Standards related to:

GB/T 40615-2021**GB/T 40615-2021: PDF in English (GBT 40615-2021) **

GB/T 40615-2021

NATIONAL STANDARD OF THE

PEOPLE’S REPUBLIC OF CHINA

ICS 29.020

CCS F 21

Guides of power system voltage stability evaluation

ISSUED ON. OCTOBER 11, 2021

IMPLEMENTED ON. MAY 01, 2022

Issued by. State Administration for Market Regulation;

Standardization Administration of the People's Republic of China.

Table of Contents

Foreword... 3

1 Scope... 4

2 Normative references... 4

3 Terms and definitions... 4

4 General principles... 5

5 Criteria for voltage stability evaluation... 6

6 Evaluation method for steady-state voltage stability... 7

7 Evaluation method for transient voltage stability... 11

8 Evaluation method for long-term voltage stability... 12

Guides of power system voltage stability evaluation

1 Scope

This document establishes the principles for evaluating the voltage stability of power

systems. It specifies the evaluation contents and methods for steady-state voltage

stability, transient voltage stability and long-term voltage stability.

This document is applicable to the calculation and analysis of voltage stability of power

systems of 110kV and above. For other voltage levels, it can refer to this document for

implementation.

2 Normative references

The following referenced documents are indispensable for the application of this

document. For dated references, only the edition cited applies. For undated references,

the latest edition of the referenced document (including any amendments) applies.

GB 38755-2019, Code on security and stability for power system

3 Terms and definitions

For the purposes of this document, the terms and definitions defined in GB 38755-2019

as well as the followings apply.

3.1 voltage stability

The ability that after the power system is subjected to small or large disturbances, the

system voltage can be maintained or recovered to the allowable range without voltage

collapse.

[Source. GB 38755-2019, 2.2.2]

3.2 voltage collapse

The process that when the system is in a state of voltage instability, the load continues

to try to obtain more power (active or reactive) by increasing the current, resulting in a

large-scale voltage drop in the system.

3.3 steady-state voltage stability

The ability that after the power system is subjected to a small disturbance, all the

busbars of the system maintain a stable voltage.

[Source. GB 38755-2019, 2.2.2.1]

3.4 transient voltage stability

The ability that after the power system is subjected to a large disturbance, all the busbars

of the system maintain a stable voltage.

[Source. GB 38755-2019, 2.2.2.2]

3.5 long-term voltage stability

The ability that after the power system is greatly disturbed, all the busbars of the system

maintain a stable voltage in the long-term process.

3.6 renewable energy station

All equipment below the grid connection point of the wind farm or photovoltaic power

station centrally connected to the power system.

NOTE. It includes transformers, busbars, lines, converters, energy storage, wind turbines,

photovoltaic power generation systems, reactive power adjustment equipment and auxiliary

equipment.

[Source. GB 38755-2019, 2.11]

3.7 multi-infeed DC short-circuit ratio; MSCR

The ratio of the short-circuit capacity of the DC-fed converter bus to the equivalent DC

power after considering the influence of other DC circuits.

[Source. GB 38755-2019, 2.6]

4 General principles

4.1 Voltage stability evaluation is an important part of power system safety and stability

evaluation. The safety and stability evaluation of the power system shall include the

voltage stability evaluation.

4.2 The voltage stability evaluation of power system generally includes steady-state

voltage stability evaluation and transient voltage stability evaluation. In the post-fault

operation mode, the long-term voltage stability evaluation shall be used when the

steady-state voltage stability margin is low or the long-term component dynamic

characteristics such as over-excitation of the unit need to be considered.

5.2 Criteria for transient voltage stability evaluation

In the transient process after the power system is subjected to large disturbances, the

load bus voltage shall recover to above 0.80p.u. within 10s (this document selects the

average rated voltage as the voltage reference value). In new energy-intensive areas, it

shall be ensured that new energy stations are not disconnected from the grid and do not

repeatedly enter low voltage ride through. Care shall be taken to distinguish the voltage

drop near the oscillation center due to generator power angle oscillation and the voltage

drop due to transient voltage instability.

5.3 Criteria for long-term voltage stability evaluation

In the long-term process after the power system is subjected to large disturbances, the

load bus voltage shall be maintained or restored to above 0.90p.u. When judging by

simulation calculation, the response of long-term dynamic components and links shall

be included to reach a new balance point.

5.4 Selection of monitoring bus

In practical application of criteria for transient and long-term voltage stability

evaluation, the voltage monitoring point shall be selected at the load bus.

