cost-effective solution for the protection of delta connected capacitor units used for individual compensation of motors. It is important to mention that fuses for motors are not suitable to protect capacitors. They are overrated to withstand motor starting current, and too slow to operate with capacitors.

**Inrush current** occurs when capacitor units are switched on. This inrush current can be significantly increased if other capacitors connected in parallel are already energized (back-to-back switching). In this case, inrush reactors are required to reduce the transient overcurrent to values acceptable for the capacitor (100 In) and the switching device.

Typical values for inrush reactors range from 50 to 400 µH.

**Auto-excitation** can occur when capacitor and motor remain permanently connected after disconnection. In this case, the motor could behave as a generator by self-excitation during the deceleration and may cause over-voltages large enough to produce capacitor failures. To prevent this phenomenon, the capacitor current must be lower than the no-load magnetizing current of the motor. A value of 90% of no-load current is recommended.

A motor’s no-load-current is typically in the range 30 to 40% of its rated value. From a practical point of view, auto-excitation is most likely to occur when the motor power factor is relatively low, under 0.8, and installation power factor objective is high, above 0.95.

**Harmonics** are high order currents leading to excessive heating of the capacitor. They have to be considered for permanent operation. Typical solution are detuned reactors.

In some cases, motor starters may produce harmonics during the start-up. Given the short duration, no specific protection will be necessary.

On the figure below is shown a typical arrangement for individual compensation of motor.

**Motor fuses** (or other protection) are rated to withstand starting, 3-7 time the motor rated current. These fuses will not protect the capacitor unit.

**Capacitor HRC fuses**are specific to protect capacitor in case of an internal fault. When determining fuse rating, inrush currents and capacitor case rupture curves must be taken into consideration. The rating should be at least 1.7 the capacitor rated current**Inrush reactors**are necessary in case of back-to-back capacitor configuration, to limit inrush current. It is the case with several motors with individual PFC or with capacitor bank on the same busbar. Single motor with no other capacitor bank in parallel can go without inrush reactors but usually, go with in case of the later additional motor.**Correct-Sizing**of capacitors is essential to avoid auto-excitation phenomenon when motor and capacitor remain connected after disconnection from the supply. It is recommended to verify that capacitor current remains below 90% of the motor no-load current. Conservative recommendation is to limit capacitor power below 30% of motor power. Otherwise, a dedicated contactor to disconnect the capacitor from the motor before the supply disconnection has to be added. But additional cost and place need to be evaluated.

See more on motor-management

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