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Prevention And Treatment Of Power Grid Voltage Accident
Jan 23, 2018

Prevention and Treatment of Power Grid Voltage Accident

Abstract: This paper mainly introduces the provisions of accident treatment rules for voltage

 accidents, explains the hazards of voltage unqualified, the causes of low and high voltage,

 the measures of voltage regulation, the voltage regulation principle of transformer substation

 and distribution substation in normal operation And grid voltage accident handling method.

Key words: grid voltage accident voltage unqualified hazard reason regulating measures accident

 treatment

First, the accident handling procedures for the provisions of the grid voltage accident

Voltage is an important indicator of power quality, accident treatment regulations: the system

of the central point (ie provincial regulation of the voltage monitoring bus, the same below)

 voltage exceeds the specified voltage curve ± 5% value and duration of more than 1 hour

 constitutes an obstacle, 2 hours as an accident; if the voltage curve exceeds the value of ± 10%

, and duration of more than 30 minutes also constitute an obstacle, more than 1 hour is also

 counted as an accident.

Second, the grid voltage failure hazards

1, for power users

All kinds of electrical equipment are designed and manufactured in accordance with the rated

 voltage, these devices run at rated voltage to achieve the best results, the voltage is too

 large to deviate from the rated value, the user will have adverse effects.

For example, lighting, its luminous efficiency, luminous flux and service life are related to

 voltage. When the voltage increases, incandescent and fluorescent lamp flux will increase,

 but the service life will be shortened; the other hand, the voltage is reduced, so that the

 luminous flux decreased, light-emitting insufficient, affecting people's vision and work 

efficiency. Asynchronous motor electromagnetic torque is proportional to the square of its 

terminal voltage, when the voltage is reduced by 10%, the motor speed decreases, the torque 

is about to be reduced by 19%. If the motor drag the mechanical load unchanged, the voltage

 decreases, the motor speed down, slip increases, the stator current also increases, increased 

heat, winding temperature increases, accelerating the aging of insulation, shortened life; when 

the terminal voltage is too Low, the motor may stop, even under heavy load can not start.

 Electric furnace and other electric equipment output is roughly proportional to the square of

 the voltage, the voltage will reduce the furnace smelting time and reduce productivity.

2, the power grid is concerned

The voltage drop will increase the power loss of the grid. Voltage is too low may also jeopardize

 the stability of power grid operation, the occurrence of voltage collapse accident. While the

 voltage is too high to affect the insulation equipment.

Therefore, it is one of the basic tasks to adjust the grid operation to ensure that the voltage

 at the user is close to the rated value.

Third, the grid voltage is low and the reasons for the high side

1, the reasons for the low voltage

(1) power supply network or distribution network structure is unreasonable, in particular,

 some rural power transmission line distance is too long, the power supply radius is too large,

 the wire cross section is too small, so that the line voltage loss is too large.

(2) reactive power power shortage or reactive power compensation equipment mismanagement,

 long-term disrepair, often disabled, so that reactive power balance damage, which is the general

 decline in power grid voltage level the root causes.

(3) transformer tap position placed unreasonable.

(4) unreasonable power wiring, overload, load power factor is low, power equipment maintenance and 

line failure, are likely to make the power grid voltage drop.

2, the reasons for the high voltage grid

With the development of modern power grid, 20-30 million KW large capacity units directly into the

 EHV power grid, and 500KV EHV line put into operation, the line charging power is greater,

 the charging power per hundred kilometers (capacitive reactive power) About 10 million Kvar,

 220KV-500KV EHV power grid in the reactive surplus, so that the main network voltage is too high.

 This is a new issue of grid development.

Fourth, the grid voltage adjustment measures

Having sufficient reactive power supplies is a necessary condition to ensure that the grid has

 a good operating voltage level. But to make all the users at the voltage quality are in line

 with the requirements, but also must use a variety of means of voltage regulation.

 On the power grid, the current means to adjust the voltage of the user terminal,

 mainly to take the following measures:

(1) appropriate selection of the transformer ratio;

(2) change the line parameters R and X and change the reactive power distribution to reduce 

the network voltage loss.

In order to ensure that the central point voltage changes do not exceed the specified range,

 in the reactive power balance of the premise, you can use the following methods of voltage

 regulation:

1, the use of adjustment transformer tap regulator

Changing the transformer ratio can raise or lower the voltage of the secondary winding.

 It is divided into two ways, namely no-load voltage regulation and on-load voltage regulation.

(1) no load regulation

The so-called no-load voltage regulation, that is, without the load regulator, this regulator

 must disconnect the power after the transformer power cut operation, change the transformer 

tap, to adjust the secondary voltage purposes. Because of the need for power outages without

 load regulation, so this method is suitable for seasonal power outage change (with) power 

station. Because can not change according to the load, flexible regulation voltage, so many

 city power network to 110KV and above transformers have been gradually using on-load tap changer.