6 Evaluation method for steady-state voltage stability

6.1 Basic requirements

6.1.1 Before calculating and evaluating steady-state voltage stability, the simulation

model and calculation conditions shall be clarified, including partition division,

generator and new energy model, maximum reactive power that generator can generate,

generator output distribution method, terminal voltage, bus voltage, DC model, DC

power adjustment method, reactive power compensation model, transformer model,

load model, load growth method.

6.1.2 Simulation calculation shall use steady-state model. Among them, the generator

generally adopts the PV node model with upper and lower limit of reactive power. The

load shall use a constant power model. Based on the actual situation, the ZIP model can

also be used. The scale configuration can be carried out. Z, I, and P represent constant

impedance, constant current, and constant power, respectively.

6.1.3 The load growth mode has a great influence on the steady-state voltage stability.

It shall be determined according to the actual situation, or a typical growth method shall

be adopted. If the P-U curve is calculated, the load growth mode is constant power

factor growth. Calculate the U-Q curve. The load growth mode is the same as the active

power and the reactive power growth. The power output shall be determined according

to the load growth and the actual output distribution of the power grid.

Figure 2 -- Normalized U-Q curves for constant source, reactance, and active

power

6.2.2 P-U curve

6.2.2.1 The load growth method is to increase the load of the area or bus through a

certain load growth method to obtain the P-U curve. This method is suitable for

evaluating the voltage stability of the load area.

6.2.2.2 Section flow growth method. The sending end adopts to increase the output of

the unit. The receiving end generally adopts the increased load. It is also possible to

reduce the output of the receiving end unit to increase the power flow of the sending

and receiving end sections to obtain the P-U curve. This method is suitable for

evaluating the maximum power transmission and reception capacity in the region.

6.2.2.3 The "inflection point" of the P-U curve is the voltage collapse point. The steady-

state voltage stability of the system is evaluated by calculating the regional active power

margin Kvp.

6.2.3 U-Q curve

6.2.3.1 The "Imaginary synchronous camera method" is to imaginatively place a

synchronous camera at the monitoring bus that is not limited by reactive power output.

The U-Q curve is obtained by changing the voltage of the bus.

6.2.3.2 The load growth method and the section load flow growth method can also be

used to obtain the U-Q curve.

6.2.3.3 The lowest point of the U-Q curve is the voltage collapse point. The static

voltage stability of the system is evaluated by calculating the reactive power margin

Kvq.

6.2.4 Multi-infeed DC short-circuit ratio

By defining the multi-infeed influence factor (MIIF), it is used to describe the strength

of the interaction between the DC subsystems of the multi-infeed AC-DC system.

Accordingly, the calculation method of the multi-infeed DC short-circuit ratio MISCRi

is obtained as formula (3).

Where,

i - The DC link number;

Saci - The short-circuit capacity of the DC infeed commutation bus i, in megavolt-

7.1.6 The renewable energy station shall adopt an electromechanical or electromagnetic

transient equivalent model that considers the topology, reactive power compensation

and station-level control in the station.

7.1.7 Flexible AC and DC transmission equipment such as flexible DC transmission

system, static var generator (SVG), controllable series compensation device (TCSC),

unified power flow controller (UPFC) shall adopt electromechanical transient model or

more detailed electromagnetic transient model.

7.1.8 A simulation calculation program that can simulate the transient process of the

power system in detail shall be used. The program shall carry out simulation analysis

for different faults and give the transient process of the power system after the fault.

For large-capacity DC drop-point power grids, when the DC response characteristics

have a great influence on the system voltage stability, the electromechanical-

electromagnetic transient hybrid simulation or full electromagnetic transient simulation

can be used for verification.

7.2 Calculation method and process

7.2.1 Determine the operation mode that needs studying. Calculate to obtain the steady-

state operation.

7.2.2 Determine the dynamic element model parameters corresponding to the operation

mode.

7.2.3 The fault set to be studied shall be selected according to GB 38755-2019.Select

the electrical quantity to be monitored.

7.2.4 Conduct time-domain simulation calculations for selected faults. Based on the

criteria for transient voltage stability evaluation, perform transient voltage stability

judgment. Determine critical fault sets and grid weaknesses.

7.2.5 Determine the transmission capacity subject to transient voltage stability through

operation mode adjustment. Propose the safety and stability control strategy and grid

optimization scheme.

8 Evaluation method for long-term voltage stability

8.1 Basic requirements

8.1.1 It is mainly used to analyze voltage stability under the action of slow-response

dynamic components and control devices, including boiler-turbine slow speed control,

generator (including controller) overcurrent and overexcitation limit, constant

temperature load, secondary control of voltage and frequency (such as on-load voltage

regulating transformer, parallel controllable capacitor).

......

Source: Above contents are excerpted from the PDF -- translated/reviewed by: www.chinesestandard.net / Wayne Zheng et al.