The development of the SA grid code on Power Quality emission

Dr. Gerhard Botha

2017/08/08

Overview

• What is the Grid Code?

• What is Power Quality?

• Power Quality Management Principles

• Differences

• Challenges

What is the Grid Code?

• The Grid Code is intended to establish the reciprocal obligations of industry participants around the use of the TS and operation of the interconnected power system (IPS).

• The Grid Code shall ensure the following:

– That accountabilities of all parties are defined

– for the provision of open access to the TS

– That minimum technical requirements

– Obligations defined

– relevant information

– cost drivers and pricing principles are transparent

• The Grid Code defines what is understood by non-discrimination through the definition of consistent and transparent principles, criteria and procedures.

… And the Distribution Code

• To set the basic rules of connecting to the Distribution System.

• To ensure that all users of the Distribution System are treated in a non-discriminatory manner.

• To specify the technical requirements to ensure the safety and reliability of the Distribution System.

What is Power Quality?

• Any deviation of the voltage waveform from a perfect sinusoid at unity magnitude

– Voltage regulation

– Flicker

– Harmonics

– Voltage dips

– Interruptions

– RVC

• Three-phase waveforms must be of equal magnitude and separated by 120°

– Voltage unbalance

Background

• Compatibility between electrical system and equipment

• NRS 048-series

– Limits for harmonics, flicker and unbalance, RVCs

– Dips, interruptions managed

– 95% of the time at 95% of sites

• Based on IEC 61000-series

• EN 50160

• Point of Common Coupling (PCC)

• Point of Connection (POC)

Background …continued

System wide compatibility principles

Apportionment

Power Quality Management

• Utilities

• Managed through license conditions– NRS 048-2

• Power Quality Directive– Draft (March 2002)

• Governance code– Responsibilities

– Processes

• Customers

Management Principles

• Utility to manage the quality of its product:

– Limits

– Planning levels

– Specify emission limits

– Communication

• Customers to manage both their

– Immunity to QOS deviations

– Emissions as specified by Utility

• Immunity and Emissions management applies to Generators/IPPs as well

• Equipment manufacturers to provide appropriate equipment

NRS048-2

NRS048-4

IEC61000-3-6/7/13

Contracts / Meetings

Differences

• What is the difference?

• Both customers of the wires business

• Treat the same– As far as possible

• Planning principles

Key Requirements

9. Power Quality

(1) The following requirements shall apply to all categories of RPPs.

(2) Power quality and voltage regulation impact shall be monitored at the POC and shall include an assessment of the impact on power quality from the RPP concerning the following disturbances at the POC:

(a) voltage fluctuations:

(i) rapid voltage changes

(ii) flicker

(b) high-frequency currents and voltages:

(i) harmonics

(ii) inter-harmonics

(iii) disturbances greater than 2 kHz.

(c) unbalanced currents and voltages:

(i) deviation in magnitude between three phases

(ii) deviation in angle separation from 120° between three phases.

(d) RPP will generally follow the supply network frequency:

(i) Any attempt by the RPP to change the supply frequency may result in severe

distortion of the voltage at the POC, PCC and other points in the network.

Key Requirements

(3) Power quality and voltage regulation impact shall be monitored at the POC.

(4) Voltage and current quality distortion levels emitted by the RPP at the POC shall not exceed the apportioned limits as determined by the relevant NSP. The calculation of these emission levels shall be based on international and local specifications. The allocation shall be fair and transparent.

(5) The RPP generator shall ensure that the RPP is designed, configured and implemented in such a way that the specified emission limit values are not exceeded.

(6) The maximum allowable voltage change at the POC after a switching operation by the RPP (e.g. of a compensation devices) shall not be greater than 2%.

(7) The RPP can assume that the network harmonic impedance at the POC will be less than 3 times the base harmonic impedance for the range of reference fault levels at the POC, i.e. the network harmonic impedance shall not exceed a harmonic impedance of:

Z h = 3 ∙𝑉𝑙𝑙2

𝑆𝑠𝑐∙ ℎ

where h is the harmonic number, V is the nominal voltage in kV, and S is the fault level in MVA.

Challenges

• Major differences between South African and European networks

– Base load generation location

– Transmission line lengths

– Overhead vs. underground

– Use of shunt capacitors

– Interconnections to other countries

• Weak networks where significant generators want to connect

– SCR

Differences

• Commercial Agreements– Performance levels

– Utility to manage risk(s), e.g. impedance envelope

– Current performance vs. Future performance

• Apportioning– RPP emission limits slightly smaller due to additional capacity created

by generator

– IEC 61000-series

• Emissions– Expect more emissions at higher frequencies, especially due to typical

generator power electronic equipment.

Challenges

• QOS is not an exact science

– Based on statistically derived values (historic from e.g. Europe)

– Statistical analysis for comparison

– Simulation models lack detail

• Process for temporary exemptions

– Principles agreed upon

– Automatic provided no significant risk to NSP

Illustration of Three-Times Impedance

• Clause 6

𝑍𝑏 = 𝑘 ∙𝑉𝑙𝑙2

𝑆𝑠𝑐• Where k = 3

• Three times applies at harmonics– Not at fundamental

• Also see NRS048-4

• Double-edged0

20

40

60

80

100

120

140

160

-100 100 300 500

Imp

ed

ance

[Ω

]

Frequency [Hz]

Zb

3*Zb

Impedance 95th Percentile(Busbars V < 132kV, simulated, base case files)

0

20

40

60

80

100

120

140

160

180

200

0 500 1000 1500 2000 2500

Zb (1x)

Zb x3

Z_env (proposal)

Z_95th (V<=132kV)

Single site comparisons

• Include contingencies

Generic envelope

0 500 1000 1500 2000 25000

20

40

60

80

100

120

140

160

180

20095th Percentile vs linear impedance

Frequency [Hz]

Norm

alis

ed I

mpedance [

Zb(5

0)=

1 ]

Zbase

2xZbase

3xZbase

Zenvnew

Zmv95

th

Zhv95

th

Voltage emission, Current emission and Impedance• Assuming network impedance at time of connection is similar to Example 2:

– Current at the 9th harmonic, less than the emission limit will result in a voltage emission exceedence.