(2) on-load voltage regulator

On-load tap-changer can switch its tap under load operation, and the range of voltage regulation 

is also larger than that of common transformer, and the voltage regulation range can reach 20% -30%

 of the rated voltage. So 110KV and above transformer is widely used, and with the second phase

 of the rural power grater end, in the rural network has also been vigorously promoted.

2, change the reactive power distribution of the power network voltage regulator

The way to change the reactive power distribution of the power network is to install the parallel

 reactive power compensation equipment at the end of the transmission line and near the user.

The reactive power compensation equipment of the electric wire netting mainly includes the

 synchronous camera, electrostatic capacitor, static compensator.

(1) the same period of the camera

At the same time, the camera is essentially a generator that can only generate reactive power. 

It supplies the inductive reactive power to the system when it is overdriven and absorbs the

 reactive power from the system when it is under-excited. So the same period to adjust

 the camera excitation, you can smoothly change the size of its reactive power and direction,

 which can be smoothly adjust the voltage in the region, can increase the voltage can also reduce 

the voltage. Simultaneous phase adjustment can be equipped with automatic excitation device

 can automatically reduce the grid voltage to increase the output of reactive power to maintain 

the system voltage. Especially when a forced excitation device, the system failure can also

 increase the power grid voltage, which is to improve the stability of the grid is beneficial.

 But the same period the camera is rotating machinery, operation and maintenance more complex,

 active loss is also large, investment costs. In China often installed in the hub substation

. In foreign countries, is now rarely used instead of static compensator.

(2) electrostatic capacitors


The electrostatic capacitor can be connected to the substation bus by triangular and star

 connection, and can only supply the reactive power of the grid, but can not absorb the reactive

 power. It supplies the reactive power QC value and the node voltage U is proportional to the

 square of the square, namely:

So the voltage drops, it provides reactive power is also reduced, so the power grid failure or

 other causes the voltage drop, the output of the reactive power is reduced, resulting in the

 grid voltage continues to decline, which is the electrostatic capacitor Of the shortcomings.

 Capacitors can be installed capacity of large or small, can be used together, but also

 decentralized installation of reactive power supply in place to reduce the line power loss and 

voltage loss. The electrostatic capacitor has less investment cost per unit capacity and less

 power loss during operation, and is also convenient for maintenance. In order to adjust the power

 of the capacitor in operation, the capacitors can be connected in groups to vary the load, group

 the inputs, and cut. It is widely used in the system substation and user power distribution.

(3) static compensator

Static compensator, also known as controllable static var compensator, is a dynamic reactive power

 compensation. Its characteristic is to controllable reactor and electrostatic capacitor

 in parallel use, the capacitor can issue reactive power, controllable reactor can absorb 

reactive power, can be adjusted in accordance with the load changes, thus making the bus voltage

 remains stable. It can quickly and smoothly adjust the voltage, easy operation and maintenance,

 power loss is small, unbalanced load changes can be done to phase compensation, the impact of 

strong adaptability of the load, most of the 500KV substation installed static compensator.

In addition, for some of the 500KV substation near the power plant, as 500KV incoming line 

charging power is larger, making the primary voltage transformer substation higher voltage,

 in order to meet the requirements of the secondary side of the voltage, only by adjusting the 

transformer Tap-changer is sometimes difficult to work, it is usually in the secondary side of

 the bus in parallel groups of inductance coil, in order to voltage fluctuations, the appropriate

 increase in reactive power and voltage loss, to buck the purpose.

3, change the line parameters R and X method voltage regulator

The voltage loss can be approximated as the longitudinal component of the voltage drop:

From the above formula, in the transmission power is constant, change the size of the parameters

 R and X, you can change the voltage loss, play a regulator role.

(1) with series capacitor compensation method of line voltage regulator

In the high-voltage grid, usually reactance X is much greater than R, with series capacitor method

, change the line reactance to reduce the voltage loss. For the load power factor is low, the 

transmission power is larger, the load fluctuation, wire cross-section of the larger lines, series

 capacitor voltage regulator, the effect is particularly significant.

(2) according to the allowable voltage loss to select the wire cross-section

In the low-voltage power grid, many users are very scattered, the capacity is not large, and the

 wire cross-section is small, the voltage loss in PR / U component accounts for a large proportion

 of parallel compensation and series compensation is not only economical and are restricted, For

 this kind of electric wire netting, change the resistance of the wire to obtain certain voltage

 regulation effect. Therefore, in the design and construction of low-voltage power grid, in

 accordance with the provisions of the allowable voltage loss, select the appropriate wire 

cross-section, is to ensure the quality of the user voltage one of the important measures,

 with the second phase of rural power grids, A large degree of implementation, rural network,

 due to small cross-section of the wire line voltage and power loss caused by a large state of the

 situation has been greatly improved.