• Eskom strengthens the network and the impedance becomes similar to Example 1:

– The current emission at the 37th exceeded the limit, will now result in a voltage emission exceedence.

h 5 9 37

V_limit

[%] 0.50% 0.10% 0.10%

I_limit

[A] 12.70 1.41 0.34

I_meas

[A] 10.00 0.50 0.70

V_meas (2)

[%] 0.19% 0.16% 0.00%

V_meas (1)

[%] 0.21% 0.00% 0.15%

Power Quality Guideline

• Included in RPP Grid Code

• Requirements for RPPs

– Not NSP / Service Provider

• Measurement principles

– IEC 61000-4-30 Class A

– 7 days

• Minimum current harmonic emission limit

– 0.1 A

Power Quality Guideline

• Explain principles

• Simulation based compliance

– Emissions (voltage and current)

– Resonance conditions created / shifted (simulated network harmonic impedance)

• Measurement based compliance

– Emissions (current and voltage)

• Full compliance

– No problems detected

• Partial compliance

– No serious problems detected, the risk to QOS levels acceptable and the IPP will be allowed to connect or continue to operate while mitigation being implemented

• Non-compliance

– Serious problems detected requiring mitigation before connection or further operation

Proving Compliance

• Cigre 468: Review of Disturbance Emission Assessment Techniques

• Small RPPs

– < 5 MVA

– IEC 61727 / IEC 61000-3-7

• Group Harmonic Distortion

𝐻𝐷𝐼𝑙𝑖𝑚𝑖𝑡𝐺𝑥 = ℎ=𝑛𝑙𝑜𝑤

𝑛𝑢𝑝

𝐼𝑙𝑖𝑚𝑖𝑡 ℎ2

Where the lower and upper bounds of the calculation are given as follows (as defined in the RPP Grid Code version 2.9):

Harmonic Distortion Group 1: nlow = 2 and nup = 13

Harmonic Distortion Group 2: nlow = 14 and nup = 25

Harmonic Distortion Group 3: nlow = 26 and nup = 39

Harmonic Distortion Group 4: nlow = 40 and nup = 50

Group Harmonic Distortion

Proving Compliance

• Zero voltage emission

• Correlation

• Voltage scaling

– 3 times impedance

Key Requirements

9. Power Quality

(1) RPPs of categories A1, A2, A3, and category B with rated power ≤ 5MVA shall comply with the requirements as detailed in Appendix 13 - RPP Power Quality Compliance Guideline.

(2) The following requirements shall apply to the following RPP categories:

(a) Category B with rated power above 5 MVA,

(b) All category C RPPs.

(3) For grid code compliance the RPP shall monitor and report on power quality using an IEC 61000-4-30 Class A power quality monitoring device. The reporting will be done to prove compliance at the POC against the NSP requirements specified in the system supply agreements. The power quality parameters to be reported on include:

(a) flicker

(b) harmonics

(c) unbalanced voltages

Key Requirements

9. Power Quality

(4) Voltage and current quality distortion levels emitted by the RPP at the POC may not exceed the apportioned limits as determined by the relevant NSP. The calculation of these emission levels shall be based on international and local specifications. The allocation shall be fair and transparent. For current harmonics, allowance is made for individual harmonics to exceed specified limits by up to 50%, provided the Group Harmonic Distortion limits for the four bandwidth groupings (specified in Appendix 13: RPP Power Quality Compliance Guideline) are met.

(5) Appendix 13 provides guidance to RPP generators on a method to prove compliance to the Power Quality requirements as set out by the NSP.

(6) The RPP generator shall ensure that the RPP is designed, configured and implemented in such a way that the specified emission limit values are not exceeded.

Key Requirements

9. Power Quality

(7) The NSP will manage any voltage harmonic compatibility level exceedances due to the network harmonic impedance at the POC being more than 3 times the base harmonic impedance for the range of reference fault levels at the POC in line with its license conditions. The 3 times base harmonic impedance is calculated using the following equation:

Z h = 3 ∙𝑉𝑙𝑙2

𝑆𝑠𝑐∙ ℎ

where h is the harmonic number, V is the nominal voltage (line-to-line) in kV, and S is the fault level in MVA. The angle of the network harmonic impedance is such that the impedance may range from fully inductive to fully capacitive.

(8) In order to assist with the maximum resonance of 3 times as per clause (6) above, no RPP may connect equipment, e.g. shunt capacitor banks, that will cause a resonance of more than 3 times at the POC at any frequency.

Key Requirements

9. Power Quality

(9) The Transmission and Distribution network service providers shall use reasonable endeavours to furnish the RPP with a reliable and continuous connection for the delivery of electrical energy up to the POC. The network operators do not guarantee that the continuity and voltage quality of the connection will always be maintained under all contingencies. It is therefore incumbent upon the RPP to take adequate measures to protect the RPP facility against any losses and/or damage arising from frequency deviations, connection/supply interruptions, voltage variations (including voltage dips), voltage harmonics, voltage flicker, voltage unbalance, voltage swells and transients, undervoltages and overvoltages in the connection. It is also incumbent upon the RPP to take such necessary measures so as not to cause any damage to the TS and DS.

Conclusions

• Significant learning curve

• Started with customer management principles

• Adapted (partially) to measurement certainty

• Guideline available to all

• Further updates