(3) with the series inductance coil to increase the line reactive power and voltage loss method

 voltage regulator

For some transmission distance, the transmission power of the 500KV line, the line of the charging

 power is larger, the entire line showing capacitive, high power factor, so that the end of the

 line voltage is high, then the line is often used in series inductance coil method ,

 The absorption line on the excess capacitive reactive power, increase the voltage loss, 

in order to achieve the purpose of voltage regulation.

4, auxiliary voltage regulation measures - by changing the generator excitation current to

 change the terminal voltage of the generator to adjust the voltage

This method is generally adjustable within ± 5% of the nominal voltage. Small power grid

 directly to the isolated power plant, the line is not long, the voltage loss is not, so 

change the generator voltage to meet the user's voltage quality requirements. But the multi-stage

 transformer power supply range of the system, only by the generator voltage regulator can not 

meet the requirements.

Fifth, the normal operation of the substation and the distribution of the voltage regulation 

principle

In normal operation, substations and power distribution stations equipped with on-load tap 

changer and reactive power compensation equipment shall voluntarily set the voltage-fluctuation

 range (ie, the voltage curve) of each voltage center point according to the provincial regulation

 (or ground adjustment) ).

From the transformer voltage load relationship, we know, with the method of adjusting the voltage

 regulator transformer tap, with the tap to rise, the transformer turns ratio is reduced,

 the secondary winding output voltage is rising, but in this Process, the transformer's own power 

loss, especially reactive power loss is also increasing. In other words, when the power grid for

 some reason caused by reactive shortfall, by adjusting the transformer tap method voltage

 regulator, will increase the reactive power shortage, resulting in other regions of the voltage


continues to decline.

Therefore, the existing on-load tap changer and reactive power compensation equipment substation

 and distribution, the normal operation of the specific voltage regulation principle is generally:


1, the voltage is reduced, the priority into the reactive power compensation equipment, 

and then transfer transformer tap, but to avoid sending the reactive power to the system.

2, the voltage increases, the priority to adjust the transformer tap, then consider the withdrawal

 of reactive power compensation equipment.

Reactive power compensation equipment to vote, cut to get the consent of their scheduling.

Sixth, the grid voltage accident handling

1, deal with the center point of the accident is too low voltage measures

(1) to make low-voltage hub near the power plant and reactive power compensation equipment with a substation to increase the reactive power output, if necessary, reduce the power plant active power (but the frequency to be qualified) to increase the reactive power. However, when the voltage of the receiving end located at the remote transmission is too low, the active power of the receiving power plant should be considered.

(2) to make other and even the whole system of the generator, adjust the camera, static compensation, static capacitors are filled with reactive power, but be careful not to make the already high central point voltage exceeds the allowable value;

If the above treatment method is invalid, the center point voltage is still too low, you should limit the electricity, if necessary, can pull the gate. The principle order of load limiting is:

Ⅰ, the limit voltage is low and ultra-regional power load;

Ⅱ, the limit load equipment overload power supply area;

Ⅲ, according to the order of the accident gate power cuts.

2, deal with the central point of the high voltage accident measures

(1) Reduce the reactive power output to a minimum in the power plant adjacent to the high voltage center point and the substation equipped with a modulator, a static compensator and an electrostatic capacitor, and the phase modulator is changed into a phase inductive operation (absorption of inductive reactive power).

(2) to make other and even system-wide generator and adjust the camera, static compensation and electrostatic capacitors are reduced reactive power output, and even the lowest, but do not make the low center of the voltage is lower than the allowable value;

(3) so that the high voltage near the center of the power plant with light load part of the unit shutdown.

In addition, in order to maintain the stability of the grid to prevent the occurrence of voltage collapse accident, the system should be set up a number of monitoring points, and the provisions of the voltage drop accident limit. When the voltage monitoring point voltage drops to the accident limit, the power plant and the substation with the camera should be used by the attendant generator and adjust the camera overload capacity to increase reactive power to maintain the voltage and report provincial tone. The provincial transfer should be rapid increase (into) the system all the spare capacity and reactive power to restore the effect of increasing the capacity of spare capacity, if necessary, cut part of the user, to increase the voltage and eliminate the equipment overload.


Add: No. 16, Road 3, Sanjiang Industry Zone, Shengzhou, Zhejiang, China

Contact:Boris Yue      

Email: boris@benoelectric.com

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